BBH^^BSHaH^Ml Hije ^, ^. ^m pbrarg OH 566 K14 t ' '. i\ i»u ""iKtlirM??!?!*.,^,!,*!^ UNIVERSITY LIBRARIES S00570810 L 32656 Date Due 23f.1ay34' may SI 19May3a mY^40r ljr 4 7S ^Apr'50 |> 15]un'S0 4 4]un52ifc gii^'ii^U 11 1:VV ) ■" ^r.c MAR - 5 1980 VI AY 2 4 «¥6 4a3i READINGS IN EVOLUTION, GENETICS, AND EUGENICS THE UNIVERSITY OP CHICAGO PRESS CHICAGO. ILLINOIS THE BAKER & TAYLOR COMPANY NEW YORK THE CAMBRIDGE UNIVERSITY PRESS LONDON THE MARUZEN-KABUSHIKI-KAISHA TOKYO, OSAKA, KYOTO, FUKUOKA, SENDAI THE MISSION BOOK COMPANY SHANGHAI READINGS IN EVOLUTION, GENETICS, AND EUGENICS By HORATIO HACKETT NEWMAN Professor of Zoology in the University of Chicago ^O'^rOV'- ^^ K<^\ ^v^; c. ^o^ THE UNIVERSITY OF CHICAGO PRESS CHICAGO, ILLINOIS Copyright 192 i By The University of Chicago All Rights Reserved Published October 192 1 Composed and Printed By The University of Chicago Press Chicago, Illinois, U.S.A. THIS VOLUME IS AFFECTIONATELY DEDICATED TO MY MOTHER 3265S PREFACE This book has been prepared to meet a specific demand, long felt here and elsewhere, for an account of the various phases of evolu- tionary biology condensed within the scope of one volume of moderate size. The present writer has now for sixteen successive years pre- sented in lecture form to large classes of students the subjects of evolution, genetics, and eugenics. Never have we been able to find a single book that would cover the required ground. In fact it has been necessary to require, or at least to recommend, as many as three books. It is believed that the present book will furnish ade- quate reading material for a major or a semester course in evolutionary biology. Some supplementary reading may be necessary in case an instructor wishes to emphasize one or more phases of the subject; but for a first course in the subject we believe that all of the essential reading material will be found within the text itself. An effort has been made to present the subject in the best peda- gogical order. After a general introduction, a rather long chapter appears in which the whole history of the development of evolution- ary science is outlined, together with the names and contributions of the leading evolutionists. Part II is a presentation of the evi- dences of organic evolution, beginning with the bodies of evidence most definite and direct, and ending with the less definite and more controversial. Part III deals with causo-mechanical theories of evolution with Darwinism as the central topic. Part I\' concerns itself with genetics or modern experimental evolution, and Part V with eugenics, or genetics as applied to human improvement. The book consists largely of excerpts, some long and some short, from both the older classical evolutionary writers and the modern writers. Our aim has been to select the most significant or character- istic passages from each author. In most cases this ideal has been attained, but it has sometimes happened that we have had to make our selection of material to meet a real need in the book, and accord- ingly have selected from an author a passage he himself might not consider particularly characteristic of his work. We have succeeded, nevertheless, in welding together out of a collection of isolated chai^ers and passages what seems to us to be a close approach to a coherent unit. Unification has been accomplished by the aid of editorial connecting passages, introductory statements, criticisms, and sum- maries. In certain cases it became necessar}^, for a variety of reasons, • • Vll viii PREFACE for the editor to write short chapters on certain topics that were not presented in the available literature in sufficiently brief compass or in sufficiently non-technical language. The one-man textbook is only too often written to emphasize the author's pet theories and is likely to be unduly biased. The present work is completely non-partisan. It consists of the writ- ings of many authors and presents many diverse theories. The student is left to balance the various views one against another and to form his own judgment. It is very unfortunate, but none the less true, that even in these scientific days, the subject of evolution has a bad name in many communities, and in many educational institutions with religious affiliations. The mistake is made of supposing that evolution and religion are diametrically opposed. The present writer has been at some pains to make it clear that evolution and religion are strictly compatible. We teachers of evolution in the colleges have no sinister designs upon the religious faith of our students. While this book is intended primarily for a college textbook, we have also had in mind the general reader. Apart from a few of the more technical details, the text seems to us very readable. The language of the great classic writers — Darwin, Wallace, Romanes, De Vries, Le Conte — is simple and lucid. Among recent biological books few are written so freshly and vividly as those of Professor J. Arthur Thomson. The clearness and scientific accuracy of Conklin, Saleeby, Guyer, Walter, Lull, Osborn, the Coulters, Downing, Shull, Tayler, Popenoe, Johnson, and others, are familiar to American biologists. Scrupulous care has been taken to verify all passages quoted, but it is hardly likely that, in so large a mass of material, all errors shall have been avoided. The author and the publishers would welcome as a favor any suggestions or corrections submitted by interested readers. A list of fifty books from which material has been quoted is given on pages 510-512. To the authors and publishers of these books and monographs we wish herewith to tender our grateful acknowledg- ments for their generosity and co-operation. A considerable amount of material for which permission to reprint had been granted fails to appear in the present volume. It is hoped to incorporate this material in an appendix to a later edition, or else to use it in the form of a small volume of supplementary readings. H. H. N. August 15, 1921 TABLE OF CONTENTS PAGE List of Illustrations xv-xviii PART I. INTRODUCTORY AND HISTORICAL Chapter I. Introduction 3 "vlVhat Organic Evolution Is — Definitions 3 ^yrhe Modern Attitude as to the Truth of the Evolution Doctrine . 5 'VWhat Organic Evolution Is Not 8 Chapter II. Historical Account of the Development of the Evolution Theory. H.H.N 10 Evolution among the Greeks 11 Post-x\ristotelians 14 The Early Theologians 14 The Revival of Science 15 The Great Naturalists of the Eighteenth Century 16 X Lamarck 18 Cuvier and Geofifroy St. Hilaire 21 Catastrophism and Uniformitarianism 22 The Reawakening of the Evolution Idea 23 Charles Darwin 24 ">■■ Summary of Darwin's Theories 25 Contemporary Opinion Regarding the Validity of Darwin's Views 27 Isolation Theories ^^ Orthogenesis Theories H Mutation or Heterogenesis Theories 36 The Rise and Vogue of Biometry 3^ \^Modern Experimental Evolution 39 ^V^Iendel's Laws -^^ Hybridization and the Origin of Species 43 Neo-Mendelian Developments 43 heredity and Sex 44 Chapter III. The Relation of Evolution to Materialism. Joseph Le Conte 4o PART II. EVIDENCES OF ORGANIC EVOLUTION Chapter IV. Is Organic Evolution an Established Principle ? H.H.N "^^ Chapter V. Evidences from Palaeontology 61 Strength and Weakness of the Evidence ^^ ix >v X TABLE OF CONTENTS _ PAGE \pther Opinions as to the Adequacy of the Evidences from Palae- ontology 62 What Fossils Are and How They Have Been Preserved .... 63 Fossils Classified 63 On the Conditions Necessary for Fossilization 64 On the Lapse of Time during Which Evolution Is Believed to Have Taken Place 67 On the Principal General Facts Revealed by a Study of the Fossils 69 Fossil Pedigrees of Some Well-known Vertebrates 70 Pedigree of the Horse 70 Pedigree of the Camels. W. B. Scott 73 Evolution of the Elephants. A. Franklin Shull 76 Chapter VI. The Evolution of Man: Palaeontology. Richard Swann Lull 81 Origin of Primates 81 Origin of Man 82 Fossil Man 84 Evidences of Human Antiquity 94 Future of Humanity 95 Chapter VII. Evidences from Geographic Distribution , . 97 Some of the More Significant Facts about the Distribution of Animals loi The Fauna of Oceanic Islands. George John Romanes . . . loi The Fauna of Madagascar and New Zealand. A. R. Wallace . no The Distribution of Marsupials. A. R.Wallace in The Distribution of Birds. A. R.Wallace *. 112 Summary of Mammalian Dispersal. Hans Gadow . . . . 114 Summary of the Argument for Evolution as Based on Geographic Distribution 115 Chapter VIII. Evidences from Classification 117 The Principles of Classification. A. F. Shull 117 The Method of Classification. Charles Darwin 120 What Is a Species ? 121 Chapter IX. Evidence from Blood Tests. W.B.Scott . . . 124 Chapter X. Evidences from Morphology (Comparative Anat- omy). George John Romanes 129 Chapter XL Evidences from Embryology 164 The Facts of Reproduction and Development 164 Outline of Animal Development. D. S. Jordan and V. L. Kellogg . 165 Chapter XII. Critique of the Recapitulation Theory. W. B. Scott 173 TABLE OF CONTENTS xi PART III. THE CAUSAL FACTORS OF ORGANIC EVOLUTION PAGE Chapter XIIL Introductory Statement 185 What We Owe to Darwin 186 Chapter XIV. The Background of Darwinism-— Adaptations. H.H.N 188 Laws of Adaptation 192 Adaptations Classified 195 Some Special Adaptations 196 Parasitism and Degeneration 197 Color in Animals 200 Chapter XV. The Background of Darwinism — Continued . . 206 The Web of Life. /. Arthur Thomson 206 Chapter XVI. Natural Selection. Charles Darwin . . . . 219 Foundation Stones of Natural Selection 219 Darwin's Own Estimate as to the Role of Natural Selection in Evolution 219 Effects of Habits and the Use or Disuse of Parts; Correlated Variation; Inheritance 220 Darwin's Idea of the Causes Responsible for the Origin of Domes- tic Races 221 Darwin's Idea of the Origin of Varieties, Species, and Genera in Nature 221 The Term "Struggle for Existence" Used in a Large Sense . . 222 Geometrical Ratio of Increase ■ ^^S Natural Selection ; Or the Survival of the Fittest 223 Sexual Selection 230 Illustrations of the Action of Natural Selection, or the Survival of the Fittest 232 Summary of Chapter on Natural Selection 233 Difficulties and Objections to Natural Selection as Seen by Darwin 236 Chapter XVH. Critique of Darwinism 245 Summary of Darwin's Natural-Selection Theory. Vernon L. Kellogg 2-^5 Objections to Darwinism 247 Defense of Darwinism ^52 General Defense of Darwinism. /. L. Tayler 253 Experimental Support of the Effectiveness of Natural Selection . 256 The Present Status of Natural Selection 2 58 The Relation of Mendelism and the Mutation Theory to Natural Selection. C. C. Nutting 258 xii TABLE OF CONTENTS PAGE Chapter XVIII. Other Theories of Species-Forming ... 263 Theories Auxiliary to Natural Selection 263 Weismann's Theory of Panmixia 263 Weismann's Theory of Germinal Selection 265 Roux's Theory of Intraselection or the Battle of the Parts . . 268 Coincident Selection or Organic Selection 268 Isolation Theories 269 Theories Alternative to Natural Selection 273 Chapter XIX. A New Composite Causo-Mechanical Theory OF Evolution (the Tetr akinetic Theory), Henry Fairfield Oshorn 275 The Energy Concept of Life 275 The Four Complexes of Energy 280 PART IV. GENETICS Chapter XX. The Scope and Methods of Genetics . . . . 287 Definitions 287 The Scope and Methods of Genetics 287 The Importance of the Cell Theory in Genetics 289 Chapter XXL The Bearers of the Heritage (an Account of the Cellular Basis of Heredity). Michael F. Guy er 290 Chapter XXII. Variation. Ernest Brown Babcock and Roy Elwood Clausen 307 Chapter XXIII. Are Acquired Characters (Modifications) Hereditary? 323 Misunderstandings as to the Question at Issue. /. Arthur Thom- son 323 The Inheritance or Non-Inheritance of Acquired Characters. Edwin Grant Conklin 330 The Other Side to the Question- 336 A Possible Mechanism for the Transmission of Acquired Characters. Michael F. Giiyer 338 Chapter XXIV. The Mutation Theory 346 New Species (Mutants) of Oenothera. Hugo De Vries .... 348 Summary of De Vries's Mutation Theory. Thomas Hunt Morgan . 355 Criticisms 359 Causes of Mutations 360 Chapter XXV. Biometry (the Statistical Study of Variation and Heredity). H.H.N 365 The Statistical Study of Variation 365 Bimodal and Multimodal Curves 368 The Coefficient of Correlation 369 Statistical Study of Inheritance. Edwin Grant Conklin . . . 370 TABLE OF CONTENTS xiii PAGE Chapter XXVI. Heredity in Pure Lines. H.H.N 376 Are Determiners (Genes) Constant or Variable ? 378 Chapter XXVII. Mendel's Laws of Heredity 380 Mendel's Life and Character. /. Arthur Thomson 380 Mendel's Discoveries. /. Arthur Thomson 380 Mendel's Explanations. /. M. Coulter and M. C. Coulter . . . 386 Illustrations of Simple Mendelian Inheritance in Both Animals and Plants. J. Arthur Thomson 393 Chapter XXVIII. The Physical Basis of Mendelism. Ernest B. Bahcock and Roy E. Clausen 401 Chapter XXIX. Neo-Mendelism in Plants. John M. Coulter and Merle C. Coulter 413 Presence and Absence Hypothesis 413 Blends 415 The Factor Hypothesis 417 Chapter XXX. Neo-Mendelian Heredity in Animals. H.H.N. 429 Illustrations of the Factor Hypothesis , . 429 The Factorial Analysis of Color in Mice 429 Different Kinds of Albinos 430 Castle's Guinea Pigs 43 1 Chapter XXXL Sex-linked and Other Kinds of Linked Inheritance in Drosophila and Other Species. William E. Castle 433 Drosophila Type and Poultry Type of Sex-linked Inheritance . . 436 Chapter XXXII. Linkage and Crossing-Over. William E. Castle 441 Chapter XXXIII. Sex Determination. H.H.N 449 Various Theories of Sex Determination 449 The Chromosomal Mechanism of Sex Determination .... 450 Sex Determination in Parthenogenetic Species 451 The Poultry Type of Sex Determination 45^ Sex Differentiation 453 PART V. EUGENICS Chapter XXXIV. The Inheritance of Human Characters, Physical and Mental. Elliot R. Downing 459 Chapter XXXV. Human Conservation. Herbert E. Walter . .473 How Mankind May Be Improved 473 More Facts Needed • -^'-^ Further Application of What We Know Necessary 474 The Restriction of Undesirable Germplasm 475 Control of Immigration 475 xiv TABLE OF CONTENTS PAGE More Discriminating Marriage Laws 477 An Educated Sentiment 477 Segregation of Defectives 478 Drastic Measures 479 The Conservation of Desirable Germplasm 480 By Subsidizing the Fit 480 By Enlarging Individual Opportunity 481 By Preventing Germinal Waste 481 Who Shall Sit in Judgment ? 482 Chapter XXXVI. Eugenics and Euthenics. Paul Popenoe and Roswell H. Johnson 484 Chapter XXXVII. The Promise of Race Culture. Caleb Williams Saleeby 497 Bibliography 510 Index 515 LIST OF ILLUSTRATIONS PAGE 1. Total Geologic Time Scale 68 2. Feet AND Teeth IN Fossil Pedigree OF THE Horse ... 72 3. Four Stages IN THE Evolution OF THE Cameline Skull . . 74 4. Four Stages in the Evolution of the Cameline Fore Foot , 7 5 5. Evolution of Head and Molar Teeth of Mastodons and Elephants 77 6. Skull of Java Ape-Man, Pithecanthropus erectus .... 87 7. Jaws of Man and of the Apes 88 8. Restoration of Prehistoric Men 90 9. Neanderthaloid Skull of La Chapelle-aux-Saints ... 91 10. Skeleton of Neanderthal Man 92 11. Skeleton OF Seal 130 12. Skeleton OF Greenland Whale 132 13. Paddle of Whale Compared with Hand of Man . . . . 133 14. Wing of Reptile, Mammal, and Bird 134 15. ^KE'LETO^ O'F Dinornis gravis 137 16. Hermit Crabs Compared with Cocoa-nut Crab .... 139 17. Rudimentary or Vestigial Hind Limbs of Python . . . 141 18. Apteryx australis 142 19. Illustrations of the Nictitating Membrane in the \' arious Animals ^47 20. Rudimentary, or Vestigial and Useless, Muscles of the Human Ear 148 21. Portrait OF a Young Gorilla i4Q 22. Lower Extremities OF a Young Child 150 23. An Infant, Three Weeks Old, Supporting Its Own Weight FOR Over Two Minutes 151 24. Sacrum OF Gorilla Compared with That OF Man . . . 152 25. Diagrammatic Outline of the Human Embryo When about Seven Weeks Old ^S3 26. Front AND Back View OF Adult Human Sacrum . . . . i53 27. Appendix vermiformis ii'a/o(/a/>/fma. . 315 54. Schematic Curves of Head Height in Hyalodaphnia as Grown IN Media OF Three Different Food Values . . 316 55. Climatic Effects UPON Plumage IN Pigeons 317 56. Effects of Injections into Ovary of Scrophularia. . . . 319 LIST OF ILLUSTJL\TIONS xvii PAGE 57. Oenothera lamarckiana 347 58. A Series Showing Oenothera lamarckiana and Several of Its Mutants Growing Side by Side 352 59. Diagram Showing in Condensed Form the Genealogy of THE Oenothera lamarckiana Family and Its Various Mutants 357 60. Some Divergent Types (Mutations) of Beetles Produced BY Subjecting the Germ Cells to External Influences 361 61. Two Plants of Onagra biennis, Showing the Effect of Injections of Zinc Sulphate into the Ovule .... 362 62. Polygon of Variation for the Total Number of Scutes in THE Nine Bands of the Armadillo 366 63. BiMODAL Polygon Plotted from Data on the Earwig . . 369 64. Correlation Table of 400 Plants of Sixty-Day Oats . 370 65. Diagram of Galton's "Law of Ancestral Inheritance" . 372 66. Scheme to Illustrate Galton's "Law of Filial Regression " 374 67. Diagram Illustrating Behavior of Chromosomes in Mendel's Cross of Tall and Dwarf Peas 388 68. Diagram Illustrating Behavior of First Hybrid Genera- tion When Inbred 389 69. Diagram Illustrating Dihybrid Ratio 392 70. Diagram Showing the Characteristic Pairing, Size Rela- tions, and Shapes of the Chromosomes of Drosophila ampelophila 402 71. Diagram of Mitosis in a Species Having Four Chromo- somes in Its Cells 4^4 72. The Reduction Division as Represented for a Species Whose Diploid Number Is Four 406 73. Diagram of Chromatin Interchange bet\\ten Homolo- gous Members of a Pair of Chromosomes 408 74. Diagram Showing Consequences of Independent Segrega- tion OF Chromosomes in Drosophila ampelophila .... 409 75. Diagram to Show Chromosome Relations in the Inherit- ance OF Sex in Drosophila ampelophila 4 1 1 76. Diagram Showing How the Original Scheme Must Be Modified to Satisfy the Presence and Absence Hypothe- sis ^'^ 77. Diagram Showing How Presence and Absence Scheme Is Actually Used *^^^ 78. Diagram Illustrating Blending Inheritance 416 xviii LIST OF ILLUSTRATIONS PAGE 79. Diagram Illustrating Complementary Factors . . . . 418 80. Diagram Illustrating Behavior OF iNinBiTORY Factor . . 421 81. Diagram Sho\\tng Some Possible Combinations in Fa When Fi OF Figure 80 Is Inbred 422 82. Diagram Showing the Heterozygote Situation . . . . 422 83. Diagram Illustrating the Action of a Supplementary Factor 423 84. Diagram Illustrating Nilsson-Ehle's Explanation of the 15:1 Ratio in F2 of Hybrid between Red- and White- grained Wheat 425 85. Another Method of Visualizing Nilsson-Ehle's 15:1 Ratio 426 86. Diagram of Nilsson-Ehle's 6s : i Ratio . . . . . . . 427 87. Sex-Linked Inheritance of White and Red Eyes in Droso- phila 434 88. Reciprocal Cross to That Shown in Figure 87 . . . .435 89. Drawing Showing the Four Pairs of Chromosomes Seen in THE Dividing Egg of Drosophila 436 90. Diagram Showing the Location in the Four Paired Chromo- somes OF Drosophila, of the Genes for Various Mendeliz- ING Characters, as Determined by Morgan 437 91. Sex-Linked Inheritance of Barred and Unbarred (Black) Plumage in Poultry 438 92. Reciprocal Cross to That Shown in Figure 91 ... . 439 93. Diagram to Show the Mechanism of Crossing Over . . . 440 94. An Armadillo Egg about Six Weeks after Fertilization, Showing the Quadruplet Foetuses 451 95. A Typical Opposite-Sexed Pair of Cattle Twins .... 455 96. A Pedigree of Brachydactylism 460 97. A Pedigree Showing Transaussion of Cataract . . . .461 98. A Pedigree Showing Heredity of Feeble-Mindedness . . 463 99. Another Pedigree Showing Heredity of Feeble-Minded- ness 464 100. Pedigree Showing Heredity of Insanity 465 loi. Pedigree Showing Heredity of Insanity and Neurotic Tendency 465 PART I INTRODUCTORY AND HISTORICAL CHAPTER I INTRODUCTION WHAT ORGANIC EVOLUTION IS — DEFINITIONS [The following selections are representative both of the older and of the newer attitudes of thinkers on the subject of organic evolution. The earlier writers were greatly impressed with the sublimity of the idea and found it in full accord with their religious faith. The later writers are less awed by the vastness of the process and hence adopt a more completely materialistic attitude. It is not necessary, how- ever, to discard one's religious beliefs in order to adopt a scientific attitude toward the problems of organic evolution.^ These points of view are well expressed in the following quotations. — Ed.] ''The world has been evolved, not created; it has arisen little by little from a sm.all beginning, and has increased through the activity of the elemental forces embodied in itself, and so has rather grown than suddenly come into being at an almighty word. What a sublime idea of the infinite might of the great Architect! the Cause of all causes, the Father of all fathers, the Ens entium! For if we could compare the Infinite it would surely require a greater Infinite to cause the causes of effects than to produce the effects themselves. "All that happens in the world depends on the forces that prevail in it, and results according to law; but where these forces and their substratum. Matter, come from, we know not, and here we have room for faith. "—Erasmus Darwin,^ as interpreted by Weismann. "When I first came to the notion, .... of a succession of extinc- tion of species, and creation of new ones, going on perpetually now, and through an indefinite period of the past, and to continue for ages to come, all in accommodation to the changes which must continue in the inanimate and habitable earth, the idea struck me as the grandest which I had ever conceived, so far as regards the attributes of the Presiding Mind."— From a letter of Sir Charles Lyell to Sir John Herschel, 1836. 1 See Joseph Le Conte, Relation of Evolution to Materialism, chap. iii. 2 From R. S. Lull, Organic Evolution (The Macmilhin Compan}-. Reprinted by permission). 3 4 READINGS IN EVOLUTION, GENETICS, AND EUGENICS *'It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so com- plex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduc- tion ; Inheritance which is almost implied by reproduction ; Variability from the indirect and direct action of the condition of hfe, and from use and disuse; a Ratio of Increase so high as to lead to a struggle for Life, and as a consequence to Natural Selection, entailing Diver- gence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is a grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, while this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved. " — Charles Darwin, Origin of Species, conclud- ing paragraph. ''Speaking broadly we find as a fact that transmutation of species through the geologic ages has been accompanied by increasing diver- gence of type, by the increased specialization of certain forms, and by the closer and closer adaptation to conditions of life on the part of the forms most highly specialized, the more perfect adaptation and the more elaborate specialization being associated with the greatest variety or variation in the environment. Accepting for this process the name organic evolution, Herbert Spencer has deduced from it the general law, that as life endures generation after generation, its character, as shown in structure and function, undergoes constant differentiation and specialization. In this view, the transmutation of species is not merely an observed process, but a primitive necessity involved in the very organization of life itself." — D. S. Jordan and V. L. Kellogg, Evolution and Animal Life (1908), p. 4. ''The Doctrine of Evolution is a body of principles and facts con- cerning the present condition and past history of the living and lifeless things that make up the universe. It teaches that natural processes INTRODUCTION 5 have gone on in the earlier ages of the world as they do to-day, and that natural forces have ordered the production of all things about which we know."— Henry Edward Crampton, The Doctrine oj Evolu- tion (1911), p. I. ''Evolution is the gradual development from the simple unorgan- ized condition of primal matter to the complex structure of the physi- cal universe; and in like manner, from the beginning of organic life on the habitable planet, a gradual unfolding and branching out into all the varied forms of beings which constitute the animal and plant kingdoms. The first is called Inorganic, the last Organic Evolution. " — Richard Swann Lull, Organic Evolution (191 7), p. 6. THE MODERN ATTITUDE AS TO THE TRUTH OF THE EVOLUTION DOCTRINE "Among that public which, though educated and intelligent, is not yet professionally scientific, there has been, of late, a widespread belief that naturalists have become very doubtful as to the truth of the theory of evolution and are casting about for some more satisfactory substitute, which shall better explain the infinitely varied and mani- fold character of the organic world. This belief is an altogether mis- taken one, for never before have the students of animals and plants been so nearly unanimous in their acceptance of the theory as they are to-day. It is true that there are still some dissentient voices, as there have been ever since the pubHcation of Darwin's 'Origin of Species,' but the whole trend of scientific opinion is strongly in favor of the evolutionary hypothesis." — ^William Berryman Scott, The Theory of Evolution, p. I. "But the biological sciences were still slower [than the physical sciences] to come to their true position as dignified science. Here was the last stronghold of the supernaturalist. Thrust out from the field of 'physical science' it was in the phenomena of life that the last stand was made by those who claim that supernatural agency intervenes in nature in such a way as to modify the natural order of events. When Darwin came to dislodge them from this, their last intrenchment, there was a fight, intense and bitter, but, like all attempts lo stay the prog- ress of human knowledge, this final struggle of the supernaturalists was foredoomed to failure. The theory of evolution has taken its place beside the other great conceptions of natural relations,^ and largely through its establishment biology has become truly a science 6 READINGS IN EVOLUTION, GENETICS, AND EUGENICS with a large group of phenomena consistently arranged and properly classified. The discussion which followed the publication of Darwin's 'Origin of Species ' lasted for nearly a generation, but it is now practi- cally closed, so far as any attempt to discredit evolution as a true scientific generalization is concerned. Scientists are no longer ques- tioning the fact of evolution ; they are busied rather with the attempt to further explore and more perfectly understand the operation of the factors that are at work to produce that development of animals and plants which we call organic evolution." — Maynard M. Metcalf, An Outline of the Theory of Organic Evolution (igii), pp. xxii-xxiii. "Biologists turned aside from general theories of evolution and their deductive application to special problems of descent, in order to take up objective experiments on variation and heredity for their own sake. This was not due to any doubts concerning the reaUty of evolution or to any lack of interest in its problems. It was a policy of masterly inactivity deliberately adopted; for further discussions concerning the causes of evolution had clearly become futile until a more adequate and critical view of existing genetic phenomena had been attained." — E. B. Wilson (address as president of the American Association for the Advancement of Science, 19 14). "The theory of development, as it was revived by Darwin nearly half a century ago, is in its modern form prevaiHngly unhistorical. True, it has forced beneath its sceptre the m.ethods of investigation of all the sciences which deal with the living world and to-day almost completely controls scientific thought And yet science does not sincerely rejoice in its conquests. Only a few incorrigible and uncritically disposed optimists steadfastly proclaim what glorious progress we have made; otherwise, in scientific as in lay circles, there prevails a widespread feeling of uncertainty and doubt. Not as though the correctness of the principle of descent were seriously questioned; rather does the conviction steadily grow that it is indispensable for the comprehension of living nature, indeed self- evident." — Gustav Steinmann (translated by W. B. Scott from Die Abstammungslehre [1908], pp. 1-2). "The many converging lines of evidence point so clearly to the central fact of the origin of forms of life by an evolutionary process that we are compelled to accept this deduction, but as to almost all the essential features, whether of cause or of mode, by which specific INTRODUCTION diversity has become what we perceive it to be, we have to confess an ignorance nearly total." — William Bateson, Problems of Genetics (1913), p. 248. ''The demonstration of evolution as a universal law of living nature is the great intellectual achievement of the nineteenth century. Evolution has outgrown the. rank of a theory, for it has won a place in natural law beside Newton's law of gravitation, and in one sense holds a still higher rank, because evolution is the universal master, while gravitation is among its many agents. Nor is the law of evolu- tion any longer to be associated with any single name, not even with that of Darwin, who was its greatest exponent. It is natural that evolution and Darwinism should be closely connected in many minds, but we must keep clear the distinction that evolution is a law, while Darwinism is merely one of the several ways of interpreting the work- . ings of this law. ''In contrast to the unity of opinion on the law of evolution is the wide diversity of opinion on the causes of evolution. In fact, the causes of the evolution of life are as mysterious as the law of evolution is certain. Some contend that we already know the chief causes of evolution, others contend that we know little or nothing of ihem. In this open court of conjecture, of hypothesis, of more or less heated controversy the names of Lamarck, of Darwin, of Weismann figure prominently as leaders of different schools of opinion; while there are others, like myself, who for various reasons belong to no school, and are as agnostic about Lamarckism, as they are about Darwinism or Weismannism, or the more recent form of Darwinism, termed Muta- tion by De Vries. "In truth, from the period of the earlier stages of Greek thought man has been eager to discover some natural cause of evolution, and to abandon the idea of supernatural intervention in the order of nature. Between the appearance of The Origin of Species, in 1859, and the present time there have been great waves of faith in one explanation and then in another: each of these waves of confidence has ended in disappointment, until finally we have reached a stage of very general scepticism. Thus the long period of evolution, experi- ment, and reasoning which began with the French natural philosopher, Buffon, one hundred and fifty years ago, ends in 1916 with the general feeling that our search for causes, far from being near completion, has only just begun. 8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS ''Our present state of opinion is this: we know to some extent how plants and animals and man evolve; we do not know why they v, evolve. We know, for example, that there has existed a more or less complete chain of beings from nomad to man, that the one-toed horse had a four-toed ancestor, that man has descended from an unknown ape-like form somewhere in the Tertiary. We know not only those larger chains of descent, but many of the minute details of these transformations. We do not know their internal causes, for none of the explanations which have in turn been offered during the last hun- dred years satisfies the demands of observation, of experiment, of reason. It is best frankly to acknowledge that the chief causes of the orderly evolution of the germ are still entirely unknown, and that our search must take an entirely fresh start." — H. F. Osborn, The Origin and Evolution of Life (Charles Scribner's Sons), 1918, pp. viii-x. WHAT ORGANIC EVOLUTION IS NOT [i. The evolution doctrine is not a creed to be accepted on faith, as are religious faiths or creeds. It appeals entirely to the logical faculties, not to the spiritual, and is not to be accepted until proved. 2. It does not teach that man is a direct descendant of the apes and monkeys, but that both man and the modern apes and monkeys have been derived from some«as yet unknown generalized primate ancestor possessing the common attributes of all three groups and lacking their specializations. 3. It is not synonymous with Darwinism, for the latter is merely one man's attempt to explain how evolution has occurred. 4. Contrary to a very widespread idea, evolution is by no means incompatible with religion. Witness the fact that the early Christian Theologians, Augustine and Thomas Aquinas, were evolutionists, and the majority of thoughtful theologians of all creeds are today in accord with the evolution idea, many of them even applying the prin- ciple to their studies of religion; for religious ideas and ideals, like other human characters, have evolved from crude beginnings and are still undergoing processes of refinement. 5. The evolution idea is not degrading. Quite the contrary; it is ennobling as is well brought out by the classic statement of Darwin on page 4 and by that of Lyell, on page 3. 6. The evolution doctrine does not teach that man is the goal of all evolutionary process, but that man is merely the present end product of one particular series of evolutionary changes. The goal INTRODUCTION 9 of evolution in general is perfection of adaptation to the conditions of life as they happen to be at any particular time. Many a highly perfected creature has reached the goal of its evolutionary course only to perish because it was too highly perfected for a particular environment and could not withstand the hardships incident to radi- cally changed world-conditions. Many evolutions therefore have been completed, while others are still awaiting the opportunity to speed up toward a new goal. 7. Evolution is therefore not entirely a thing of the past. Obvi- ously some species, including Man perhaps, are nearly at the end of their physical evolution, but there are always certain generalized plastic types awaiting the next great opportunity for adaptive speciali- zation. — Ed.] CHAPTER II HISTORICAL ACCOUNT OF THE DEVELOPMENT OF THE EVOLUTION THEORY H. H. Newman The chief sources of material for the present chapters are: Osborn's From the Greeks to Darwin} and Judd's The Coming of Evolution.'^ Professor Osborn studies the evolution of the evolution idea as a biologist would investigate the evolution of a group of species, using all of the available sources of evidence at his disposal. The fragments of ancient writing and the crude imaginings of early natural philoso- phers are the fossils of the evolution idea, many of them ancestors of modern principles; fragments of ancient or discarded ideas that still persist, though irrelevant to modern thought, are the vestigial structures that proclaim kinship between the past and the present; parallelisms between the development of ideas in the minds of inde- pendent thinkers do not prove plagiarism, but indicate common descent from the same ancestral ideas. This whole history is an important chapter in the story of human evolution in general, for it deals with the evolution of a characteristic human faculty — that of appreciating the broad relations that exist between the past and the present. This faculty has evolved as truly as has an organic system such as the nervous system, and is unques- tionably closely bound up with the latter. The evolution theory is a vast fabric of interrelated and inter- dependent facts and principles. The fabric has been gradually woven out of separate threads and now stands strong though flexible, with strands reaching into all sciences and tending to unify all science. It was only after the lesser ideas came to be clearly apprehended that it was possible for the master minds of Lamarck and of Darwin to weave them together into a consistent fabric and to bring the facts together under the one great conception, that of organic evolution. Classification was a science, comparative anatomy had made much progress, the principles of embryology were fairly well understood, "■ H. F. Osborn, From the Greeks to Darwin (The Macmillan Company, 1908). » John W. Judd, The Coming of Evolution (Cambridge University Press, 191 1). 10 HISTORICAL ACCOUNT OF EVOLUTION THEORY ii much palaeontological discovery had been made, before it was found that the facts from these sources all pointed to one general principle, and only one, that master-principle "organic evolution." We shall now trace the development of the evolution idea from its inception among the Greeks to its present status, and shall first give a brief account of Greek evolution. EVOLUTION AMONG THE GREEKS The early Greek thinkers were sea people. ''Along the shores and in the waters of the blue Aegean," says Osborn, "teeming with what we now know to be the earliest and simplest forms of animals and plants, they founded their hypotheses as to the origin and succession of life The spirit of the Greeks was vigorous and hopeful. Not pausing to test their theories by research, they did not suffer the disappointments and delays which come from one's own efforts to wrest truths from Nature. " The Greeks were anticipators of Nature. Their speculations out- stripped the facts; in fact were usually made with "eyes closed to the facts." Their theories were inextricably bound up with current mythology, were naive, vague, and, from our modern point of view, ridiculous; yet they contained many grains of truth and were the germs out of which grew the saner ideas of subsequent thinkers, Thales (624-548 B.C.) was the first of the Greeks to theorize about the origin of life. "He looked upon the great expanse of mother ocean and declared water to be the mother from which all things arose, and out of which they exist." This idea anticipates the modern idea of the aquatic or marine origin of life, and also the present idea as to the indispensability of water in all vital processes. Anaximander (611-547 B.C.) has been called the prophet of Lamarck and of Darwin. While his theories were highly mythical in character, he conceived the idea of a gradual evolution from a formless or chaotic condition to one of organic coherence. He saw vaguely the idea of transformation of aquatic species into terrestrial, even deriving man from aquatic fishlike men (mythical mermen) who were able to emerge from the water only after they had undergone the necessary changes required for land life. This idea involves that of adaptation, one of the cornerstones of the modern evolutionary structure. Anaximenes (588-524 B.C.), a pupil of Anaximander, "found in air the cause of all things. Air, taking the form of soul, imparts life, motion, and thought to animals. " It is questionable whether this is a 12 READINGS IN EVOLUTION, GENETICS, AND EUGENICS prophecy of the importance of oxygen and oxidation in vital processes. Anaximenes also introduced the idea of abiogenesis (spontaneous generation of living substance), his idea being that animals and plants arose out of a primordial terrestrial slime wakened into life by the sun's heat. This primordial terrestrial slime is perhaps a prophecy of Oken's "Urschleim" or of protoplasm. Xenophanes (576-480 B.C.), probably another pupil of Anaxi- mander, ''agreed with his master so far as to trace the origin of man back to the transition period between the fluid or water and solid or land stages of the development of the earth." He was the first to recognize fossils as the remains of animals once alive, and to see in them proof that once the seas covered the entire surface of the earth. Heraclitus (535-475 B.C.), the first of a group of physicists, was the great proponent of the philosophy of change. He was imbued with the idea that all was motion, that nothing was fixed. "Everything was perpetually transposed into new shapes." Although HeracUtus did not apply his ideas to living creatures and their evolutions, his philosophy was influential in molding the ideas of his successors. Empedocles (495-435 B.C.) '' took a great stride beyond his predeces- sors, and may justly be called the father of the Evolution idea He beHeved in Abiogenesis, or spontaneous generation, as the explana- tion of the origin of life, but that Nature does not produce the lower and higher forms simultaneously or without an effort. Plant life comes first, and animal life developed only after a long series of trials." He thought that all creatures arose through the fortuitous combina- tion of scattered and miscellaneous parts which were attracted or repelled by the forces of love or hate (the two great forces in Nature). Thus arose every sort of combination of parts, some more or less har- monious and complete, others with ill-assorted organization, lacking in some parts, double or triple in others. Some of these combinations could not survive, because of their incompleteness and incongruity, but ''other forms arose which were able to support themselves and multiply." This is a sort of vague prophecy of the survival of the fittest or of natural selection. Four sparks of truth may be found in Empedocles' philosophy, "first, that the development of life was a gradual process; second, that plants were evolved before animals; third, that imperfect forms were gradually replaced (not succeeded) by perfect forms; fourth, that the natural cause of the production of perfect forms was the extinction of the imperfect." HISTORICAL ACCOUNT OF EVOLUTION THEORY 13 Democritus (b.450 B.C.), said to have been the first comparative anatomist, contributed to the substructure of evolution the idea of the ''adaptation of single structures and organs to certain purposes." Anaxagoras (500-428 B.C.) was the first of the Greeks '' to attribute the adaptations of Nature to Intelligent Design, and was thus the founder of Teleology," an idea that has played a retarding function in the history of evolution. "With Aristotle (384-322 B.C.) we enter a new world," says Osborn. *'He towered above his predecessors, and by the force of his genius created Natural History." The evolution idea took a great step forward with Aristotle and reached a stage beyond which it did not go for many centuries. He covered nearly the whole field, touching upon most of the foundation stones of the complex problem. His ideas, like those of all the Greeks, were often vague and, in the light of present knowle'dge, incoherent; but, considering the meager factual background with which he had to work he had a surprising grasp of the whole situation. Some of his principal ideas were: 1. He had a clear idea of laws of Nature (''Necessity"), and attributed all evolutionary changes to natural causes. 2. He opposed the ideas of Empedocles as to the fortuitous origin of adaptive characters, and favored the idea of intelligent design in nature. He was therefore a teleologist. 3. Hence he rejected the hypothesis of the survival of the fittest, because it was based on chance. 4. He "had substantially the modern conception of the Evolution of Hfe, from a primordial soft mass of living matter. " 5. He had an idea of a linear phylogenetic series, beginning with plants, then plant-animals, such as sponges and sea anemones, then animals with sensibility, and thence by graded stages up to Man. 6. "He perceived the unity of type in certain classes of animals, and considered rudimentary organs as tokens whereby Nature sustains this unity. " 7. "He anticipated Harvey's doctrine of Epigenesis in embryonic development." 8. "He fully perceived the forces of hereditary transmission, of the prepotency of one parent or stock, and of Atavism and Reversion. " 9. He is the father of that ancient fallacy called "prenatal influ- ences," and believed in the inheritance of acquired characters, as is shown in the following passage : 14 READINGS IN EVOLUTION, GENETICS, AND EUGENICS " Children resemble their parents not only in congenital characters, but in those acquired later in life. For cases are known where parents have been marked by scars and children have shown traces of these scars at the same points; a case is also reported from Chalcedon in which a father had been branded with a letter, and the same letter somewhat blurred and not sharply defined appeared upon the arm of the child." POST-ARISTOTELIANS With Aristotle the evolution idea reached a high watermark and thereafter the tide steadily declined. Pliny, Epicurus, Lucretius, and others kept the idea alive, but added nothing of importance to Aristotle's contribution. Lucretius (99-55 B.C.) appears to have been chiefly a follower of Empedocles in so far as his ideas as to the origin of animals are con- cerned. He ignored Aristotle and his much more advanced phi- losophy of Nature, finding the earlier, more mythical conceptions better suited to poetic expression. He was not truly an evolutionist, for he believed that all animals and plants arose fully formed from the earth. Lucretius is of importance chiefly as a retarding factor, for his ideas were accepted and admired even up to the eighteenth century; witness Milton's immortal verse: ''The Earth obey'd, and straight, Op'ning her fertile womb, teem'd at a birth Innumerous living creatures, perfect forms, Limb'd and full grown. " THE EARLY THEOLOGIANS The evolution idea made no progress from the time of Aristotle until the revival of learning in the Middle Ages. The chief inhibiting factor was the church, which favored traditional knowledge and the special-creation idea in its most literal form. Yet the early theo- logians, such as Gregory, Augustine, and Thomas Aquinas, were open- minded about the evolution idea and attempted to reconcile it with the scriptural account of creation. ''Gregory of Nyssa (331-396 a.d.) taught," says Osborn, ''that Creation was potential. God imparted to matter its fundamental properties and laws. The objects and completed forms of the Universe developed gradually out of chaotic material. " HISTORICAL ACCOUNT OF EVOLUTION THEORY ' 15 Augustine (353-430 a.d.) conceived the idea, now so generally adopted by theologians, that the biblical account of creation is alle- gorical. "In explaining the passage 'In the beginning God created heaven and the earth,' he says: "In the beginning God made the heaven and the earth, as if this were the seed of the heaven and the earth, although as yet all the matter of heaven and of earth was in confusion, but because it was certain that from this the heaven and the earth would be, therefore the material itself is called by that name. " Thomas Aquinas (1225-74), who wrote much later and was one of the leading church authorities, satisfied himself with merely expound- ing Augustine: "As to the production of plants, Augustine holds a different view, .... for some say that on the third day plants were actually produced, each in its kind — a view favoured by the superficial reading of Scripture. But Augustine says that the earth is then said to have brought forth grass and trees Qausaliter; that is, it then received the power to produce them. For in those first days .... God made creation primarily or causaliter, and then rested from His work." THE REVIVAL OF SCIENCE During the long centuries until the awakening of science in the Middle Ages the evolution idea smouldered along in the minds of a few thinkers, but it was only when a few daring spirits broke the trammels of scholasticism and began once more to give free rein to observation and speculation that the idea once more burst into flame and began its second great period of advance. A small group of natural philosophers, scarcely more scientific in their methods than the Greeks, were the first to revive interest in the evolution idea. Of these the names of Bacon, Descartes, Leib- nitz, and Kant are the most famous. Francis Bacon (1561-1626) did much to revive the vogue of Aris- totelian ideas. He also added some new ideas: (i) that the muta- bility of species was the result of the accumulation of variations; (2) that variations of an extreme kind, equivalent to "mutations," some- times occur; (3) that new species might arise by a degenerative process from old species. Emanuel Kant (17 24-1 804) was purely a philosopher, not an observing naturalist, but he profited by the writings of the contem- porary naturalists, especially those of Buff on and Maupertius. His l5 READINGS IN EVOLUTION, GENETICS, AND EUGENICS general ideas of evolution were comprehensive and summed up the best features of all preceding writers, but he did not contribute any- thing new to the pressing problem of the causes of evolution. Real progress was not to be made through further speculation. What was most needed was facts, and it was the task of the naturalists to furnish these. The earliest of the eighteenth-century naturalists were still anticipators of Nature in that their theories outran their facts. Of these the names of Bonnet and Oken are the best known. Bonnet (1720-93) was an evolutionist only in the sense that he believed that the adult organism is present in the egg and evolves from it by a process of unfolding or expansion. He was a zoological observer of some note, however, and made some of the most important contributions of his time to the general subject. He believed "that the globe had been the scene of great revolutions, and that the chaos described by Moses was the closing chapter of one of these; thus the Creation described in Genesis may be only a resurrection of animals previously existing." This theory admits of no progress and is scarcely worthy of the name evolution. Oken (1776-1851) is known chiefly for his "Urschleim" doctrine and his ideas of cells as vesicular units of life. According to him, "Every organic thing has arisen out of slime and is nothing but slime in various forms. This primitive slime originated in the sea from inorganic matter." These ideas are purely speculative, but suggest our modern ideas of protoplasm and cells. THE GREAT NATURALISTS OF THE EIGHTEENTH CENTURY Three great names stand out above all the rest during this period : those of Linnaeus, Buffon, and Erasmus Darwin. Linnaeus (1707-78) was the father of taxonomy. He contributed facts rather than theories; he invented our present system of binomial nomenclature of both animals and plants, and a great many of his generic and specific names still persist. Unfortunately he was an ardent advocate of the special-creation idea, holding that all of the true species were created as they are known today, except that new combinations may have arisen through hybridization or through degeneration. His influence wasgreat, but was reactionary and proved a serious hindrance to the progress of the evolution idea. Btiffon (1707-88), born the same year as Linnaeus, has been recognized as the father of the modern applied form of the evolution idea. He attempted to explain particular cases on an evolutionary HISTORICAL ACCOUNT OF EVOLUTION THEORY 17 basis. He lived at a time when it was dangerous to express views that might be interpreted as unorthodox, and this may account for the apparent lack of conviction in his own ideas; for he wavered between special creation and evolution. His chief contribution is the idea of the direct influence of the environment in the modification of the structure of animals and plants and the conservation of these modifi- cations through heredity. This seems to imply that he believed in the inheritance of acquired characters. He expressed himself as believing that climate has had a direct effect in the production of various races of man, that new varieties of animals have been formed through human intervention (an idea implying artificial selection), that similar results are produced by geographic migration and through isolation. He expressed the view that there is a great struggle for existence among animals and plants to prevent overcrowding and to maintain the balance of Nature. This appears to be an anticipation of Malthus' ideas on population, which were so influential in shaping the theories of Charles Darwin and of Wallace. While many of his ideas appear to be highly advanced for his time, his special applications are open to serious criticism. He reasons, for example, that the pig as it exists at present could not have been formed on any original complete and perfect plan, but seems to have been formed as a compound from other animals. It has useless parts which could hardly have been a part of a perfect plan as originally conceived. He thought that "the ass is a degenerate horse, and the ape a degenerate man. " On the whole Buff on was not a strong advocate of evolution and his influence was far from being as important as some recent writers appear to believe. Erasmus Darwin (1731-1802), grandfather of Charles Darwin, was a physician, a naturalist, and a minor poet. Undoubtedly he transmitted to his grandson his thoughtful habit and love of science and was influential in shaping his ideas on evolution. The elder Darwin's theories as to the causes of evolution closely paralleled those of Lamarck, his distinguished contemporary in France, but it is now very generally conceded that the ideas of the two men were independently derived from similar materials. Erasmus Darwin laid little emphasis on the direct action of the environment, which had been Buffon's main dependence, and dwelt on the internal origin of adap- tive characters. ''All animals," he said, ''undergo transformations which are in part produced by their own exertions, in response to 1 8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS pleasures and pains, and many of these acquired forms or propensities are transmitted to their posterity." One could ask for no clearer statement of the idea that acquired characters are inherited. The fierceness of the struggle for existence was clearly recognized by Dr. Darwin. He considers that this struggle is beneficial to Nature as a whole because it checks the too rapid increase of life. One step farther in the argument, and he would have arrived at the idea of the survival of the fittest, but he never took that step. He agreed with the early Christian fathers in his belief that the powers of development were implanted within the first organisms by the Creator and that subsequent evolution of adaptive characters went on without further- divine intervention. The power of improvement rests within the creature's own organizations and is due to his own efforts. The effects of these efforts, he believes, are transmitted to offspring so that there might be a cumulative effect throughout many generations of the results of effort. Erasmus Darwin was perhaps the first to express clearly the ideas that millions of years have been required for the processes of organic evolution and that all life arose from one primordial protoplasmic mass. He writes as follows: "From thus meditating upon the minute portion of time in which many of the above changes have been produced, would it be too bold to imagine, in the great length of time since the earth began to exist, perhaps millions of ages before the commencement of the history of mankind, that all warm-blooded animals have arisen from one living filament, which the first great Cause imbued with animality, with the power of acquiring new parts, attended with new propensities, directed by irritations, sensations, volitions, and associations, and thus possess- ing the faculty of continuing to improve by its own inherent activity, and of delivering down these improvements by generation to pos- terity, world without end ? " LAMARCK Lamarck (1744-182 9), the greatest of French evolutionists, is now looked upon as ''the founder of the complete modern Theory of Descent. " Osborn considers him '' the most prominent figure between Aristotle and Darwin. One cannot compare his Philosophie zoologique with all previous and contemporary contributions to the evolution theory or learn the extraordinary difficulties under which he laboured, and that his work was put forth only a few years after he had turned HISTORIC.\L ACCOUNT OF EVOLUTION THEORY 19 from Botany to Zoology, without gaining the greatest admiration for his genius. No one has been more misunderstood, or judged with more partiality by over or under praise. The stigma placed upon his writ- ings by Cuvier, who greeted every fresh edition of his words as a 'nouvelle folie,' and the disdainful illusions to him by Charles Darwin (the only writer of whom Darwin ever spoke in this tone) long placed him in the light of a purely extravagant, speculative thinker. Yet, as a fresh instance of the certainty with which men of science finally obtain recognition, it is gratifying to note the admiration which has been accorded to him in Germany by Haeckel and others, by his countrymen, and by a large school of American and English writers of the present day; to note, further, that his theory was finally taken up and defended by Charles Darwin himself, and that it forms the very heart of the system of Herbert Spencer. " ^ Lamarck's main theory of evolution was expressed by him in the ^ form of his four ''laws": I. ''Life, by its proper forces, continually tends to increase the volume of every body which possesses it, and to increase the size of its parts, up to a limit which brings it about." II. "The production of a new organ in the animal body results from the supervention of a new want which continues to make itself felt, and a new movement which this want gives rise to and maintains." III. "The development of organs and their powers of action are constantly in ratio to the employment of these organs." IV. "Everything which has been acquired, impressed upon, or changed in the organization of individuals during the course of their life is preserved by generation and transmitted to new individuals which have descended from those which have undergone these changes. It is about the last " law " that the controversy rages, for it upholds the idea that acquired characters are inherited, now known as the "Lamarckian doctrine." A somewhat more specific statement of Lamarck's theory of evolution may be summed up in the following list of factors which he considered as playing an essential role in evolution. 1. "Favorable circumstances attending changes of environment, soil, food, temperature, etc., supposed to act directly in the case of plants, indirectly in the case of animals and man." 2. "Needs, new physical wants or necessities induced by the changed conditions of life. Lamarck believed that change of habits 20 READINGS IN EVOLUTION, GENETICS, AND EUGENICS may lead to the origination or modification of organs; that changes of function also modify or create new organs. By changes of environ- ment animals become subjected to new surroundings, involving new ways and means of living. Thus, certain land birds, driven by neces- sity to obtain their food in the water, gradually assumed characters adapting them for swimming, wading, or for searching for food in the shallow water, as in the case of the long-necked kinds. " 3. ''Use and disuse. To use an organ is to develop it; not to use it is to eventually lose it. The anterior limbs of birds became capable of sustained flight through use; the hind limbs of whales are lost through disuse, etc." 4. ''Competition. Nature takes precautions not to overcrowd the earth. The stronger and larger living things destroy the smaller and weaker. The smaller multiply very rapidly, the larger slowly. A physiological balance is maintained. ■' 5. "The transmission of acquired characters. The advantages gained by every individual as the result of the structural changes resulting from use or disuse are handed down to its descendants who begin where the parent leaves off, and so are able to continue the pro- gression or retrogression of the character." 6. "Cross-breeding. Tf when any peculiarity of form or any defects whatsoever are acquired, the individuals in this case, always pairing, they will produce the same peculiarities, and if for successive generations confined to such unions, a special distinct race will then be formed. But perpetual crosses between individuals which have not the same peculiarities of form result in the disappearance of all the peculiarities acquired by the particular circumstances.'" 7. "Isolation. 'Were not man separated by distances of habita- tion, the mixtures resulting from crossing would obliterate the general characters which distinguish different nations.' This thought is expressed in his account of the origin of men from apes, and is not applied to living things in general. " In addition to his theories as to the causes of evolution, Lamarck was the first to present the idea of the tree of life, or phylogenic tree, as a mode of representing animal relationships. All previous classifi- cations 'had been based on the idea of a single linear phylogenetic series, each lower group being supposedly ancestral to a higher group, and all in a single chain. We may best sum up Lamarck's work and influence in the words of Osborn: HISTORICAL ACCOUNT OF EVOLUTION THEORY 21 ''Lamarck, as a naturalist, exhibited exceptional powers of defini- tion and description, while in his philosophical writings upon Evolu- tion, his speculation far outran his observations, and his theory suffered from the absurd illustrations which he brought forward in support of it His critics spread the impression that he believed animals acquired new organs simply by wishing for them. His really sound speculation in Zoology was also injured by his earlier thoroughly worthless speculation in Chemistry and other branches of science. Another marked defect was, that Lamarck was completely carried away with the belief that his theory of the transmission of acquired characters was adequate to explain all the phenomena. He did not, like his contemporaries, Erasmus Darwin and Goethe, perceive and point out, that certain problems in the origin of adaptations were still left wholly untouched and unsolved His arguments are, in most cases, not inductive, but deductive, and are frequently found not to support his law but to postulate it. "It is now a question whether Lamarck's factor is a factor in Evolution at all! If it prove to be no factor, Lamarck will sink gradually into obscurity as one great figure in the history of opinion. If it prove to be a real factor, he will rise into a more eminent position than he now holds, — into a rank not far below Darwin." CUVIER AND GEOFFROY ST. HILAIRE Georges Cuvier (1769-183 2) deserves especial mention as one of the strongest negative factors in the development of the evolution idea. He was, first of all, an opponent of Lamarck, and, second, of evolution in general. He ranged himself with Linnaeus as a special creationist and advocated the idea of fixity of species. "All the beings, " said he, "belonging to one of these forms (perpetual since the beginning of all things, that is, the Creation) constitute what we call species." So able was Cuvier and so much in favor at the French court that he succeeded in throwing Lamarck's views into disrepute and thus greatly retarded the progress of evolution. He was brilliant as a comparative anatomist and palaeontologist and will long be known for his discoveries in these fields. E. Geoff roy St. Hilaire (17 72-1844) did his best to defeat the retarding influence of Cuvier. The two engaged in a long and bitter controversy over the evolution idea. While not a supporter of Lamarckism proper, he was a thoroughgoing evolutionist, favoring 22 READINGS IN EVOLUTION, GENETICS, AND EUGENICS the doctrine of Buffon, that the direct action of the environment was the sole cause of evolution. He also, in a sense, anticipated De Vries, in that he believed that new species might be formed by transmutation or sudden large variations occurring in one generation. "Hence the underlying causes of transformations," he said, "were profound changes induced in the egg by external influences, accidents as it were, regulated by law. " The controversy between Cuvier and St. Hilaire was a losing one for the latter. The cards were stacked against him and after him the evolution idea was retired to comparative obscurity until revived by Charles Darwin. CATASTROPHISM AND UNIFORMITARIANISM The development of the science of geology had a profound influence upon that of evolution. The prevailing theories as to historical geology during the Middle Ages involved the idea of "catastrophism. " According to this view all important changes in the earth's crust represented sudden radical transformations, involving earthquakes, volcanic outbursts, floods, sudden upliftings of submerged areas, or equally sudden submergence of land bodies. From these ideas natu- rally grew the related idea of great, world-wide destructions of animals and plants, followed by re-creation of new faunas and floras. Cuvier, for example, interpreted the more or less distinct fossil strata as being the result of a series of tremendous cataclysms, the last of which had been the great deluge of Scripture, in which Noah figured prominently. He thought that at each cataclysm great floods of water had covered the earth, that the existing animals had been buried in mud and thus preserved as fossils, and that a new creation followed each cataclysm. The great strength of this conception was that it appeared to give scientific support to both special creation and the Mosaic account of the "Flood." As compared with the pure evolutionary conception, this alternative was highly acceptable to the church and was pro- claimed as orthodox. The Scotch philosopher and geologist, Hutton, who lived during the last half of the eighteenth century, combated the idea of catastrophism by advocating the doctrine of "uniformitari- anism," a view involving the idea that past changes on the earth were the result of the same sort of gradual changes as are observed to be taking place today — in brief, that there has been a strict uni- formity of change throughout the entire period of geologic history. There may have been, according to this view, local catastrophes, HISTORICAL ACCOUNT OF EVOLUTION THEORY 23 such as volcanic outbursts, earthquakes, and floods, but the main trend of change has been slow and constant, due largely to erosion and allied phenomena. This view had practically no influence on the ideas of the time and for a long period the idea of catas- trophism triumphed over the more truly evolutionary view of uni- formitarianism; thus the evolution idea was destined to lie dormant till revived by Charles Darwin. THE REAWAKENING OF THE EVOLUTION IDEA A number of important influences paved the way for the rehabili- tation of the evolution idea at the hands of the younger Darwin. Which of these was the most important it is difficult to say. Prob- ably Charles Lyell's Principles of Geology and Malthus' On Population were the most suggestive works that Darwin encountered. He was also doubtless influenced by Robert Chambers' Vestiges of Natural History of Creation which appeared in 1844. Charles Lyell (i 797-1875) so successfully rehabilitated the doctrine of uniformitarianism in geology that it became very generally accepted, thus paving the way for a more favorable consideration of the idea of organic evolution. Charles Darwin as a very young man took Lyell's Principles of Geology with him on his voyage on the '' Beagle " and read it with the greatest devotion, as is evidenced by his dedication of the journal of his voyage: "To Charles Lyell, Esq., F.R.S., this second edition is dedicated with grateful pleasure, as an acknowledgment that the chief part of whatever scientific merit this Journal and other works of the author may possess, has been derived from studying the well-known, admirable Principles of Geology.'' Malthus' influence on Darwin's ideas is well expressed by Judd as follows : "Fifteen months after this 'systematic inquiry' began [referring to Darwin's exhaustive working over of his notes taken during his voyage on the 'Beagle'], Darwin happened to read the celebrated work of Malthus 'On Population' for amusement, and this served as a spark falling on a long prepared train of thought. The idea that as animals and plants multiply in geometrical progression, while the suppHes of food and space to be occupied remain nearly constant, and that this must lead to a struggle for existence of the most desperate kind, was by no means new to Darwin, for the elder De Candolle, Lyell, and others had enlarged upon it; yet the facts with regard to 24 READINGS IN EVOLUTION, GENETICS, AND EUGENICS the human race, so strikingly presented by Malthus, brought the whole question with such vividness before him that the idea of 'Natural Selection' flashed upon Darwin's mind." CHARLES DARWIN (1809-82) Charles Darwin is without question the foremost figure in the development of the evolution idea and probably in the development of science in general. The publication of his book, The Origin of Species, in 1859, was the most important event in biological history. As has been already shown, Darwin's chief ideas had been anticipated not by one but by several of his predecessors. Nevertheless, he was the first to furnish a really adequate proof of the fact of evolution and his causo-mechanical theory to explain the method of evolution was supported by a mass of systematically arranged data such as has been paralleled neither before nor since. Darwin was the first evolu- tionist effectively to employ the inductive method, that of everywhere seeking facts first and then devising theories to fit the facts. He never allowed speculation to outstrip observation, as nearly all of his predecessors had done, but made theory await the amassing of facts in its support, until the accumulation of the latter seemed almost to speak out the theory of themselves. Our greatest debt to Darwin is due to his establishment of the factual basis of evolution; his selection theory was relatively of minor significance in so far as its value in the development of the evolution idea was concerned. Yet this latter theory gained the widest acceptance among the scientifically inclined during the entire post-Darwinian period. It has been viciously assailed on all sides and has tottered repeatedly under the attacks of well-trained adversaries. Some of the weaker elements of the theory have given way under stress, and the whole selection factor as a primary causal factor in evolution has been seriously called into question; but since Darwin's time the fact of evolution has been almost universally accepted. The story of Darwin's life is almost a romance. '' Born in 1809, " says Lull,'^ ''this emancipator of human minds from the shackles of slavery to tradition saw the light of day upon the very day that ushered in the life of Abraham Lincoln, the emancipator of human bodies from a no more real physical bondage. Darwin studied first at Edinburgh, but finding medicine unsuited to his tastes, entered Christ's College, Cambridge, as a candidate for the church. His love ' Richard Swann Lull, Organic Evolution (The Macmillan Company, 191 7). PMOPERTY LIBRARY HISTORICAL ACCOUNT OF EVOLUTION THEORY 25 of Nature, however, dominated all other interests and shortly after graduation an opportunity came to join the ship 'Beagle' as naturalist in a voyage of exploration around the world. The five years spent upon this memorable journey, the narrative of which is so admirably set forth in the book, A Naturalist's Voyage around the World, resulted in the accumulat on of the first of Darwin's great series of observations, the final decision to devote his life to zoological research, and the beginning of that illness which made him a life-long invalid. This last factor necessitated a retired life and thus proved of indirect bene- fit, as it enabled him to accomplish the immense amount of work which he did without being impeded by the distractions of a public career." SUMMARY OF DARWIN's THEORIES Since two subsequent chapters are to be devoted to Darwinism, only an outline of Darwin's theories need be presented in the present historical account. Although Darwin was an all-round biologist and gave attention to practically every phase of evolutionary biology, he is known espe- cially for his selection theories'. There are three of these : the theory of artificial selection, the theory of natural selection, and the theory of sexual selection. -. a) Artificial selection. — According to Darwin the commonest method of producing, under human culture, new races of animals and plants is that of selection. The breeder selects from among the highly variable individuals of a parent-race those which possess the begin- nings of desired modifications, and he breeds them together, expecting that the offspring will show the desired character, some in a more highly perfected condition, others in a less. The ones that vary favorably are again selected for breeding stock, and the same process is carried on until the desired character has been perfected. Although we now know that this is far from being a typical experi- ence among breeders, it appeared to Darwin to be so typical that he transferred the selection idea from the breeder to Nature, making Nature the selecting agency responsible for the production of natural wild species. His argument is as follows: b) Natural selection. — The following factors are involved : ^ I. All animals and plants tend to multiply in geometrical ratio. 2. There is not food or room for a much larger number of animals and plants than now exist. 26 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 3. All members of a species vary in many if not all directions. 4. Those that vary in the more favorable directions, so as better to fit them to meet the conditions of life, survive in larger numbers than those varying in less favorable directions. This is Spencer's " survival of the fittest." 5. The survivors of one generation become the parents of the next and, therefore, the more favorable characters are passed on more largely than the less favorable. 6. There is in each generation a slow but definite approach toward complete adaptation to life-conditions. 7. Variations neither useful nor harmful would not be affected by natural selection, and would be left either as fluctuating variations or as polymorphic characters. c) Sexual selection. — This theory was offered to supplement that of natural selection, because Darwin considered the latter as inade- quate to explain the facts of sexual dimorphism, or secondary sexual characters. The theory is as follows: There is always a contest among males for possession of females, in which the inferior males are eliminated either because they are, on the one hand, less courageous or weaker or less well equipped with weapons of combat, or because, on the other hand, the more attractive males, whether on account of colors, odors, phosphorescence, behavior, etc., would succeed in winning mates from those less endowed. Thus would be enhanced the sexual dimorphism until it reaches extremes in many cases that are truly remarkable. The name of Alfred Russell Wallace (1822-1913) will always be associated with that of Charles Darwin as co-author of the theory of natural selection. Wallace at the age of twenty-six went on a natural- istic expedition, primarily for collecting specimens from new regions. He covered almost the same ground as did Darwin in his voyage on the ''Beagle." Wallace had read Lyell's Principles of Geology, Malthus' 0« Population, Chambers' Vestiges of Creation. While in Sarawak he tells. us: "I was quite alone with one Malay boy as cook, and during the evenings and wet days, I had nothing to do but to look over my books and ponder over the problem which was rarely absent from my thoughts. " While thus engaged the idea of natural selection came to him as though by a sudden flash of insight. When the idea was still in process of formation he wrote it out on thin paper and mailed it to Darwin, stating that he considered the idea new and asking Darwin to show it to Lyell, who had expressed interest in a HISTORICAL ACCOUNT OF EVOLUTION THEORY 27 former paper of Wallace. The ideas were expressed under the title On the Tendency of Varieties to Depart Indefiititely from the Original Type, and it proved to be an unusually^ concise and lucid statement of the main points of the natural-selection theory. Darwin at once wrote to Lyell as follows: "I never saw a more striking coincidence; if Wallace had my MS sketch, written in 1842, he could not have made a better short abstract ! Even his terms now stand as heads of my chapters. Please return to me the MS which he does not say he wishes me to publish but I shall, of course, at once write and offer to send it to any journal. So all my originality, whatever it may amount to, will be smashed, though my book, if it ever have any value, will not be deteriorated, as all the labour consists in the application of the theory. I hope you will approve of Wallace's sketch, that I may tell him what to say." Lyell insisted that Darwin publish an abstract of his own work simultaneously with that of Wallace, and this course was carried out. Darwin's generosity was equaled by that of Wallace who wrote, in 1870: "I have felt all my life and still feel the most sincere satisfaction that Mr. Darwin had been at work long before me, and that it was not left for me to attempt to write The Origin of Species. I have long since measured my own strength and know well that it would be quite unequal to the task." Still later he wrote: ''I was then (and often since) the 'young man in a hurry,' he [Darwin] the painstaking student, seeking ever the full demonstration of the truth he had discovered, rather than to achieve immediate personal fame." One must perforce admit the nobility of character of both men; but there can be no serious competition between the two for the honor of being called the originator of the natural-selection theory. CONTEMPORARY OPINION REGARDING THE VALIDITY OF DARWIN's VIEWS At first Darwin was inclined to believe that the selection factor was all-sufficient to account for the origin of species, as well as that of adaptations; but as time passed he modified his earlier more sanguine views and came to the conclusion " that natural selection has been the main but not the exclusive means of modification." Many of his followers went to such extremes in their advocacy of the all-sufficiency of natural selection as would not have met with Darwin's approval. 28 READINGS IN EVOLUTION, GENETICS, AND EUGENICS "The first effect of Darwin's works," says McFarland/ ''was to carry the world of science by storm, but at the same time to arouse intense hostihty on the part of the theologians who found the theory of descent .... incompatible with the doctrines of Creation. In this conflict Darwin took no part, but was championed by Huxley, while Bishop Wilberforce led the opposition. The battle was long and bitter, there was much acrimonious writing on both sides, but the theory of descent — the doctrine of evolution — was found to be invulnerable and at present the theologians themselves have accepted it and even make use of it in their own work. " But as the years flew by the Darwinian doctrines began to meet with assaults from the scientists themselves, who, having endeavored to prove their validity, began to find them inadequate to the require- ments of expanding knowledge. The question was asked, 'What is the origin of the fittest ?' Given the fittest, we easily understand how it is perpetuated, but how does it arise ? In the striking phrase of someone: 'Natural selection might explain the survival of the fittest but fails to account for the arrival of the fittest!' " Darwin's main supporters during the most trying controversial period were Herbert Spencer and Thomas H. Huxley. Herbert Spencer (i 820-1 903) was an extremely able supporter of the general theory of evolution, but was more definitely an advocate of Lamarckism than of natural selection. His role was that of a champion of the whole philosophy of evolution as opposed to special creation, and it was largely due to his forceful writings that Darwinism won the battle against dogmatism. Spencer tried to explain the structure of protoplasm (living substance) on a physicochemical basis. He thought of the structural units of protoplasm as compa- rable with the molecules of chemical compounds, each local region of the protoplasm in the organism being made up of different kinds of units, which he called "physiological units. " This conception of the physical basis of organic structure had a considerable influence in shaping Darwin's ideas and was probably the basis of the latter's provisional theory of "pangenesis." This theory was probably the first consistently worked out theory of the mechanics of heredity. It was thought that every part of the body is continually giving off its particular kind of units ("gemmules") into the blood. These gem- mules are transported by the blood stream to all parts of the body and ^ J. McFarland, Biology, General and Medical (The Macmillan Company, 1918). HISTORIC.\L ACCOUNT OF EVOLUTION THEORY 29 collect in the germ cells. This was supposed to account for the fact that from the germ cell will develop an organism like the parent in various details. If a part of the body was modified through func- tioning or through changed environment, it would have modified gemmules, which, in turn, would go to the germ cells and carry over the modification to the next generation. This theory was not satis- factory even to Darwin and is now only of historical interest. Spencer is best known in the history of the evolution theory as an ardent neo-Lamarckian. He states his belief as follows: '^ Change of function produces change of structure; it is a tenable hypothesis that changes of structure so produced are inherited. " This idea prevailed until it was cast down by Weismann. Thomas Henry Huxley (1825-95), ^^^ of the keenest, most analyti- cal thinkers of the nineteenth century, not only defended the general doctrine of evolution against Bishop Wilberforce and his aids, but was an able investigator in the fields of comparative anatomy and embry- ology. "At the British Association at Oxford in i860," says Judd, ''after an American professor had indignantly asked 'Are we a fortuitous concourse of atoms?' as a comment on Darwin's views, Dr. Samuel Wilberforce, the Bishop of Oxford, ended a clever but flippant attack on the Origin by enquiring of Huxley, who was present as Darwin's champion, if it 'was through his grandfather or his grand- mother that he claimed his descent from a monkey ? ' "Huxley made the famous and well-deserved retort : 'I asserted — and I repeat — that a man has no reason to be ashamed of having an ape for his grandfather. If there were an ancestor whom I should feel ashamed of recalling, it would rather be a man — a man of restless and versatile intellect — who not content with success in his own sphere of activity, plunges into scientific questions with which he has no real acquaintance, only to obscure them by aimless rhetoric, and distract the attention of his hearers from the real point at issue by eloquent digressions and skilled appeals to religious prejudice!' "Huxley himself accepted the theory of Natural Selection — but not without some important reservations — these, however, did not prevent him from becoming its most ardent and successful champion. Darwin used to acknowledge Huxley's great service to him in under- taking the defense of the theory — a defense which his own hatred of controversy and state of health made him unwilling to undertake — by laughingly calling him 'my general agent' while Huxley himself in replying to the critics, declared he was 'Darwin's bulldog.'" 30 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Ernst Haeckel (1834-1919) was one of the earliest and most influential followers of Darwin in Germany. In his Generelle Mor- phologie, published in 1866, seven years after the Origin of Species first appeared, he applied the doctrine of evolution, and especially the theory of natural selection, to the whole field of vertebrate mor- phology. Beyond question Haeckel overapplied the theory and in a sense weakened its influence by his rather uncritical use of materials. His writings have been translated into most languages and ''are popularly believed to represent the best scientific thought on the matter. " Biologists today, however, are apt to look askance at Haeckel's works and to consider that they did more harm than good to Darwinism. August Weismann (1834-1914) was the first really original evolutionist after Darwin. Like other thinkers of his time, he realized that further progress in the knowledge of the causal basis of evolution lay in further investigation of the causes of variation and the physical basis of heredity. Weismann has been classed as a neo-Darwinian because he was a strong advocate of some form of selection, but his "selection" was not the selection of Darwin. Realizing that the greatest weakness of the natural-selection theory lay in its inadequacy as an originator of variations, he proposed the "germinal-selection" theory. He contended that all heritable variations have their origin in the germ cell, and therefore that a new type of organism arises only from a changed type of germ cell. The germinal-selection theory stands out in striking contrast with Darwin's "pangenesis" theory. The former is centrifugal, the latter centripetal. "Determiners" of new characters, according to Weismann, arise in the germ plasm and work outward to all parts of the developing body; while the "gem- mules," Darwin's equivalent of determiners, originate in the body tissues and are carried to the germ cells in each generation. Accord- ing to Weismann, there is a struggle among the determiners for the available food and favorable positions in the germ cell, and those that receive the most food and the best positions gain an initial advantage, so that they are able to initiate the development of larger or more perfectly adapted organs. The descendants through cell division of these favored determiners are in a position to compete with other determiners on a more favorable footing in each succeeding generation, so that the character represented by them steadily increases in a linear or definitely directed fashion until it reaches the state of complete adaptation or fitness. Such a character may even continue its direct line of advance beyond the point of maximum fitness and result in HISTORICAL ACCOUNT OF EVOLUTION THEORY 31 what are known as overspecializations. The theory therefore would, if well founded, account not only for the initial stages of new adaptive characters, but also for overspecializations, two phenomena that natural selection was unable to account for. Not only were pro- gressive evolutionary changes explained by germinal selection, but regressive changes seemed to be .even more readily accounted for on this basis. In the struggle among determiners in the germ cell some of the less favored units would be handicapped at the outset by insufficient food or unfavorable position and would produce smaller or less effective structures. Progressively, from generation to generation, these weakened determiners would lose ground and become less and less successful in competition until they were weaklings among determiners and would be able to initiate only degenerate or vestigial structures, or else would die out and lose their place altogether, thus accounting for total losses of structure. This theory does not exclude natural selection, but rather increases its importance, for every structure that arises to the threshold of utility or disutility meets the winnowing process of natural selection. The fitter individuals survive in the long run and these perpetuate the germ cells in which the successful determiners reside. A slightly different explanation of degenerating structures in- volves the principle of ''panmixia. " According to this idea, changing environmental conditions may render certain adaptive organs of lessened value or of no value, as would be the case in the eyes of cave animals. In different individuals the eye determiners would vary in their success in competition with other determiners, and since natural selection would no longer put a premium on perfect eyes, all grades of eyes would be equally inherited and gradually the poorer or degenerate eyes would become more numerous, till, finally, there would be no good eyes in the race. Thus it will be seen that the germinal-selection theory was auxiliary to natural selection and tended to support the latter at two of its weakest points. But the supporting theory itself has the fundamental weakness of lacking a factual basis. It is purely hypothetical and cannot be put to an experimental test. Every time an objection to the theory was raised an auxiliary h>T)othesis was added to explain away the difficulty, till finally it fell to the ground through sheer top-heaviness, unable further to support its intricate structure of interrelated hypotheses. A much more valuable and lasting contribution of Weismann was his theory of "germinal continuity" and of the ''apartness of the germ plasm. " The whole theory has come to be known as the " germ-plasm 32 READINGS IN EVOLUTION, GENETICS, AND EUGENICS theory," which forms the framework of nearly all of our modern genetics. According to this view the germ plasm is immortal in that it is perpetuated from generation to generation through the instrumentality of mitotic cell division, each germ cell being the prod- uct of the division of a previous germ cell back to the first germ cell that arose at the dawn of life. Thus a germ cell cannot be a product of the soma, but the soma is the product of germ cells. The soma loses its generalized characters and specializes in various ways. Once specialized, soma cells are believed to have lost their capacity to play a germinal role. Specialization means mortality. Thus the relation- ship between parent and offspring is not that the parent gives rise to the offspring, but that the same germ plasm gives rise to both parent and offspring. The logical conclusion to which this line of reasoning leads is that the changes in the soma, no matter how produced, are helpless to produce any effect upon the germ plasm, since germ cells come only from germ cells and not from soma cells. Consequently Weismann led the assault against Lamarckism and won the day so conclusively that even in these modern times few biologists have the temerity to express aloud any definite belief in the inheritance of acquired charac- ters. Weismann's germ-plasm idea is the cornerstone of modern genetics, though there are some forward-looking biologists who, looking at things with a physiological bias, cannot make themselves believe in the total independence of any tissue — even the sacred germ plasm. Weismann's influence was very great, especially during the last decade of the nineteenth century, and his theories gave rise to an immense amount of research, chiefly of a cytological and embryo- logical character. ISOLATION THEORIES Among the theories subsidiary to natural selection as an aid to species forming are the various isolation theories. One of the weak- nesses inherent in natural selection had to do with the probable swamping out of new types by promiscuous breeding with the more numerous individuals of the older types. "Anything," says Metcalf, ''which divides a species into groups, which do not freely interbreed, is said to segregate (isolate) the members of the species into these sub- divisions." Some American writers, especially Jordan and Kellogg, Gulick, and Crampton, have dealt with the isolation factor in evolution and believe HISTORICAL ACCOUNT OF EVOLUTION THEORY 33 that it is a major factor of as great importance in species forming, or nearly so, as natural selection. But the prevailing opinion seems to be that isolation is really a kind of selection, more like artificial selection than anything else, which separates out certain pure lines and prevents promiscuous interbreeding. Various agents are known to produce isolation by erecting barriers to interbreeding between groups of individuals within a species. These segregative factors may be geographical, climatic, reproductive, physiological, or, in plants, the result of soil diversity. Thus a mountain range, on the two sides of which a species migrates, effectively separates the species into two independent groups. Heat, cold, moisture, etc., separate others. Reproductive incompatibility between new and older types is equally effective, as is assortative mating of like with like. Like natural selec- tion, isolation has nothing to do with the origin of new types, but merely aids in the preservation of types when once formed. Were there not spontaneous variations among animals and plants, there would be nothing to isolate. Therefore isolation plays only an auxiliary role, helping to preserve new races once they are formed. ORTHOGENESIS THEORIES ''The orthogenetic evolution theories of various authors, based upon the assumed occurrence of variations in determinate lines or directions (a restricted and determinate variation as compared with the nearly infinite, fortuitous, and indeterminate variation assumed in the selection theories), are of several types. The mention of two will reveal pretty well the more important characters of all. Not a few biologists have always believed in the existence of a sort of mystic, special vitalistic force or principle by virtue of which determination and general progress in evolution is chiefly fixed. Such a capacity, inherent in living matter, seems to include at once possibility of pro- gressive or truly evolutionary change. Not all evolution is in a single direct line, to be sure; ascent is not up a single ladder or along a single geological branch, but these branches are few (as indeed we actually know them to be, however the restriction may be brought about) and the evolution is always progressive, that is, toward what we, from an anthropocentric point of view, are constrained to call higher and higher or more ideal Hfe stages and conditions. "Other naturalists also seeming to see this source of determinate or orthogenetic evolution, but not inclined to surrender their dis- belief in vitalism, in forces over and beyond the familiar ones of the 34 READINGS IN EVOLUTION, GENETICS, AND EUGENICS physicochemical world, have tried to adduce a definite causomechani- cal explanation of orthogenesis. The best and most comprehensive types of this explanation are those essentially Lamarckian in principle, in which the direct influence of environmental conditions, the direct reactions of the life stuff to stimuli and influences from the world outside, are the causal factors in such an explanation. But while every naturalist will grant that such factors do change and control in a considerable degree the life of the individual, most see no mechan- ism or means of extending this control directly to the species." The above-quoted paragraphs from Jordan and Kellogg^ will serve to place before the reader the general ideas involved in the orthogenesis conception. A brief account of the various special theories of orthogenesis follows: Carl von NdgeWs ideas of orthogenesis involve a belief in a sort of mystical principle of progressive development, a something, quite intangible, that exists in organic nature, which causes each organism, to strive for or at least make for specialization or perfect adaptation. This idea of an inner driving and directing force reminds one of the "entelechy" of Driesch, or Bergson's '^ creative evolution." Nageli believed that animals and plants would have developed essentially as they have without any struggle for existence or natural selection. This form of orthogenesis theory, then, is alternative to natural selection. Theodore Ewicr''s theory of orthogenesis is more scientific and less mystical than Nageli's. He believed that lines of evolution were not miscellaneous and haphazard, but were confined to a few definite directions, determined at their initial stages not by natural selection but by the laws of organic growth, aided by the inheritance of acquired characters. A new character makes a beginning as would the first step in a slow chemical change, or series of such changes, and it must go through to a fixed end, under given conditions, just as surely as does the chemical process. Only when a given character or line of evolu- tion results in the production of a ver>' positive advantage or dis- advantage to the species does natural selection step in to interfere with orthogenesis. The causes of orthogenesis are said "to lie in the effects of external influences, climate, nutrition, or the given constitu- tion of the organism." Actual species-forming, or the breaking-up into specific units of- the orthogenetic lines of change, depends, according to Eimer, upon ^ Jordan and Kellogg, Evolution and Animal Life (D. Appleton and Company). HISTORICAL ACCOUNT OF EVOLUTION THEORY 35 three factors: a standstill or cessation of development on the part of some lines; sudden development by leaps (practically mutations); and hindrance or difficulty of reproduction (the type of thing that Romanes emphasized as physiological isolation ten years later). Eimer illustrated his theories by the evolution of color patterns in lizards and those on the wings of butterflies. In both he believed that longitudinal stripes were primitive, that rows of dots followed these which were in turn followed by crossbands, reticular patterns, and finally by solid coloration. This hypothetical phylogenetic order is more or less closely paralleled by the ontogenetic order, in the lizards at least. It will be noted that Elmer's theory places natural selection in a subordinate position, but does not dismiss it altogether, as is done by Nageli. It aids natural selection in explaining adaptations in that it furnishes for natural selection various characters of selective value, which may be either perpetuated or eliminated according to their utility. E. D. Cope, a leading American palaeontologist of the past cen- tury, had an orthogenetic theory involving his ideas of "bathmism" (growth force), ''kinetogenesis" (direct effect of use and disuse and environmental influence), and ''archaesthetism" (influence of primi- tive consciousness). It may be said that his ideas were Lamarckian throughout. In common with the majority of palaeontologists of later date — Osboj-n, WiUiston, Hyatt, Smith, and others — Cope felt the need of some factor other than natural selection to explain the apparent steady progress of characters in definitely directed lines as seen in the fossils. It is natural therefore that palaeontologists almost universally lay hold of both Lamarckian and orthogenesis ideas. Charles Otis Whitman, who, until his death over ten years ago, was considered the leading American zoologist, had strong leanings toward orthogenesis. In one of his few publications he says: "Natural selection, orthogenesis, and mutation appear to present fundamental contradictions; but I believe that each stands for truth, and reconciHation is not far distant. The so-called mutations of Oenothera are indubitable facts; but two leading questions remain to be answered. First, are these mutations now appearing, as is agreed, independently of variation, nevertheless the products of variations th^ took place at an earlier period in the history of these plants ? Secondly, if species can spring into existence at a single leap, without the assist- , ance of cumulative variations, may they not also originate with such 36 READINGS IN EVOLUTION, GENETICS, AND EUGENICS assistance ? That variation does issue a new species, and that natural selection is a factor, though not the only factor, in determining results, is, in my opinion, as certain as that grass grows although we cannot see it grow. Furthermore, I believe I have found indubitable evidence of species-forming variation advancing in a definite direction (ortho- genesis), and likewise of variations in various directions (amphi- genesis). If I am not mistaken in this, the reconciliation for natural selection, and orthogenesis is at hand." In concluding this brief account of orthogenesis, it should be said that definitely directed evolution is now believed to be one of the laws of organic evolution, but that we have no clear ideas as yet as to what are its underlying causes. Therefore orthogenesis is not a causo- mechanical theory of evolution at all. MUTATION OR HETEROGENESIS THEORIES The theory of "mutations" is associated with the name of Hugo De Vries, the well-known Dutch botanist; that of "heterogenesis," with the name of H. Korchinsky, a Russian. Though Korchinsky anticipated De Vries by several years, his work was not supported by the large amount of experimental data that characterized that of the great Dutch worker. The relative claims for recognition as the founder of the mutation theory are almost on a par with those of Darwin and Wallace for the natural- selection theory. Both Darwin and De Vries held .back their theo- ries until they appeared to be adequately supported by personally collected facts. There is a striking parallelism between the ideas and conclusions of De Vries and those of Korchinsky, and since this is true a resume of De Vries's better-known work will serve to give the essentials of the whole conception. De Vries began his genetic experiments by a study of the variations of plants in the field. After learning their normal variability in nature, he transferred them to the experimental garden and there attempted to improve them by selection. He found that the improved living conditions due to better soil and cultivation induced a wider range of variability in size, luxuriance, and fecundity. Such variations were plus or minus in their character, fluctuating about a mean or average. It was exactly this type of variability that Darwin empha- sized as the raw material of evolution; but De Vries found by experi- ment that selection had no permanent hereditary effect when based HISTORICAL ACCOUNT OF EVOLUTION THEORY 37 to fluctuating variations, since the latter were merely somatic responses on variable growth conditions. This negative finding led him to renewed interest in discontinuous or saltatory variations as the only alternative to fluctuating or continuous variations. He looked far and wide among species of wild plants for a species that might exhibit a significant amount of saltatory variation and finally discovered in the evening primrose {Oenothera lamarckiana) what seemed to exhibit exactly the hoped-for characteristics. This large, stately plant with conspicuous yellow blooms had escaped from cultivation and was growing wild in the fields. In addition to a large number of plants that showed only minor differences among them- selves, De Vries found several individuals growing among the typical individuals which differed not merely in degree but in kind. These were as different as distinct varieties, and, when the seeds were planted in the garden they bred true to their kind. The only ques- tion now was whether they had actually arisen from typical parents. To test this possibility, seeds of several typical plants were planted in the garden; the result being not only a repetition of the pecuHar types observed in the field, but of about a dozen other true breed- ing types with well-marked differences from the parent-species and among themselves. These new types De Vries considered as new elementary species and he called them "mutants." They came into existence suddenly in one generation and, as a rule, bred true. Whatever factors were responsible for mutations, the seat of origin must have been in the germ cell and not in the soma. Consequently they were inherited fully from the start. The same mutations occurred in considerable numbers and in successive years. In one case a given mutation occurred only once in eight years of observation. Some mutants were robust and successful, others were weak and incapable of living under natural conditions, others were sterile. On the basis of these results, which are reported in detail in chapter xxiv, De Vries came to the conclusion that evolution was based upon the sudden appear- ance of new varieties or elenientary species and not upon the natural selection of fluctuating variations. The mutation theory compared and contrasted with the natural selection theory. — It will be recalled that the raw material upon which natural selection works is the minute individual or continuous varia- tion that is universal in all living forms and is known to be largely somatic in character and due to differences in environment. Darwin SS READINGS IN EVOLUTION, GENETICS, AND EUGENICS did not distinguish between somatic and germinal variations. The essential feature of mutations is that they are germinal in origin and therefore come forth full-fledged in the first generation arising from the changed germ. Darwin recognized '^saltatory variations" or ''sports/' which are mutations, but did not consider them of suffi- ciently frequent occurrence to furnish an adequate material for selection. De Vries, on his side, did not discard the principle of selection, but showed that selection acted as between mutants, serving to elimi- nate those which are unfit and allowing the sufficiently fit to survive alongside the parent-types. According to Darwin's view, the new t\^es arose only at the expense of the old, for only through the elimina- tion of the old (less fit) types could the new types progress toward further fitness. Darwin's view was ill suited to explain the origin of new distinct types, because the process of selection proceeded by imperceptible steps. De Vries's view gives us distinctly different, pure breeding types at once that, if isolated, would be new elementary species from the first. In conclusion it may be said that the mutation theory was at first intended as a substitute for natural selection, but that later the selection idea was adopted as a directive principle, guiding mutations toward adaptiveness. THE RISE AND VOGUE OF BIOMETRY No historical account of the development of the evolution idea would be complete without a statement of the role played by biometry in the study of evolutionary data. Biometry is the statistical study of variation and heredity. During the last decade of the nineteenth century it became obvious to those who had followed the progress of the subject that farther advance toward the solution of the problem of the causes of evolution must come from a better under- standing of variation and heredity, the two fundamental factors involved. Three main modes of attack were developed during these years: the statistical (biometry), the experimental (chiefly breeding work), and the microscopical (cytology or the study of the minute structure of the germ cells). Sir Francis Gallon, a cousin of Charles Darwin, was the founder of biometry. He applied certain already understood principles that had been developed mainly in the study of the laws of chance to the study of variations, and, by comparing the boiled-down formulas HISTORICAL ACCOUNT OF EVOLUTION THEORY 39 resulting from his computations of parental generations with those of offspring, he arrived at two laws of heredity: the law of filial regres- sion, and that of ancestral shares of inheritance. The essence of the first was that the offspring of exceptional parents tend to regress toward mediocrity in proportion to the degree of parental excep- tionalness. The second law was really explanatory of the first, for it was found that the offspring inherit not only from parents, but from the various grades of ancestors, and it was the pulldown of a miscel- laneous ancestry that made for regression toward mediocrity. It appeared that half of the hereditary influence could be assigned to parents, half of the remainder to grandparents, half of the remaining remainder to great-grandparents, and so on down the line. Karl Pearson, a pupil and follower of Galton, has carried the study of biometry to a more highly refined state. His attempt has been to apply to the study of evolution the precise quantitative methods which are used in physics and in chemistry. While much of Pearson's work is far beyond the range of the average professional biologist today, it is extremely useful as a tool in handling data in which great accuracy is demanded. Frequently, however, the methods are far too refined for the material, and much time is wasted in handling crude data by means of highly refined instruments of measurement and ultra- accurate mathematical methods. On the whole the contributions of biometry to our understanding of the causes of evolution are rather disappointing. About the only clean-cut finding has been the discovery that some variations are continuous and others discontinuous. The former are capable of being expressed in a single curve with a single mode, while the latter are expressed in bimodal or polymodal curves. If material is homo- geneous to start with it is likely to give monomodal curves, but if it is heterogeneous, its heterogeneity will be revealed by the plural modes. In a subsequent connection (chapter xxv) some further account of the details of biometry will be presented. We must for the present be content with having placed biometry in its setting as one step in the advance of the evolution idea. MODERN EXPERIMENTAL EVOLUTION ''While De Vries," says Castle,^ ''was engaged in his studies of the evening primrose he hit upon an idea far more important, as most biologists now believe, than the idea of mutation, though De Vries ^ W. E, Castle, Genetics and Eugenics (Harvard University Press, 1920), p. 82. 40 READINGS IN EVOLUTION, GENETICS, AND EUGENICS himself, both before and since, has seemed to regard it as of minor importance. He called this the Haw of splitting of hybrids.^ The same law, it is claimed, was independently discovered about the same time by two other botanists, Correns in Germany, and Tschermak in Austria. Further, historical investigations made by De Vries showed that the same law had been discovered and clearly stated many years previously by an obscure naturalist of Briinn, Austria, named Gregor Mendel, and we have now come to call this law by his name, MendeVs Law. ^lendel was so little known .when his discovery was published that it attracted little attention from scientists and was soon forgotten, only to be unearthed and duly honored years after the death of its author. Had Mendel lived forty years later than he did, he would doubtless have been a devotee of biometry, for he had a mathematical type of mind and his discovery of a law of hybridization was due to the fact that he applied to his biological studies methods of numerical exactness which he had learned from algebra and physics. In biology he was an amateur, being a teacher of the physical and natural sciences in a monastic school at Briinn. Later he became head of the monastery and gave up scientific work, partly because of other duties, partly because of failing eyesight." There had been plant-hybridizers before Mendel, but their lack of exactness in technique had prevented them from discovering the law of segregation or splitting of hybrids. Joseph Gottlieb Kolrenter (1783-1806), who really belonged to the period of Lamarck, barely missed making the discovery that was afterward made by Mendel. The salient features of his work are according to Castle:^ " I. Kolreuter established the occurrence of sexual reproduction in plants by showing that hybrid offspring inherit equally from the pollen plant and the seed plant. ''2. He showed that hybrids are commonly intermediate between their parents in nearly all characters observed, such for example as size and shape of parts. ''3. Many hybrids are partially or wholly sterile, especially when the parents are very dissimilar (belong to widely distinct species). Such hybrids often exceed either parent in size and vigor of growth. "4. Kolreuter did not observe the regular splitting of hybrids which Mendel and De Vries record, but some of his successors did, particularly Thomas Knight (1799) and John Goss (1822) in England, » Op. clL, p. 80, HISTORICAL ACCOUNT OF EVOLUTION THEORY 41 who were engaged in crossing the garden peas with a view to producing more vigorous and productive varieties, and Naudin (1862) in France, who made a comprehensive survey of the facts of hybridization in plants and came very near to expressing the generaUzation which Mendel reached four years later." Mendel's law "The earliest experimental investigations of heredity," says Locy^ in a concise summary of Mendel's work, ''were conducted with 'plants, and the first epoch-making results were those of Gregor Mendel (1822-1884), a monk and later abbot, of an Augustinian monastery at Briinn, Austria. In the garden of the monastery, for eight years before publishing his results, he made experiments on the inheritance of individual (or unit) characters in twenty-two varieties of garden peas. Selecting certain constant and obvious characters, as color, and form of seed, length of stem, etc., he proceeded to cross these pure races, thus producing hybrids, and thereafter, to observe the results of self-fertilization among the hybrids. ''The hybrids were produced by removing the unripe stamens of certain flowers and later fertilizing them by ripe pollen from another pure breed having a contrasting character. The results showed that only one of a pair of unit characters appeared in the hybrid of the next generation, while the other contrasting character lay dormant. Thus, in crossing a yellow-seeded with a green-seeded pea, the hybrid genera- tion showed only yellow seeds. The character thus impressing itself on the entire progeny was called dominant^ while the other that was held in abeyance was designated recessive. "That the" recessive color was not blotted out was clearly demon- strated by allowing the hybrid generation to develop by self-fertiliza- tion. Under these circumstances a most interesting result was attained. The filial generation, derived by self-fertilization among the hybrids, produced plants with yellow and green seeds, but in the ratio of three yellow to one green. All green-seeded individuals and one-third of the yellow proved to breed true, while the remaining two thirds of the yellow-seeded plants, when self-fertilized, produced yellow and green seeds in the ratio of three to one. "Subsequent breedings gave an unending series of results similar to those obtained with the first filial generation. ^ William A. Locy, The Main Currents of Zoology (Henry Holt & Company, 1918), pp. 37-39. 42 READINGS IN EVOLUTION, GENETICS, AND EUGENICS ''This great principle of alternative inheritance was exhibited throughout the extensive experiments of Mendel, and it is now recog- nized as one of the great biological discoveries of the nineteenth century." The essential feature of Mendel's discovery was not the phenome- non of dominance, for relatively few instances of pure dominance have been discovered; but it was the phenomenon of segregation. By segregation is meant that although determiners for opposed heredi- tary characters derived from diverse parental sources may unite in a common germ plasm for one generation, they segregate out pure, or unmodified by their association together, in the next and subsequent generations. This law of segregation depends on the idea that the germ cell is composed of bundles of separately inheritable unit charac- ters, which may be paired or grouped, shuffled and redealt like cards, so as to give an infinite number of permutations and combinations without affecting the unit determiners themselves. From the evolutionary standpoint it is supposed that new unit characters arise by mutations and are fully hereditary. They cannot be swamped out by interbreeding unless they are recessive, for they will dominate the old characters. Even recessive characters could be perpetuated by segregation, or by the union of two individuals possess- ing tlie determiner in the recessive condition as well as the dominant. Thus a knowledge of the behavior of unit characters in heredity reveals part of the mechanism for conserving new characters if they are advantageous or even sufficiently fit to survive. New types or species might arise through processes of hybridiza- tion and the survival of individuals possessing the most favorable combinations of characters. "Evolution from this point of view," says Morgan,^ "has consisted largely in introducing (by mutations) new factors that influence characters already present in the animal or plant. "Such a view gives us a somewhat different picture of evolution from the old idea of a ferocious struggle between the individuals of a species with the survival of the fittest and the annihilation of the less fit. Evolution assumes a more peaceful aspect. New advantageous characters survive by incorporating themselves into the race, improv- ing it and opening to it new opportunities. In other words, the emphasis may be placed less on the competition between the indi- ^ T. H. ]\Iorgan, A Critique of the Theory of Evolution (Princeton University Press, 1916), pp. 87, 88. HISTORICAL ACCOUNT OF EVOLUTION THEORY 43 viduals of a species (because the destruction of the less fit does not in itself lead to anything that is new) than on the appearance of new characters and modifications of old characters that become incorpo- rated in the species, for on these depends the evolution of the race." HYBRIDIZATION AND THE ORIGIN OF SPECIES As a consequence of the great interest aroused by Mendel's hybridization experiments the question has arisen as to the role of hybridization in organic evolution. Certain it is that a vast number of animal and plant races now existing are mixed or hybrid in nature and are continually sphtting up into various Mendelian segregates. How many pure races are there today ? Some authors think that no variable races today are pure. Lotzy goes so far as to claim and attempt to prove that unit characters are fixed and that the only source of variation is hybridization, or amphimixis. Biologists today would not be willing to go thus far with Lotzy, but it seems beyond question that hybridization has played an important role in the pro- duction of very many groups now living. It is of interest to recall that Linnaeus, though a special creationist, admitted the possibility of the origin of new species by hybridization. NEO-MENDELIAN DEVELOPMENTS Since the rediscovery of Mendel's paper by De Vries and its perusal by thousands of biologists the world over, Mendelian breeding experi- ments with all manner of animals and plants has been the ruling passion of geneticists. Among the leading neo-Mendelians are Bate- son, Morgan, Castle, Correns, East, Hurst, Shull, Tschermak, and the pupils of these. Perhaps the first two mentioned, Bateson and Morgan, have con- tributed most largely to an understanding of the intricacies of the Mendelian operations. Bateson has become so imbued with the idea that all mutations are the result of the loss of factors that he proposes the hypothesis that ''evolution has taken place through the steady loss of inhibiting factors," as Morgan puts it. "Living matter was stopped down, so to speak, at the beginning of the world. As the stops are lost, new things emerge. Living matter has changed only in that it becomes simpler." It is quite probable that Bateson, in pro- posing so radical a view, intended to be taken only half-seriously. Apart from this, his best-known expression of opinion, Bateson is the 44 READINGS IN EVOLUTION, GENETICS, AND EUGENICS author of a large amount of fine work in genetics and will rank high in the history of the subject. T. H. Morgan, our leading American geneticist, is best known for his researches into the mechanism of Mendelian inheritance. Through the statistical study of ratios and linkages of characters in the fruit fly Drosophila, it has been possible to chart the localities of the deter- miners or genes of at least 150 mutant characters. He has shown that four linked groups of genes exist, corresponding to the four kinds of chromosomes of the germ cells; one of these groups is sex-linked and is therefore to be assigned to the X-chromosome of the mutant male. Two other large groups are to be located in the two large autosomes, and one very small group is assumed to be located in the microsome. Not only have characters, or their determiners, been assigned to given chromosomes, but they have been located in a linear series on a given chromosome. So accurately have these loci been determined that they may be used to predict unknown breeding ratios. It would seem that when a theory serves so well that it may be used to predict the results of experiments, such a theory must be founded on facts. Morgan and his collaborators in genetics are now convinced that they have discovered the actual mechanism of heredity in the behavior of the chromosomes in maturation and fertilization and that it is unex- pectedly simple. Their views have aroused considerable opposition, but they have met successfully all attacks up to the present. If it be true that the actual machinery of variation and heredity has been dis- covered, we are farther along in our understanding of the causo- mechanical basis of evolution than we could have hoped to be at so early a date. HEREDITY AND SEX Since Darwin's theory of sexual selection, sex has been a compli- cating factor in evolutionary theories, and one of the chief advances of the present century has been in connection with the factors con- trolling sex determination and sex differentiation. The evolution of sex has also been a subject for considerable research. It now appears that sex is an inherited Mendelian character, the determiner of which is carried in a definite chromosome or group of chromosomes. Cytological examination of germ cells, under the able leadership of E. B. Wilson, has now made it certain that sex, if not directly the result of the presence or absence of specific chromo- somes, at least is absolutely correlated with such chromosomes. It appears, however, that the sex which is settled by the chromosome HISTORICAL ACCOUNT OF EVOLUTION THEORY 45 mechanism at the time of fertihzation may or may not reaUze its normal somatic differentiation, depending upon the presence or absence of the proper environment. Cases are on record in which an individual germinally determined as a female may be caused to develop the secondary sexual characters of the male, or even to pro- duce sperms instead of eggs. A great deal of extremely interesting work on sex control and sex reversals has been done within the last half-dozen years and new discoveries are being made almost daily. In fact, it might be said that the genetic study of sex marks the high-tide level of modern genetic advance. CONCLUDING REMARKS Now that we have traced the evolution of the science of organic evolution from its crude beginnings among the Greeks up to the present, we are in a position to go back and make a systematic study of some of the more important phases of evolutionary science. Charles Darwin found it necessary to prove the fact of organic evolu- tion before attempting to discover- its causes. His method of proof was to marshal a great array of facts which agree with the idea of descent with modification; and we shall follow Darwin's method in the subsequent chapters dealing with the evidences of evolution. Note. — In the first half of the present historical account many short passages are presented in quotation marks without mentioning the source of the quotation. In all such cases it will be understood that these passages are from H. F. Osborn's book, From the Greeks to Darwin (The Macmillan Company). CHAPTER III THE RELATION OF EVOLUTION TO MATERIALISM^ Joseph Le Conte It is seen in the sketch given in the previous chapter that, after every struggle between theology and science, there has been a read- justment of some beliefs, a giving up of some notions which really had nothing to do with religion in a proper sense, but which had become so associated with religious belief as to be confounded with the latter — a giving up of some line of defense which ought never to have been held because not within the rightful domain of theology at all. Until the present the whole difficulty has been the result of misconception, and Christianity has emerged from every struggle only strengthened and purified, by casting off an obstructing shell which hindered its growth. But the present struggle seems to many an entirely different and far more serious matter. To many it seems no longer a struggle of theology, but of essential religion itself — a deadly life-and-death struggle between religion and materialism. To many, both skeptics and Christians, evolution seems to be synonymous with blank mate- rialism, and therefore cuts up by the roots every form of religion by denying the existence of God and the fact of immortality. That the enemies of religion, if there be any such, should assume and insist on this identity, and thus carry over the whole accumulated evidence of evolution as a demonstration of materialism, although wholly unwar- ranted, is not so surprising; but what shall we say of the incredible folly of her friends in admitting the same identity! A little reflection will explain this. There can be no doubt that there is at present a strong and to many an overwhelming tend- ency toward materialism. The amazing achievements of modern science; the absorption of intellectual energy in the investigation of external nature and the laws of matter have created a current in that direction so strong that of those who feel its influence — of those who do not stay at home, shut up in their creeds, but walk abroad in the light of modern thought — it sweeps away and bears on its bosom all ^ From J. Le Conte, Evolution (copyright 1888). Used by special permission of the publishers, D. Appleton & Company. 46 THE RELATION OF EVOLUTION TO MATERIALISM 47 but the strongest and most reflective minds. Materialism has thus become a fashion of thought; and, hke all fashions, must be guarded against. This tendency has been created and is now guided by science. Just at this time it is strongest in the department of biology, and especially is evolution its stronghold. This theory is supposed by many to be simply demonstrative of materialism. Once it was the. theory of gravitation which seemed demonstrative of materialism. The sustentation of the universe by law seemed to imply that Nature operates itself and needs no God. That time is passed. Now it is evolution and creation by law. This will also pass. The theory seems to many the most materialistic of all scientific doctrines only because it is the last which is claimed by materialism, and the absurdity of the claim is not yet made clear to many. The truth is, there is no such necessary connection between evo- lution and materialism as is imagined by some. There is no dif- ference in this respect between evolution and any other law of Nature. In evolution, it is true, the last barrier is broken down, and the whole domain of Nature is now subject to law; but it is only the last; the march of science has been in the same direction all the time. In a word, evolution is not only not identical with materialism, but, to the deep thinker, it has not added a feather's weight to its proba- bility or reasonableness. Evolution is one thing and materialism quite another. The one is an established law of Nature, the other an unwarranted and hasty inference from that law. Let no one imagine, as he is conducted by the materialistic scientist in the paths of evo- lution from the inorganic to the organic, from the organic to the animate, from the animate to the rational and moral, until he lands, as it seems to him, logically and inevitably, in universal material- ism — let no such one imagine that he has walked all the way in the domain of science. He has stepped across the boundary into the domain of philosophy. But, on account of the strong tendency to materialism and the skilful guidance of his leaders, there seems to be no such boundary; he does not distinguish between the induc- tions of science and the inferences of a shallow philosophy; the whole is accredited to science, and the final conclusion seems to carry with it all the certainty which belongs to scientific results. The fact that these materiaHstic conclusions are reached by some of the foremost scientists of the present day adds nothing to their probabihty. In a question of science, viz., the law of evolution, their authority is deservedly high, but in a question of philosophy, viz., 48 READINGS IN EVOLUTION, GENETICS, AND EUGENICS materialism, it is far otherwise. If the pure scientists smile when theological philosophers, unacquainted with the methods of science, undertake to dogmatize on the subject of evolution, they must pardon the philosophers if they also smile when the pure scientists imagine that they can at once solve questions in philosophy which have agitated the human mind from the earliest times. I am anxious to show the absurdity of this materialistic conclusion, but I shall try to do so, not by any labored argument, but by a few simple illustra- tions. 1. It is curious to observe how, when the question is concerning a work of Nature, we no sooner find out how a thing is made than we immediately exclaim: "It is not made at all, it became so of itself!" So long as we knew not how worlds were made, we of course con- cluded they must have been created, but so soon as science showed how it was probably done, immediately we say we were mistaken — they were not made at all. So also, as long as we could not imagine how new organic forms originated, we were willing to believe they were created, but, so soon as we find that they originated by evolution, many at once say: ''We were mistaken; no creator is necessary at all." Is this so when the question is concerning a work of man? Yes, of one kind — viz., the work of the magician. Here, indeed, we believe in him, and are delighted with his work, until we know how it is done, and then all our faith and wonder cease. But in any honest work it is not so ; but on the contrary, when we under- stand how it is done, stupid wonder is changed into intellectual delight. Does it not seem, then, that to most people God is a mere wonder-worker, a chief magician ? But the mission of science is to show us how things are done. Is it any wonder, then, that to such persons science is constantly destroying their superstitious illusions ? But if God is an honest worker, according to reason — i.e., according to law — ought not science rather to change gaping wonder into intelligent delight, superstition into rational worship ? 2. Again, it is curious to observe how an old truth, if it come only in a new form, often strikes us as something unheard of, and even as paradoxical and almost impossible. A little over thirty years ago a little philosophical toy, the gyroscope, was introduced and became very common. At first sight, it seems to violate all mechanical laws and set at naught the law of gravitation itself. A heavy brass wheel, four to five inches in diameter, at the end of a horizontal axle, six or eight inches long, is set rotating rapidly, and then the free end of the THE RELATION OF EVOLUTION TO MATERIALISM 49 axis is supported by a string or otherwise. The wheel remains suspended in the air while slowly gyrating. What mysterious force sustains the wheel when its only point of support is at the end of the axle, six or eight inches away ? Scientific and popular literature were flooded with explanations of this seeming paradox. And yet it was nothing new. The boy's top, that spins and leans and will not fall, although solicited by gravity, so long as it spins, which we have seen all our lives without special wonder, is precisely the same thing. Now, evolution is no new thing, but an old familiar truth; but, coming now in a new and questionable shape, lo, how it startles us out of our propriety ! Origin of forms by evolution is going on everywhere about us, both in the inorganic and the organic world. In its more familiar forms, it had never occurred to most of us that it was a scientific refutation of the existence of God, that it was a demonstra- tion of materialism. But now it is pushed one step farther in the direction it has always been going — it is made to include also the origin of species — only a little change in its form, and lo, how we start! To the deep thinker, now and always, there is and has been the alterna- tive — materialism or theism. God operates Nature or Nature operates itself; but evolution puts no new phase on this old question. For example, the origin of the individual by evolution. Everybody knows that every one of us individually became what we now are by a slow process of evolution from a microscopic spherule of protoplasm, and yet this did not interfere with the idea of God as our individual maker. Why, then, should the discovery that the species (or first individuals of each kind) originated by evolution destroy our belief in God as the creator of species ? 3. It is curious and very interesting to observe the manner in which vexed questions are always finally settled, if settled at all. All vexed questions — i.e., questions which have taxed the powers of the greatest minds age after age — are such only because there is a real truth on both sides. Pure, unmixed error does not five to plague us long. Error, when it continues to live, does so by virtue of a germ of truth contained. Great questions, therefore, continue to be argued pro and con from age to age, because each side is in a sense — i.e., from its own point of view — true, but wrong in excluding the other point of view; and a true solution, a true rational philosophy, will always be found in a view which combines and reconciles the two partial, mutually excluding views, showing in what they are true and in what they are false — explaining their differences by transcending 50 READINGS IN EVOLUTION, GENETICS, AND EUGENICS them. This is so universal and far-reaching a principle that I am sure I will be pardoned for illustrating it in the homeliest and tritest fashion. I will do so by means of the shield with the diverse sides, giving the story and construing it, however, in my own way. There is, appar- ently, no limit to the amount of rich marrow of truth that may be extracted from these dry bones of popular proverbs and fables by patient turning and gnawing. We all remember, then, the famous dispute concerning the shield, with its sides of different colors, which we shall here call white and black. We all remember how, after vain attempts to discover the truth by dispute, it was agreed to try the scientific method of investi- gation. We all remember the surprising result. Both parties to the dispute were right and both were wrong. Each was right from his point of view, but wrong in excluding the other point of view. Each was right in what he asserted, and each wrong in what he denied. And the complete truth was the combination of the partial truths and the elimination of the partial errors. But we must not make the mis- take of supposing that truth consists in compromise. There is an old adage that truth lies in the middle between antagonistic extremes. But it seems to us that this is the place of safety, not of truth. This is the favorite adage, therefore, of the timid man, the time-server, the fence-man, not the truth-seeker. Suppose there had been on the occasion mentioned above one of these fence-philosophers. He would have said: "These disputants are equally intelligent and equally valiant. One side says the shield is white, the other that it is black; now truth lies in the middle; therefore, I conclude the shield is gray or neutral tint, or a sort of pepper-and-salt. " Do we not see that he is the only man who has no truth in him? No; truth is no hetero- geneous mixture of opposite extremes, but a stereoscopic combination of two surface views into one solid reality. Now, the same is true of all vexed questions, and I have given this trite fable again only to apply it to the case in hand. There are three possible views concerning the origin of organic forms whether individual or specific. Two of these are opposite and mutually excluding; the third combining and reconciling. For example, take the individual. There are three theories concerning the origin of the individual. The first is that of the pious child who thinks that he was made very much as he himself makes his dirt-pies; the second is that of the street-gamin, or of Topsy, who says: "I was not made at all, I growed''; the third is that of most intelligent THE RELATION OF EVOLUTION TO MATERIALISM 51 Christians — i.e., that we were made by a process of evolution. Observe that this latter combines and reconciles the other two, and is thus the more rational and philosophical. Now, there are also three exactly corresponding theories concerning the origin of species. The first is that of many pious persons and many intelligent clergymen, who say that species were made at once by the Divine hand without tiatural process. The second is that of the materialists, who say that species were not made at all, they were derived, '^they growed." The third is that of the theistic evolutionists, who think that they were created by a process of evolution — who believe that making is not incon- sistent with growing. The one asserts the divine agency, but denies natural process; the second asserts the natural process, but denies divine agency; the third asserts divine agency by natural process. Of the first two, observe, both are right and both wrong; each view is right in what it asserts, and wrong in what it denies — each is right from its own point of view, but wrong in excluding the other point of view. The third is the only true rational solution, for it includes, combines, and reconciles the other two; showing wherein each is right and wherein wrong. It is the combination of the two partial truths, and the elimination of the partial errors. But let us not fail to do perfect justice. The first two views of origin, whether of the indi- vidual or of the species, are indeed both partly wrong as well as partly right; but the view of the pious child and of the Christian con- tains by far the more essential truth. Of the two sides of the shield, theirs is at least the whiter and more beautiful. But, alas! the great bar to a speedy settlement of this question and the adoption of a rational philosophy is not in the head, but in the heart — is not in the reason, but in pride of opinion, self-conceit, dogmatism. The rarest of all gifts is a truly tolerant, rational spirit. In all our gettings let us strive to get this, for it alone is true wisdom. But we must not imagine that all the dogmatism is on one side, and that the theological. Many seem to think that theology has sl'' pre- emptive right'' to dogmatism. If so, then modern materialistic science has "jumped the claim.'' Dogmatism has its roots deep-bedded in the human heart. It showed itself first in the domain of theology, because there was the seat of power. In modern times it has gone over to the side of science, because here now is the place of power and fashion. There are two dogmatisms, both equally opposed to the true rational spirit, viz., the old theological and the new scientific. The old clings fondly to old things, only because they are old; the new grasps eagerly 52 READINGS IN EVOLUTION, GENETICS, AND EUGENICS after new things, only because they are new. True wisdom and true philosophy, on the contrary, tries all things both old and new, and holds fast only to that which is good and true. The new dogmatism taunts the old for credulity and superstition; the old reproaches the new for levity and skepticism. But true wisdom perceives that they are both equally credulous and equally skeptical. The old is credulous of old ideas and skeptical of new; the new is skeptical of old ideas and credulous of new. Both deserve the unsparing rebuke of all right- minded men. The appropriate rebuke for the old dogmatism has been already put in the mouth of Job in the form of a bitter sneer : "No doubt ye are the people, and wisdom shall die with you." The appropriate rebuke for the new dogmatism, though not put into the mouth of any ancient prophet, ought to be uttered — I will under- take to utter it here. I would say to these modern materialists, ''No doubt ye are the men, and wisdom and true philosophy were horn with you." Let it be observed that we are not here touching the general ques- tion of the personal agency of God in operating Nature. This we shall take up hereafter. All that we wish to insist on now is that the process and the law of evolution does not differ in its relation to materialism from all other processes and laws of Nature. If the sustentation of the universe by the law of gravitation does not disturb our belief in God as the sustainer of the universe, there is no reason why the origin of the universe by the law of evolution should disturb our faith in God as the creator of the universe. If the law of gravitation be regarded as the Divine mode of sustentation, there is no reason why we should not regard the law of evolution as the Divine process of creation. It is evident that if evolution be materialism, then is gravitation also materialism; then is every law of Nature and all science materialism. If there be any difference at all, it consists only in this : that, as already said, here is the last line of defense of the supporters of supernatural- ism in the realm of Nature. But being the last line of defense — the last ditch — it is evident that a yielding here implies not a mere shifting of line, but a change of base; not a readjustment of details only, but a reconstruction of Christian theology. This, I believe, is indeed necessary. There can be little doubt in the mind of the thoughtful observer that we are even now on the eve of the greatest change in traditional views that has taken place since the birth of Christianity. But let no one be greatly disturbed thereby. For then, so now, change comes not to destroy but to fulfil all our dearest THE RELATION OF EVOLUTION TO MATERIALISM 53 hopes and aspirations; as then, so now, the germ of living truth has, in the course of ages, become so encrusted with meaningless traditions which stifle its growth that it is necessary to break the shell to set it free; as then, so now, it has become necessary to purge religious belief of dross in the form of trivialities and superstitions. This has ever been and ever will be the function of science. The essentials of religious faith it does not, it cannot, touch, but it purifies and ennobles our conceptions of Deity, and thus elevates the whole plane of religious thought. PART II EVIDENCES OF ORGANIC EVOLUTION CHAPTER IV IS ORGANIC EVOLUTION AN ESTABLISHED PRINCIPLE ? H. H. Newman 1. Is there definite proof of organic evolution ? 2. If so, what is the nature of the proof ? 3. What are the evidences of evolution, and in what ways do these bear witness that evolution has occurred and is still occurring ? Before presenting in any detail the several bodies of data that constitute the "evidences of evolution," let us anticipate a little by attempting to answer the three questions just propounded. I. Reluctant as he may be to admit it, honesty compels the evolutionist to admit that there is no absolute proof of organic evolution. But, for that matter, there is no absolute proof of any- thing that depends on records of past events. We have no absolute proof that Caesar or Napoleon once lived, or fought, or conquered. All we have are the accounts left by the historians which we accept without question because they are the products of human thought and imagination. There is no absolute proof for either of the more or less directly opposed theories of the origin of the material universe: the ''nebular hypothesis" of Laplace, and the " planetesimal hypothesis" of Chamberlin and Moulton. Both of these theories rest upon exactly the same types of evidences as does the theory of organic evolu- tion, viz., the amassing of facts which appear to be explicable on the assumption that the one or the other theory is true. If all of the facts are in accord with it, and none are found that are incapable of being reconciled with it, a working hypothesis is said to have been advanced to the rank of a proved theory. As yet it is impossible to say that either of these theories as to the origin of the universe has been proved. Yet there is much less popular opposition to the acceptance of these theories as facts than there is to the general theory of organic evolu- tion. Similarly, there are certain widely accepted theories of the origin of the present conditions of the earth's crust, and its liquid and gaseous envelopes. The accepted theory, as given us by Hut ton and especially by Lyell, is essentially an evolutionary theory and depends for its proof on almost exactly the same types of evidence as does that 57 58 READINGS IX EVOLUTION, GENETICS, AND EUGENICS of organic evolution. The basis of the accepted theory of geological evolution is the " uniformitarian doctrine" of Lyell, which assumes that the key to the past lies in the present, that the changes that are going on today are of the same order and kind as those of the past, and, finally, that there is neither beginning nor end to the earth's evolutionary history, but that a slow and orderly development has gone on and will continue indefinitely. The proof of this conception consists of an array of facts derived from a study of the earth's crust, including its stratified structure, of traces of animal and plant life preserved in the rocks, of observed changes in continental contours going on today, of erosion going on in coasts and streams, and of a considerable array of facts derived from a study of other worlds than ours in the making. The theory of geologic evolution meets with scarcely any opposition today, although its foundations are no more securely based than are those of organic evolution. In a sense the proofs of the atomic, ionic, and electron theories are even less absolutely estabhshed than is that of organic evolution, because no one has ever seen nor ever can see an atom, an ion, or an electron. Chemical and physical facts are rationalized by assuming the existence of these units with their various properties. The only evidences of the existence of atoms, ions, and electrons appear in the facts that, on the assumption that they exist, the whole array of observed chemical and physical phenomena are rationalized and bound together into a coherent, consistent, and intelligible system. In other words, with the atomic, ionic, and electron theories chemistry and physics are highly rational sciences; without these theories the phenomena of physics and chemistry would be a hopeless hodgepodge. Yet who would say that these fundamental theories are absolutely proved ? The only type of proof of phenomena that cannot be directly observed or that pertain to the remote past is circumstantial proof. By analogy we conclude that certain changes took place thus and so in the past because we observe similar changes going on today. Every past event has left a trace, and it is the task of the historian, anti- quarian, or evolutionist to discover and to interpret these traces. Some- times the traces exist as vestiges in modern life and are meaningless unless related to their origin in the past. The task of the student of organic evolution is to gather all of the traces of past changes both in living creatures today and in the preserved remains of creatures of the remote past. A collection of traces of evolution involves many IS ORGANIC EVOLUTION ESTABLISHED ? 59 apparently unrelated bodies of phenomena. There are evidences of evolution in the grouping of animals into phyla, classes, orders, families, genera, species, varieties, and races; in the homologies that exist in general structure and in particular organs between different groups of animals and plants; in the orderly process of ontogeny or embryonic development of the individual; in actual blood relation- ship, based upon chemical reactions; on the succession of extinct animals and plants found as fossils imbedded in the geologic strata; in the present geographical distribution of the various groups of animals and plants, in the light of data derived from a study of geological changes; and finally, in experimental evolution, which involves the observation under experimental control of changes in organisms and the origin of new varieties or elementary species. 2. The nature of the proof of organic evolution, then, is this: that, using the concept of organic evolution as a working hypothesis it has been possible to rationalize and render intelligible a vast array of observed phenomena, the real facts upon which evolution rests. Thus classification (taxonomy), comparative anatomy, embryology, palaeontology, zoogeography and phytogeography, serology, genetics, become consistent and orderly sciences when based upon evolu- tionary foundations, and when viewed in any other way they are thrown into the utmost confusion. There is no other generalization known to man which is of the least value in giving these bodies of fact any sort of scientific coherence and unity. In other words, the working hypothesis works and is therefore acceptable as truth until overthrown by a more workable hypothesis. Not only does the hypothesis work, but, with the steady accumulation of further facts, the weight of evidence is now so great that it overcomes all intelligent opposition by its sheer mass. There are no rival hypotheses except the outworn and completely refuted idea of special creation, now retained only by the ignorant, the dogmatic, and the prejudiced. 3. In answer to the question, ''What are the evidences of evolution and in what ways do these bear witness that evolution has occurred and is still occurring?" we may present an ordered hst of subjects that are to be taken up serially in detail. In connection with each of these bodies of evidence the character of their witness-bearing will be discussed. Some of the evidences are more direct and freer from purely inter- pretative construction than others. Some evidences are primary and foundational; some are in themselves rather inconclusive, but serve 6o REx\DIXGS IN EVOLUTION, GENETICS, AND EUGENICS to confirm other facts, and, when reinforced by other evidences, are themselves strongly substantiated. Perhaps the crowning evidence of the truth of evolution is that all of these diverse bodies of phenomena invariably support one another and all point in the same direction and to the same conclusion, viz., that organic evolution is a fact. In presenting the evidences of evolution, those evidences that are believed to furnish the most direct proof are discussed first and those whose evidence is subsidiary and confirmatory are dealt with later. The order of treatment, therefore, will be as follows: I. Palaeontology — the evidence afforded by a study of the dis- tribution in time (vertical distribution in the earth's strata) of the fossil remains of extinct animals and plants. II. Geographic distribution — the evidence afforded by present (also, to some extent, past) horizontal distribution of contemporaneous animals and plants. III. Classification — the evidence that the present groups of animals and plants have arisen by ''descent with modification," which is an evolutionary conception. IV. Comparative anatomy {homologies and vestigial structures) — • the evidence derived from the fact that structures in unlike organisms have a common plan and mode of origin; that changes have occurred which are in some way related to changes of habit or of environment. V. Serology {blood-transfusion tests) — the evidence that the chemical specificity of the blood parallels taxonomic specificity. VI. Ejnbryology {the doctrine of recapitulation) — the evidence that the embryonic development of the individual follows the main outlines of the evolutionary history of its ancestors. VII. Experimental evolution {genetics) — evidences that heritable variations can be produced experimentally and that these are of the same general character as those which occur spontaneously in Nature. (This material will be presented in some detail in Part IV of this book.) CHAPTER V EVIDENCES FROM PALAEONTOLOGY STRENGTH AND WEAKNESS OF THE EVIDENCE [The word palaeontology means literally the science of ancient life. Practically, it is the study of the fossil remains of extinct animals and plants, including any traces of their existence, such as footprints, impressions in slate, clay, or coal. The evidence from the fossils has definite elements of strength in that it deals with actual organisms that formerly inhabited the earth's surface. Many of these species must have left descendants, some of which are doubtless living in a modified condition today. Palaeontology should be able either strongly to support or to contradict the idea of evolution. If its data accord with the evolution idea and are opposed to the special creation idea, the fossils may be said to be evidences of evolution. The weakness of the study of fossils lies in the fact that extremely few samples of the living forms that have existed in the past have been preserved, and of those that have been preserved only a very small percentage have been dug up and studied by capable scientists. Many types of animals and plants, moreover, are soft and capable of preservation only under such exceptional conditions that but a rare specimen here and there over the world, scattered through various widely separated strata, has been found. Only very common or abundant types are likely to have been preserved and discovered, for the chances of an uncommon form being preserved would be small and the further chances of these infrequently preserved specimens being found woul d be infinitely smaller. The great majority of fossil remains are fragmentary or preserved very incompletely, so that only the hard parts have come down to us. There are, of course, many important exceptions to this rule, and these are our chief reliance in interpreting ancient life. That Darwin fully realized the vulnerable points in the palaeonto- logical record is shown by the following quotation from the Origin of Species: — Ed.] ''I look at the geological record as a history of the world imper- fectly kept and written in a changing dialect; of this history we possess 6i 62 READINGS IN EVOLUTION, GENETICS, AND EUGENICS the last volume alone, relating only to two or three countries. Of this volume only here and there a short chapter has been preserved; and of each page only here and there a few lines. Each word of the slowly changing language, more or less different in the successive chapters, may represent the forms of life which are entombed in our successive formations and which falsely appear to us to have been abruptly introduced." OTHER OPINIONS AS TO THE ADEQUACY OF THE EVIDENCES OF PALAEONTOLOGY ''The primary and direct evidence in favour of evolution can be furnished only by palaeontology. The geological record, so soon as it approaches completeness, must, when properly questioned, yield either an affirmative or a negative answer: if Evolution has taken place there will its mark be left; if it has not taken place there will lie its refutation." — T. H. Huxley. "The geological record is not so hopelessly incomplete as Darwin believed it to be. Since The Origin of Species was written our knowl- edge of that record has been enormously extended, and we now possess no complete volumes, it is true, but some remarkably full and illumi- nating chapters. The main significance of the whole lies in the fact ihdii, just in proportion to the completeness of the record is the unequivocal character of its testimony to the truth of the evolutionary theory. ^^ — W. B. Scott. ''On the other hand, matters have greatly improved since Darwin wrote his oft-cited Chapter X; many lands then geologically unknown have been explored and many of the missing chapters and paragraphs in the history of life have been brought to light. The most ancient biologically intelligible period of the earth's history is called the Cambrian and, compared with the succeeding periods, the Cambrian has always been poor in fossils, great areas and thicknesses of rocks being entirely barren. No one could doubt that our knowledge of Cambrian life was most incomplete and inadequate. A few years ago Dr. C. D. Walcott, Secretary of the Smithsonian Institution, dis- covered in the Canadian Rockies a most marvelous series of Cambrian fossils of an incredible delicacy and beauty of preservation, which have thrown a flood of new and unexpected light into very dark places. It is clear that the Cambrian seas swarmed with a great variety and profusion of life, but that in only a few places, so far known to us, EVIDENCES FROM PALAEONTOLOGY 63 * were conditions such that these delicate creatures could be preserved. It is not possible to say how far the difficulty caused by the imperfec- tion of the geological record will be removed by the progress of dis- covery. Even as matters stand to-day, the astonishing fact is that so much has been preserved, rather than that the story is so incom- plete. Notwithstanding all the difficulties, the palaeontological method remains one of the most valuable means of testing the theory of evolution, because certain chapters in the history of life have been recorded with a minuteness that is really very surprising." — W. B. Scott, Theory of Evolution, (The Macmillan Company. Re- printed by permission). WHAT FOSSILS ARE AND HOW THEY HAVE BEEN PRESERVED *' Fossils are only animals and plants which have been dead rather longer than those which died yesterday." — T. H. Huxley. "Fossils are either actual remains of bones or other parts preserved intact in soil or rocks, or else, and more commonly, parts of animals which have been turned into stone, or of which stony casts have been made. All such remains buried by natural causes are called fossils." — Jordan and Kellogg. FOSSILS CLASSIFIED [Class I. The actual remains of recently extinct animals and plants which have been buried or surrounded by some sort of preserv- ing material constitute the first type under consideration. Such remains have undergone little or no change of the original organic matter into inorganic. Thus we find the complete bodies of great hairy mammoths frozen in the arctic ice. These are so well preserved that dogs have fed upon their flesh. Nearly a thousand species of extinct insects, including many ants, have been obtained practically intact from amber, a form of petrified resin. Innumerable mollusk shells, teeth of sharks, pieces of buried logs, bones of animals buried in asphalt lakes and bogs, have been found in a well-preserved condition. Class 2. Petrified fossils. — The process of petrification involves the replacement, particle for particle, of the organic matter of a dead animal or plant by mineral matter. So completely is the finer structure preserved that microscopic sections of preserved tissues, especially of plants, have practically the same appearance as sections .made from living organisms. Various mineral materials have been employed in petrification, such as quartz, hmestone, or iron pyrites. 64 READINGS IN EVOLUTION, GENETICS, AND EUGENICS * Class 3. Casts and impressions. — Very frequently the animal or plant has been buried in mud or has lain on a soft mud flat only long enough to have left its impress in the plastic material. Sub- sequently the entire organism has decayed and been dissolved away, and its place has been taken by a mineral deposit. Thus only the external appearance has been preserved, as would be the case in making plaster-of-paris casts. Sometimes traceries of soft-bodied animals have been left upon forming slate or coal that are almost as accurate in detail as a lithograph. Perhaps the most remarkable fossils known are those found by Professor Charles D. Walcott in the marine oily shales of British Columbia. A large number of soft-bodied invertebrates of Cambrian age have been found so wonderfully preserved that not only are the external features revealed, but sometimes even the details of the internal organs may be seen through the transparent integu- ment. Some authorities include among fossils such traces of extinct life as footprints, utensils and tools of extinct man, and even the vestiges of archaic sea beaches. Perhaps this is stretching the definition of the term "fossil" too far. — Ed.] ON THE CONDITIONS NECESSARY FOB, EOSSILIZATION "Examination and study of the rocks of the earth reveal the fact that fossils or the remains of animals and plants are found in certain kinds of rocks only. They are not found in lava, because 'lava comes from volcanoes and rifts in the earth's crust, as a red-hot, viscous liquid, which cools to form a hard rock. No animal or plant caught in a lava stream will leave any trace. Furthermore, fossils are not found in granite, nor in ores of metals, nor in certain other of the common rocks. Many rocks are, like lava, of igneous origin; others, like granite, although not originally in the melted condition, have been so heated subsequent to their formation, that any traces of animal or plant remains in them have been obliterated. Fossils are found almost exclusively in rocks which have been formed by the slow deposition in water of sand, clay, mud, or lime. The sediment which is carried into a lake or ocean by the streams opening into it sinks slowly to the bottom of the lake or ocean and forms there a layer which gradually hardens under pressure to become rock. This is called sedimentary rock, or stratified rock, because it is composed of sedi- EVIDENCES FROM PALAEONTOLOGY 65 ment, and sediment always arranges itself in layers or strata. In ' sedimentary or stratified rocks fossils are found. The commonest rocks of this sort are limestone, sandstone, and shales. Limestone is formed chiefly of carbonate of lime; sandstone is cemented sand, and' shales, or slaty rocks, are formed chiefly of clay. ''The formation of sedimentary rocks has been going on since land first rose from the level of the sea; for water has always been wearing away rock and carrying it as sediment into rivers, and rivers have always been carrying the w^orn-off lime and sand and clay downward to lakes and oceans, at the bottoms of which the particles have been piled up in layers and have formed new rock strata. But geologists have shown that in the course of the earth's history there have been great changes in the position and extent of land and sea. Sea bottoms have been folded or upheaved to form dry land, while regions once land have sunk and been covered by lakes and seas. Again, through great foldings in the cooling crust of the earth, which resulted in depression at one point and elevation at another, land has become ocean and ocean land. And in the almost unimaginable period of time which has passed since the earth first shrank from its hypo- thetical condition of nebulous vapor to be a ball of land covered with water, such changes have occurred over and over again. They have, however, mostly taken place slowly and gradually. The principal seat of great change is in the regions of mountain chains, which, in most cases, are simply the remains of old folds or wrinkles in the crust of the earth. "When an aquatic animal dies, it sinks to the bottom of the lake or ocean, unless, of course, its flesh is eaten by some other animal. Even then its hard parts will probably find their way to the bottom. There the remains will soon be covered by the always dropping sedi- ment. They are on the way to become fossils. Some land animals also might, after death, get carried by a river to the lake or ocean, and find their way to the bottom, where they, too, will become fossils, or they may die on the banks of the lake or ocean and their bodies , may get buried in the soft mud of the shores. Or, again, they are often trodden in the mire about salt springs or submerged in quick- sand. It is obvious that aquatic animals are far more likely to be . preserved as fossils than land animals. This inference is strikingly proved by fossil remains. Of all the thousands and thousands of kinds of extinct insects, mostly land animals, comparatively few speci- mens are known as fossils. On the other hand, the shell-bearing 66 READINGS IN EVOLUTION, GENETICS, AND EUGEN^CS mollusks and crustaceans are represented in almost all rock deposits which contain any kind of fossil remains." — Jordan and Kellogg/ [The study of geology teaches us that the earth's outer zones have undergone within the period of vertebrate history numerous profound changes which in general we may term climatic changes. There have been periods of continental subsidence, accompanied by ocean-floor elevations, during which great continental plains have been covered with comparatively shallow seas. The marine faunas of the seas have migrated into these shallows and representatives of them have been buried in sediment. When the reverse change has occurred and the continental plain has been again elevated, the sedimentation of the shallow-sea period forms a great rocky stratum laden with marine fossils. Between periods of subsidence millions of years elapsed, and therefore a break in the continuity of the entombed fossils is to be expected. Discontinuity between the fossil faunas in adjacent strata is the invariable rule. Were it not for this periodicity of subsidence and elevation there would be no boundaries between consecutive geologic strata. In addition to the methods of fossilization mentioned, a few others deserve notice. Many animals of the arid plains have been fossilized by becoming imbedded in dust or sand drifts which have piled up against rocky outcrops or have filled in dried-up arroyos. Some very valuable fossils have been recovered from asphaltic deposits as the result of animals falling into liquid or semiliquid lakes or pools of asphalt. Not only are external organs preserved with precision, but even delicate internal structures, such as the brains or the viscera of verte- brates, have been found in such a perfectly natural shape that the comparative anatomy could be worked out with confidence. On the whole, then, we must conclude that the earlier, pessimism regarding the inadequacy and insufficiency of fossil data is giving way before a steadily increasing optimism, due to the very rapid advance in technique and the surprisingly abundant discoveries of the modern palaeontologist. The more enthusiastic of the new school of fossil- hunters do not despair of ultimately bringing to light all of the really essential links in the chain of evidence necessary to place the evolution theory beyond the reach of controversy. — Ed.] ^ From D. S. Jordan and V. L. Kellogg, Evolution and Animal Life (copy- right 1907). Used by special permission of the publishers, D. Appleton & Company. EVIDENCES FROM P.\LAEONTOLOGY 67 ON THE LAPSE OF TIME DURING WHICH EVOLUTION IS BELIEVED TO HAVE TAKEN PLACE "Independently of our not finding fossil remains of such infinitely numerous connecting links [referring to the objection that all steps in the evolution of modern types should be revealed in the fossils], it may be objected that time cannot have sufficed for so great an amount of organic change, all changes having been effected slowly. It is hardly possible for me to recall to the reader who is not a practical geologist, the facts leading the mind feebly to comprehend the lapse of time. He w^ho has read Sir Charles Lyell's grand work on the Principles of Geology^ which the future historian will recognize as having produced a revolution in natural science, and yet does not admit how vast have been the past periods of time, may at once close this volume. Not that it suffices to study the Principles of Geology, or to read special treatises by different observers on separate forma- tions, and to mark how each author attempts to give an inadequate idea of the duration of each formation, or even of each stratum. We can best gain some idea of past time by knowing the agencies at work, and learning how deeply the surface of the land has been denuded, and how much sediment has been deposited. As Lyell has well remarked, the extent and thickness of our sedimentary formations are the result and the measure of the denudation which the earth's crust has elsewhere undergone. Therefore a man should examine for him- self the great piles of superimposed strata, and watch the rivulets bringing down the mud, and the waves wearing away the sea-cliffs, in order to comprehend something about the duration of past time, the monuments of which we see all around us." — Charles Darwin, Origin of Species. "In 1862," says Schuchert,^ "the physicist. Lord Kelvin .... held that as our planet was continually losing energy in the form of heat, the globe was a molten mass somewhere between 20,000,000 and 400,000,000 years ago, with a probabihty of this state occurring about 98,000,000 years ago. Finally in 1897 he concurred in Clarence King's conclusion that the globe was a molten mass about 24,000,000 years ago. Both of these conclusions, however, were wrought out under the Lap- lacian hypothesis, and now many geologists hold that the earth never was molten. While geologists have not been able to fit their evidence into so short a time, they have ever since been trying to keep their ^ C. Schuchert, Texi-Book of Geology, Part II, Historical Geology (191 5). 68 READINGS IN EVOLUTION, GENETICS, AND EUGENICS o (J MILLIONS ■ OF o «c AGE OF MAN s o QUARTERNARY AGE YEARS O UJ OF z ui TERTIARY m MAMMALS u 1- z < ■ v> UPPER 9— < UJ CRETACEOUS >- o o o AGE o N z = kJ LOWER CRETACEOUS . 1 OF REPTILES U) (COMANCHEAN) kj a. (t UI >- It ' oT LJ JURASSIC lO- o s K < u. • < DC TRIASSIC S .0 - AGE u o s PERMIAN Sis PENNSYLVANIAN 15- OF (UPPER SSh AMPHIBIANS UJ CARBONIFEROUS) O (J . < MISSISSIPPIAN lEG - UJ Q . UJ >• u (LOWER CARBONIFEROUS) - o i"s O 20- § AGE OF N O g UJ DEVONIAN 1 2 " 3 FISHES UJ < < Q- ^ £ >- u 2 SILURIAN i ° o z - o U) "^ 25- 1- g ORDOVICIAN u . < AGE o UJ cc OF < "^n — INVERTEBRATES a. >• ae w CAMBRIAN •SKJ MILLIONS - OF o KEWEENAWAN YEARS U o i^ j£ si ANIMIKIAN 35- N S2 < LJ EVOLUTION OF UJ 2 HURONIAN = 5 w INVERTEBRATES 2 < .« ■•" Q 1^ d >■ a. y u s: (0 O o ALGOMIAN (E in 0-zP 40- o o oc oe < UJ -s ee a. a B a o H o I— ( EVIDENCES FROM PALAEONTOLOGY 69 estimates within the bounds of Lord Kelvin's older calculations. Wal- cott, in 1893, on the basis of the stratigraphic record and the known discharge of sediment by rivers, concluded that 70,000,000 years had elapsed since sedimentation began in the Archeozoic. Sir Archibald Giekie places the time at 100,000,000 years, and most geologists have tried, although with difficulty, to fit the record within these estimates. ^' Since the discovery of radium, all of the calculations previously made have been set aside by the new school of physicists, and now the geologists are told they can have 1,000,000,000 or more years as the time since the earth attained its present diameter Even if finally it shall turn out that the physicists have to reduce their estimates as to the age of certain minerals and rocks, geologists nevertheless appear to be on safer ground in accepting their estimates than those based either on sedimentation, chemical denudation, or loss of heat by the earth." [The last decade has seen the demise of the outworn objection to evolution based on the idea that there has not been time enough for the great changes that are believed by evolutionists to have occurred • Given 100,000,000 or 1,000,000,000 years since life began, we can then allow 1,000,000 years for each important change to arise and establish itself. We can also understand why it is that so little change can be noted in the majority of wild animals and plants within the historic period. A thousand years in the development of the race is like a second in the development of an individual and, though no one can notice any change in a growing creature in a second or a minute, very radical changes can be noted in an hour or a day or a year. We cannot see any movement in an hour hand of a clock, but it moves with certainty around the dial in a relatively short time. There is there- fore no shortage of time. Evolution may have been infinitely slow, but time has been infinitely long. The accompanying time scale shows the lapse of time and the distribution in time of the main groups of animals (Fig. i). — Ed.] ON THE PRINCIPAL GENERAL FACTS REVEALED BY A STUDY OF THE FOSSILS [i. None of the animals or plants of the past are identical with those of the present. The nearest relationship is between a few species of the past and some living species which have been placed in the same families. 70 READINGS IX E\'OLUTION, GENETICS, AND EUGENICS 2. The animals and plants of each geologic stratum are at least generically different from those of any other stratum, though belonging in some cases to the same families or orders. 3. The animals and plants of the oldest (lowest) geologic strata represent all of the existing phyla, except the Chordata, but the representatives of the various ' phyla are relatively generalized as compared with the existing types. 4. The animals and plants of the newest (highest) geologic strata are most like those of the present and help to link the present with the past. 5. There is, in general, a gradual progression toward higher types as one proceeds from the lower to the higher strata. 6. Many groups of animals and plants reached the climax of specialization at relatively early geologic periods and became extinct. 7. Only the less specialized relatives of the most highly specialized types survived to become the progenitors of the modern representa- tives of their group. 8. It is very common to find a new group arising near the end of some geologic period during which vast climatic changes were taking place. Such an incipient group almost regularly becomes the domi- nant group of the next period, because it developed under the changed conditions which ushered in the new, period and was therefore especially favored by the new environment. 9. The evolution of the vertebrate classes is more satisfactorily shown than that of any other group, probably because they represent the latest phylum to evolve, and most of their history coincides with the period within which fossils are known. 10. Most of the invertebrate phyla had already undergone more than half of their evolution at the time when the earliest fossil remains were deposited. — Ed.] FOSSIL PEDIGREES OF SOME WELL-KNOWN VERTEBRATES PEDIGREE OF THE HORSE [Of all fossil pedigrees that of the horse is most often mentioned in evolutionary literature. The main facts have been known for about forty years, and there is a rather general consensus of opinion as to the history as a whole. It appears practically certain that the horse family (Equidae) arose from a group of primitive five-toed ungulates or hoofed mammals called Condylarthra that lived in Eocene times. EVIDENCES FROM PAI^AEONTOLOGY 71 No particular member of this extinct group has been found that fulfils all the requirements of a primitive horse ancestor, so the chances are that the real ancestral condylarthran has not been discovered.' — Ed.] ''The course of their [Equidae] evolution," says Dendy,' ''has evidently been determined by the development of extensive, dry, grass-covered, open plains on the American continent. In adap- tation to life on such areas structural modification has proceeded chiefly in two directions. The limbs have become greatly elongated and the foot uplifted from the ground, and thus adapted for rapid flight from pursuing enemies, while the middle digit has become more and more important and the others, together with the ulna and the fibula, have gradually disappeared or become reduced to mere vestiges. At the same time the grazing mechanism has been gradually perfected. The neck and head have become elongated so that the animal is able to reach the ground without bending its legs, and the cheek teeth have acquired complex grinding surfaces and have greatly increased in length to compensate for the increased rate of wear. As in so many other groups, the evolution of these special characters has been accompanied by gradual increase in size. Thus EoJiippus, of Lower Eocene times, appears to have been not more than eleven inches high at the shoulder, while existing horses measure about sixty-four inches, and the numerous intermediate genera for the rriost part show a regular progress in this respect. "All these changes have taken place gradually, and a beautiful series of intermediate forms indicating the different stages from EoJiip- pus to the modern horse [Eqmis] have been discovered. The sequence of these stages in geological time exactly fits in with the theory that each one has been derived from the one next below it by more perfect adaptation to the conditions of life. Numerous genera have been described, but it is not necessary to mention more than a few." ["The first indisputably horselike animal appears to have been Hyracotherium,'' of the Lower Eocene of Europe. Another Lower Eocene form is Eohippus, which lived in North America, probably having migrated across from Asia by the Alaskan land connection which was in existence at that time. In Eohippus the fore foot had four completely developed hoofed digits and a "thumb" reduced to a splint bone; in the hind foot the great toe had entirely disappeared and the Httle toe is represented by a vestigial structure or splint bone. ^ Arthur Dendy, Outlines of Evolutionary Biology (D. Appleton & Company, 1916), 72 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Then came in succession Orohippus, of the Upper Eocene, Mesohippus of the Lower Eocene, Pliohippiis of the Upper Phocene, and finally Equus : Qua- ternary and Recent. Pliohippus : Pliocene. * Protohippus : Lower Plio- cene. Miohippus : Miocene. Mesohippus : Lower Mio- cene. Orohippus Eocene. Fig. 2. — Feet and teeth in fossil pedigree of the horse. {After Marsh.) a, Bones of the fore foot; h, bones of the hind foot; c, radius and ulna; d, fibula and tibia; e, roots of a tooth; / and g, crowns of upper and lower teeth. Equus of the Quaternary and Recent. Other genera might be men- tioned, but the history of this series has been pictured in a classic EVIDENCES FROM P.\LAEONTOLOGY 73 diagram by Marsh, and in this (Fig. 2) the reader may trace upward from Orohippus to Equus the steady changes in fore and hind feet, bones of the forearm, bones of the lower leg, and the grinding teeth of upper and lower jaws. So definitely and clearly has the horse pedigree been worked out that, according to Dendy, ''the palaeontological evidence amounts to a clear demonstration of the evolution of the horse from a five-toed ancestor along the lines indicated above." For a long time the palaeontological series of the horse was un- rivaled by other vertebrate types, but now we have almost equally complete series for several other modern types, notably the camels and the elephants. We shall present herewith accounts of the pedi- gree of the camels by Professor Scott, and that of the elephants by Professor Shull. And, to conclude the vertebrate pedigrees, we shall present in the next chapter that of man as given by Professor Lull. In extenuation of the use of vertebrate material to the exclusion of invertebrate, the present writer has only this to offer, that verte- brate material is more intelligible to the non-biological reader and is more in his own field of knowledge and interest. — Ed.] PEDIGREE OF THE CAMELS^ W. B. SCOTT There remains one family of mammals with which it is necessary to deal and that is the camel tribe. This family has two well-defined subdivisions, the camels of the Old World and the llamas, guanacos, etc., of South America. For a very long time, the family was entirely confined to North America and did not reach its present homes until the Pliocene epoch of the Tertiary period. The skeleton of a Patago- nian guanaco may be taken as the starting point of our inquiry. In this animal the third incisor and the canine are retained in the upper jaw, all the incisors and the canine in the lower. The anterior two grinding teeth have been lost and the others are moderately high- crowned. The skull is broad and capacious behind, narrow and tapering in front. The neck is long and its vertebrae very curiously modified. The limbs are long and slender and have undergone nearly the same modifications as in the horses; the ulna is greatly reduced, interrupted in the middle and its separated portions are fused with the radius. In the hind leg the shaft of the fibula has been completely ^ From W. B. Scott, The Theory of Evolution (copyright 1917)- Used by special permission of the publishers, The Macmillan Company. 74 READINGS IN EVOLUTION, GENETICS, AND EUGENICS suppressed; the upper end fuses with the tibia, while the lower remains as a small separate bone, wedged in between the tibia and the heel- bone. The feet are very long and slender, with two toes in each; the Fig. 3. — Four stages in the evolution of the cameline skull. A, Protylopiis Upper Eocene; B, Poebrotheriiim, Lower Eocene; C, Procamelus, Upper Miocene, Z>, guanaco, Recent. {From Scott.) long bones of the foot are co-ossified to form a '^ cannon-bone, " the very young skeleton showing that this co-ossification does actually take place. The toes proper are free, giving the "cloven hoof," but the hoofs are very small and the weight is carried upon a soft, thick pad. EVIDENCES FROM PALAEONTOLOGY 75 IT JH JF M Fig. 4. — Four stages in the evolution of the cameline fore foot. A , Protylopus, Upper Eocene; B, Poehrothcrium, Lower Eocene; C, Procamelus, Upper JMiocene; Z), guanaco, Recent. {From Scott.) 76 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Were there time enough to do so, we might trace the development of this family backward, step by step, through all the many stages between the Pleistocene and the Upper Eocene in quite as unbroken sequence and in as full detail as can be done for the horses. We must, however, pass over all the intermediate steps and consider the ances- tral camels of the Upper Eocene. These were very little animals, hardly larger than a jack rabbit, which had the full complement of teeth, 44 in total number, and all with very low crowns. The limbs, and especially the feet, are relatively short, the ulna is complete and separate, as is also the fibula; there are four toes in each foot, though the lateral pair of the hind foot are extremely slender, and there is no co-ossification to form cannon-bones. The hoofs are well developed, in form like those of an antelope, so that there can have been no pad. For the present, the line cannot be carried back of the Upper Eocene, the probable ancestors from the middle and Lower Eocene being, as yet, represented only by fragmentary specimens. In addition to this main stem of cameline descent which resulted in the modern species, there were two short-lived side branches which should be mentioned. One, ending in the Lower Miocene, was the series descriptively called '' gazelle-camels, " small animals with very long and slender legs, evidently swift runners. The other series, the so-called "giraffe-camels," terminated in the Upper Miocene; these were browsers and display an increasing stature, especially in the length of the neck and fore limbs. They adapted themselves to the growing aridity of the western plains. EVOLUTION OF THE ELEPHANTS^ A. FRANKLIN SHULL The mastodon-elephant series shows a larger number of obvious changes than most of the other series named, all of these changes except that of the body having to do with features of the head. From the numerous specimens of elephant-like forms available, the following are selected (following Lull) as probably representing a direct line of evolution: Moeritheritim from the Upper Eocene of Egypt; Palaeomastodon from the Lower Oliogocene of Egypt, also from India; Trilophodon from the Miocene of Europe, Africa, and North America; Mastodon from the Pliocene and Pleistocene of ^ From A. F. Shull, Principles of Animal Biology (copyright 1920). Used by special permission of the publishers. The McGraw-Hill Book Company. EVIDENCES FROM P.\LAEONTOLOGY 77 North America, Europe and Asia; Stegodon from the Phocene of southern Asia; and Elephas from the Pleistocene of the Americas, Europe, and Asia, as well as the living elephants of Asia and Africa. Fig. 5.— Evolution of head and molar teeth of mastodons and elephants. A, A', Elephas, Pleistocene; B, Stegodon, Pliocene; C, C , Mastodon, Pleistocene; D, D', Trilophodon, Miocene; E, E' , Falaeomastodon, Oligocene; F, F' , Moc- ritherium, Eocene. {From Lull.) 78 READINGS IN EVOLUTION, GENETICS, AND EUGENICS A study of Figure 5 in connection with the following account will dis- close the more striking steps of evolution. These forms differed from one another in a number of features, but the differences between any member of the series and the one that precedes or that which follows were so small that the series is obviously a continuous one. Moerithe- rium was very different from the modern elephant, but the inter- mediate forms completely bridged the gap. The series exhibits an enormous increase in size of body, changes in the form and size of the teeth, a reduction in the number of teeth, an alteration in the method of tooth succession, the enlargement of certain teeth to become tusks, the elongation and subsequent shortening of the lower jaw, the development of the upper lip and nose into a proboscis, and an increase in the height of the skull through the development of large cavities in the substance of the • bone. These features are described in the several forms seriatim. Moeritherium. — The earliest animal recognized as belonging to the elephant series, Moeritherium by name, was recovered from the late Eocene and early Oligocene deposits of northern Egypt. It was slightly over three feet in height. The features suggesting elephantine affinities are the high posterior portion of the skull (Fig. S, F); composed of somewhat cancellate bone, that is, bone containing open spaces; the elongation of the second pair of incisors in each jaw to form short tusks; the indication of transverse ridges on the molar teeth (Fig. 5, F) ; and the position of the nasal openings some distance back of the tip of the upper jaw, indicating probably a prehensile upper lip. There were 24 teeth, and the neck was long enough to enable the animal to put its head to the ground. It probably fed upon tender shoots and swamp vegetation. Palaeomastodon. — This form also lived in Egypt, but has recently been found in India. It dates from early Oligocene time. Palaeo- mastodon was of somewhat larger size than the preceding form, the posterior part of the skull was distinctly higher (Fig. 5, E') — with a greater development of cancellate bone, and the neck was somewhat shortened. The upper incisors of the second pair were more elongated as tusks and bore a band of enamel on their front surfaces. The lower second incisors were present, but not enlarged. All other incisors and the canines had disappeared. The molar teeth (£) resembled those of Moeritherium but were larger. The lower jaw was considerably elongated, and the total number of teeth was still high (26). The nasal openings had receded until they were just in front of the eyes, EVIDENCES FROM PALAEONTOLOCxY yg • which is beheved to indicate the existence of a short proboscis extending at least to the tips of the tusks. Trilophodon. — Trilophodon, a great migrant and consequently wide-spread over several continents as stated above, exhibited in several respects a striking advance over Palaeomastodon; but this advance was in the main in the same direction as was indicated by the change from Moeriiherium to Palaeomastodon. Trilophodon was a huge animal, nearly as large as modern Indian elephants. The tusks were considerably longer (Fig. 5, D') and still bore a band of enamel. The molar teeth were large and greatly reduced in number, so that only two were present at any one time on each side of each jaw. The surface of these teeth bore a somewhat larger number of transverse crests (Fig. 5, D) than were present in the earlier forms. The lower jaw was enormously elongated, so that it projected as far forward as the tusks. The great weight of the lower jaw and tusks was associated with a considerable development of cancellate bone in the skull, to which the supporting muscles of the neck were attached. Presumably there was a proboscis which extended to or beyond the tips of the tusks and lower jaw. Mastodon. — The mastodons on the whole represent a line of development which became extinct; but in their incipient stages they appear to have given rise to the succeeding forms leading to the elephants. The body was somewhat larger than that of Trilophodon, being about the size of the Indian elephant. The tusks {C) were much elongated (9 feet or more), but the lower jaw was greatly short- ened and the lower incisor teeth were reduced or wanting. The molar teeth (Fig. 5, C) were scarcely more complex than earlier forms, and numbered two on each side of each jaw. They were still crushing teeth, and the food must have been tender twigs and succulent plants; indeed, remains of such objects have been found in the region of the stomach of the fossil mastodons. Stegodon.— This animal is of interest chiefly because the molar teeth bore five or six well-defined transverse ridges (Fig. 5, ^). These ridges were due to plates of enamel extending up through the tooth, and inclosing a substance known as dentine. Over the enamel in an unworn tooth was a thin coat of a third substance called cement, but there was not much of this substance between the ridges. In the latter respect Stegodon differed, as is pointed out below, from the elephants and mammoths. On the whole, Stegodon was intermediate between the mastodons and elephants. 8o READINGS IN EVOLUTION, GENETICS, AND EUGENICS Elephas. — In this genus are included a number of extinct forms (the mammoths) from three or four continents, and the Hving ele- phants. The extinct forms, though called mammoths, were not large animals, being no larger than the Indian elephant of today, and not so large as the living African species. Some of the features of the elephants , their size, the short neck, the long proboscis, and the heavy- tusks are matters of common observation. The skull' is very high and short (Fig. 5, A'). The height is due chiefly to the development of cancellate bone, not to the enlargement of the brain, which is still quite small. As stated above, the high skull affords the necessary leverage for the muscles that support the weight of the tusks. The molar teeth are distinctly grinding teeth (Fig. 5, A). Each tooth bears a number of transverse ridges, about ten in the African elephant and two dozen or more in the Indian species. These ridges are worn down by the chewing of harsh food, so that the upper surface displays a number of flattened tubular plates of enamel inclosing dentine and bound together by cement. A tooth is completely worn out by use, and is replaced by another. The method of replacement, however, is peculiar. While the tusks (incisors) are of two sets, one following the other like milk and permanent teeth of other mammals, the grinders succeed one another in continuous fashion. There are never more than two visible grinders on each side of each jaw. As they wear out they move forward in the jaw, and are replaced by new teeth appearing behind. New molars thus enter at intervals of two to four years in young elephants, and at intervals of 15 to 30 years in later life. If an elephant lives long enough (60 years or more) it develops a total of 28 teeth, including tusks, but has not more than ten (often less) at any one time. Correlated with the nature of the teeth of the elephants are their food and chewing habits. Whereas the ancestral forms whose molars bore prominent elevations lived on twigs and tender herbage which they crushed in mastication, the mammoths with their flattened tooth surfaces devoured grasses, sedges, and other harsh vegetation which they ground with lateral motion of the teeth upon one another. In this respect modern elephants are like the mammoths. In the changes described above is found one of the most beautiful and best established evolutionary series with which the palaeontolo- gist is acquainted. Only a few others equal or approach it in clearness and completeness. CHAPTER* VI THE EVOLUTION OF MAN: PALAEONTOLOGY^ Richard Swann Lull ORIGIN OF PRIMATES Stock. — There is but little doubt that two important orders of modern mammals, the Carnivora and the Primates, had a common origin, diverging mainly along lines determined by a dietary contrast, as the former have become more strictly flesh-eating or predaceous, the latter largely fruit-eating and as a consequence more completely arboreal. Back of each group lie as annectant forms the Insectivora, not perhaps such as are alive to-day, as all these are highly specialized along diverse lines, but generalized insectivores possessing, because of their primitiveness, a wider range of potential adaptation. Mat- thew is ''disposed to think of these, our distant ancestors, at the dawn of the Tertiary, as a sort of hybrid between a lemur and a mongoose, rather catholic in their tastes, living among and partly in the trees, with sharp nose, bright eyes, and a shrewd little brain behind them, looking out, if you will, from a perch among the branches, upon a world that was to be singularly kind to them and their descendants." Thus we can define the stock as a relatively large-brained arboreal insectivore, of primitive but adaptable dentition, and especially of progressive mentality. Time. — The time of primate origin must have been not later than basal Eocene, as primates, clearly definable as such, are found in the Lower Eocene rocks of both Europe and North America. Place. — The simultaneous appearance of the primate in the Old World and the New gives rise to the same conclusions as to their place of origin and their migrations thence as with other modernized mammals. It suffices now to say that their ancestral home was boreal Holarctica, probably within the limits of the present continent of Asia, whence they migrated southward along the three great continental radii. The impelling cause of this migration was the increasing northern cold, before which the boreal limitations of the tropical forests retreated, carrying with them the primates which, in ^ From R. S. Lull, Organic Evolution (copyright 191 ?)• Used by special permission of the publishers, The Macmillan Company. 81 82 READINGS IN EVOLUTION, GENETICS, AND EUGENICS general, are utterly dependent upon such an environment for their sustenance. Geologic record. — Primates are found in the North American sediments from Lower to Upper Eocene time, when they became extinct. Thus, while their remains constitute a relatively large per- centage of the total fauna of the Eocene, primates are utterly unknown on this continent from that time until the coming of man. In Europe the record is similar except that the extinction occurred at a somewhat later date, the Oligocene. Furthermore, they reappear in Europe in the Lower Miocene, at the time of the proboscidean migration out of Africa, whence these primates may also have come. Their second European extinction was in the Upper Pliocene shortly before the first appearance of mankind. But in southern Asia, Africa, and South America the evolution of primates seems to have been continuous since the first great southward migration. The evidence, however, is not so much the historical documents as the presence of primates in those places at the present time, the fossil record is not entirely lacking although highly incom- plete. The South American monkeys may have had their origin in the ancient North American primates, or more doubtfully, the stock may have come by way of Africa. Scott inclines toward the latter view although he says the evidence is by no means conclusive. ORIGIN OF MAN Stock. — According to W. K. Gregory, the stock from which man arose was some big-brained anthropoid related most nearly to the chimpanzee-gorilla group, an assumption based upon anatomical evidences, in spite of wide differences in habitus and consequent adaptation. Place. — Evidences point to central Asia as the place of descent from the trees of the human precursor, the reasons for this belief being several. First, it was central for migrations elsewhere; Europe, on the other hand, where the most conclusive, in fact almost the exclusive evidence for fossil man is found, is too small an area for the divergent evolution of the several human species. Second, Asia is contiguous to the oldest known human remains, which, as we shall see, were found in Java. Third, it was the seat of the oldest civilizations, not only of the existing nations which, like the Chinese, trace their recorded history back to a hoary antiquity, but of nations which preceded them by thousands of years, and whose records have not yet come to light. THE EVOLUTION OF MAN 83 This antiquity vastly exceeds that of the nations of Europe or of the Americans or of Africa. Fourth, central Asia is the source of almost all of our domestic animals, many of which have been subjected to human will and control for thousands of years, and this is equally true of many of our domestic plants. This is not due to the fact that man first reached civilization in Asia, but rather that he chose for his com- panions the highest and best of their several evolutionary lines, and Asia was the place of all others upon earth where the evolution in general of organic life reached its highest development in late Cenozoic time (Williston). Fifth, climatic conditions in Asia in the Miocene or early Pliocene were such as to compel the descent of the prehuman ancestor from the trees, a step which was absolutely essential to further human development. Impelling cause. — We look for a geologic cause back of this most momentous crisis in the evolution of humanity and we find it in conti- nental elevation and consequent increasing aridity of climate, espe- cially to the northward of the Himalayas. With this increased aridity and tempering of tropical heat came the dwindling of the forested areas suitable to primate occupancy. Barrell has suggested that this diminution left residual forests comparable to the diminishing lakes and ponds of the Devonian, which upon final desiccation compelled their denizens to become terrestrial or perish. The dwindling of the residual forests would have an effect upon the tree-dwellers which may be expressed in precisely the same words. Once upon the ground the effect upon even a conservative type — and the primates in general, where constant conditions prevail, are slow of change — would be the rapid acquisition of such adaptations as were necessary to insure sur- vival under the new conditions. The other man-like apes had, unfortunately for their further evolution, reached a region where tropical forests continued to be available and hence have retained their arboreal life and with it a stagnation of progress. The result has been, at any rate on the part of the three larger forms, a degeneracy from the estate of their common ancestry with mankind; the gibbons seem to have deteriorated less, while terrestrial man has risen to the summit of primate evolution. Time. — The time of the descent is not later than early Pliocene nor earHer than Miocene time; when the terrestrial ape-man became what we would call human was perhaps later, but certainly during the Pliocene, which makes the age of man as such measurable in terms of hundreds of thousands of years! 84 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Significance of the descent from trees. — As a result of the descent from the trees, certain definite factors were called into play, each of which had its effect on the further evolution. Briefly enumerated, these are: (i) Assumption of the erect posture; (2) liberation of the hands from their ancient locomotor function to become organs of the mind; (3) loss of the easily obtainable food of the tropical forests, necessitating the search for sustenance, both plant and animal, and man became a hunter; (4) need of clothing with increasing inclemency of the weather, especially during the long winters; (5) freedom from climatic restrictions — when an omnivorous diet and clothing were acquired man was no longer limited to one definite habitat and the result was dispersal; (6) the development of communal life, rendered possible by the terrestrial habitat. Primates are at best gregarious, submitting, as in the gorilla, to the leadership of the strongest male, but it is only by communal life with its attendant division of labor that man can rise above the level of utter savagery. Evolutionary changes. — -Human evolutionary changes which are recorded are: more erect posture, shorter arms, perfection of thumb opposability, reduction of muzzle and of size of teeth, loss of jaw power, development of chin prominence, increase in skull capacity, diminution of brow-ridges, diminution in strength of zygo- matic or temporal arch, increase in size and complexity of brain, especially frontal lobes, development of articulate speech. FOSSIL MAN Fossil remains of man are found under two conditions, in river valley deposits and in limestone caverns which served first as a dwelling-place and later as a sepulture. Of these the caverns have been by far the most productive, but they contain only the remains of the later races, as the caverns according to Penck did not become available for human occupancy before middle Pleistocene time. The rarity of human fossils may be explained, first, by the various burial customs which seldom are sufficiently perfect to preclude the possibility of alternate wetting and drying or of rapid oxidation, both of which are prohibitive of fossilization. If man lived and died in the forests the chances for his fossilization, in common with other forest creatures, was very remote, for the remains of such are almost invari- ably destroyed by other animals, by dampness, or by fungi, and rarely attain a natural burial in sediment. If, on the other hand, he dwelt THE EVOLUTION OF MAN 85 in the open, the chances of so shrewd a creature being caught in the flood waters and thus buried in sediment were not very great. However we account for it, the fact remains that reUcs of ancient man are rare and are valued accordingly. In North America. — Repeated instances of seemingly ancient man have been brought to light in North America, such as the " Cale- veras skull" of the California gold-bearing gravels, which was satirized by Bret Harte; the Nebraska 'Xoess man," and those of the Trenton gravels; none of which, with the possible exception of the last- men- tioned, has proved to be really old in the geologic sense. Indirect evidence of human antiquity, that is, the association of North Ameri- can man with animals which are now extinct, while very rare, has been reported in at least two highly authentic instances. The first of these was at Attica, New York, and is attested by Doctor John M. Clarke, the New York state geologist. Four feet below the surface of the ground, in a black muck, he found the bones of the mastodon (Masto- don americajtus) , and 12 inches below this, in undisturbed clay, pieces of pottery and thirty fragments of charcoal. The charcoal may have been of natural origin, but the presence of the pottery seems conclu- sive. The other instance was that of the remains of a herd of extinct bison {Bison antiquus) found near Smoky Hill River, Logan County, Kansas, and thus described by Professor Williston: An "arrow-head was found underneath the right scapula of the largest skeleton, embedded in the matrix, but touching the bone itself. The skeleton was lying upon the right side The bone bed when cleared off .... contained the skeletons of five or six adult animals, and two or three younger ones, together with a foetal skeleton within the pelvis of one of the adult skeletons. The animals had evidently all perished together, during the winter. There was no possibility of the accidental intrusion of the arrow-head in the place where found It must have been within the body of the animal at the time of death, or have been lying on the surface beneath its body." What at this writing is claimed to be another genuine case of such an association, this time of the actual human bones, has just been announced from Florida. This find, which has been reported by State Geologist Sellards, was made at Vero, eastern Florida, in 1913. The fossil human bones are from two incomplete skeletons and are found in strata which also contain remains of the following extinct species: Elephas columbi, Eqiius leidyi, a fox, a deer, the ground-sloth, Megalonyx jefersoni, and the American mastodon. 86 READINGS IN EVOLUTION, GENETICS, AND EUGENICS In South America. — A number of finds have been recorded from South America, notably by the late Florentino Ameghino of Buenos Aires, who contributed so largely to our knowledge of South American prehistoric life. An expert from Washington, Doctor Ales Hrdlicka, has studied with the utmost care the locality and character of each of these finds in the Western World, and has expressed the opinion that none is of an antiquity greater than that of the pre-Columbian Indians. Further evidence lies in the uniformity of type, except for minor distinctions, of all native American peoples. There is no such racial differentiation as that seen in the Old World, and the argument is that there has not been time for such a deployment. The area and condi- tions as an adaptive radiation center are surely ample. In Africa. — The only African relics thus far reported are those of prehistoric cultures, comparable to those of Southern Europe, in certain caverns of the Barbary States. There has also been reported from Oldoway ravine, German East Africa, a human skeleton of undoubted antiquity. It is described, however, as being neither a very early nor a primitive type. In Asia. — Asia has given us in Pithecanthropus the oldest known relic of the Hominidae, found at Trinil in the island of Java. Osborn says: "It is possible that within the next decade one or more of the Tertiary ancestors of man may be discovered in northern India among the foothills known as the SiwaHks. Such discoveries have been heralded, but none have thus far been actually made. Yet Asia will probably prove to be the center of the human race. We have now discovered in southern Asia primitive representatives or relatives of the four existing types of anthropoid apes, namely, the gibbon, the orang, the chimpanzee, and the gorilla, and since the extinct Indian apes are related to those of Africa and of Europe, it appears probable that southern Asia is near the center of the evolution of the higher primates and that we may look there for the ancestors not only of prehuman stages like the Trinil race but of the higher and truly human types." In Europe.— It is in Europe, however, that the tale of human prehistory is the most complete, not only through the happy accident of preserval, but because it has been much more thoroughly explored than has the Asiatic evolutionary center. The latter, however, holds the greatest hopes for future exploration since, as we have emphasized, Europe is too small to be an adaptive radiation center and European THE EVOLUTION OF MAN 87 prehistoric man represents waves of migration from the greater continent. Nevertheless the European record has enabled us to name and define a number of distinct human species, and here the record of the cultural evolution of man is also unusually complete. Hence Euro- pean chronology is taken as a standard in describing discoveries from any portion of the world. CHRONOLOGICAL TABLE (Adapted from Osborn, 19 15) Postglacial Time 25,000 years Upper Palaeolithic culture Cro-Magnon man Fourth Glacial Stage (Wiirm, Wisconsin) 50,000 years Close of Lower Palaeolithic culture Neanderthal man Third Interglacial Stage 150,000 years Beginning of Lower Palaeolithic culture Piltdown and pre-Neanderthaloid men Third Glacial Stage (Riss, lUiftoian) 175,000 years Second Interglacial Stage 375,000 years Heidelberg man Second Glacial Stage (Mindel, Kansas) 400,000 years First Interglacial Stage 475,000 years Pithecanthropus, ape-man First Glacial Stage (Giinz, Nebraskan) 500,000 years Pithecanthropus. — The Java ape-man, Pithecanthropus erectus (Figs. 6 and 7, A), was discovered in Trinil,. on the Solo orBengawan River in central Java, in 1894. The type consists of a calvarium or skull cap, a left thigh bone, and two / 1 / upper molar teeth. The / ^ I ^ skull is characterized by its /^\ — ^^ ^~-— — ^ v limited capacity, about two- / V — f—^^^^y^-^p ^ \ thirds that of man ; and by if/\/rn[^T23I the low flat forehead and beetling brows. Hence not only was the brain limited in its total size, but this I'^^'- 6.-Skull of Java ape-man, FUycan- . „ , ^ , thropiis erectus. {From Lull, after Dubois.) was especially true of the frontal lobes, which, as we have seen, are the seat of the higher intel- lectual faculties. Thus, as Osborn says, although touch, taste, and 88 READINGS IN EVOLUTION, GENETICS, AND EUGENICS vision were well developed there was a limited faculty for profiting by experience and accumulated tradition. The femur associated with the skull is remarkable for its length and slight curvature as compared with the primitive Neanderthal race of Europe and indicates a creature fully as erect and nearly as tall as the average European of today, the height being estimated at 5 feet 7 inches as compared with 5 feet 3 inches for the Nean- derthals and 5 feet 8 inches, the average height of modem males. The erect posture of course implies the liberation of the hands from any part in the locomotor function. The teeth are somewhat ape-like, but are more human than are those of the gibbon, and the human mode of mastication has been acquired. Certain authorities have tried to prove that Pithecanthropus is nothing but a large gibbon, but the weight of authority considers it prehuman, though not in the line of direct development into humanity. It is neverthe- less a highly important transi- tional form. Associated with the Pithe- cmUhropus remains are those of a number of the contem- porary animals which fix the date as either of the Upper Pho- cene or lowermost Pleistoceen period, which being rendered in terms of years gives an esti- mated age of about 500,000! Fig. 7. — Jaws, left outer aspect, of A, chimpanzee, Pan, sp.; B, fossil chimpanzee, Pan veins, found in association with Pilt- down man; C, Heidelberg man, Homo heidelbergensis; D, modern man, H. sapiens. {From Lnll, after Woodward.) THE EVOLUTION OF MAN 89 Heidelberg man. — Homo heidelhergensis, the Heidelberg man, represents the oldest recorded European race, geologically speaking. The type was discovered in 1907 in river sands, 79 feet below the surface, at Mauer, near Heidelberg, South Germany. The relic consists of a perfect lower jaw with the dentition (Fig. 7, C). The description by the discoverer. Doctor Schoetensack, follows (from Osborn) : ''The mandible shows a combination of features never before found in any fossil or recent man. The protrusion of the lower jaw just below the front teeth (the chin prominence) which gives shape to the human chin is entirely lacking. Had the teeth been absent it would have been impossible to diagnose it as human. From a fragment of the symphysis of the jaw it might well have been classed as some gorilla-like anthropoid, while the ascending ramus resembles that of some large variety of gibbon. The absolute certainty that these remains are human is based on the form of the teeth — molars, pre- molars, canines, and incisors are all essentially human and although somewhat primitive in form, show no trace of being intermediate between man and the anthropoid apes but rather of being derived from some older common ancestor. The teeth, however, are small for the jaw; the size of the border would allow for the development of much larger teeth. We can only conclude that no great strain was put on the teeth, and therefore the powerful development of the bones of the jaw was not designed for their benefit. The conclusion is that the jaw, regarded as unquestionably human from the nature of the teeth, ranks not far from the point of separation between man and the anthropoid apes. In comparison with the jaws of the Neanderthal races .... we may consider the Heidelberg jaw as pre-Neander- thaloid; it is, in fact, a generalized type." Associated with the Heidelberg jaw is an extensive warm-climate fauna: straight-tusked elephant (E. antiquus), Etruscan rhinoceros, primitive horse, bison, wild cattle (urus), bear, lion, and so on, all of which aid in establishing the date of the jaw as Second Interglacial and its age, conservatively estimated, at from 300,000 to 375,000 years. The cultural evolution of Heidelberg man is indicated by the presence of eoliths, flint implements of the crudest workmanship, if indeed their apparent fashioning is not merely the result of use. Neanderthal man. — The original specimen of the Neanderthal man. Homo neanderthalensis or primigenius (Figs. 8, 9, 10) was dis- covered in 1856 not far from Diisseldorf in Rhenish Prussia. Here the valley of the Diissel forms the deep Neanderthal ravine, whose go READINGS IX EVOLUTION, GENETICS, AND EUGENICS limestone walls are penetrated by caverns, in one of which the remains were'found. What was doubtless a perfect skeleton at the time of its o m <^ i ;-i "O U ^ V. -l a o o bf) )-H . ^ -O OJ ^. -o «0 ?^ a « ^ 1/2 to '--1 -C *^ ft! £-1- O ;-! 3 -4— > t— » w OJ M— 1 M— > p^ o o c CO CO S B o c o G •5 ^ discovery was so injured by its finders that only a portion of it, which is now preserved in the Provincial Museum at Bonn, was saved. " This prophet of an unknown race was for a time utterly without honor THE EVOLUTION OF MAN 91 though of course the subject of a most heated controversy, being con- sidered as non-human, or, as Virchow beUeved, owing its distinctive characters to disease. The sagacity of Huxley threw true hght upon the problem, though it was not until the mute testimony of other representatives of the race (the men of Spy) was offered that even Huxley's masterful conception of the Neanderthal characters was taken as an accepted fact. Professor Huxley's descrip- tion of the Neanderthal type is classic. He says: ''The anatomical char- acters of the skeletons bear out conclusions which are not flattering to the appear- ance of the owners. They were short of stature but powerfully built, with strong, curiously curved thigh bones, the lower ends of which are so fashioned that they must have walked with a bend at the knees. Fig. 9. — Neanderthaloid skull of L Chapelle-aux-Saints {Homo ncandcrlhalcnsis {From Lull, after Boide.) Their long depressed skulls had very strong brow-ridges; their lower jaws, of brutal depth and solidity, sloped away from the teeth down- wards and backwards in consequence of the absence of that especially characteristic feature of the higher type of man, the chin prominence." Subsequently several more specimens have come to light, at Spy in Belgium, at Krapina in Croatia, at Le Moustier, La Chapelle-aux- Saints and La Ferrassie in France, and at Gibraltar, which, while differing in various details, effectually serve to establish the race, whose main characteristics are: Heavy, overhanging brows, retreating fore- head, long upper lip; jaw less powerful than that of the Heidelberg man but very thick and massive; chin generally strongly receding but in process of forming; dentition extraordinarily massive in the La Chapelle specimen, whereas in those of Spy the teeth are small. The skull in many characteristics is nearer to the anthropoids than to modern man. The brain is large and its volume is surely human, but the pro- portions are again less like those of recent man than like the anthro- poids. The chest is large and robust, the shoulders broad, and 92 READINGS IX EVOLUTION, GENETICS, AND EUGENICS the hand large, but the fingers are relatively short, the thumb lacking the range of movement seen in modern man. The knee was some- what bent, the leg powerful, with a short shin and clumsy foot, clearly not of cursorial adaptation. The curve of the bent leg was correlated with a similar curvature of the spine, so that the man could not stand fully erect, as he lacked the fourth or cervical curvature of Homo sapiens. The average stature was 5 feet 3 inches, with a range from 4 feet 10.3 inches to 5 feet 5.2 inches, partly sex differences. Neanderthal man lived in Eu- rope from the Third Interglacial stage through the Fourth Glacial, a duration of thousands of years, and then became extinct, from twenty to twenty-five millenniums ago. He seems to have been an actual lineal successor of the man of Heidelberg, but was throughout his long career an unprogressive static race. One of the most remarkable features in connection with this race, however, was the very reverent way in which the dead were buried, with an abun- dance of ornaments and finely worked flints. This can have but one interpretation, the awakening within this ancient type of the instinctive belief in immortality! Piltdown m.an. — In 191 2 was announced the discovery of a very ancient man from the Thames gravels at Piltdown, Sussex, England. Here again the skull was injured and partly lost, so that the question of its proper restoration has been the subject of considerable contro- versy. The material consists of portions of the cranial walls, nasal bones, a canine tooth, and part of a lower jaw. The brain-case in this instance is typically human, except for the remarkably thick cranial walls. The forehead is high and lacks the superorbital ridges of Neanderthal man and Pithecanthropus. While the skull is of com- FiG. 10. — Skeleton of Neanderthal man. A , Homo neanderthalensis, com- pared with that of a living native Australian; B, Homo sapiens, the latter the lowest existing race, (From Lull, after Woodward.) THE EVOLUTION OF MAN 93 paratively high human type, the associated jaw and canine tooth clearly are not, and some difficulty was met in explaining their evolu- tionary discrepancy. That has apparently been answered, however, by the conclusion that the association of the material is purely acci- dental and that the jaw not only does not belong with the skull, but that it is not even human but is that of a fossil chimpanzee. That being the case, there seems to be no reason for the exclusion of the Piltdown man, who has been named Eoanthropus dawsoni, from the direct line of human ancestry. The specimen is not, perhaps, so surely dated as are those of the other European races, but it is associated with a warm-climate fauna and is generally considered to belong to the Third Interglacial stage — from 100,000 to 150,000 years old, and hence vastly more ancient than the more primitive Homo neonder- thalensis. (See Fig. "j, B.) Cro-Magnon man. — The original finds of the men of the Cro- Magnon race. Homo sapiens, were made at Gower, Wales, and at Aurignac, France. In the latter place seventeen skeletons came to light in 1852, but were buried in the village cemetery and thus lost to science, and not until 1868, when five more skeletons were discovered at Cro-Magnon, France, was the race established. These individuals, an old man, two young men, a woman and a child, are thus the types of the race. This magnificent race is thus characterized : Skull large but narrow, with a broad face, hence disharmonic. Facial angle equalling the highest type of Homo sapiens. Jaw thick and strong, with a narrow but very prominent chin. Forehead high and orbital ridges reduced. Brain not only of high type but very large, that of the women exceeding the average male of to-day. The stature of the old man was 6 feet 4.5 inches; the average for males being 6 feet 1.5 inches, for women 5 feet 5 inches, a great dis- parity. The lower segments of the limbs were long, in contrast with the Neanderthal type, hence the men of Cro-Magnon were swift- footed, while those of Neanderthal were slow. Osbom says: ''The wide, short face, the extremely prominent cheekbones, the spread of the palate and a tendency of the upper cutting teeth and incisors to project forward, and the narrow, pointed chin recall a facial type which is best seen to-day in tribes living in Asia to the north and to the south of the Himalayas. As regards their stature the Cr6-:Magnon race recall the Sikhs living to the south of the Himalayas. In the disharmonic proportions of the face, that is, the combination of broad cheekbones and narrow skull, they resemble the Eskimo. The 94 READINGS IN EVOLUTION, GENETICS, AND EUGENICS sum of the Cro-jMagnon characters is certainly Asiatic rather than African, whereas in the Grimaldis (of which specimens have been found in association with Cro-Magnons at the Grotte des Enfants, Mentone) the sum of the characters is decidedly negroid or African." The Cro-Magnons again show by their elaborate burial customs how old and well founded is the belief in life after death. They are supposed to be the people who left on the walls of the caverns of France and Spain the marvelous examples of Upper Palaeolithic art of which Professor Osborn's book gives so adequate a description. They lived for a while contemporaneously with the men of Neanderthal and may have contributed somewhat to the final extinction of the latter. In the course of time, however, they too declined, although to this day survivors of the race may be seen in Dordogne, at Landes, near the Garonne in Southern France, and at Lannion in Brittany. Osborn says: The decHne of the Cro-Magnons, with their artistic culture, ''may have been par tly due to environmental causes and the abandon- ment of their vigorous nomadic mode of life, or it may be that they had reached the end of a long cycle of psychic development We know as a parallel that in the history of many civilized races a period of great artistic and industrial development may be followed by a period of stagnation and decline without any apparent environmental cause." Europe was repopulated after Cro-Magnon decline by later invaders from the Asiatic realm, the so-called Mediterranean narrow- headed and the Alpine 'broad-headed types, etc., probably differen- tiated in Asia in early Palaeolithic times. The repopulation took place in the Upper Palaeolithic. EVIDENCES OF HUMAN ANTIQUITY Great variation. — These, briefly summarized, are, first, great variation. If man is monophyletic, that is, derived from a single prehuman species, and there is no reason to believe otherwise, he must be old, for while the adaptations to ground-dwelling after the descent from the trees were doubtless relatively rapidly acquired, the differen- tiation into the various races, due perhaps largely to climatic influ- ences rather than to any notable environmental change, must have been slowly attained. As corroborative evidence we have but to point to the mural paintings on Egyptian monuments, dating back THE EVOLUTION OF MAN 95 several thousand years, in which are depicted the Ethiopian, Caucasian, and the Hke, which are in some instances striking Ukenesses of the present-day Egyptians. Universal distribution is, in animals, another mark of antiquity: in man, it is probably less so because of his greater intelligence. And yet before transportation had become a science man's spread over land and sea was extremely slow. High intelligence as compared with that of the anthropoids is also a mark of antiquity, for the brain, especially the type of brain found in the higher human races, must have been very slow of development. Our study of fossil man shows this. Communal life, division of labor and all of the complicated interactions which it brings about, and the development of law and religions all have taken time. When we realize that Babylonian texts, twice as remote as the patriarch Abraham, give evidence of highly perfect laws and of a civilization which must have antedated their production by centuries, we gain another yet more emphatic im- pression of human antiquity. Add to all this the palaeontological evidence of man's association with various genera and numerous successive species of prehistoric animals of which he alone survives, and the evidence is complete. FUTURE OF HUMANITY Because of his intelligence and communal co-operation man is no longer subject to the laws which govern the adaptation of animals to their environment. Osborn's law of adaptive radiation, which, as we have seen, applies equally well to the insects, reptiles, and mam- mals, fails in its application to mankind; and yet man has become as thoroughly adapted to speed, flight, to the fossorial and aquatic as they; but his adaptation is artificial and to a very small extent only alTects his physical frame, while theirs is natural and the stamp of environment is deeply impressed upon the organism. Man's physical evolution has virtually ceased, but in so far as any change is being effected, it is largely retrogressive. Such changes are: Reduction of hair and teeth, and of hand skill; and dulling of the senses of sight, smell, and hearing upon which active creatures depend so largely for safety. That sort of charity which fosters the physi- cally, mentally, and morally feeble, and is thus contrary to the law of natural selection, must also in the long run have an adverse effect upon the race. 96 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Man is hardly as yet subject to Malthus' law, for while he is increasing more rapidly than any other animal, owing largely to the care of the young which makes the expectation of life of the new-born relatively very high, his migratory ability, but above all his intelli- gence, save him from the application of the law. A single new dis- covery such as that of electricity may increase his food supply and other life necessities several fold. His future evolution, in so far as it is progressive, will be mental and spiritual rather than physical, and as such will be the logical conclusion of the marvelous results of organic^evolution. CHAPTER VII EVIDENCES FROM GEOGRAPHIC DISTRIBUTION [Just as palaeontology may be said to be a study of the vertical distribution (distribution in time) of organisms, so geographic distribu- tion may be called a study of the horizontal distribution of organisms, on the earth's surface at any given time (spatial distribution). We are chiefly to be concerned with the present spatial distribution of animal and plant species, but equally interesting studies have been and still may be made of the horizontal or contemporaneous existence of extinct forms. Much new knowledge has been gained by combining the data of palaeontology with those of geographic distribution. In fact, neither field can be studied profitably without recourse to the other. This fact was clearly perceived by J. A. Thomson in his little manual on Evolution when he combined the two types of evidence in one chapter under the title "Evidences of Evolution from Explorer and Palaeontologist." It was a consideration of the present and of the past distribution of Edentates that led Charles Darwin to his first clear concept of descent with modification. In his voyage on the "Beagle" he found that present-day Edentates (armadillos, sloths, anteaters), a very peculiar group of archaic mammals, are practically confined to South America. When he also found that the only fossil Edentates, resem- bling but also differing from the existing types, are also confined to South America, he easily arrived at the only inference permitted by the facts: that the present Edentates are the modified descendants of the Edentates of the past. The following quotations from both an older and a recent writer will give the reader a clear idea of the ways in which the general facts of geographic distribution bear witness to the truth of the evolutionary principle. — Ed.] "The theory," says Wallace,' "which we may now take as estab- lished — that all the existing forms of life have been derived from other forms by a natural process of descent with modification, and that this same process has been in action during past geological time — should ^ From A. R. Wallace, Darwinism (1889). Used by special permission of the publishers, The Macmillan Company. 97 98 READINGS IX EVOLUTION, GENETICS, AND EUGENICS enable us to give a rational account not only of the peculiarities of form and structure presented by animals and plants, but also of their grouping together in certain areas, and their general distribution over the earth's surface. " In the absence of any exact knowledge of the facts of distribution, a student of the theory of evolution might naturally anticipate that all groups of allied organisms would be found in the same region, and that, as he travelled farther and farther from any given centre, the forms of life would differ more and more from tho^e which prevailed at the starting-point, till, in the remotest regions to which he could penetrate, he would find an entirely new assemblage of animals and plants, altogether unlike those with which he was familiar. He would also anticipate that diversities of climate would always be associated with a corresponding diversity in the forms of life. "Now these anticipations are to a considerable extent justified. Remoteness on the earth's surface is usually an indication of diversity in the fauna and flora, while strongly contrasted climates are always accompanied by a considerable contrast in the forms of life. But this correspondence is by no means exact or proportionate, and the converse propositions are often quite untrue. Countries which are near to each other often differ radically in their animal and vegetable productions; while similarity of climate, together with moderate geographical proximity, are often accompanied by marked diversi- ties in the prevailing forms of life. Again, while many groups of animals — genera, families, and sometimes even orders — are confined to limited regions, most of the families, many genera, and even some species are found in every part of the earth. An enumeration of a few of these anomalies will better illustrate the nature of the problem we have to solve. "As examples of extreme diversity, notwithstanding geographical proximity, we may adduce Madagascar and Africa, whose animal and vegetable productions are far less alike than are those of Great Britain and Japan at the remotest extremities of the great northern continent; while an equal, or perhaps even a still greater, diversity exists between Australia and New Zealand. On the other hand, Northern Africa and South Europe, though separated by the Mediterranean Sea, have faunas and floras which do not differ from each other more than do the various countries of Europe. As a proof that similarity of climate and general adaptability have had but a small part in determining the forms of life in each country, we have the fact of the enormous increase EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 99 of rabbits and pigs in Australia and New Zealand, of horses and cattle in South America, and of the common sparrow in North America, though in none of these cases are the animals natives of the countries in which they thrive so well. And lastly, in illustration of the fact that allied forms are not always found in adjacent regions, we have the tapirs, which are found only on opposite sides of the globe, in tropical America and the Malayan Islands; the camels of the Asiatic deserts, whose nearest allies are the llamas and alpacas of the Andes; and the marsupials, only found in Australia and- on the opposite side of the globe in America. Yet, again, although mammalia may be said to be universally distributed over the globe, being found abundantly on all the continents and on a great many of the larger islands, yet they are entirely wanting in New Zealand, and in a considerable number of other islands which are, nevertheless, per- fectly able to support them when introduced. ''Now most of these difficulties can be solved by means of well- known geographical and geological facts. When the productions of remote countries resemble each other, there is almost always conti- Quity of land with similarity of climate between them. When adjacent countries differ greatly in their productions, we find them separated by a sea or strait whose great depth is an indication of its antiquity or permanence. When a group of animals inhabits two countries or regions separated by wide oceans, it is found that in past geological times the same group was much more widely distributed, and may have reached the countries it inhabits from an intermediate region in which it is now extinct. We know, also, that countries now united by land were divided by arms of the sea at a not very remote epoch, while there is good reason to believe that others now entirely isolated by a broad expanse of sea were formerly united and formed a single land area. There is also another important factor to be taken account of in considering ho\y animals and plants have acquired their present peculiarities of distribution, — changes of chmate. We know that quite recently a glacial epoch extended over much of what are now the temperate regions of the northern' hemisphere, and that consequently the organisms which inhabit those parts must be, comparatively speaking, recent immigrants from more southern lands. But it is a yet more important fact that, down to middle Tertiary times at all events, an equable temperate climate, with a luxuriant vegetation, extended to far within the Arctic circle, over what are now barren t wastes, covered for ten months of the year with snow and ice. The 100 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Arctic zone has, therefore, been in past times capable of supporting almost all of the forms of life of our temperate regions; and we must take account of this condition of things whenever we have to specu- late on the possible migration of organisms between the old and new continents." "Many of the facts of distribution," says Shull,' "are capable of interpretation by the assumption that evolution has operated with the other factors. If each kind of animal has arisen from a pre-existing kind, then each group of related animals must have had an ancestral form, and if the component parts of the groups are widespread the range of the ancestral form may be considered to be the center of dispersal of the group. The facts of distribution can apparently be interpreted only on this basis. "Accepting evolution, along with the other factors which can be recognized, the method of distribution is generally conceived to be as follows. The ancestral form tends to spread in all directions. In some directions it is limited by unfavourable conditions either through- out its life or for some time. In other directions it extends its range. Anywhere within its range new types of individuals may arise through the process of evolution. These new types may be fitted to occupy new regions, and if they are formed near the limits of the range they may find opportunity to spread into areas which are inaccessible to the unaltered members of the species. Thus may arise recognizably distinct forms coincident in range with certain environmental condi- tions. If particular forms, or the individuals of a single form, are accidentally (or possibly by sporadic migration) transferred across barriers the distribution of the group becomes discontinuous. If these processes have been going on for a long time, that is, if the common ancestors of a group of forms existed long ago, the range may have had time to become very extensive, or its discontinuity very marked. If, contrariwise, the ancestors were comparatively recent, the range is likely to be much smaller. For this reason, groups that have diverged far enough to have attained the rank of families are on the whole more widespread than those so nearly allied as to be con- sidered genera. Should the environment become altered within a given range, the occupying form might be driven from it or destroyed . ^ From A. F. Shull, Principles of Animal Biology (copyright 1920). Used by special permission of the pubhshers, The McGraw-Hill Book Company. EVIDENCES FROM GEOGRAPHIC DISTRIBUTION loi If the environment in a region adjoining a range should change in a favourable manner, the range might be extended at that point without any alteration on the part of the animals. ''The. distribution of animals is inferred to be in harmony with this method, which involves, it will be noted, the factors of migration, evolution, physiological and morphological dependence upon the environment, the diversity and changeableness of the earth's surface, and extinction; and in this manner are explained the differences in geographical position, differences in size of range, differences in the continuity of range and the fact that ranges are at first continuous, differences in physical and biological conditions which characterize the ranges of different forms, and the geographical proximity of apparently related forms." SOME OF THE MORE SIGNIFICANT FACTS ABOUT THE DISTRIBUTION OF ANIMALS THE FAUNA OF OCEANIC ISLANDS^ GEORGE JOHN ROMANES Turning now from aquatic organisms to terrestrial, the body of facts from which to draw is so large, that I think the space at my dis- posal may be best utilized by confining attention to a single division of them — that, namely, which is furnished by the zoological study of oceanic islands. In the comparatively limited — but in itself extensive — class of facts thus presented, we have a particularly fair and cogent test as between the alternative theories of evolution and creation. For where we meet with a volcanic island, hundreds of miles from any other land, and rising abruptly from an ocean of enormous depth, we may be quite sure that such an island can never have formed part of a now submerged continent. In other words, we may be quite sure that it always has been what it now is — -an oceanic peak, separated from all other land by hundreds of miles of sea, and therefore an area supplied by nature for the purpose, as it were, of testing the rival theories of creation and evolution. For, let us ask, upon these tiny insular specks of land what kind of life should we expect to find ? To this question the theories of special creation and of gradual evolution would agree in giving the same answer up to a certain point. For both theories would agree in supposing that these islands would, at all ^ From G. J. Romanes, Darwin and after Darwin (copyright 1892). Used by special permission of The Open Court Publishing Company. I02 READINGS IN EVOLUTION, GENETICS, AND EUGENICS events in large part, derive their inhabitants from accidental or occa- sional arrivals of wind-blown or water-floated organisms from other countries — especially, of course, from the countries least remote. But, after agreeing upon this point, the two theories must part company in their anticipations. The special-creation theory can have no reason to suppose that a small volcanic island in the midst of a great ocean should be chosen as the theatre of any extraordinary creative activity, or for any particularly rich manufacture of peculiar species to be found nowhere else in the world. On the other hand, the evolution theory would expect to find that such habitats are stocked with more or less peculiar species. For it would expect that when any organisms chanced to reach a wholly isolated refuge of this kind, their descendants should forthwith have started upon an independent course of evolu- tionary history. Protected from intercrossing with any members of their parent species elsewhere, and exposed to considerable changes in their conditions of life, it would indeed be fatal to the general theory of evolution if these descendants, during the course of many genera- tions, were not to undergo appreciable change. It has happened on two or three occasions that European rats have been accidentally imported by ships upon some of these islands, and even already it is observed that their descendants have undergone a slight change of appearance, so as to constitute them what naturalists call local varieties. The change, of course, is but slight, because the time allowed for it has been so short. But the longer the time that a colony of a species is thus completely isolated under changed condi- tions of life the greater, according to the evolution theory, should we expect the change to become. Therefore, in all cases where we happen to know, from independent evidence of a geological kind, that an oceanic island is of very ancient formation, the evolution theory would expect to encounter a great wealth of peculiar species. On ihe other hand, as I have just observed, the special-creation theory can have no reason to suppose that there should be any correlation between the age of an oceanic island and the number of peculiar species which it may be found to contain. Therefore, having considered the principles of geographical distri- bution from the widest or most general point of view, we shall pass to the opposite extreme, and consider exhaustively, or in the utmost possible detail, the facts of such distribution where the conditions are best suited to this purpose — that is, as I have already said, upon oceanic islands, which may be metaphorically regarded as having been EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 103 formed by nature for the particular purpose of supplying naturalists with a crucial test between the theories of creation and evolution. The material upon which my analysis is to be based will be derived from the most recent works upon geographical distribution — espe- cially from the magnificent contributions to this department of science which we owe to the labours of Mr. Wallace. Indeed, all that follows may be regarded as a condensed filtrate of the facts which he has collected. Even as thus restricted, however, our subject matter would be too extensive to be dealt with on the present occasion, were we to attempt an exhaustive analysis of the floras and faunas of all oceanic islands upon the face of the globe. Therefore, what I propose to do is to select for such exhaustive analysis a few of what may be termed the most oceanic of oceanic islands — that is to say, those oceanic islands which are most widely separated from main- lands, and which, therefore, furnish the most unquestionable of test cases as between the theories of special creation and genetic descent. Azores. — A group of volcanic islands, nine in number, about 900 miles from the coast of Portugal, and surrounded by ocean depths of 1,800 to 2,500 fathoms. There is geological evidence that the origin of the group dates back at least as far as Miocene times. There is a total absence of all terrestrial Vertebrata, other than those which are known to have been introduced by man. Flying animals, on the other hand, are abundant: namely, 53 species of birds, one species of bat, a few species of butterflies, moths and hymenoptera, with 74 species of indigenous beetles. All these animals are unmodified European species, with the exception of one bird and many of the beetles. Of the 74 indigenous species of the latter, 36 are not found in Europe; but 19 are natives of Madeira or the Canaries, and 3 are American, doubtless transplanted by drift-wood. The remaining 14 species occur nowhere else in the world, though for the most part they are allied to other European species. There are 69 known species of land-shells, of which 37 are European, and 32 peculiar, though all allied to European forms. Lastly, there are 480 known species of plants of which 40 are peculiar, though allied to European species. , Bermudas. — A small volcanic group of islands, 700 miles from North Carolina. A though there are about 100 islands in the group, their total area does not exceed 50 square miles. The group is sur- rounded by water varying in depth from 2,500 to 3,800 fathoms. The I04 READINGS IN EVOLUTION, GENETICS, AND EUGENICS only terrestrial Vertebrate (unless the rats and mice are indigenous) is a lizard allied to an American form, but specifically distinct from it, and therefore a solitary species which does not occur anywhere else in the world. None of the birds or bats are peculiar, any more than in the case of the Azores; but, as in that case, a large percentage of the land-shells are so — namely, at least one quarter of the whole. Neither the botany nor the entomology of this group has been worked out; but I have said enough to show how remarkably parallel are the cases of these two volcanic groups of islands situated in different hemispheres but at about the same distance from large continents. In both there is an extraordinary paucity of terrestrial Vertebrata, and of any peculiar species of bird or beast. On the other hand, there is in both a marvellous wealth of peculiar species of insects and land-shells. Now these correlations are all abundantly intelligible. It is a difficult matter for any terrestrial animal to cross 900, or even 700 miles of ocean: therefore only one lizard has succeeded in doing so in one of the two parallel cases; and living cut off from intercrossing with its parent form, the descendants of that lizard have become modified so as to constitute a peculiar species. But it is more easy for large flying animals to cross those distances of ocean: consequently, there is only one instance of a peculiar species of bird or bat — namely, a bull-finch in the Azores, which, being a small land-bird, is not hkely ever to have had any other visitors from its original parent species coming over from Europe to keep up the original breed. Lastly, it is very much more easy for insects and land-moUusca to be conveyed to such islands by wind and floating timber than it is for terrestrial mammals, or even than it is for small birds and bats; but yet such means of transit are not sufficiently sure to admit of much recruiting from the mainland for the purpose of keeping up the specific types. Consequently, the insects and the land-shells present a much greater proportion of peculiar species — namely, one half and one fourth of the land-shells in the one case, and one eighth of the beetles in the other. All these cor- relations, I say, are abundantly inteUigible on the theory of evolution; but who shall explain, on the opposite theory, why orders of beetles and land-mollusca should have been chosen from among all other animals for such superabundant creation on oceanic islands, so that in the Azores alone we find no less than 32 of the one and 14 of the other ? And, in this connection, I may again allude to the peculiar species of beetles in the island of Madeira, Here there are an enor- mous number of peculiar species, though they are nearly all related to, EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 105 or included under the same genera, as beetles on the neighboring conti- nent. Now, as we have previously seen, no less than 200 of these species have lost the use of their wings. Evolutionists explain this remarkable fact by their general laws of degeneration under disuse, and the operation of natural selection, as will be shown later on; but it is not so easy for' special creationists to explain why this enormous number of peculiar species of beetles should have been deposited on Madeira, all aUied to beetles on the nearest continent, and nearly all deprived of the use of their wings. And similarly, of course, with all the peculiar species of the Bermudas and the Azores. For who will explain, on the theory of independent creation, why all the peculiar species, both of animals and plants, which occur on the Bermudas should so unmistakably present American affinities, while those which occur on the Azores no less unmistakably present European affinities ? But to proceed to other, and still more remarkable, cases. The Galapagos Islands. — This archipelago is of volcanic origin, situated under the equator between 500 and 600 miles from the West Coast of South America. The depth of the ocean around them varies from 2,000 to 3,000 fathoms or more. This group is of peculiar interest, from the fact that it was the study of its fauna which first suggested to Darwin's mind the theory of evolution. I will, therefore, begin by quoting a short passage from his writings upon the zoological relations of this particular fauna. "Here almost every product of the land and of the water bears the unmistakable stamp of the American continent. There are twenty-six land birds; of these, twenty-one, or perhaps twenty- three, are ranked as distinct species, and would commonly be assumed to have been here created; yet the close affinity of most of these birds to American species is manifest in every character, in their habits, gestures, and tones of voice. So it is with the other animals, and with a large pro- portion of the plants, as shown by Dr. Hooker in his admirable Flora of this archipelago. The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, distant several hundred miles from the continent, feels that he is standing on American land. Why should this be so ? Why should the species which are supposed to have been created in the Galapagos Archipelago, and nowhere else, bear so plainly the stamp of affinity to those created in America ? There is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the several classes are associated together, which closely resembles the io6 READINGS IX EVOLUTION, GENETICS, AND EUGENICS conditions of the South American coast; in fact, there is a considerable dissimilarity in all these respects. On the other hand, there is a con- siderable degree of resemblance in the volcanic nature of the soil, in the climate, height, and size of the islands, between the Galapagos and Cape de Verde Archipelagoes; but what an entire and absolute difference in their inhabitants! The inhabitants of the Cape de Verde Islands are related to those of Africa, like those of the Galapagos to America. Facts such as these admit of no sort of explanation on the' ordi- nary view of independent creation; whereas in the view here main- tained it is obvious that the Galapagos Islands would be likely to receive colonists from America, and the Cape de Verde Islands from Africa; such colonists would be liable to modification — the principle of inheritance still betraying their original birthplace. " The following is a synopsis of the fauna and flora of this archi- pelago, so far as at present known. The only terrestrial vertebrates are two peculiar species of land- tortoise, and one extinct species; five species of lizards, all peculiar — two of them so much so as to constitute a peculiar genus; — and two species of snakes, both closely allied to South American forms. Of birds there are 57 species, of which no less than 38 are peculiar; and all the non-peculiar species, except one, belong to aquatic tribes. The true land-birds are represented by 31 species, of which all, except one, are peculiar; while more than half of them go to constitute peculiar genera. Moreover, while they are all unquestionably allied to South American forms, they present a beautiful series of gradations, "from perfect identity with the conti- nental species, to genera so distinct that it is difficult to determine with what forms they are most nearly allied; and it is interesting to note that this diversity bears a distinct relation to the probabilities of, and facilities for, migration to the islands. The excessively abund- ant rice-bird, which breeds in Canada, and swarms over the whole United States, migrating to the West Indies and South America, visiting the distant Bermudas almost every year, and extending its range as far as Paraguay, is the only species of land-bird which remains completely unchanged in the Galapagos; and we may therefore con- clude that some stragglers of the migrating host reach the islands sufficiently often to keep up the purity of the breed" [Wallace]. Again, of the thirty peculiar land-birds, it is observable that the more they dift'er from any other species or genera on the South American continent, the more certainly are they found to have their nearest relations among those South American forms which have the EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 107 more restricted range, and therefore the least Hkely to have found their way to the islands with any frequency. The insect fauna of the Galapagos Islands is scanty, and chiefly composed of beetles. These number 35 species, which are nearly all peculiar, and in some cases go to constitute peculiar genera. The same remarks apply to the twenty species of land-shells. Lastly, of the total number of flowering plants (332 species) more than one half (174 species) are peculiar. It is observable in the case of these peculiar species of plants — as also of the peculiar species of birds — that many of them are restricted to single islands. It is also observable that with regard both to the fauna and flora, the Galapagos Islands as a whole are very much richer in peculiar species than either the Azores or Bermudas, notwithstanding that both the latter are considerably more remote from the nearest continents. This differ- ence, which at first sight appears to make against the evolutionary interpretation, really tends to confirm it. For the Galapagos Islands are situated in a calm region of the globe, unvisited by those periodic storms and hurricanes which sweep over the North Atlantic, and which every year convey some stragghng birds, insects, seeds, etc., to the Azores and Bermudas. Notwithstanding their somewhat greater isolation geographically, therefore, the Azores and Bermudas are really less isolated biologically than are the Galapagos Islands; and hence the less degree of peculiarity on the part of their endemic species. But, on the theory of special creation, it is impossible to understand why there should be any such correlation between the prevalence of gales and a comparative inertness of creative activity. And, as we have seen, it is equally impossible on this theory to under- stand why there should be a further correlation between the degree of peculiarity on the part of the isolated species, and the degree in which their nearest allies on the mainland are there confined to narrow ranges, and therefore less likely to keep up any biological communi- cation with the islands. St. Helena. — 'A small volcanic island, ten miles long by eight wide, situated in mid-ocean, 1,100 miles from Africa, and 1,800 from South America. It is very mountainous and rugged, bounded for the most part by precipices, rising from ocean depths of 17,000 feet, to a height above the sea-level of nearly 3,000. When first discovered it was richly clothed with forests; but these were all destroyed by human agency during the i6th, 17th, and i8th centuries. The records of civili- zation present no more lamentable instance of this kind of destruction. io8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS From a merely pecuniary point of view the abolition of these pri- meval forests has proved an irreparable loss; but from a scientific point of view the loss is incalculable. These forests served to harbour countless forms of life, which extended at least from the Miocene age, and which, having found there an ocean refuge, survived as the last remnants of a remote geological epoch. In those days, as Mr. Wallace observes, St. Helena must have formed a kind of natural museum or vivarium of archaic species of all classes, the interest of which we can now only surmise from the few remnants of those remnants, which are still left among the more inaccessible portions of the mountain peaks and crater edges. These remnants of remnants are as follows: There is a total absence of all indigenous mammals, reptiles, fresh-water fish, and true land-birds. There is, however, a species of plover, allied to one in South Africa; but it is specifically distinct, and therefore peculiar to the island. The insect life, on the other hand, is abundant. Of beetles, no less than 129 species are believed to be aboriginal, and, with one single exception, the whole number are peculiar to the island. "But in addition to this large amount of specific peculiarity (perhaps unequalled anywhere else in the world) the beetles of this island are remarkable for their generic isolation, and for the altogether exceptional proportion in which the great divisions of the order are represented. The species belong to 39 genera, of which no less than 25 are peculiar to the island; and many of these are such isolated forms that it is impossible to find their allies in any particular country" [Wallace]. More than two-thirds of all the species belong to one group of weevils — a circumstance which serves to explain the great wealth of beetle-population, the weevils being beetles which live in wood, and St. Helena having been originally a densely wooded island. This circumstance is also in accordance with the view that the peculiar insect fauna has been in large part evolved from ancestors which reached the island by means of floating timber; for, of course, no explanation can be suggested why special creation of this highly peculiar insect fauna should have run so disproportionately into the production of weevils. About two-thirds of the whole number of beetles, or over 80 species, show no close affinity with any existing insects, while the remaining third have some relations, though often very remote, with European and African forms. That this high degree of peculiarity is due to high antiquity is further indicated, according to our theory, by the large number of species which some of the types comprise. Thus, the 54 species of Cossonidae may be EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 109 referred to three types ; the 1 1 species of Bemhidium form a group by themselves; and the Heteromera form two groups. ''Now, each of these types may well be descended from a single species, which origi- nally reached the island from some other land; and the great variety of generic and specific forms into which some of them have diverged is an indication, and to some extent a measure, of the remoteness of their origin" [Wallace]. But, on the counter-supposition that all these 128 peculiar species were separately created to occupy this particular island, it is surely unaccountable that they should thus present such an arborescence of natural affinities amongst themselves. Passing over the rest of the insect fauna, which has not yet been sufficiently worked out, we next find that there are only 20 species of indigenous land-shells — which is not surprising when we remember by what enormous reaches of ocean the land is surrounded. Of these 20 species no less than 13 have become extinct, three are allied to Euro- pean species, while the rest are so highly peculiar as to have no near allies in any other part of the globe. So that the land-shells tell exactly the same story as the insects. Lastly, the plants likewise tell the same story. The truly indige- nous flowering plants are about 50 in number, besides 26 ferns. Forty of the former and ten of the latter are peculiar to the island, and, as Sir Joseph Hooker tells us, "cannot be regarded as very close specific allies of any other plants at all." Seventeen of them belong to peculiar genera, and the others all differ so markedly as species from their congeners, that not one comes under the category of being an insular form of a continental species. So that with respect to its plants, no less than with respect to its animals, we find that the island of St. Helena constitutes a little world of unique species, allied among themselves, but diverging so much from all other known forms that in many cases they constitute unique genera. Sandwich Islands. — These are an extensive group of islands, larger than any we have hitherto considered — the largest of the group being about the size of Devonshire. The entire archipelago is vol- canic, with mountains rising to a height of nearly 14,000 feet. The group is situated in the middle of the North Pacific, at a distance of considerably over 2,000 miles from any other land, and surrounded by enormous ocean depths. The only terrestrial vertebrates are two lizards, one of which constitutes a peculiar genus. There are 24 aquatic birds, five of which are peculiar; four birds of prey, two of which are peculiar; and 16 land-birds, all of which are peculiar. no READINGS IN EVOLUTION, GENETICS, AND EUGENICS Moreover, these i6 land-birds constitute no less than lo peculiar genera, and even one peculiar family of five genera. This is an amount of peculiarity far exceeding that of any other islands, and, of course, corresponds with the great isolation of this archipelago. The only other animals which have here been carefully studied are the land- shells, and these tell the same story as the birds. For there are no less than 400 species which are all, without any exception, peculiar; while about three-quarters of them go to constitute peculiar genera. Again, of the plants, 620 species are believed to be endemic; and of these 377 are peculiar, yielding no less than 39 peculiar genera. THE FAUNA OF MADAGASCAR AXD NEW ZEALAND^ A. R. WALLACE The two exceptions just referred to are- Madagascar and New Zealand, and all the evidence goes to show that in these cases the land connection with the nearest continental area was very remote in time. The extraordinary isolation of the productions of Madagascar — almost all the most characteristic forms of mammalia, birds, and reptiles of Africa being absent from it — renders it certain that it must have been separated from that continent very early in the Tertiary, if not as far back as the latter part of the Secondary period; and this extreme antiquity is indicated by a depth of considerably more than a thousand fathoms in the Mozambique Channel, though this deep portion is less than a hundred miles wide between the Comoro Islands and the main- land. Madagascar is the only island on the globe with a fairly rich mammalian fauna which is separated from a continent by a depth greater than a thousand fathoms; and no other island presents so many peculiarities in these animals, or has preserved so many lowly organised and archaic forms. The exceptional character of its pro- ductions agrees exactly with its exceptional isolation by means of a very deep arm of the sea. New Zealand possesses no known mammals and only a single species of batrachian; but its geological structure is perfectly conti- nental. There is also much evidence that it does possess one mammal, although no specimens have been yet obtained. Its reptiles and birds are highly peculiar and more numerous than in any truly oceanic island. Now the sea which directly separates New Zealand from Australia is more than 2,000 fathoms deep, but in a north-west direc- ^ From A. R. Wallace, Darwinism (copyright 1889). Used by special permis- sion of the publishers, The Macmillan Company. EVIDENCES FROM GEOGRAPHIC DISTRIBUTION iii tion there is an extensive bank under i,ooo fathoms, extending to and including Lord Howe's Island, while north of this are other banks of the same depth, approaching towards a submarine extension of Queensland on the one hand, and New Caledonia on the other, and altogether suggestive of a land union with Australia at some very- remote period. Now the peculiar relations of the New Zealand fauna and flora with those of Australia and of the tropical Pacific Islands to the northward indicate such a connection, probably during the Cre- taceous period; and here, again, we have the exceptional depth of the dividing sea and the form of the ocean bottom according well with the altogether exceptional isolation of New Zealand, an isolation which has been held by some naturalists to be great enough to justify its claim to be one of the primary Zoological Regions. THE DISTRIBUTION OF MARSUPIALS' A. R. WALLACE This singular and lowly organised type of mammals constitutes almost the sole representative of the class in Australia and New Guinea, while it is entirely unknown in Asia, Africa, or Europe. It reappears in America, where several species of opossums are found; and it was long thought necessary to postulate a direct southern con- nection of these distant countries, in order to account for this curious fact of distribution. When, however, we look to what is known of the geological history of the marsupials the difiiculty vanishes. In the Upper Eocene deposits of Western Europe the remains of several animals closely allied to the American opossums have been found; and as, at this period, a very mild climate prevailed far up into the arctic regions, there is no difficulty in supposing that the ancestors of the group entered America from Europe or Northern Asia during early Tertiary times. But we must go much further back for the origin of the Australian marsupials. All the chief types of the higher mammalia were in existence in the Eocene, if not in the preceding Cretaceous period, and as we find none of these in Australia, that country must have been finally separated from the Asiatic continent during the Secondary or Mesozoic period. Now during that period, in the Upper and the Lower Oohte and in the still older Trias, the jaw-bones of numerous small mammalia have been found, forming eight distinct genera, which ' From A. R. Wallace, Darwinism (copyright 1889). Used by special per- mission of the publishers, The Macmillan Company. 112 READINGS IN EVOLUTION, GENETICS, AND EUGENICS are believed to have been either marsupials or some allied lowly forms. In North America also, in beds of the Jurassic and Triassic formations, the remains of an equally great variety of these small mammalia have been discovered; and from the examination of more than sixty speci- mens, belonging to at least six distinct genera. Professor Marsh is of the opinion that they represent a generalised type, from which the more specialised marsupials and insectivora were developed. From the fact that very similar mammals occur both in Europe and America at corresponding periods, and in beds which represent a long succession of geological time, and that during the whole of this time no fragments of any higher forms have been discovered, it seems probable that both the northern continents (or the larger portion of their area) were then inhabited by no other mammalia than these, with perhaps other equally low types. It was, probably, not later than the Jurassic age when some of these primitive marsupials were able to enter Australia, where they have since remained almost com- pletely isolated; and, being free from the competition of higher forms, they have developed into the great variety of types we now behold there. These occupy the place, and have to some extent acquired the form and structure of distinct orders of the higher mammals — the rodents, the insectivora, and the carnivora — while still preserving the essential characteristics and lowly organisation of the marsupials. At a much later period — probably in late Tertiary times — the ances- tors of the various species of rats and mice which now abound in Australia, and which, with the aerial bats, constitute its only forms of placental mammals, entered the country from some of the adjacent islands. For this purpose a land connection was not necessary, as these small creatures might easily be conveyed among the branches or in the crevices of trees uprooted by floods and carried down to the sea, and then floated to a shore many miles distant. That no actual land connection with, or very close approximation to, an Asiatic island had occurred in recent times, is sufflciently proved by the fact that no squirrel, pig, civet, or other widespread mammal of the Eastern hemisphere has been able to reach the Australian continent. THE DISTRIBUTION OF BIRDS^ A. R. WALLACE These vary much in their powers of flight, and their capability of traversing wide seas and oceans. Many swimming and wading birds ^ From A. R. Wallace, Darwinism (copyright 1891). Used by special per- mission of the publishers, The Macmillan Company. EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 113 can continue long on the wing, fly swiftly, and have, besides, the power of resting safely on the surface of the water. These would hardly be limited by any width of ocean, except for the need of food; and many of them, as the gulls, petrels, and divers, find abundance of food on the surface of the sea itself. These groups have a wide distri- bution across the oceans; while waders — expecially plovers, sandpipers, snipes, and herons — are equally cosmopolitan, travelling along the coasts of all the continents, and across the narrow seas which separate them. Many of these birds seem unaffected by climate, and as the organisms on which they feed are especially abundant on arctic, tem- perate, and tropical shores, there is hardly any limit to the range even of some of the species. Land-birds are much more restricted in their range, owing to their usually limited powers of flight, their inability to rest on the surface of the sea or to obtain food from it, and their greater specialisation, which renders them less able to maintain themselves in the new coun- tries they may occasionally reach. Many of them are adapted to live only in woods, or in marshes, or in deserts; they need particular kinds of food or a limited range of temperature; and they are adapted to cope only with the special enemies or the particular group of competi- tors among which they have been developed. Such birds as these may pass again and again to a new country, but are never able to establish themselves in it; and it is this organic barrier, as it is termed, rather than any physical barrier, which, in many cases, determines the presence of a species in one area and its absence from another. We must always remember, therefore, that, although the presence of a species in a remote oceanic island clearly proves that its ancestors must at one time have found their way there, the absence of a species does not prove the contrary, since it also may have reached the island, but have been unable to maintain itself, owing to the inorganic or organic conditions not being suitable to it. This general principle applies to all classes of organisms, and there are many striking illus- trations of it. In the Azores there are eighteen species of land-birds which are permanent residents, but there are also several others which reach the islands almost every year after great storms, but have never been able to establish themselves. In Bermuda the facts are still more striking, since there are only ten species of resident birds, while no less than twenty other species of land-birds, and more than a hundred species of waders and aquatics are frequent visitors, often in great numbers, but are never able to estabHsh themselves. 114 READINGS IN EVOLUTION, GENETICS, AND EUGENICS SUMMARY OF MAMMALIAN DISPERSAL* HANS GADOW Australia as the earliest great mass of land permanently severed from the rest is in almost undisturbed possession of the lowest mam- mals. It is the sole refuge of the monotremes, and the marsupials have narrowly escaped a similar fate. They take us to the next independent continent, South America. This had three chances, or epochs, of being stocked with mammals. Within the Cretaceous period it seems to have received its marsupial stock from the north, the pro- genitors of all modern marsupials. A second influx during the early Tertiary brought edentates and rodents as its first Placentals from Africa, and those queer Ungulates, the Toxodonts and Pyrotheria, unless we prefer to look upon these Eocene extinct orders as truly aboriginal to South America, when this was still continuous with the ancient Brazil- Afro-Indian Gondwanaland. The third and last inroad came once more from the north, when with the close of the Miocene permanent connection with North America was re-established. This brought the modern odd-toed and pair-toed Ungulates, with dogs, cats and bears in their wake, and lastly man. There remains the huge North World. Eurasia and North America have always formed a wide circumpolar ring, which repeatedly broke and joined again. Whatever group of terrestrial creatures was developed in the eastern, Asiatic, half, was sure to turn up in the western, and vice versa. Lastly, the mysterious African continent. It began originally as the centre of the ancient equatorial South World; it has lost these con- nections and has become joined to the northland, after many vicissi- tudes. It is therefore most difficult to apportion its fauna rightly; moreover for fossils it is almost a blank, except Egypt. It must have had some share in the evolution of mammals, like edentates, rodents, insectivores, hyrax, elephants, sirenians and lemurs, all groups with an ancient stamp. But what share it had, against Eurasia, in the development of say ungulates, carnivores, monkeys, we do not know. Not much is likely to have originated in Europe; the elephants, rhinos, hippos, lions and hyaenas were migrants rather from than to Africa, rarely across some Mediterranean bridge, usually by Asia Minor. The more dominant forms of our present fauna have originated, to use an expression of Darwin's, "in the larger areas and more efficient * From Hans Gadow, Wanderings of Animals (1913), Cambridge University Press EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 115 workshops of the north," and the balance is in favour of Asia as the cradle of modern mammals. Is it an idle dream to think of the future ? A survey of the past reveals the vanishing of whole faunas from extensive countries, which were then repeopled by other forms from elsewhere. What has happened before, may happen in times to come. Countless groups, once flourishing, are no more; many others have had their day and are now on the decline, whilst others are flourishing now, are even in the increase and seem to have a future before them. Such favoured assemblies are the toads and frogs, lizards and snakes, Passerine birds and rodents, mostly the small-sized members of their tribes; the days of giants are past. All this has happened in the natural course of events, without the influence of man, who only within most recent times has become the most potent and destructive factor to the ancient faunas of the world. SUMMARY OF THE ARGUMENT FOR EVOLUTION AS BASED ON GEOGRAPHIC DISTRIBUTION [On the hypothesis of special creation or on any other hypothesis except evolution that has even been suggested, the extremely intricate patchwork of animal and plant distribution remains an unsolvable picture puzzle, without rhyme or reason. When this puzzle is attacked with the aid of the evolutionary idea, the key to the whole maze is furnished and the difficulties clear up with remarkable ease. The whole hodgepodge makes sense and we can understand many pre- viously irreconcilable facts. In no field does the working hypothesis of evolution work to such advantage as in this field. On the basis that a species arises at one place, spreads out over large areas, becoming modified as it goes, that new species are formed from old through modification after isolation from the parent-stock, how do the facts of distribution look when examined in detail ? 1. Cosmopolitan groups, those with the widest distribution, are those to whom no barriers are sufficient to check migration, e.g., strong fliers, Man, earthworms carried by Man. 2. Restricted groups are usually those to which barriers are readily set up and are frequently the last remnants of a formerly successful fauna or flora, which continue to survive only in some restricted area where the conditions are rather more favorable than elsewhere. Ii6 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 3. The study of the distribution of species belonging to a single genus reveals that the more primitive or generalized species occupy a central position and the most specialized species are at the outer boundaries of the distributional area. 4. The faunas and floras of continental islands are just what we should expect on the basis that there was at one time a land connection with the nearest continent; that at this time the faunas and floras were the same on both island and continent; that, later, the continent and island were separated by an impassable barrier of ocean; and that the inhabitants of the two bodies evolved separately. 5. The faunas and floras of oceanic islands are like those of the nearest mainland and are of those types, for the most part, that might most readily have been blown or carried on floating debris. 6. The conclusions arrived at by students of geographic distribu- tion, past and present, as to the existence of former land connections, now broken, are borne out by the independent findings of geologists and geographers. — Ed.] CHAPTER VIII EVIDENCES FROM CLASSIFICATION THE PRINCIPLES OF CLASSIFICATION^ A. F. SHULL The International Code. — Some of the essential features of the International Code are as follows. The first name proposed for a genus or species prevails on the condition that it was published and accompanied by an adequate description, definite or indication, and that the author has applied the principles of binomial nomenclature. This is the so-called law of priority. The tenth edition of the Sytema Naturae of Linnaeus is the basis of the nomenclature. The author of a genus or species is the person who first pubhshes the same in connec- tion with a definition, indication or description, and his name in full or abbreviated is given with the name; thus, Bascanian anthonyi Stejneger. In citations the generic name of an animal is written with a capital letter, the specific and subspecific name without initial capital letter. The name of the author follows the specific name (or subspecific name if there is one) without intervening punctuation. If a species is transferred to a genus other than the one under which it was first described, or if the name of a genus is changed, the author's name is included in parentheses. For example, Bascanion anthonyi Stejneger should now be written Coluber anthonyi (Stejneger), the ge- neric name of this snake having been changed. One species constitutes the type of the genus; that is, it is formally designated as typical of the genus. One genus constitutes the type of the subfamily (when a subfamily exists), and one genus forms the type of the family. The type is indicated by the describer or if not indicated by him is fixed by another author. The name of a subfamily is formed by adding the ending -inae, and the name of a family by adding -idae to the root of the name of the type genus. For example, Colubrinae and Colubri- dae are the subfamily and family of snakes of which Coluber is the type genus. The basis of classification. — Early systematists largely employed superficial characters to differentiate and classify animals, and their ' From A. F. ShuU, Principles of Animal Biology (copyright 1920). Used by special permission of The McGraw-Hill Book Company, 117 Ii8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS classifications were thus largely artificial and served principally as convenient methods of arrangement, description and cataloging. Since the time of the development of the theory of descent with modifications by Lamarck (1809) and Darwin (1859), there has been an attempt to base the classification on relationships. Very nearly related animals are put into the same species. They are related because they descend from a common ancestry, and that common ancestry could not in most cases have been very ancient, otherwise evolution within the group would have occurred and the species would have been split into two or more species. Species that are much alike are included in one genus, being thus marked off from the species of another genus. The similarity of the species of a genus is held to indicate kinship, but since there is greater diversity among the indi- viduals of a genus than among the members of a species, the common stock from which the species of a genus have sprung must have existed at an earlier time, in order that evolution could bring about the degree of divergence now observed. In like manner, a family is made up of genera, and their likeness is again a sign of affinity. But to account for the greater difference between the extreme individuals belonging to a family, evolution must have had more time, that is, the common source of the members of a family must have antedated the common source of the individuals of a genus. Orders, classes, and phyla are similarly regarded as having sprung from successively more remote ancestors, the time differences being necessary to allow for the differ- ences in the amount of evolution. This statement is in general correct. However, since evolution has probably not proceeded at the same rate at all periods, nor in all branches of the animal kingdom at any one time, the time relations of the groups of high or low rank must not be too rigidly assigned. Thus certain genera, in which evolution has been slow, are probably much older than some families in which evolution has been rapid. It is not improbable, also, that some genera are quite as old as the families which include them; but in no case can they be older. Furthermore, different groups are classified by taxonomists of different temperaments, so that groups of a given nominal rank may be much more inclusive (and hence older) in one branch of the animal kingdom than in another. On the whole, nevertheless, the groups of higher rank have sprung from ancestry more remote than that of the groups of lower rank. The means of recognizing the kinship implied in classification permit some differences of opinion. It is recognized that likeness in EVIDENCES FROM CLASSIFICATION 119 structural characters is the chief clue to affinities. However, the evidential value of similarity in one or several structures unaccom- panied by the similarity of all parts is to be distrusted, since animals widely separated and dissimilar in most characters may have certain other features in common. Thus, the coots, phalaropes and grebes among birds have lobate feet but, as indicated by other features, they are not closely related; and there are certain lizards (Amphisbaenidae) which closely resemble certain snakes (Typholopidae) in being blind, limbless, and having a short tail. The early systematists were very liable to bring together in their classification analogous forms, that is, those which are functionally similar; or animals which are super- ficially similar. In contrast with the early practice, the aim of taxonomists at the present time is to group forms according to homol- ogy, which is considered an indication of actual relationship. Since a genetic classification must take into consideration the entire animal, the search for affinities becomes an attempt to evaluate the results of all morphological knowledge, and it is also becoming evident that other things besides structure may throw light upon relationships. The fossil records, geographical distribution, ecology and experi- mental breeding may all assist in establishing affinities. The method of taxonomy. — It is evident that before the relation- ships of animals can be determined the forms must be known, for unknown forms constitute breaks in the pedigrees of the groups to which they belong. Moreover, as pointed out above, the structural characters, variation and distribution must be known before a form can be placed in the proper place in a genetic system. For these reasons an important part of systematic work is the description of forms and an analysis of their differences. After the Linnaean system was adopted zoologists attacked this virgin field and for many years "species making" predominated. Even at the present time when other aspects of zoology have come to receive relatively more attention it is an interesting fact that the analytical method prevails in systematic studies, and taxonomy suffers from, and in part merits, the criticism that it is a mere cataloging of forms and ignores the higher goal of investigation, namely, the discovery of the course of evolution. Many systematists, however, recognize that the ultimate purpose of taxonomic work is to discover the relationships as well as the differences between the described forms in order that the course of evolution may be determined. In other words, it is appreciated that while analytical studies are necessary they are only preliminary, and i20 READINGS IN EVOLUTION, GENETICS, AND EUGENICS that upon their results must be built synthetic studies, if taxonomy is to fulfil its purpose. THE METHOD OF CLASSIFICATION CHARLES DARWIN^ Naturalists, as we have seen, try to arrange the species, genera, and families in each class, on what is called the Natural System. But what is meant by this system ? Some authors look at it merely as a scheme for arranging together those living objects which are most alike, and for separating those which are most unlike; or as an artificial method of enunciating, as briefly as possible, general propositions, — that is, by one sentence to give the characters common, for instance, to all mammals, by another those common to all carnivora, by another those common to the dog-genus, and then, by adding a single sentence, a full description is given of each kind of dog. The ingenuity and utiUty of this system are indisputable. But many naturalists think that something more is meant by the Natural System; they believe that it reveals the plan of the Creator; but unless it be specified whether order in time or space, or both, or what else is meant by the plan of the Creator, it seems to me that nothing is thus added to our knowledge. Expressions such as that famous one by Linnaeus, which we often meet with in a more or less concealed form, namely, that the characters do not make the genus, but that the genus gives the charac- ters, seem to imply that some deeper bond is included in our classifica- tions than mere resemblance. I believe that this is the case, and that community of descent — the one known cause of close similarity in organic beings — is the bond which, though observed by various degrees of modification, is partially revealed to us by our classifications. Let us now consider the rules followed in classification, and the difficulties which are encountered on the view that classification either gives some unknown plan of creation, or is simply a scheme for enunciating general propositions and of placing together the forms most like each other. It might have been thought (and was in ancient times thought) that those parts of the structure which determined the habits of life, and the general place of each being in the economy of nature, would be of very high importance in classification. Nothing can be more false. No one regards the external similarity of a mouse to a shrew, of a dugong to a whale, of a whale to a fish, as of any ^ From The Origin oj Species. EVIDENCES FROM CLASSIFICATION 121 importance. These resemblances, though so intimately connected with the whole life of the being, are ranked as merely "adaptive or analogical characters " : but to the consideration of these resemblances we shall recur. It may even be given, as a general rule, that the less any part of the organisation is concerned with special habits, the more important it becomes for classification. As an instance: Owen, in speaking of the dugong, says, "The generative organs, being those which are most remotely related to the habits and food of an animal, I have always regarded as affording very clear indications of its true affinities. We are least likely in the modifications of these organs to mistake a merely adaptive for an essential character." With plants how remarkable it is that the organs of vegetation, on which their nutrition and life depend, are of little signification; whereas the organs of reproduction, with their product the seed and embryo, are of paramount importance! So again in formerly discussing certain morphological characters which are not functionally important, we have seen that they are often of the highest service in classification. This depends on their constancy throughout many allied groups; and their constancy chiefly depends on any slight deviations not having been preserved and accumulated by natural selection, which acts only on serviceable characters. WHAT IS A SPECIES? "Each kind of animal or plant, that is, each set of forms which in the changes of the ages has diverged tangibly from its neighbors, is called a species. There is no absolute definition for the word species. The word kind represents it exactly in common language, and is just as susceptible to exact definition. The scientific idea of species does not differ materially from the popular notion. A kind of tree or bird or squirrel is a species. Those individuals which agree very closely in structure and function belong to the same species. There is no absolute test, other than the common judgment of men competent to decide. Naturalists recognize certain formal rules as assisting in such a decision. A series of fully intergrading forms, however varied at the extremes, is usually regarded as forming a single species. There are certain recognized effects of chmate, of climatic isolation, and of the isolation of domestication. These do not usually make it necessary to regard as distinct species the extreme forms of a series concerned."^ ^ From D. S. Jordan and V. L. Kellogg, Evolution and Animal Li/e. 122 READINGS IN EVOLUTION, GENETICS, AND EUGENICS "The term 'species' was thus defined by the celebrated botanist De Candolle: 'A species is a collection of all the individuals which resemble each other more than they resemble anything else, which can by mutual fecundation produce fertile individuals, and which repro- duce themselves by generation, in such a manner that we may from analogy suppose them all to have sprung from one single individual.' And the zoologist Swainson gives a somewhat similar definition : 'A species, in the usual acceptation of the term, is an animal which, in a state of nature, is distinguished by certain peculiarities of form, size, colour, or other circumstances, from another animal. It propagates, after its kind, individuals perfectly resembling the parent; its pecu- liarities, therefore, are permanent.' " ^ [As will have become apparent, the fundamental assumption underlying classification is that the closest fundamental similarities between animals (or plants) are found in the forms most closely related and that the greatest differences are found in those forms which are unrelated or at best very distantly related. The assumption implies the idea of descent with modification, which is no more nor less than evolution. Using this evolutionary basis, we can arrive at an extremely satisfactory classification both of living and of extinct forms; and there is no other basis of classification that works. The question might well be asked whether it is possible to test the validity of the assumption that degrees of resemblance vary directly with closeness of blood relationship ? Two direct tests of this may be and have been made. The closest of blood relatives possible are individuals that have been derived by the dividing of a single egg. Armadillo^ quadruplets have been shown to be thus derived, and detailed studies of the closeness of resemblance existing between members of a given set indicate that they are vastly more alike than are the simultaneously born offspring of animals which give birth to several young, but in which each young is derived from a separate egg. If we use the index of correlation to indicate the degree of similarity between individuals we find that ordinary brothers or sisters are only about 50 per cent alike, while armadillo quadruplets are over 90 per cent alike. Identical or duplicate twins in human beings are believed to have an origin from one egg, after the fashion of the armadillo, ^ From A. R. Wallace, Daricinism. 2 See H. H. Newman, The Biology of Twins (191 7), University of Chicago Press. EVIDENCES FROM CLASSIFICATION 123 though the proof has not been forthcoming. Everyone is famihar with the remarkable similarity, amounting almost to identity, between such twins. Thus we are able to show that the closest blood relation- ship known is associated with the closest resemblance. The next degree of resemblance is between members of the same family, brothers, sisters, cousins, etc., and we do not hesitate to explain this resemblance as due to blood relationship. In this we merely accept the known principles of heredity. The second direct test of the validity of the assumption that degrees of resemblance run parallel with degrees of blood relationship is found in connection with '' blood- transfusion tests." This evidence, as presented by Professor Scott, forms the substance of the next chapter. — Ed.] CHAPTER IX EVIDENCE FROM BLOOD TESTS^ « W. B. Scott Here may be conveniently considered the very interesting and significant blood tests which have been made in the last fifteen years by various physiologists and especially by Dr. George H. F. Nuttall, of the University of Cambridge. Though there are several methods of making these tests, the "precipitation method" employed by Dr. Nuttall will be quite sufficient for the ends sought in these lec- tures. The method and significance of the tests can best be explained bv taking as an example human blood, which, of course, has been most extensively and minutely studied, because of its legal importance as well as its scientific interest. Ordinary chemical analysis is unable to determine the differences in blood-composition between various animals, but that there were important differences had long been understood. This was shown by the fact that, in performing the operation for the transfusion of blood, it was not practicable to substitute animal for human blood, since the former might cause serious injury to the patient. ' The precipitation method of making blood tests is as follows: Freshly drawn human blood is allowed to coagulate or clot, which it will do in a few minutes, if left standing in a dish, and then the serum is drained away from the clot. Blood-serum is the watery, almost colourless part of the blood, which remains after coagulation. Small quantities of this serum are injected, at intervals of one or two days, into the veins of a rabbit and cause the formation in the rabbit's blood of an anti-body, analogous to the anti-toxin which is produced in the blood of a horse by the injection of diphtheria virus. After the last injection the rabbit is allowed to live for several days and is then killed and bled, the blood is left until it clots and the serum drained off and preserved. The serum obtained thus from a rabbit is called ''anti-human" serum and is an exceedingly deUcate test for human blood, not only when the latter is fresh, but also when it is in the form of old and dried blood-stains, or even when the blood is ^ From W. B. Scott, The Theory of Evolution (copyright 1917)- Used by special permission of the publishers, The Macmillan Company. 124 EVIDENCE FROM BLOOD TESTS 125 putrid. Stains, for example, are soaked in a very weak solution of common salt and, if necessary, the blood solution is filtered until it is quite limpid and clear. Into the blood solution a few drops of the anti-human serum are conveyed and, if the stains are of human blood, a white precipitate is formed and thrown down, but if the stains are of the blood of some domestic animal, such as a pig, sheep, or fowl, no such reaction follows. In the same manner as above described, we may prepare anti-pig, anti-horse, anti-fowl, etc., etc., sera by injecting the fresh-drawn serum of a pig, horse, fowl, or any other animal into the rabbit, instead of human blood-serum. In some countries, notably in Germany and Austria, this test has already been adopted by the courts of justice and has been found extremely useful in the detection of crime. Further investigation showed that these blood tests might be employed to determine the degrees of relationship between different animals, for, although a prompt and strong reaction is usually obtained only from the blood of the same species as that from which the original injection into the rabbit was taken, the blood of nearly allied species, such as the horse and donkey, for example, gives a weaker and slower precipitation. By using stronger solutions and allowing more time, quite distant relationships may be brought out. Nuttall and his collaborator, Graham-Smith, made many thousands of such experi- ments bearing upon the problems of relationship and classification and it is of great significance to note that their highly interesting and important results contain few surprises, but, in almost all cases, merely serve to confirm the conclusions previously reached by other methods, such as comparative anatomy and palaeontology. It will be instructive to quote some of these results, the quotations being taken from "Blood Immunity and Blood Relationship, by G. H. F. Nuttall, including Original Researches by G. L. Graham-Smith and T. S. P. Strangeways, " Cambridge, 1904. ''In the absence of palaeontological evidence the question of the interrelationship amongst animals is based upon similarities of struc- ture in existing forms. In judging of these similarities, the subjective element may largely enter." ''The very interesting observations upon the eye made by Johnson also demonstrate the close relationships between the Old World forms and man, the macula lutea tending to disappear as we descend in the scale of New World Monkeys and being absent in the Lemurs. The results which I published upon my tests with precipitins directly supported this evidence, for the reactions 126 READINGS IN EVOLUTION, GENETICS, AND EUGENICS obtained with the bloods of Simiidae (i.e., Man-Hke Apes) closely resemble those obtained with human blood, the bloods of Cercopithe- cidae (Old World Monkeys) came next, followed by those of Cebidae and Hapalidae (New World Monkeys and Marmosets) which gave but slight reactions with anti-human serum, whilst the blood of Lemuroidea gave no indication of blood-relationship." "A perusal of the pages relating to the tests made upon the many bloods I have examined by means of precipitating anti-sera, will very clearly show that this method of investigation permits of our drawing certain definite conclusions. It is a remarkable fact .... that a common property has persisted in the bloods of certain groups of animals throughout the ages which have elapsed during their evolution from a common ancestor, and this in spite of differences of food and habits of life. The persistence of the chemical blood-relationship between the various groups of animals serves to carry us back into geological times, and I beheve we have but begun the work along these lines, and that it will lead to valuable results in the study of various problems of evolution." The general conclusions on interrelationships, so far as they are of particular interest for our purpose, reached by Nuttall and Graham- Smith as the result of many thousands of blood tests, may be summa- rized as follows: I. If sufficiently strong solutions be used and time enough be allowed, a relationship between the bloods of all mammals is made evident. . 2. The degrees of relationship between man, apes and monkeys have already been noted. 3. Anti-carnivore sera show ''a preponderance of large reactions amongst the bloods of Carnivora, as distinguished from other Mam- malia; the maximum reactions usually take place amongst the more closely related forms in the sense of descriptive zoology." 4. Anti-pig serum gives maximum reactions only with the bloods of other species of the same family, moderate reactions those of rumi- nants and camels, and moderate or slight reactions with those of whales. Anti-llama serum gives a moderate reaction with the blood of the camel, and the close relationship between the deer family and the great host of antelopes, sheep, goats and oxen is clearly demonstrated. 5. An ti- whale serum gives maximum reactions only with the bloods of other whales and slight reactions with those of pigs and ruminants. EVIDENCE FROM BLOOD TESTS 127 6. A close relationship is shown to exist between all marsupials, with the exception of the Thylacine, or so-called Tasmanian Wolf. 7. Strong anti-turtle serum gives maximum reactions only with the bloods of turtles and crocodiles; with those of lizards and snakes the results are almost negative. With the egg-albumins of reptiles and birds a moderate reaction is given. 8. Anti-lizard serum produces maximum results with the bloods of lizards and reacts well with those of snakes. 9. These experiments indicate that there is a close relationship between lizards and snakes, on the one hand, turtles and crocodiles on the other. They further indicate that birds are more nearly allied with the turtle-crocodile series than with the lizard-snake series, results for which palaeontological studies had already prepared us. - 10. ''Tests were made by means of anti-sera for the fowl and ostrich upon 792 and 649 bloods respectively. They demonstrate a similarity in blood constitution of all birds, which was in sharp con- trast to what had been observed with mammalian bloods, when acted upon by anti-mammahan sera. Differences in the degree of reaction were observed, but did not permit of drawing any conclusions." II. I have already called attention to the fact that the prob- lematical Horseshoe-crab is indicated by its embryology to be related to the air-breathing spiders and scorpions rather than to the marine Crustacea. It is of exceptional interest to learn that embryology is supported by the results of the blood tests. It must not be supposed that there is any exact mathematical ratio between the degrees of relationship indicated by the blood tests and those which are shown by anatomical and palaeontological evidence. Any supposition of the kind would be immediately nega- tived by the contrast between the blood of mammals and that of birds. It could hardly be maintained that an ostrich and a parrot are more nearly allied than a wolf and a hyena and yet that would be the inference from the blood tests. Like all other anatomical and physiological characters, the chemical composition of the blood is subject to change in the course of evolution and these developmental changes do not keep equal pace in all parts of the organism. It is the rule rather than the exception to find that one part of the structure advances much more rapidly than other parts, such as the teeth, the skull, or the feet. The human body is, fortunately for us, of rather a primitive kind, while the development of the brain is far superior to that of any other mammal and this great brain development has 128 READINGS IN EVOLUTION, GENETICS, AND EUGENICS necessitated a remodeling of the skull. On the other hand, the skeleton, limbs, hands and feet are but slightly specialized. In the elephant tribe, so far as we can trace them back in time, there has been little change, save in size, in the structure of the body or limbs , while the teeth and skull have passed through a series of remarkable changes. It is for this reason that it is unsafe to found a scheme of classification, which is meant to be a brief expression of relationship, upon a single character, for the result is almost invariably misleading. The results of blood tests must be critically examined and checked by a comparison with the results obtained by other methods of investiga- tion, but after every allowance has been made, these tests are very remarkable. The blood tests have brought very strong confirmation to the theory of evolution and from an entirely unexpected quarter; they come as near to giving a definite demonstration of the theory as we are likely to find, until experimental zoology and botany shall have been improved and perfected far beyond their present state. CHAPTER X EVIDENCES FROM MORPHOLOGY (COMPARATIVE ANATOMY)^ GEORGE JOHN ROMANES The theory of evolution supposes that hereditary characters admit of being slowly modified wherever their modification will render an organism better suited to a change in its conditions of life. Let us, then, observe the evidence which we have of such adaptive modifi- cations of structure, in cases where the need of such modification is apparent. We may begin by again taking the case of the whales and porpoises. The theory of evolution infers, from the whole structure of these animals, that their progenitors must have been terrestrial quadrupeds of some kind, which gradually became more and more aquatic in their habits. Now the change in the conditions of their life thus brought about would have rendered desirable great modifica- tions of structure. These changes would have begun by affecting the least typical — that is, the least strongly inherited — structures, such as the skin, claws, and teeth. But, as time went on, the adaptations would have extended to more typical structures, until the shape of the body would have become affected by the bones and muscles required for terrestrial locomotion becoming better adapted for aquatic locomotion, and the whole outhne of the animal more fish-Hke in shape. This is the stage which we actually observe in the seals, where the hind legs, although retaining all their typical bones, have become shortened up almost to rudiments, and directed backwards, so as to be of no use for walking, while serving to complete the fish-like taper of the body (Fig. ii). But in the whales the modification has gone further than this so that the hind legs have ceased to be apparent externally, and are only represented internally — and even this only in some species — by remnants so rudimentary that it is difficult to make out with certainty the homologies of the bones; moreover, the head and the whole body have become completely fish-Hke in shape (Fig. 12). But profound as are these alterations, they affect only ^ From G. J. Romanes, Darwin and after Darwin (copyright 1892). Used by special permission of the publishers, The Open Court Publishing Company. 129 130 READINGS IN EVOLUTION, GENETICS, AND EUGENICS to en o C/2 O EVIDENCES FROM MORPHOLOGY 131 those parts of the organism which it was for the benefit of the organism to have altered, so that it might be adapted to an aquatic mode of existence. Thus the arm, which is used as a fin, still retains the bones of the shoulder, fore-arm, wrist, and fingers, although they are all enclosed in a fin-shaped sack, so as to render them useless for any purpose other than swimming (Fig. 13). Similarly, the head, although it so closely resembles the head of a fish in shape, still retains the bones of the mammalian skull in their proper anatomical relations to one another; but modified in form so as to offer the least possible resistance to the water. In short, it may be said that all the modifi- cations have been effected with the least possible divergence from the typical mammalian type, which is compatible with securing so perfect an adaptation to a purely aquatic mode of life. . Now I have chosen the case of the whale and porpoise group, because they offer so extreme an example of profound modification of structure in adaptation to changed conditions of life. But the same thing may be seen in hundreds and hundreds of other cases. For instance, to confine our attention to the arm, not only is the limb modified in the whale for swimming, but in another mammal — the bat — it is modified for flying, by having the fingers enormously elongated and overspread with a membranous web. In birds, again, the arm is modified for flight in a wholly different way — the fingers here being very short and all run together, while the chief expanse of the wing is composed of the shoulder and forearm. In frogs and lizards, again, we find hands more like our own; but in an extinct species of flying reptile the modification was extreme, the wing having been formed by a prodigious elongation of the fifth finger, and a membrane spread over it and the rest of the hand (Fig. 14). Lastly, in serpents the hand and arm have disappeared altogether. Thus, even if we confine our attention- to a single organ, how wonderful are the modifications which it is seen to undergo, although never losing its typical character. Everywhere we find the distinction between homology and analogy which was explained in the last chapter — the distinction, that is, between correspondence of structure and correspondence of function. On the one hand, we meet with structures which are perfectly homologous and yet in no way analogous; the structural elements remain, but are profoundly modified so as to perform wholly different functions. On the other hand, we meet with structures which are perfectly analogous, and yet in no way homologous; totally different structures are modified 1^2 READINGS IN EVOLUTION, GENETICS, AND EUGENICS V c4 u tn 0) bO u ed •— 1 c<3 a o a ^ o ^ (n a> IH c3 en ••H ^ '« a a> ^ ■*j «+-! o en 4> fl O Xi >» >>4 c3 •4-> a (U a •^H -d 3 «-i (U ^ H oj .a en -tJ ^-^ rt K c:; «ii 8 "-^i M fe. M V V yi^ c* Ji > 'eS ,c! ^ ^ 5J rt o ^ OJ *<* c ^ • ^H 'v ^ ^ ?5 OJ l-i -13 u • D O C . c- -^ ^ _i h c4 M I- c 3 3 C/5 ,5 m o X x> ^ c •" ?; CI vi-i CA I40 READINGS IN EVOLUTION, GENETICS, AND EUGENICS ordinary crab to a hermit-crab in all the respects previously pointed out. Next, there must have been the change back again from a hermit-crab to an ordinary crab, so far as living without the necessity of a mollusk-shell is concerned. From an evolutionary ppint of view, therefore, we appear to have in the existing structure of Birgus a morphological record of all these changes, and one which gives us a reasonable explanation of why the animal presents the extraordinary appearance which it does. But, on the theory of special creation, it is inexplicable why this land-crab should have been formed on the pattern of a hermit-crab, when it never has need to enter the shell of a mollusk. In other words, its peculiar structure is not especially in keeping with its present habits, although so curiously allied to the similar structure of certain other crabs of totally different habits, in relation to which the peculiarities are of plain and obvious significance. I will devote the remainder of this chapter to considering another branch of the argument from morphology, to which the case of Birgus serves as a suitable introduction: I mean the argument from rudi- mentary structures. Throughout both the animal and vegetable kingdoms we con- stantly meet with dwarfed and useless representatives of organs, which in other and alUed kinds of animals and plants are of large size and functional utility. Thus, for instance, the unborn whale has rudi- mentary teeth, which are never destined to cut the gums; and throughout its life this animal retains, in a similarly rudimentary condition, a number of organs which never could have been of use to any kind of creature save a terrestrial quadruped. The whole anatomy of its internal ear, for example, has reference to hearing in air, as Hunter long ago remarked, ''is constructed upon the same principle as in the quadruped"; yet, as Owen says, "the outer open- ing and passage leading therefrom to the tympanum can rarely be affected by sonorous vibrations of the atmosphere, and indeed they are reduced, or have degenerated, to a degree which makes it difficult to conceive how such vibrations can be propagated to the ear-drum during the brief moments in which the opening may be raised above the water." Now, rudimentary organs of this kind are of such frequent occur- rence, that almost every species presents one or more of them — • usually, indeed, a considerable number. How, then, are they to be accounted for ? Of course the theory of descent with adaptive modi- fication has a simple answer to supply — namely, that when, from EVIDENCES FROM MORPHOLOGY 141 changed conditions of life, an organ which was previously useful becomes useless, it will be suffered to dwindle away in successive generations, under the influence of certain natural causes which we shall have to consider in future chapters. On the other hand, the theory of special creation can only maintain that these rudiments are formed for the sake of adhering to an ideal type. Now, here again the former theory appears to be triumphant over the latter; for, without waiting to dispute the wisdom of making dwarfed and useless structures merely for the whimsical motive assigned, surely if such a f^ifK KJ A'>OllA£flrAIKy h///p-Ll/ylBS A . \lEfjT, Fig. 17. — Rudimentary or vestigial hind limbs of python, as exhibited in the skeleton and on the external surface of the animal. Drawn from nature, \ nat. size. {From Romanes.) method were adopted in so many cases, we should expect that in con- sistency it would be adopted in all cases. This reasonable expectation, however, is far from being realized. We have already seen that in numberless cases, such as that of the fore-limbs of serpents, no vestige of a rudiment is present. But the vacillating policy in the matter of rudiments does not end here ; for it is shown in a still more aggravated form where within the limits of the same natural groups of organisms a rudiment is sometimes present and sometimes absent. For instance, although in nearly all the numerous species of snakes there are no vestiges of limbs, in the Python we find very tiny rudiments of the hind-limbs (Fig. 17). Now, is it a worthy conception of Deity that, while neglecting to maintain his unity of ideal in the case of 142 READINGS IN EVOLUTION, GENETICS, AND EUGENICS nearly all the numerous species of snakes, he should have added a tiny rudiment in the case of the Python — and even in that case should have maintained his ideal very inefficiently, inasmuch as only two limbs, instead of four, are represented ? How much more reasonable is the naturalistic interpretation; for here the very irregularity of their appearance in different species, which constitutes rudimentary structures one of the crowning difficulties to the theory of special design, furnishes the best possible evidence in favour of hereditary Fig. 1 8. — Apteryx australis. Drawn from life in the Zoological Gardens, I nat. size. The external wing is drawn to a scale in the upper part of the cut. The surroundings are supplied from the most recent descriptions. {From Romanes.) descent; seeing that this irregularity then becomes what may be termed the anticipated expression of progressive dwindling due to inutility. Thus, for example, to return to the case of wings, we have already seen that in an extinct genus of bird, Dinornis, these organs were reduced to such an extent as to leave it still doubtful whether so much as the tiny rudiment hypothetically supplied to Figure 15 was present in all the species. And here is another well-known case of another genus of still existing bird, which, as was the case with Dinornis, occurs only in New Zealand (Fig. 18). Upon this island there are no four-footed enemies — either existing or extinct — to escape from which the wings of birds would be of any service. Conse- EVIDENCES FROM MORPHOLOGY U3 quently we can understand why on this island we should meet with such a remarkable dwindling away of wings. Similarly, the logger-headed duck of South America can only flap along the surface of the water, having its wings considerably reduced though less so than the Apteryx of New Zealand. But here the interesting fact is that the young birds are able to fly perfectly well. Now, in accordance with a general law to be considered in a future chapter, the life-history of an individual organism is a kind of con- densed recapitulation of the life-history of its species. Consequently, we can understand why the little chickens of the logger-headed duck are able to fly like all other ducks, while their parents are only able to flap along the surface of the water. Facts analogous to this reduction of wings in birds which have no further use for them, are to be met with also in insects under similar circumstances. Thus, there are on the island of Madeira somewhere between 500 and 600 species of beetles, which are in large part peculiar to that island, though related to other — and therefore presumably parent — species on the neighboring continent. Now, no less than 200 species — or nearly half the whole number — are so far deficient in wings that they cannot fly. And, if we disregard the species which are not peculiar to the island — that is to say, all the species which likewise occur on the neighboring continent, and therefore, as evolu- tionists conclude, have but recently migrated to the island, — we find this very remarkable proportion. There are altogether 29 peculiar genera, and out of these no less than 2t, have all their species in this condition. Similar facts have been recently observed by the Rev. A. E. Eaton with respect to insects inhabiting Kerguelen Island. All the species which he found on the island — viz., a moth, several flies, and numerous beetles — he found to be incapable of flight; and therefore, as Wallace observes, "as these insects could hardly have reached the islands in a wingless state, even if there were any other known land inhabited by them, which there is not, we must assume that, like the Madeiran insects, they were originally winged, and lost their power of flight because its possession was injurious to them" — Kerguelen Island being " one of the stormiest places on the globe, " and therefore a place where insects could rarely afford to fly without incurring the danger of being blown out to sea. Here is another and perhaps an even more suggestive class of facts. 144 READINGS IN EVOLUTION, GENETICS, AND EUGENICS It is now many years ago since the editors of Silliman's Journal requested the late Professor Agassiz to give them his opinion on the following question. In a certain dark subterranean cave, called the Mammoth Cave, there are found some peculiar species of blind fishes. Now the editors of Silliman's Journal wished to know whether Profes- sor Agassiz would hold that these fish had been specially created in these caves, and purposely devoided of eyes which could never be of any use to them; or whether he would allow that these fish had prob- ably descended from other species, but, having got into the dark cave, gradually lost their eyes through disuse. Professor Agassiz, who was a believer in special creation, allowed that this ought to constitute a crucial test as between the two theories of special design and heredi- tary descent. "If physical circumstances," he said, "ever modified organised human beings, it should be easily ascertained here." And eventually he gave it as his opinion, that these fish "were created under the circumstances in which they now live, within the limits over which they now range, and with the structural peculiarities which now characterise them." Since then a great deal of attention has been paid to the fauna of this Mammoth cave, and also to the faunas of other dark caverns, not only in the New, but also in the Old World. In the result, the following general facts have been fully established. 1. Not only fish, but many representatives of other classes, have been found in dark caves. 2. Wherever the caves are totally dark, all the animals are blind. 3. If the animals live near enough to the entrance to receive some degree of light, they may have large and lustrous eyes. 4. In all cases the species of blind animals are closely allied to species inhabiting the district where the caves occur; so that the blind species inhabiting the American caves are closely allied to American species, while those inhabiting European caves are closely allied to European species. 5. In nearly all cases structural remnants of eyes admit of being detected, in various degrees of obsolescence. In the case of some of the crustaceans of the Mammoth cave the foot-stalks of the eyes are present, although the eyes themselves are entirely absent. Now, it is evident that all these general facts are in full agreement with the theory of evolution, while they offer serious difficulties to the theory of special creation. As Darwin remarks, it is hard to imagine conditions of Ufe more similar than those furnished by deep EVIDENCES FROM MORPHOLOGY 145 limestone caverns under nearly the same climate in the two continents of America and Europe; so that, in accordance with the theor>^ of special creation, very close similarity in the organizations of the two sets of faunas might have been expected. But, instead of this, the affinities of these two sets of faunas are with those of their respective continents — as of course they ought to be on the theory of evolution. Again, what would have been the sense of creating the useless foot- stalks for the imaginary support of absent eyes, not to mention all the other various grades of degeneration in other cases ? So that, upon the whole, if we agree with the late Professor Agassiz in regarding these cave animals as furnishing a crucial test between the rival theories of creation and evolution, we must further conclude that the whole body of evidence which they now furnish is weighing on the side of evolution. So much, then, for a few special instances of what Darwin called rudimentary structures, but what may be more descriptively desig- nated — in accordance with the theory of descent — obsolescent or vestigial structures. It is, however, of great importance to add that these structures are of such general occurrence throughout both the vegetable and animal kingdoms that, as Darwin has observed, it is almost impossible to point to a single species which does not present one or more of them. In other words, it is almost impossible to find a single species which does not in this way bear some record of its own descent from other species; and the more closely the structure of any species is examined anatomically, the more numerous are such records found to be. Thus, for example, of all organisms that of man has been most minutely investigated by anatomists; and therefore I think it will be instructive to conclude this chapter by giving a list of the more noteworthy vestigial structures which are known to occur in the human body. I will take only those which are found in adult man, reserving for the next chapter those which occur in a transitory manner during earlier periods of his Hfe. But, even as thus restricted, the number of obsolescent structures which we all present in our own person is so remarkable, that their combined testimony to our descent from a quadrumanous ancestry appears to me in itself conclusive. I mean, that even if these structures stood alone, or apart from any more general evidences of our family relationships, they would be sufficient to prove our parentage. Nevertheless, it is desirable to remark that of course these special evidences which I am about to detail do not stand alone. Not only is there the general analogy 146 READINGS IN EVOLUTION, GENETICS, AND EUGENICS furnished by the general proof of evolution elsewhere, but there is likewise the more special correspondence between the whole of our anatomy and that of our nearest zoological allies. Now the force of this latter consideration is so enormous that no one who has not studied human anatomy can be in a position to appreciate it. For without special study it is impossible to form any adequate idea of the intricacy of structure which is presented by the human form. Yet it is found that this enormously intricate organisation is repeated in all its details in the bodies of the higher apes. There is no bone, muscle, nerve, or vessel of any importance in the one which is not answered to by the other. Hence there are hundreds of thousands of instances of the most detailed correspondence, without there being any instances to the contrary, if we pay due regard to vestigial characters. The entire corporeal structure of man is an exact anatomical copy of that which we find in the ape. My object, then, here is to limit attention to those features of our corporeal structure which, having become useless on account of our change in attitude and habits, are in the process of becoming obsolete, and therefore occur as mere vestigial records of a former state of things. For example, throughout the vertebrated series, from fish to mammals, there occurs in the inner corner of the eye a semi- transparent eye-lid, which is called the nictitating membrane. The object of this structure is to sweep rapidly, every now and then, over the external surface of the eye, apparently in order to keep the surface clean. But although the membrane occurs in all classes of the sub-kingdom, it is more prevalent in some than in others — e.g., in birds than in mammals. Even, however, where it does not occur of a size and mobility to be of any use, it is usually represented, in animals above fishes, by a functionless rudiment, as here depicted in the case of man (Fig. 19). Now the organisation of man presents so many vestigial structures thus referring to various stages of his long ancestral history, that it would be tedious so much as to enumerate them. Therefore I will yet further limit the list of vestigial structures to be given as examples, by not only restricting these to cases which occur in our own organisa- l^ion; but of them I shall mention only such as refer us to the very last stage of our ancestral history — viz., structures which have become obsolescent since the time when our distinctively human branch of the family tree diverged from that of our immediate forefathers, the Quadrumana. EVIDENCES FROM MORPHOLOGY 147 J^/^X JMf^pT Fig. 19. — Illustrations of the nictitating membrane in the various animals named, drawn from nature. The letter N indicates the membrane in each case. In man it is called the plica semilunaris and is represented in the two lower drawings under this name. In the case of the shark {Galeus), the muscular membrane is shown as dissected. {From Romanes.) 148 READINGS IN EVOLUTION, GENETICS, AND EUGENICS I. Muscles of the external ear. — These, which are of large size and functional use in quadrupeds, we retain in a dwindled and useless condition (Fig. 20). This is likewise the case in anthropoid apes; but in not a few other Quadrumana (e. g., baboons, macacus, magots^ etc.) degeneration has not proceeded so far, and the ears are voluntarily movable. Fig. 20. — Rudimentary, or vestigial and useless, muscles of the human ear. {From Romanes, after Gray) 2. Panniculus carnosis. — A large number of the mammalia are able to move their skin by means of subcutaneous muscle, as we see, for instance, in a horse, when thus protecting himself against the sucking of flies. We, in common with the Quadrumana, possess an active remnant of such a muscle in the skin of the forehead, whereby we draw up the eyebrows; but we are no longer able to use other considerable remnants of it, in the scalp and elsewhere, — or more correctly it is rarely that we meet with persons who. can. But most of the Quadrumana (including the anthropoids) are still able to do so. EVIDENCES FROM :\IORPHOLOGY 149 There are also many other vestigial muscles, which occur only in a small percentage of human beings, but which, when they do occur, present unmistakable homologies with normal muscles in some of the Quadrumana and still lower animals. 3. Feet. — It is observable that in the infant the feet have a strong reflection inwards, so that the soles in considerable measure face one another. This peculiarity, which is even more marked in the embryo than in the infant, and which becomes gradually less and Fig. 21. — Portrait of a young gorilla. {From Romanes, after Hartmann.) less conspicuous even before the child begins to walk, appears to me a highly suggestive peculiarity. For it plainly refers to the condition of things in the Quadrumana, seeing that in all these animals the feet are similarly curved inwards, to facilitate the grasping of branches. And even when walking on the ground apes and monkeys employ to a great extent the outside edges of their feet, as does also a child when learning to walk. The feet of a young child are also extraordinarily mobile in all directions, as are those of apes. In order to show these points, I here introduce comparative drawings of a young ape and the 150 READINGS IN EVOLUTION, GENETICS, AND EUGENICS lower extremities of a still younger child. These drawings, moreover, serve at the same time to illustrate two other vestigial characters, which have often been previously noticed with regard to the infant's foot. I allude to the incurved form of the legs and the lateral exten- sion of the great toe, whereby it approaches the thumb-like character of this organ in the Quadrumana. As in the case of the incurved position of the legs and feet, so in this case of the lateral extensibility of the great toe, the peculiarity is even more marked in embryonic Fig. 22. — Lower extremities of a young child. Drawn from life, when the mobile feet were for a short time at rest in a position of extreme inflection. {From Romanes.) than in infant life. For, as Professor Wyman has remarked with regard to the foetus when about an inch in length, ''The great toe is shorter than the others; and, instead of being parallel to them, is projected at an angle from the side of the foot, thus corresponding with the permanent condition of this part in the Quadrumana." So that this organ, which, according to Owen, "is perhaps the most characteristic peculiarity of the human structure," when traced back to the early stages of its development, is found to present a notably less degree of peculiarity. EVIDENCES FROM MORPHOLOGY 151 4. Hands. — Dr. Louis Robinson has recently observed that the grasping power of the whole human hand is so surprisingly great at birth, and during the first few weeks of infancy, as to be far in excess of present requirements on the part of a young child. Hence he con- cludes that it refers us to our quadrumanous ancestry — the young of anthropoid apes being endowed with similar powers of grasping, in order to hold on to the hair of the mother when she is using her arms for the purposes of locomotion. This inference appears to me justifiable, Fig. 23. — An infant, three weeks old, supporting its own weight for over two minutes. The attitude of the lower limbs, feet, toes, is strikingly simian. Repro- duced from an instantaneous photograph, kindly given for the purpose by Dr. L. Robinson. {From Romanes.) inasmuch as no other explanation can be given of the comparatively inordinate muscular force of an infant's grip. For experiments showed that very young babies are able to support their own weight, by holding on to a horizontal bar, for a period varying from one half to more than two minutes. With his kind permission, I here reproduce one of Dr. Robinson's instantaneous, and hitherto unpub- lished, photographs of a very young infant. This photograph was taken after the above paragraph (3) was written, and I introduce it here because it serves to show incidentally— and perhaps even better than the preceding figure— the points there mentioned with regard 152 READINGS IN EVOLUTION, GExNETICS, AND EUGENICS to the feet and great toes. Again, as Dr. Robinson observes, the attitude, and the disproportionately large development of the arms as compared with the legs give all the photographs a striking resem- blance to a picture of the chimpanzee ''Sally" at the Zoological Gardens. For " invariably the thighs are bent nearly at right angles to the body, and in no case did the lower limbs hang down and take the attitude of the erect position." He adds, ''In many cases no sign of distress is evinced, and no cry uttered, until the grasp begins to give way." MAN Gorilla Fig. 24. — Sacrum of gorilla compared with that of man, showing rudimentary tail bones of each. Drawn from nature. (From Romanes.) 5. Tail. — The absence of a tail in man is popularly supposed to constitute a difficulty against the doctrine of his quadrumanous descent. As a matter of fact, however, the absence of an external tail in man is precisely what this doctrine would expect, seeing that the nearest allies of man in the quadrumanous series are likewise destitute of an external tail. Far, then, from this deficiency in man constituting any difficulty to be accounted for, if the case were not so — i.e., if man did possess an external tail, — the difficulty would be to understand how he had managed to retain an organ which had been renounced by his most recent ancestors. Nevertheless, as the anthro- EVIDENCES FROM MORPHOLOGY 153 poid apes continue to present the rudimentary vestiges of a tail in a few caudal vertebrae below the integuments, we might well expect to find a similar state of matters in the case of man. And this is just Fig. 25. — Diagrammatic outline of the human embryo when about seven weeks old, showing the relations of the limbs and tail to the trunk, (After Allen Thompson.) r, the radial, and n, the ulnar, border of the hand and forearm; t, the tibial, and/ the fibular, border of the foot and lower leg; ati, ear; s, spinal cord; v, umbilical cord; b, bronchial gill slits; c, tail. (From Romanes.) S\ifj\^-s>]/o\Js be dvf^roHf^ dotc/di^ fc ^ ^ Lid. fosr.sjtkp-docM Z./C Fig. 26. — Front and back view of adult human sacrum, showing abnormal persistence of vestigial tail muscles. (From Romanes.) what we do find, as a glance at these two comparative illustrations will show (Fig. 24). Moreover, during embryonic life, both of the anthropoid apes and of man, the tail much more closely resembles 154 READINGS IN EVOLUTION, GENETICS, AND EUGENICS that of the lower kinds of quadrumanous animals from which these higher representatives of the group have descended. For at a certain stage of embryonic life the tail, both of apes and of human beings, is Fig. 27. — Appendix vermiformis in orang and in man, //, ilium; Co, colon; C, coecum; [F, a window cut in the wall of the coecum; ^a;^;, the appendix. {From Romanes.) Man F(ETAL Fig. 28. — The same, showing variation in the orang. {From Romanes.) actually longer than the legs (see Fig. 25). And at this stage of development, also, the tail admits of being moved by muscles which later on dwindle away. Occasionally, however, these muscles persist, and are then described by anatomists as abnormalities. The illustra- EVIDENCES FROM MORPHOLOGY 155 tions on page 153 (Fig. 26) serve to show the muscles in question, when thus found in adult man. 6. Vermiform appendix of the coecum. — This is of large size and functional use in the process of digestion among many herbivorous animals; while in man it is not only too small to serve any such purpose, but is even a source of danger to life — many persons dying every year from inflammation set up by the lodgement in this blind tube of fruit-stones, etc. In the orang it is longer than in man (Fig. 27), as it is also in the human foetus proportionally compared with the adult (Fig. 28). In some of the lower herbivorous animals it is longer than the entire body. Like the vestigial structures in general, however, this one is highly variable. Thus Figure 28 serves to show that it may some- times be almost as short in the orang as it normally is in man — both the human subjects of this illustration having been normal. 7. Ear. — Mr. Darwin writes: ^'The celebrated sculptor, Mr. Woolner, informs me of one little peculiarity in the external ear, which he has often observed both in men and women The peculiarity consists in a little blunt point, projecting from the inwardly folded margin, or helix. When present, it is developed at birth, and according to Professor Ludwig Meyer, more frequently in man than in woman. Mr. Woolner made an exact model of one such case, and sent me the accompanying draw- ing [Fig. 29] The helix obviously con- sists of the extreme margin of the ear folded inwards; and the folding appears to be in /f^ some manner connected with the whole external ear being permanently pressed backwards. In many monkeys, which do not stand high in the order as baboons and some species of macacus, the upper portion of the ear is slightly pointed, and the margin is not at all folded inwards; but if the margin were to be thus folded, a slight point would necessarily pro- ject towards the centre In Figure 30 is shown an accurate copy of a photograph of the foetus of an orang (kindly sent me by Dr. Nitsche), in which it may be seen how different the ])oinled outhne of the car is at this period from its adult condition, when it bears a close general Fig. 29. — Human ear, modeled and drawn by Mr. Woolner. a, the pro- jecting point. {From Ro- manes.) 156 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Fig. 30. — Foetus of an ©rang. Exact copy of a photograph, showing the form of ear at this early stage. {From Romanes.) resemblance to that of man (including even the occasional appear- ance of the projecting point shown in the preceding woodcut). It is evident that the folding over of the tip of such an ear, unless it is changed greatly during its further develop- ment, would give rise to a point projecting inwards."' The woodcut on page 157 (Fig. 31) serves still further to show vestigial resemblances between the human ear and that of apes. The last two figures illustrate the general resemblance between the nor- mal ear of foetal man and the ear of an adult orangoutang. The other two figures on the lower line are intended to exhibit occasional modifica- tions of the adult human ear, which approximate simian characters somewhat more closely than does the normal type. It will be observed that in their comparatively small lobes these ears resemble those of all the apes; and that while the outer margin of one is not unlike that of the Barbary ape, the outer margin of the other follows those of the chimpanzee and orang. Of course it would be easy to select individual human ears which present either of these characters in a more pronounced degree; but these ears have been chosen as models because they present both characters in conjunction. The upper row of figures likewise shows the close similarity of hair-tracts, and the direction of growth on the part of the hair itself, in cases where the human hair happens to be of an abnormally hirsute character. But this particular instance (which I do not think has been previously noticed) introduces us to the subject of hair, and hair-growth, in general. 8. Hair. — Adult man presents rudimentary hairs over most parts of the body. Wallace has sought to draw a refined distinction between this vestigial coating and the useful coating of quadrumanous animals, in the absence of the former from the human back. But even ^ Descent of Man (2d ed.), pp. 15-16. EVIDENCES FROM MORPHOLOGY 157 c^ o i-i 3 •_> C C o Ui e4 en 3 (J u v: *Sb fO O 158 READINGS IN EVOLUTION, GENETICS, AND EUGENICS this refined distinction does not hold. On the one hand, the com- paratively hairless chimpanzee which died last year in the Zoological Gardens {T. calvus) was remarkably denuded over the back; and, on the other hand, men who present a considerable development of hair over the rest of their bodies present it also on their backs and shoul- ders. Again, in all men the rudimentary hair on the upper and lower arm is directed towards the elbow — a peculiarity which occurs nowhere else in the animal kingdom, with the exception of the anthropoid apes and a few American monkeys,- where it presumably has to do with arboreal habits. For, when sitting in trees, the orang, as observed by Mr. Wallace, places its hands above its head with its elbows pointing downwards; the disposition of hair on the arms and fore-arms then has the effect of thatch in turning the rain. Again, I find that in all species of apes, monkeys, and baboons which I have examined (and they have been numerous), the hair on the backs of the hands and feet is continued as far as the first row of phalanges; but becomes scanty, or disappears altogether, on the second row; while it is invariably absent on the terminal row. I also find that the same peculiarity occurs in man. We all have rudimentary hair on the first row of phalanges, both of hands and feet: when present at all, it is more scanty on the second row; and in no case have I been able to find any on the terminal row. In all cases these peculiarities are congenital, and the total absence or partial presence of hair on the second pha- langes is constant in different species of Quadrumana. For instance , it is entirely absent in all the chimpanzees, which I have examined, while scantily present in all the orangs. As in man, it occurs in a patch midway between the joints. Besides showing these two features with regard to disposition of hair on the human arm and hand, the woodcut on page 159 (Fig. 32) illustrates a third. By looking closely at the arm of the very hairy man from whom the drawing was taken, it could be seen that there was a strong tendency towards a whorled arrangement of the hairs on the backs of the wrists. This is likewise, as a general rule, a marked feature in the arrangement of hair on the same places in the gorilla, orang, and chimpanzee. In the specimen of the latter, however, from which the drawing was taken this characteristic was not well marked. The downward direction of the hair on the backs of the hands is exactly the same in man as it is in all the anthropoid apes. Again, with regard to hair, Darwin notices that occasionally there appears in man a few hairs in the eye- brows much longer than the others; and that they seem to be EVIDENCES FROM MORPHOLOGY 159 y^f/KL t CM'Mf^^^^^* Fig. 32. — Hair tracts on the arms and hands of man, as compared with those of the chimpanzee. Drawn from Hfe. {From Romanes.) i6o READINGS IN EVOLUTION, GENETICS, AND EUGENICS representative of similarly long and scattered hairs which occur in the chimpanzee, macacus, and baboons. Lastly, it may be here more conveniently observed than in the next chapter on Embryology, that at about the sixth month the human foetus is often thickly coated with somewhat long dark hair over the entire body, except the soles of the feet and palms of the hands, which are likewise bare in all quadrumanous animals. This covering, which is called the lanugo, and sometimes extends even to the whole fore- head, ears, and face, is shed before birth. So that it appears to be useless for any purpose other than that of emphatically declaring man a child of the monkey. 9. Teeth. — Darwin writes: "It appears as if the posterior molar or wisdom teeth were tending to become rudimentary in the more civilized races of man. These teeth are rather smaller than the other molars, as is likewise the case with the corresponding teeth in the chimpanzee and orang; and they have only two separate fangs They are also much more liable to vary, both in structure and in the period of their development, than the other teeth. In the Melanian races, on the other hand, the wisdom-teeth are usually furnished with three separate fangs, and are usually sound (i.e., not specially liable to decay); they also differ from the other molars in size, less than in the Caucasian races." Now, in addition to these there are other respects in which the dwindling condition of wisdom-teeth is manifested — particularly with regard to the pattern of their crowns. Indeed, in this respect it would seem that even in the anthropoid apes there is the beginning of a tendency to degeneration of the molar teeth from behind forwards. For if we compare the three molars in the lower jaw of the gorilla, orang, and chimpanzee, we find that the gorilla has five well-marked cusps on all three of them; but that in the orang the cusps are not so pronounced, while in the chimpanzee there are only four of them on the third molar. Now in man it is only the first of these three teeth which normally presents five cusps, both the others presenting only four. So that, comparing all these genera together, it appears that the number of cusps is being reduced from behind forwards; the chimpanzee having lost one of them from the third molar, while man has not only lost this, but also one from the second molar, — and it may be added, likewise partially (or even totally) from the first molar, as a frequent variation among civilized races. But, on the other hand, variations are often met with in the opposite direction, where the EVIDENCES FROM MORPHOLOGY i6i second or the third molar of man presents five cusps — in the one case following the chimpanzee, in the other the gorilla. These latter varia- tions, therefore, may fairly be regarded as reversionary. For these facts I am indebted to the kindness of Mr. C. S. Tomes. 10. Perforations of the humerus. — The peculiarities which we have to notice under this heading are two in number. First, the supra-condyloid foramen is a normal feature in some of the lower Quadrumana (Fig. 34), where it gives passage to the great nerve of /^aK. Fig. 33. — Molar teeth of lower jaw in gorilla, orang, and man. Drawn from nature, nat. size. (From Romanes.) the forearm, and often also to the great artery. In man, however, it is not a normal feature. Yet it occurs in a small percentage of cases — viz., according to Sir W. Turner, in about one per cent, and therefore is regarded by Darwin as a vestigial character. Secondly, there is inter-condyloid foramen, which is also situated near the lower end of the humerus, but more in the middle of the bone. This occurs, but not constantly, in apes, and also in the human species. From the fact that it does so much more frequently in the bones of ancient — and also of some savage — races of mankind (viz. in 20 to 30 per cent of cases), Darwin is disposed to regard it also as a vestigial feature. l62 READINGS IN EVOLUTION, GENETICS, AND EUGENICS On the other hand, Prof. Flower tells me that in his opinion it is but an expression of impoverished nutrition during the growth of the bone. II. Flattening of Tibia. — In some very ancient human skeletons there has also been found a lateral flattening of the tibia, which rarely occurs in any existing human beings, but which appears to have been usual among the earliest races of mankind hitherto discovered. According to Broca, the measurements of these fossil human tibiae resemble those of apes. Moreover, the bone is bent and strongly JAVAI7 LOR{S CAPWCHIi;. Fig. 34. — Perforations of the humerus (supra-condyloid foramen) in three species of Quadrumana where it normally occurs, and in man, where it does not normally occur. Drawn from nature. {From Romanes.) convex forwards, while its angles are so rounded as to present the nearly oval section seen in apes. It is in association with these ape-like human tibiae that perforated humeri of man are found in greatest abundance. On the other hand, however, there is reason to doubt whether this form of tibia in man is really a survival from his quadrumanous ancestry. For, as Boyd-Dawkins and Hartmann have pointed out, the degree of flattening presented by some of these ancient human bones is greater than that which occurs in any existing species of anthropoid ape. Of course the possibility remains that the unknown species of ape from which man descended may have had its tibia more flattened than is now observable in any of the existing species. Never- EVIDENCES FROM MORPHOLOGY 163 theless, as some doubt attaches to this particular case, I do not press it — and, indeed, only mention it at all in order that the doubt may be expressed. Similarly, I will conclude by remarking that several other instances of the survival of vestigial structures in man have been alleged, which are of a still more doubtful character. Of such, for example, are the supposed absence of the genial tubercle in the case of a very ancient jaw-bone of man, and the disposition of valves in human veins. From the former it was argued that the possessor of this very ancient jaw-bone was probably speechless, inasmuch as the tubercle in existing man gives attachment to muscles of the tongue. From the latter it has been argued that all the valves in the veins of the human body have reference, in their disposition, to the incidence of blood-pressure when the attitude of the body is horizontal, or quadrupedal. Now, the former case has already broken down, and I find that the latter does not hold. But we can well afford to lose such doubtful and spurious cases, in view of all the foregoing unquestionable and genuine cases of vestigial structures which are to be met with even within the limits of our own organization — and even when these limits are still further limited by selecting only those instances which refer to the very latest chapter of our long ancestral history. CHAPTER XI EVIDENCES FROM EMBRYOLOGY THE FACTS OF REPRODUCTION AND DEVELOPMENT [It is now definitely known that all living creatures are mortal, at least as individuals, but they all have the capacity of continuing their life by the reproduction of offspring. This physical immortality is based upon an actual transmission from parent to offspring of some material substance which is so organized chemically as to be fully representative of the race or stock to which the parent belongs. Reproduction may be asexual or sexual. In asexual development a new individual may be produced by a process oi fission (dividing the parent into two or more parts, each of which has the capacity to develop into a whole new individual) ; by budding (the production of new individuals by means of outgrowths of the parent-body) ; or by giving off spores or eggs capable of development without fertiliza- tion (parthenogenesis). In sexual reproduction two kinds of parent- individuals exist: one a female which is capable of giving off relatively large single cells, called eggs (ova) ; and the other a male, which is capable of producing minute, usually motile cells, called spermatozoa. A union of ovum and spermatozoon is usually necessary before the ovum can begin its development. It is the sexual method of repro- duction that will chiefly concern us here, and, for present purposes, we may omit any further mention of the various asexual methods. An ovum may be conceived of as an individual of some definite species or race reduced to the very lowest terms. It exhibits the characteristic cell structure, consisting of cytoplasm and nucleus, cell membrane, nuclear membrane, usually a centrosome (Fig. 43). Further details as to the minute structure of the nucleus are given in chapter xxvii, where the mechanism of Mendelian heredity is dealt with. — Ed.] ''The reproductive cells from the two sexes," says Wright,' ''have very different appearances. In mammals, the ovum is a relatively large, spherical cell, just visible to the naked eye. ^ From Sewall Wright, Principles of Livestock Breeding, United States Depart- ment of Agriculture, Bulletin No. 905. 164 EVIDENCES FROM EMBRYOLOGY 165 "In birds, the yolk of an egg is really a single ovum, distended to an enormous size by food material. The sperm cell is very much smaller and can be seen well only with a high-power microscope. It is something like a tadpole in shape, having a small cell body, containing a little nucleus, and attached to this a long, whiplike process which beats rapidly while the cell is alive, enabling it to seek out and unite with the large passive egg in the act of fertilization. Enormous num- bers of sperm cells are produced by the male, but only one takes part in fertilization. After the first has penetrated the membrane of an egg cell, a change takes place in the latter which prevents the entrance of others. "The sperm activates certain formerly inert substances in the egg and the new combination cell (the zygote) starts almost at once to produce a new individual." OUTLINE OF ANIMAL DEVELOPMENT^ D, S. JORDAN AND V. L. KELLOGG The embryonic development is from the beginning up to a certain point practically alike, looked at in its larger aspect, for all the many- celled animals. That is, there are certain principal or constant characteristics of the beginning development which are present in the development of all many-celled animals. The first stage or phenome- non of development is the simple fission of the germ cell into halves (Fig. 35, b). These two daughter cells next divide so that there are four cells (c) ; each of these divides, and this division is repeated until a greater or lesser number (varying with the various species or groups of animals) of cells is produced. These cells may not all be of the same size, but in many cases they are, no structural differentiation whatever being apparent among them. The phenomenon of repeated division of the germ cell is called cleavage, and this cleavage is the first stage of development in the case of all many-celled animals. The germ or embryo in some animals consists now of a mass of few or many undifferentiated primitive cells lying together and usually forming a sphere (Fig. 35, e), or perhaps separated and scattered through the food yolk of the egg. The next stage of development is this: the cleavage cells arrange themselves so as to form a usually hollow sphere or ball, the cells lying side by side to * From D. S. Jordan and V. L. Kellogg, Evolution and Animal Life (copyright 1907). Used by special permission of the publishers, D. Appleton & Company. 1 66 ^READINGS IN EVOLUTION, GENETICS, AND EUGENICS form the outer circumferential wall of this hollow sphere (/). This is called'the hlastiila or blastoderm stage of development, and the embryo itself is called the blastula or blastoderm. This stage also is common to all the many-celled animals. The next stage in embryonic develop- ment is formed by the bending inward of a part of the blastoderm cell layer, as shown in (g) (or the splitting off inwardly of cells from a special part of the blastula cell layer). This bending in may produce a small depression or groove; but whatever the shape or extent of the sunken-in part of the blastoderm, it results in distinguishing the blastoderm layer into two parts, a sunken-in or inner portion called Fig. 35. — First stages in the embryonic development of the pond snail, Lymnaeus. a, egg cell; b, first cleavage; c, second cleavage; d, third cleavage; e, after numerous cleavages; /, blastula — in section; g, gastrula just forming — in section; h, gastrula completed — in section. (From Jordan and Kellogg, after Rabl.) the endoblast and the other unmodified portion called the ectohlast. Endo- means within, and the cells of the endoblast often push so far into the original blastoderm cavity as to come into contact with the cells of the ectoblast and thus obliterate this cavity {h). This third well-marked stage in the embryonic development is called the gastrula stage, and it also occurs in the development of all or nearly all many- celled animals. In the case of a few of the simple many-celled animals the embryo hatches — that is, issues from the egg at the time of or very soon after reaching the gastrula stage. In the higher animals, however, develop- ment goes on within the egg or within the body of the mother until the embryo becomes a complex body, composed of many various EVIDENCES FROM EMBRYOLOGY 167 tissues and organs. Almost all the development may take place within the egg, so that when the young animal hatches there is necessary little more than a rapid growth and increase of size to make it a fully developed mature animal. This is the case with the birds; a chicken just hatched has most of the tissues and organs of a full-grown fowl, and is simply a little hen. But in the case of other animals the young hatches from the egg before it has reached such an advanced stage of development; a young starfish or young crab or young honeybee just hatched looks very different from its parent. It has yet a great deal of development to undergo before it reaches the structural condition of a fully developed and fully grown starfish or crab or bee. Thus the development of some animals is almost wholly embryonic develop- ment — that is, development within the egg or in the body of the mother — while the development of other animals is largely post- embryonic, or larval development, as it is often called. There is no important difference between embryonic and postembryonic develop- ment. The development is continuous from egg cell to mature animal, and whether inside or outside of an egg it goes on regularly and uninter- ruptedly. The cells which compose the embryo in the cleavage stage and blastoderm stage, and even in the gastrula stage, are apparently all similar; there is little or no differentiation shown among them. But from the gastrula stage on, development includes three important things; the gradual differentiation of cells into various kinds to form the various kinds of animal tissues; the arrangement and grouping of these cells into organs and body parts; and finally the developing of these organs and body parts into the special condition characteristic of the species of animal to which the developing individual belongs. From the primitive undifferentiated cells of the blastoderm, develop- ment leads to the special cell types of muscle tissue, of bone tissue, of nerve tissue; and from the generalized condition of the embryo in its early stages, development leads to the specialized condition of the body of the adult animal. Development is from the general to the special, as was said years ago by von Baer, the first great student of development. A starfish, a beetle, a dove, and a horse are all alike in their beginning — that is, the body of each is composed of a single cell, a single structural unit. And they are all alike, or very much alike, through several stages of development; the body of each is first a single cell, then a number of similar undifferentiated cells, and then a 1 68 READINGS IN EVOLUTION, GENETICS, AND EUGENICS blastoderm consisting of a single layer of similar undifferentiated cells. But soon in the course of development the embryo begins to differ, and as the young animals get further and further along in the course of their development, they become more and more different until each finally reaches its fully developed mature form, showing all the great structural differences between the starfish and the dove, the beetle and the horse. That is, all animals begin development apparently alike, but gradually diverge from each other during the course of develop- ment. There are some extremely interesting and significant things about this divergence to which attention should be given. While all animals are apparently alike structurally at the beginning of development, so far as we can see, they do not all differ noticeably at the time of the first ^divergence in development. The first divergence in development is to be noted between two kinds of animals which belong to different great groups or classes. But two animals of different kinds, both belonging to some one great group, do not show differences until later in their development. This can best be understood by an example. All the butterflies and beetles and grasshoppers and flies belong to the great group or class of animals called Insecta, or insects. There are many different kinds of insects, and these kinds can be arranged in subor- dinate groups (orders), such as the Diptera, or flies, the Lepidoptera, or butterflies and moths, and so on. But all have certain structural characteristics in common, so that they are comprised in one great class — the Insecta. Another great group of animals is known as the Vertebrata, or backboned animals. The class Vertebrata includes the fishes, the batrachians, the reptiles, the birds and the mammals, each composing a subordinate group, but all characterized by the possession of a backbone or, more accurately speaking, of a notochord, a back- bonelike structure. Now, an insect and a vertebrate diverge very soon in their development from each other; but two insects, such as a beetle and a honeybee, or any two vertebrates, such as a frog and a pigeon, do not diverge from each other so soon. That is, all vertebrate animals diverge in one direction from the other great groups, but all the members of the great group keep together for some time longer. Then the subordinate groups of the Vertebrata, such as the fishes, the birds, and the others, diverge, and still later the different kinds of animals in each of these groups diverge from each other. That the course of development of any animal from its beginning to fully developed adult form is — in all its essentials — fixed and certain EVIDENCES FROM EMBRYOLOGY 169 is readily seen. All rabbits develop in the same way; every grass- hopper goes through the same developmental changes from single egg cell to the full-grown, active hopper as every other grasshopper of the same kind — that is, development takes place according to certain natural laws; the laws of animal development. These laws may be roughly stated as follows: All many-celled animals begin life as a single cell, the fertilized egg cell; each animal goes through a certain orderly series of developmental changes which, accompanied by growth leads the animal to change from a single cell to the many-celled, com- plex form characteristic of the species to which the animal belongs; this development is from simple to complex structural condition; the development is the same for all individuals of one species. While all animals begin development similarly, the course of development in the different groups soon diverges, the divergence being of the nature of a branching, like that shown in the growth of a tree. In the free tips of the smallest branches we have represented the various species of animals in their fully developed condition, all standing more or less clearly apart from each other. But in tracing back the development of any kind of animal we soon come to a point where it very much resembles or becomes apparently identical with the development of some other kind of animal, and, in addition, the stages passed through in the developmental course may very much resemble the fully devel- oped, mature stages of lower animals. To be sure, any animal at any stage in its existence differs absolutely from any other kind of animal, in that it can develop into only its own kind of animal. There is ^ something inherent in each developing animal that gives it an identity of its own. Although in its young stages it may be hardly distin- guishable from some other kind of animal in similar stages, it is sure to come out, when fully developed, an individual of the same kind as its parents were or are. A very young fish and a very young sala- mander are almost indistinguishably alike, but one is sure to develop into a fish and the other into a salamander. This certainty of an embryo to become an individual of a certain kind is called the law oV heredity. Viewed in the light of development, there must be as great a difference between one egg and another as between one animal and another, for the greater difference is included in the less. The significance of the developmental phenomena is a matter about which naturalists have yet very much to learn. It is believed, how- ever, by practically all naturalists that many of the various stages in the development of an animal correspond to or repeat, in many I70 READINGS IN EVOLUTION, GENETICS, AND EUGENICS fundamental features at least, the structural condition of the animal's ancestors. Naturalists believe that all backboned or vertebrate Fig. 36. — Stages in the development of the prawn, Peneus potimirlum. A Nauplius larva; B, first zoea stage; C, second zoea stage. (From Jordan and Kellogg, after Fritz M tiller.) Fig. 37. — Later stages in the development of the prawn, Peneus potimiriiim. D, Mysis stage; £, adult stage. {From Jordan and Kellogg.) EVIDENCES FROM EMBRYOLOGY 171 animals are related to each other through being descended from a common ancestor, the first or oldest backboned animal. In fact, it is because all these backboned animals — the fishes, the batrachians, the reptiles, the birds, and the mammals — have descended from a common ancestor that they all have a backbone. It is believed that the descendants of the first backboned animal have in the course of many generations branched off little by little from the original type until there came to exist very real and obvious differences among the back- boned animals — differences which among the living backboned animals are familiar to all of us. The course of development of an individual animal is believed to be a very rapid and evidently much condensed and changed recapitulation of the history which the species or kind of animal to which the developing individual belongs has passed through in the course of its descent through a long series of gradually changing ancestors. If this is true, then we can readily understand why a fish and a salamander, a tortoise, a bird, and a rabbit, are all much alike, as they really are, in their earlier stages of development, and gradually come to differ more and more as they pass through later and later developmental stages. A crab has a tail in one of its developmental stages, so that at that time it looks Hke and really is like the Fig. 38.— Metamorphosis of a mature stage of some tailed crustacean barnacle, Lc/>a5. a, larva; 6, adult. i-i r ^ A 1 1 I'll! (From Jordan and Kellogg.) like a crayfish. A barnacle, which looks a little like a crayfish or crab in its ma- ture stage, is hardly to be distinguished in its immature life from a young crab or lobster. Sacculina, which is a still more degenerate crustacean, is only a sort of feeding sac with rootlet-like processes projecting into the body of the host crab on which it lives as a parasite, but the young free-swimming Sacculina is essentially like a barnacle, crayfish, or crab in its young stage. However, it is obvious that this recapitulation or repetition of ancestral stages is never perfect, and it is often so obscured and modi- fied by interpolated adaptive stages and characters that but little of an animal's ancestry can be learned from a scrutiny of its development. 172 READINGS IN EVOLUTION, GENETICS, AND EUGENICS The fascinating biogenetic law of Miiller and Haeckel summed up in the phrase, ^^ ontogeny is a recapitulation of phytogeny, ^^ must not be too heavily leaned on as a support for any speculations as to the phyletic affinities of any species or group of species of organisms. ''Embryology is an ancient manuscript with many of the sheets lost, others displaced, and with spurious passages interpolated by a later hand." CHAPTER XII CRITIQUE OF THE RECAPITULATION THEORY' W. B. SCOTT Embryology is the study of the development of the individual organism from its beginning in the egg to the attainment of the adult condition. This individual development is called ontogeny and the question of the relation of ontogeny to the ancestral history of the species, or phytogeny, constitutes one of the main problems of embr\^- ology. Around this problem many controversies have raged, contro- versies which have by no means arrived at a definite solution, even to-day. Thirty years ago the "recapitulation theory" was well-nigh universally accepted, according to which the individual development, or ontogeny, was regarded as an abbreviated repetition of the ances- tral history of the species, or phylogeny. Haeckel called this theory the "fundamental biogenetic law" and upon it he established his whole "History of Creation." Nowadays, that "fundamental law" is very seriously questioned and by some high authorities is altogether denied. However, even those who take this extreme position con- cerning the recapitulation theory see in the facts of embryology one of the strongest supports of the doctrine of evolution. It was very early recognized that the recapitulation theory could not be applied with literal exactness, but was subject to certain important exceptions and qualifications. I. That the history must have been enormously abbreviated. After three weeks of incubation the tiny speck of protoplasm, which forms a circular mark on the yolk of a hen's egg, is developed into a fully formed chick, ready for hatching and able in large degree to take care of itself. On the other hand, the evolution of birds from their invertebrate ancestors, through the fishes, amphibians, and reptiles, the separation of the gallinaceous stock from other birds and the differentiation of this particular species were extremely slow processes, extending through unnumbered millions of years. Admitting reca- pitulation to the fullest extent, it is evidently a physical impossibility ^ From W. B. Scott, The Theory of Evolution (copyright 191 7). Used by- special permission of the publishers, The Macmillan Company, 173 174 READINGS IN EVOLUTION, GENETICS, AND EUGENICS that it should be a perfect repetition of phylogeny; very much of the long story must of necessity be omitted. 2. Through all the stages of development the embryo must be rendered able to live and grow and thrive through adaptation to its surroundings and changes in its environment. In some animals development takes place within the body of the mother; in others the embryo is protected by the hard egg-shell, as in birds, while the eggs of certain fishes and many invertebrates float freely in the sea and are almost without protection. Such differences in environment necessi- tate differences in the mode of development, while the presence or absence of a large amount of inert food-material, or yolk, exerts a great influence in determining the steps of ontogeny. 3. Many animals pass through a larval stage of development, in which the immature young leads an independent and self-sustaining existence, during which it is very different in appearance and structure from its adult parents. Familiar instances of this mode of develop- ment are to be found in the tadpole, which is the larva of the frog, and the caterpillar, the larva of a butterfly. Larvae are fully subject to the struggle for existence and must adapt themselves to their environ- ment and to changes in that environment, exactly as do adults, if they are to survive. In this way many changes are introduced into the ontogeny which can have no phylogenetic significance. It is found in several known instances, that nearly allied species, living under different conditions, have quite different modes of ontogeny, though their ancestral history must have been substantially identical. In one and the same species of marine worms, for example, which inhabits both the warm Mediterranean and the cold waters of the North Sea, the larva of the northern form is quite distinct from that of the southern. In attempting to interpret the meaning of embryological facts, it is thus necessary to distinguish sharply between those features which are derived from a long inheritance, and are therefore called palingenetic, from those which have been secondarily introduced in response to the changing needs of embryonic or larval life. These secondary features are termed cenogenetic. ''If we are compelled to admit that cenogenetic characters are intermingled with palingenetic, then we cannot regard ontogeny as a pure source of evidence regarding phyletic relationships. Ontogeny accordingly becomes a field in which an active imagination has full scope for its dangerous play, but in which positive results are by no means everywhere to be obtained. To attain such results, the palin- THE RECAPITULATION THEORY 175 genetic and cenogenetic phenomena must be sifted apart, an operation which required more than one critical grain of salt. On what grounds shall this critique be based ? Assuredly not by way of a vicious circle on the ontogeny again; for if cenogenetic characters are present in one case, who will guarantee that a second case, used for a comparison with the first, does not Hkewise appear in cenogenetic disguise ? If it once be admitted that not everything in development is palingenetic, that not every ontogenetic fact can be accepted at its face value, so to speak, it follows that nothing in ontogeny is immediately available for the critique of embryonic development. .The necessary critique must be drawn from another source." These remarks of Gegenbaur's were called forth by the state of wild speculation into which embryological work had fallen. As there were no generally accepted canons of interpretation for the facts of embryological development, different writers interpreted these facts in the most divergent and contradictory manner, resulting in a chaotic confusion, which led to a strong reaction against the whole method, though there can be little doubt that this reaction has gone too far. "It must be evident to any candid observer, not only that the embryological method is open to criticism, but that the whole fabric of morphology, so far as it rests upon embryological evidence, stands in urgent need of reconstruction. For twenty years embryological research has been largely dominated by the recapitulation theory; and unquestionably this theory has illuminated many dark places and has solved many a perplexing problem that without its aid might have remained a standing riddle to the pure anatomist. But while fully recognizing the real and substantial fruits of that theory, we should not close our eyes to the undeniable fact that it, like many another fruit- ful theory, has been pushed beyond its legitimate limits. It is largely to an overweening confidence in the validity of the embryological evidence that we owe the vast number of the elaborate hypothetical phylogenies which confront the modern student in such bewildering confusion. The inquiries of such a student regarding the origin of any of the great principal types of animals involve him in a labyrinth of speculation and hypothesis in which he seeks in vain for conclusions of even an approximate certainty." Many other equally vigorous and well-deserved criticisms of the embryological method might be cited, but it should be emphasized that these criticisms are all directed against the application of the method to the solution of definite and concrete problems of descent and 176 READINGS IN EVOLUTION, GENETICS, AND EUGENICS relationship.' None of them denies and many strongly affirm that embryology affords some of the strongest and most convincing evi- dence in favor of the evolutionary theory. Let us examine some of this evidence. To begin with, it should be noted that, in following out the ontogeny or individual develop- ment, the observer witnesses the formation of something new, not merely the enlargement and unfolding of a pre-existing organism, though the theory of preformation, which was widely accepted in the eighteenth century, looked upon ontogeny precisely in that way, as the growth of a germ which was the miniature of the parent. Such a theory was possible only before the development of microscopic technique had enabled the observer to detect the actual successive steps of change. The egg is a single cell, with the nucleus and all the parts of other undifferentiated cells, though it may be enormously enlarged by the presence of food- yolk. In the hen's egg this food-yolk is quite inert and the activity of development is confined to the minute disc of protoplasm on the outside of the yolk, while in the frog's egg the yolk is disseminated, though not uniformly, throughout the egg and in the mammaUan egg, which is microscopic in size, there is no yolk. It is a very remarkable fact that all of the vertebrated animals, fishes, amphibians, reptiles, birds and mammals, however different their habits and modes of life, have a mode of ontogeny which is of even more characteristically and unmistakably the same plan than is the type of their adult structure, which was described in the last chapter. The egg, or the active portion of it, divides in a definite and regular manner into a very large number of cells, which arrange them- selves in definite layers, an outer and an inner, and within these layers cell-aggregates form incipient organs, which, step by step, take on the adult condition. Not only is the plan and type of development essentially similar throughout the whole phylum of the vertebrates, but, in accordance with the recapitulation theory, many structural features which are permanent in lower forms appear in the embryos of higher and more advanced types. In the latter, however, these features are transitory and, in the course of development, they either disappear, or are so modified as to be very different, sometimes unrecog- nizable, in the adults. At a certain stage of the ontogeny the embryo of a mammal has gill-pouches like a fish, the skeletal supports of the gill-pouches, the arteries and veins which supply them with blood, the structure of the heart, in short, the entire plan of the circulatory system is fish-like. THE RECAPITULATION THEORY 177 At a later stage most of the gill-pouches have been obliterated, but one is retained and converted into the Eustachian canal, which connects the throat with the middle ear, inside of the ear-drum. Similarly, the embryological evidence shows that the lungs of air-breathers have been derived from the swim-bladder of fishes, a conclusion which had already been reached by comparative anatomy, for in a remarkable A B ^ Fig. 39. — Embryos in corresponding stage of development of shark (A), fowl (B), and man (C); g, gill slits. {From Scott.) group, known as the Dipnoi or lung-fishes, the air-bladder is utilized for purposes of respiration. It has been objected that, while embryology may prove relation- ship within a single type, it fails to demonstrate any connection between different types, but this is not altogether true. The Tuni- cata, a curious group of marine animals once referred to the Mollusca, are shown by their ontogeny to be related to the vertebrates and the same is true of certain marine worms {Balanoglossus). Indeed, most modern zoologists have adopted a scheme of classification, in which 1 78 READINGS IN EVOLUTION, GENETICS, AND EUGI:NICS the type Chordata includes not only the true vertebrates, but also the Lancelet {Amphioxiis) , the tunicates, and Bala no gloss us; this scheme is founded upon the embryological evidence. Among the inverte- brates even more remarkable examples have been observed. Such radically different types as the segmented worms and the shell- fish (Mollusca) are brought into relationship by their ontogeny and their closely similar types of larvae, as are also, though less distinctly, the brachiopods or lamp-shells, and the Bryozoa. The Horseshoe- crab, or King-crab, so abundant along our Atlantic coast, was long of uncertain affinities; originally referred to the Crustacea, largely because of its marine habits of life, embryology makes much more probable its relationship to the air-breathing scorpions and spiders, a result which has been examined previously from another point of view in connection with blood-tests. Even before the publication of Darwin's Origin of Species one of the great stumbling blocks in the way of the theory of special crea- tion was the existence in a great many animals of rudimentary organs, or such as are so far reduced and atrophied as to be of no service to their possessors. An analogy employed by my lamented friend, Mr. Richard Lydekker, may be advantageously repeated here. Let us suppose that a screw-steamer, with longitudinal shaft leading aft from the engine-room to the stern, where it carries the propeller, should, on close examination, reveal many signs that it has originally been a ''side- wheeler," or paddle-boat. Recognizable remnants of paddle- boxes, of bearings for a transverse shaft, and the like, are found; what would be the inevitable conclusion ? No one would maintain that a naval architect, in possession of his senses, in constructing a screw- steamer would deliberately introduce features which are useful and appropriate only in a paddle-boat. The only reasonable explanation would be that the vessel had originally been built as a paddle-boat and had subsequently been converted into a screw-steamer and in the conversion it had not been found necessary completely to eradicate all traces of the original construction. Obviously, the same reasoning applies to rudimentary organs. The only satisfactory explanation of such useless remnants is that their possessors are descendants of ancestors in which those organs were fully functional. It seems quite absurd to assume that, in a separately and specially created animal, useless structures, reminiscent of other animals in which the same structures are useful and valuable, should be included, merely to indicate ideal relationships and community of plan. THE RECAPITULATION THEORY 179 It was sought to break the force of this very serious objection to the theory of special creation by saying that apparently useless organs may nevertheless have functions which are still unknown to us and may be revealed by future discovery. In certain cases, like that of the thyroid gland in the neck, this contention has been justified, but there are many others to which it does not apply. For example, in the great and varied whale-tribe (order Cetacea) which includes the right, or whalebone, whales, the sperm-whales, the porpoises, dolphins, etc., the forelimbs have been converted into swimming paddles, but the hind limbs appear to have vanished completely, leaving no externally visible trace. Internally, however, recognizable remnants of the hind limb-bones may be found in various stages of reduction, which differ in the different members of the order. In the Greenland Right Whale the hip-bone, thigh-bone and shin-bone are indicated; in the Fin whale only the hip-bones and a minute rudiment of the thigh-bone are to be found; in the toothed whales only an almost unrecognizable remnant of the hip-bone is left and in one of the dolphins even that has dis- appeared. Similarly, the snakes have lost their limbs completely, so far as external appearance is concerned, and in most members of the group no trace of Hmbs is to be found on dissection, but in certain snakes the rudiments of limbs are to be detected. Leaving aside all preconceptions, which is the more probable explanation of such phenomena, the theory of special creation or the theory of evolution ? Even if it were admitted that all rudimentary organs and struc- tures found in the adult have a certain unknown use and value, no one could maintain this with regard to the countless instances of structures which are developed in the embryo, but disappear entirely before birth. It is possible to mention but a very few of such instances out of the great number that have already been observed and recorded, but these few will sufifice to illustrate the principle involved. "Examples of this may be cited from the most widely different groups: in the embryo of insects, especially of beetles, pairs of legs are formed within the egg, not only on the head and thorax, but also on the abdomen, but while those on the head are transformed into mouth-parts, those on the thorax are farther developed in their joint- ing and musculature to be locomotive legs, those on the abdomen are again resorbed. In many fresh-water, worms, the eggs of which are laid in a cocoon, from which they are hatched as a finished, minute, crawling worm, larval organs are nevertheless formed, which recall those of the Trochophore,the larva of the original worms, which swims i8o READINGS IN EVOLUTION, GENETICS, AND EUGENICS ■freely in the sea. However, these larval organs .... are never properly functional, since no actually free-swimming larva is developed but the embryo merely floats in the albuminous fluid of the cocoon. '^ A particularly beautiful example is offered by the whales in their embryological development, which has been thoroughly studied by Kukenthal. In the adult condition they show only the anterior extremities, but in the embryo the posterior pair, with their skeletal parts, are formed,but are afterwards completely atrophied. Although they are mammals, in the adult condition they have absolutely no covering of hair, since in their aquatic life another and more effective protection against loss of heat is given by means of a thick layer of blubber; only a few coarse bristles, partly with particular functions, have persisted on a few parts of the body. But in the embryo a dense covering of hair is formed, which is later transformed in a peculiar manner and atrophied. Further, a series of whales have no teeth in the adult condition, but only the well-known, eel-trap-like, horiiy plates, from which whale-bone is produced. Nevertheless, in the embryo there is a dentition of numerous teeth, which are, however, resorbed, without ever piercing the gum."^ Throughout the great group of the ruminants, which includes the oxen, buffaloes, bison, sheep, goats, antelopes, deer and giraffes, the collar-bone is invariably lacking, since it is superfluous on account of the exclusively locomotive manner in which the fore legs are employed. In the embryo sheep the collar-bone is established and even, to some extent ossified, but is subsequently resorbed and disappears entirely. No doubt, the collar-bone will be found in many other embryo rumi- nants, when the proper examination shall have been made, but its demonstrated presence in the foetal sheep is sufficiently striking. In the higher mammals the number of teeth was originally 44, or 11 on each side of both upper and lower jaws, but in most of the modern or existing groups of these higher mammals this number has been very considerably reduced through the suppression of certain teeth. We have every reason to believe that the ancestors of the forms with reduced dentition possessed teeth in full numbers and that there has actually been a loss of teeth in the course of descent. This conclusion is abundantly confirmed by the facts of embry.ology. Take, for example, the great group of the gnawing mammals or Rodentia, in which the front teeth or incisors, above and below, are reduced to one on each side, except in the rabbits. The incisors are chisel-shaped and ^ Otto Maas, Die Ahstammungslchrc, pp. 273-74. THE RECAPITULATION THEORY i8i are faced with hard enamel, so that the action of the upper teeth upon " the lower keeps the cutting edges extremely sharp; these teeth do not form roots, but continue to grow throughout the lifetime of the animal. Between the chisel-like incisors and the grinding teeth, there is a long toothless gap, which, we assume, was, in the ancestors of the rodents, occupied by the second and third incisors, the canine and two or more grinders. This conclusion is justified by the facts of embryology; for instance, in the embryo of the squirrel several of the missing teeth are begun as distinct tooth-germs, but fail to develop, never cut the gum and are resorbed before birth. All available evidence points to the conclusion that birds are descended from reptiles, a conclusion which is especially strengthened by the facts of palaeontology and will be examined more at length in the following lecture. Such a descent explains many otherwise puzzling features in the ontogeny of birds, in which reptilian charac- teristics appear in transitory fashion and are either modified so as to take on typically bird-like character, or are suppressed altogether. A remarkable example of this is the formation of rudimentary teeth in certain embryonic birds, followed by their resorption and disappear- ance before hatching. It can hardly be contended that these rudimentary structures, which are confined to the embryonic stages of development and of which no trace remains in the adult, are so indispensable to the processes of ontogeny, that they were specially created to serve this temporary purpose. For such a contention there is not a particle of evidence and the theory of evolution, which regards these structures as useless remnants, due to inheritance from ancestors in which the structures are functional, offers much the most satisfactory solution of the problem that has yet been suggested. Embryology further shows that evolution is not invariably an advance from lower and simpler to higher and more complex t>pes, but may be by way of degeneration and degradation. The adoption of a parasitic mode of life is very apt to cause such degradation, and some very remarkable instances of the degene ration of parasites have been observed. An instructive example that may be cited is that of Sacculina, a nondescript creature that is parasitic on certain species of crabs. The parasite is attached to the body of its victim, under- neath the tail, by means of root-hke fibre s which penetrate and ramif\' throughout the interior of the crab. The root-like fibres absorb nutri- ment and convey it to the body of the parasite, which is reduced to a 1 82 READINGS IN EVOLUTION, GENETICS, AND EUGENICS mere bag, without appendages, muscles, nervous system, sensory- apparatus, digestive tract, or any determinable organs save those of reproduction. The creature has the power of assimilating the nutri- tive juices which are conveyed to it by the root-like filaments from the body of its host, and the power of reproduction, and it must have some respiratory and excretory capacity, though there are neither gills nor glands. From an examination of the adult parasite alone, it would be quite impossible to classify it and determine the type and class to which it should be referred, but embryology solves the problem. From the egg is hatched a free-swimming larva, which has jointed append- ages, nervous, muscular and digestive systems and, in short, clearly belongs to that group of the Crustacea which includes the barnacles. This is degeneration carried nearly to the utmost possible extreme and yet the individual development shows the derivation of this otherwise problematical parasite and the steps through which it passed in its deterioration. It was stated above that several distinguished naturalists alto- gether reject the recapitulation theory as a means of interpreting the facts of embryology. They do this on the ground that, inasmuch as changes and innovations in form or structure must arise in the germ- plasm, at the very beginning of ontogeny, there is no reason why such changes might not involve the whole course of embryological develop- ment. To my mind this a priori objection to the recapitulation theory is quite without force in view of the great body of observed facts, but there is no time to enter upon a discussion of such an abstract and difficult problem. For our present purpose, however, it is important to note that these objectors are staunch evolutionists and find in the community of mode in ontogeny between different classes of organ- isms one of the strongest arguments in support of the evolutionary doctrine. PART III THE CAUSAL FACTORS OF ORGANIC EVOLUTION CHAPTER XIII INTRODUCTORY STATEMENT H. H. Newman Any investigation of the causes of evolution must be preceded by a survey of the facts to be explained. Some of the principal facts which must be taken into account have already been placed before the reader in the preceding section dealing with evidences of evolution. If there were no other good reason for dealing with those materials before beginning a discussion of causal theories of evolution, the peda- gogical reason would be sufficient, because, until there is something to explain, the necessity for an explanation does not arise. We are of course aware that some writers prefer to deal with the facts of palaeon- tology, geographic distribution, classification, comparative anatomy embryology, etc., after a discussion of the causes of evolution. Their avowed reason for this order of treatment is that the net results of a discussion of the causes underlying evolution may be used as a means of more fully analyzing the facts. This is indeed true, but it is also true that facts should come first and explanations afterward. As a final step, the facts profitably may be re-examined in the light of causal hypotheses. One of the outstanding facts of animate nature is the phenomenon of adaptation. No naturalist has failed to note and marvel at the adaptiveness or fitness of organisms to their environment and that of parts of organisms for particular functions or activities. One of the most difficult problems in evolution is the problem of the origin and the perfection of adaptations, and most causal theories of evolution have been aimed largely at an explanation of adaptation. Consequently, before we enter upon a formal discussion of the causal theories we shall introduce an outline of some of the main facts about adaptations. By way of introduction it should also be pointed out that the causes of evolution are not all of equal value. Some of the causes are to be conceived of as primary, others as secondary, or even tertiary. Variation, for example, is absolutely primary in importance. Without variation, change, which is the very essence of evolution, would of course be impossible. Not less important is heredity; for unless there be some factor which fixes variation so that it becomes a racial asset, 185 1 86 READINGS IN EVOLUTION, GENETICS, AND EUGENICS there can be no real racial progress; and evolution is nothing more or less than racial, as opposed to individual, progress. So obvious did this seem that Charles Darwin accepted as axiomatic the general facts of variation and heredity and proceeded at once to a discussion of the directive factors of evolution. Since variation and heredity are now universally conceded to be primary factors, and selection, the Lamarckian factor, isolation, orthogenesis, etc., as secondary or guiding factors, it would seem more natural to proceed first to a discussion of variation and heredity. So much of our present knowledge of variation and heredity, however, is dependent upon the background furnished by Darwin that it seems to us a more effective pedagogical order to consider that vast and intricate conception of evolution which was first given life and unity by Charles Darwin, and has come now to be known as '^ Darwinism." Just how broad the scope of Darwin's work and how important a role he played in the development of evolutionary biology is indicated in the following appreciation of Darwin which we have summarized largely from the admirable statement in Professor J. Arthur Thom- son's book Darwinism and Human Life. WHAT WE OWE TO DARWIN 1. The web of life — the idea of linkages, interdependencies, cor- relations in the living world. The idea is essentially ecological and has been expressed elsewhere as "organic equilibrium." 2. The struggle for existence — the inevitable consequence of Mal- thus' idea of overproduction. This struggle is both inter- and intra- specific, or may be a mere struggle against fate or against hard condi- tions of inorganic environment. 3. Variability of living creatures — an idea derived from the study of changes under domestication and of diversity among wild individuals belonging to the same species. 4. Natural selection — the central idea which is to be studied pres- ently. 5. Vindication of the evolution idea. — Darwin was the first effec- tively to marshal the evidences of evolution in sufficient force to com- pel the acceptance of the fact of evolution. Much that has already been presented under the head of "Evidences of Evolution" belongs to Darwin. The placing of the fact of evolution on a sure foundation is believed by many to have been Darwin's principal contribution to science. INTRODUCTORY STATEMENT 187 6. The descent arid ascent of Man — '' a recognition of man's solidar- ity with the rest of creation, of his afhUation to a Simian stock — that man and anthropoid apes are collateral branches from a common Pri- mate stock which remains hidden in obscurity." 7. Liberation of intelligence. — ''The Origin of Species has proved a veritable Magna Charta of intellectual liberties, for, as no other single document before or since, it has released the thoughts of man from the trammels of unreasoned conservatism and dogmatism." — H. E. Crampton. 8. Ideal of scientific mood and method. — As Professor T. H. Morgan says, " It is the spirit of Darwinism, not its formulae, that we proclaim as our best heritage." Darwin was the first great evolutionist to use the inductive method, that of first securing an abundance of facts and then formulating theories to explain the facts. The above-stated eight points give us an idea of the broader con- cept of Darwinism. Today the term "Darwinism" has come to acquire a much restricted and a technical meaning. To the modern evolutionist Darwinism has come to be practically synonymous with "natural selection," or at least with the general principle of "selec- tion," some phases of which are termed "neo-Darwinism." Before we can adequately enter upon a study of Darwin's most characteristic causal theory of evolution — the natural-selection theory — it is almost imperative for us to know something of the background out of which this conception arose. Already we have presented in our survey of the evidences of evolution an array of facts most of which were known to Darwin and in accord with which he developed his causal theories. But we cannot afford to overlook the now well-known fact that what Darwinism chiefly aims to explain are the phenomena of adaptation and the web of life. These phenomena are to be conceived of as the background of Darwinism and will be dealt with as such in the next chapters. CHAPTER XIV THE BACKGROUND OF DARWINISM ADAPTATIONS H. H. Newman "The adaptation of every species of animal and plant to its en\'ironment," says Jordan and Kellogg/ "is a matter of everyday observation. So perfect is this adaptation in its details that its main facts tend to escape our notice. The animal is fitted to the air it breathes, the water it drinks, the food it finds', the climate it endures, the region which it inhabits. All its organs are fitted to its functions: all its functions to its environment. Organs and functions are alike spoken of in a half-figurative way as concessions to environment. And all structures and powers are in this sense concessions, in another sense, adaptations. As the loaf is fitted to the pan, or the river to its bed, so is each species fitted to its surroundings. If it were not so fitted, it would not live. But such fitness on the vital side leaves large room for variety in characters not essential to the life of the animal. " The authors quoted above appreciate what is perhaps the most significant fact about adaptations: that the adaptations are to a large extent molded by the environment and therefore fit the environment. So long as the environment remains uniform, a given species will remain unchanged, except for minor fluctuations and occasional mutations; but if the environment changes, sometimes even slightly, the development of the individual responds in such a way as to give a radically different end product. So we may conclude that a large part of the fitness of the organism to the environment is due to the fact that the development of each individual is molded by the environment so as to fit it. Thus some at least of the apparent mystery of adaptations is dispelled. When we think of the fitness of the organism to the environment we take an entirely one-sided view of the matter, for if the organism fits the environment, no less certainly must the environment fit the organism. This idea of the "fitness of the environment" has been ^ From D. S. Jordan and V. L. Kellogg, Evolution and Animal Life. i88 THE BACKGROUND OF DARWINISM— ADAPTATIONS 189 admirably discussed by Professor Lawrence J. Henderson in a stimu- lating volume.^ Henderson points out that the environment, no less than organ- isms, has had an evolution. The particular environmental complex as it exists today is absolutely unique. There is hardly an element of the effective environment that could be changed without causing the extinction of hfe or at least a transformation of it so profound that it might not be life at all as we know life. Water, for example, has a dozen unique properties that condition life. Carbon dioxide could not be replaced by any other substance. The properties of the ocean are so beautifully adjusted to life that we marvel at the exactness of its fitness. Finally, the chemical properties of carbon, hydrogen, and oxygen, the most abundant elements, are equally unique and unre- placeable. In brief, given the environment as it is, life could not be other than it is. The evolution of the environment and the evolution of organisms have gone hand in hand, or perhaps we might better say hand in glove, for this better expresses the idea of mutual fitness. Within the realm of the general environment as conceived by Henderson there are almost innumerable special environments due to particular combinations of the various environmental units. Within the aquatic environment, for example, there are variations such as differences in salinity, varying from extreme saltiness to almost total lack of salt; there are inshore conditions and open-sea conditions; there are surface conditions and those at relatively great depths; and there are great differences due to temperature. Similarly on land, there are surface conditions, subterranean conditions, caves, deserts, forests, plains, mountains, arctic, tropical conditions, and many others. No two areas on land are precisely similar in all respects. All of this makes for a corresponding multiplicity of animal and plant forms. In the case of plants the action of the environment is remarkably direct; 'for the plant cannot get away from a fixed environment. If the environment undergoes material change, the plant's only response is a structural one. For example, if plants that are accustomed to a rela- tively humid climate are grown in the desert they develop numerous xerophytic adaptations such as small leaves with greatly diminished transpiration surface, a thick epidermis, hairs, or spines, small stature, deep-root system, and other similar protections against the inimical desert conditions. Similarly, plants accustomed to grow in relatively ^ L. J. Henderson, The Fitness of the Environment, 19 13. IQO READINGS IN EVOLUTION, GENETICS, AND EUGENICS dry soil, if grown in soil that is covered over with water, will produce aquatic leaves and roots and undergo appropriate changes in epidermis and loss of supporting tissues, for plants that are buoyed up by water need little support. Animals, on the other hand, are for the most part not so intimately related to a local environment as are plants. They are characteristi- cally mobile creatures with varying capacities for wandering about and selecting the habitat that best suits them. ''By virtue of being unlike or possessing different properties," says Shelford,^ ''the various animal species require different conditions for the best adjustment of their internal processes. For example, the carp lives in shallow and muddy ponds and rivers, while the brook trout lives only in clear swift streams. These two organisms are able to move about and find places to which they are suited. The differ- ences between them are clearly indicated by the differences in the habitats which they prefer. "By observation and by experimentation it has been shown that animals select their habitats. By this we do not mean that the animal reasons, but that selection results from regulating behavior. The animal usually tries a number of situations as the result of random movements, and stays in the set of conditions in which its physiological processes are least interfered with. This process is called selection by trial and error. If animals are placed in situations where a number of conditions are equally available, they will almost always be found liv- ing in or staying most of the time in one of the places. The only reason to be assigned for this unequal or local distribution of the ani- mals is that they are not in physiological equilibrium in all the places. However, some animals move about so much that it is with some difficulty that we determine what their true habitats are." This idea of habitat preference and habitat selection is extremely important for a correct understanding of adaptation, or the fitness of organisms to environments. Much of the observed fitness may be due to the fact that an organism has chosen out of a wide range of environ- ments the one that best suits it. We cannot in such a case say that the environment has had a direct influence in shaping the organism any more than we could say that, when a man tries on various shoes and finds a pair to fit, he has been responsible for the fitness of the shoes. Many special adaptations may be explained through habitat choice. Thus animals such as the duckbill platypus, the lung-fishes, ^ V. E. Shelf ord, Animal Communities in Temperate America (1913). THE BACKGROUND OF DARWIxNISM— ADAPTATIONS 191 and others whose teeth are replaced by bony or chitinous plates that are used for crushing the hard shells of molluscs and crustaceans, may not confidently be said to have developed these crushing appliances and to have abandoned the use of teeth in adaptation to a habit of feeding upon hard-shelled prey; but rather it seems more likely that the loss of teeth and the development of crushers occurred through a degenerative process incident to racial senescence and that the pos- session of the crushing equipment enabled them to avail themselves of a new type of food, formerly unavailable to them. The organic environment. — In his admirable chapter entitled ''The Web of Life," which we shall quote entire. Professor Thomson has given us a vivid picture of vast systems of interdependencies that exist throughout the organic world. No species, no creature, lives to itself alone; it is intimately tied up with a host of other creatures with interwoven destinies. Thus one species of animal is adapted to live upon certain plants or other animals, which in turn may be dependent upon still other animals or plants. The elimination of one species may cause the elimination or the radical change of a dependent species. We cannot afford ever to forget this great truth of the oneness of nature. It is the keynote of life and of evolution. Adaptation due partly to functional activity. — It is a commonplace which needs no special demonstration to say that organs improve through use and deteriorate through disuse. Many organs, then, which in the adult condition appear to us to be so admirably adapted to perform certain duties, must be thought of as having been gradually molded by functioning during the entire period of individual development. If the motor nerve running to a limb bud of a growing embryo be severed at an early stage and no secondary nerve connection be established, the limb will continue to grow up to a certain point, but, in its paralyzed condition, will be incapable of exercising its functions and will cease to develop. A certain amount of development will therefore be seen to be independent of functioning, but full develop- ment of functional efficiency is obtained only through functioning. ''The relation between structure and function in an organism," says Professor Child,^ "is similar in character to the relation between the river as an energetic process and its banks and channel. From the moment that the river began to produce structural configurations in its environment, the products of its activity accumulated in certain ^ C. M. Child, "Regulatory Processes in Organisms," Jour. Morp/i., Vol. XXII (1911). 192 READINGS IN EVOLUTION, GENETICS, AND EUGENICS places and modified its flow It moulds its banks and bottom, forming here a bar, there an island, here a bay, there a point of land, but still flowing on, though its course, its speed, its depth, the character of the substances which its carries in suspension or in solution, all are altered, built up by its own past activity." According to this view, structure is simply the resultant of the interaction of function and environment, of functional activity. Though perhaps a little extreme for most of us, this view is, we beUeve, essentially correct. We are prone to overemphasize structure in our discussions of adaptation and evolution and to lay too little stress upon the energy side of development. Certainly no structure is ever formed without proto- plasmic activity of a very definite sort, and in this sense adaptations are to be thought of as the results of functioning. Why, then, do we claim to be astonished at the effective way in which certain organs accompUsh their functions, when functioning has taught them their task ? LAWS OF ADAPTATION Adaptations have been variously classified by different writers. Perhaps the most significant classification is that of Osborn, which is based on their supposed evolutionary origin. According to this writer and others there are two categories of adaptations to environ- mental conditions: the first has to do with the tendency of unrelated species to assume similar structures under similar environmental conditions; the second has to do with the tendency of related species to assume dift'erent adaptive structures under different environmental conditions. In both categories the environment appears to be the determining factor. (i) A good example of the first category, which illustrates wh^t Osborn calls "the law of convergence or parallelism of form," is seen in the tendency of many aquatic types of vertebrates to assume the fishlike form. As is well shown in Fig. 40, the shark (a fish), the ichthyosaur (an extinct aquatic reptile), and the porpoise (a marine mammal), all possess the same fusiform body best adapted for speed under water, the same types of locomotor structures, consisting of the great propeller fin (caudal fin) and the steering and balancing fins, the dorsal fins and paired fins. Apart from these superficial adapta- tions for swift locomotion in the water, the three types are pro- foundly different. The shark breathes with gills, the reptile and mammal with lungs, the fish and reptile are cold-blooded, the THE BACKGROUND OF DARWINISM— ADAPTATIOXS i 93 Fig. 40. — Three aquatic types of vertebrate, to illustrate convergent adapta- tion of three wholly unrelated forms of marine life. All three show the fusiform body, median and paired fins, though the skeletal structures are radically differ- ent. A, shark (Pisces); B, ichthyosaur (Reptilia); C, porpoise (Mammalia). {From Newman^ after Osborn.) 194 READINGS IN EVOLUTION, GENETICS, AND EUGENICS mammal warm-blooded. The internal anatomy of the three differs fundamentally in every detail. A list of other types of convergence will more adequately illustrate the law. Flying and parachuting animals occur among nearly all vertebrate and some invertebrate classes. Planes of some sort are found for supporting the body in the air. The plane is made in various ways in different groups, but functions much the same in all of them. Running animals of various classes have long legs, and a tendency to stand on the toes. There is also in several unrelated groups the tendency to reduce the number of toes, the culmination of which is seen in the one-toed horses. Climbing animals are all provided with clinging appendages of some sort, including such structures as hooked claws, prehensile fingers or tail, suction pads on the feet, and other similar adaptations. Burrowing animals have, as a rule, extra-heavy shoulder girdle and strong fore limbs with heavy gouging claws. Many of them also are blind or nearly so, as befits life in dark underground passages. Desert-dwelling animals as a rule are provided with heavy scales, spines, or armor, to prevent excessive loss of moisture and as a protection against spiny plants. They also usually have burrowing habits enabling them to escape the extremes of heat and cold. Cave animals are usually blind or nearly so and are relatively pale in color, sometimes without any pigmentation. Deep-sea animals of many sorts have phosphorescent orgains by means of which they either attract their prey or find their way about the dark sea floor. Some of these organs, called "lanterns," can be used as searchlights. The eyes of deep-sea fish are either enormously large or are "telescope eyes," adapted for sensing light of low intensities. Ant-eating animals, belonging to several distinct groups, are heavily armored against the attacks of ants, have strong claws for digging up ant galleries, have long snouts or beaks with a long sticky tongue for capturing ants, and an arrangement of the glottis to prevent ants from crawling into the lungs. 2. There are almost innumerable examples of the law of divergence of form, which is called also the law of adaptive radiation. Almost every successful class or order of vertebrate animals, for example, has members that have adjusted themselves to all of the main modes of living. Thus among lizards, for example, there are primitive THE BACKCxROUND OF DARWINISM— ADAPTATIONS 195 running forms that prefer the surface Kfe and swift motion; subter- ranean burrowing types that sometimes are limbless like snakes, and are blind; many arboreal or climbing types; a few volant or flying types; a few ant-eating types; and several more or less completely aquatic types. Each of these types has the customary adaptations for its own mode of life. We see, then, that whether divergent structures are molded into a semblance of similarity to fit a definite environment, or whether similar structures are modified in diverse ways to fit various divergent environments, the adaptation is related very definitely to the environ- ment and to the functional life of the organism. No wonder, then, that so many biologists consider that the ehvironment has been a molding force in the evolution of adaptations. One of the most interesting discussions of adaptations is that of Weismann, who, it appears, is greatly impressed with the uni- versality of adaptation. His thesis apparently is that if we had complete knowledge of the field of biology, we would discover that everything is adaptive, and that many structures or habits that now appear to us useless or non-adaptive, would be found to have a definite value to the organisms possessing them. Some authors take the opposite extreme and claim that adaptation has been merely read into a vast number of so-called adaptive structures and that when these structures shall have been adequately investigated they will be found to lack the value imputed to them by uncritical observers. Some- where between these extreme views lies the truth. Thus we see that a certain amount of adaptation is inevitable and needs only a formal physiological explanation. The fitness of the environment, habitat selection, and the relationship that exists between function and structure, are adequate explanations for the general fact of adaptation and thus take away much of the mystery that has shrouded the concept of fitness. There are, however, very many types of special adaptation which do not yield so readily to the general explanation given. Some of the most important of these will be de- scribed below. ADAPTATIONS CLASSIPIED Adaptations are variously classified by different authors, and that of Jordan and Kellogg is as good as any: " {a) food-securing; {b) self- defense; (c) defense of young; {d) rivalry; (e) adjustment to sur- roundings." 196 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Some very common adaptations may belong to several of these categories at once. Thus the sharp teeth and hooked claws of car- nivorous mammals serve equally well for food-securing, for self- defense, for defense of young, and for rivalry. Similarly, the horns of deer and other ungulates are equally adapted for self-defense, defense of young, and rivalry. There can be no especial advantage, in this connection, in present- ing a detailed review of adaptations of the sorts given in the foregoing classification; therefore we shall confine our efforts to a description of a few typical adaptations about which the greatest controversy has raged. SOME SPECIAL ADAPTATIONS The electric organ of the torpedo, a widely distributed elasmo- branch fish, consists of a sort of honeycomb-like structure on each side of the head. This structure acts as a storage battery and is capable of storing up electricity of considerable voltage. The animal is capable of giving a very distinct shock to an attacker and can thus defend itself quite effectively. There is also an electric eel, native to the waters of Paraguay and Brazil, that is able to give severe shocks to bathers or to horses driven through the streams. A type of catfish native to the river Nile has a similar electric equipment. In all of these cases the storage battery is made up of modified voluntary muscles and is of considerable size. The mammary glands of mammals are skin glands usually with well-defined ducts leading to the surface and terminating in teats. These glands are quite voluminous and serve admirably the purpose of feeding new-born young until the latter are able to use the more varied food normal to the adult. In the lowest mammals, the monotremes or egg-laying mammals, these glands are relatively poorly developed and diffuse; also they are known to be developed through a regional specialization of sweat glands. In the true mammals or Eutheria the glands are modified sebaceous or oil glands and may be seen to develop from the same embryonic rudiments as the latter. The marsupial pouch of the kangaroo and its allies is a pocket- like fold of the integument, folded forward or backward over the region of the abdomen in which are located the mammary glands. This pouch is used as a shelter for the tiny immature larval foetuses. Hartmann has recently described a very striking piece of behavior in connection with the birth of young opossums. The young are born THE BACKGROUND OF DARWINISM— ADAPTATIOXS 197 in an exceedingly immature state and looking like tiny pink grubs. They crawl under their own power, by means of a swimming-like motion, through the hairs of the mother's abdomen, till they reach the pouch. This they enter unaided and each tiny larva finds for itself a slender tubular teat, which it swallows and holds in place by a specially adapted hold-fast mouth. The young remains attached fixedly to this teat for some weeks, feeding almost constantly on milk. After a long interval the teat is released, the mouth metamorphoses into the adult form and the young feeds only at intervals, as do the young of other mammals. This complex of adaptive structures and instincts is among the most remarkable in the annals of biology. Nest-making instincts in birds represent, on the behavior side, adaptations of extraordinary perfection. Some nests are built with the greatest care and precision, others represent a relatively crude and slovenly performance. Some nests are made of twigs, fibres, and mud, others of mud alone, still others are hollowed out in clay or sand banks, and some are made in holes in the ground. In any case, the type of nest is highly specific and due to a hereditary instinct; for birds receive no instruction in nest-making. Before bringing to a close this brief list of particularly noteworthy adaptations let us recall to mind the series of special adaptations listed as examples of the laws of adaptation, such as aquatic, arboreal, cur- sorial, flying, burrowing, ant-eating, and, especially, adaptations of deep-sea animals. PARASITISM AND DEGENERATION A vast number of animals and plants have given up the active" search for food and have taken up the relatively easy habits of para- sitism. In adaptation to this life certain structures have developed and many of the characters found in independent, free-roving crea- tures have disappeared or become reduced to mere vestiges. Thus the more completely dependent or parasitic an animal becomes, the more completely does it lose its organs of locomotion and its sense organs such as eyes, auditory organs, tentacles, etc. Some animals are free-living when young or in the larval condition and only settle down to a parasitic Hfe when near the end of the life-cycle; other animals are parasitic only when young or larval and become inde- pendent in the adult condition; still others are parasitic throughout the entire life-cycle and pass from host to host without any interval of independent life. Some of these complete parasites pass one phase of the life-cycle on one species of host and the remainder on another igS READINGS IN EVOLUTION, GENETICS, AND EUGENICS species of host. Thus the liver fluke in the adult condition lives in the gall bladder of the sheep, while the early larvae live within the body cavities of a species of land snail. The transfer from host to host in this case must be a procedure involving many chances of failure to a very few chances of success, and, in adaptation to these vicissitudes, the number of eggs and larvae produced by a single adult individual runs up into the milUons. The classic case of extreme parasitic degeneration is that of Sacculina. The young larva of Sacculina is a typical entomostracan crustacean larva which swims about and leads a free life for a time, but soon attaches itself by means of its antennae to a hair pit of a crab, a small hole in the latter's armor. The internal tissues of the larva then undergo degenerative processes and are reduced to an almost fluid mass of embryonic cells, which flow through the hair pore of the crab, and into the latter's lymph spaces. The small mass of cells then rounds up and is carried about with the circulation of the crab's blood until it comes to a favorable place of lodgment, usually the wall of the intestine just back of the stomach. Here it flattens out and sends rootlike branches almost all over the crab's body, like a malignant tumor in its invasion of foreign tissues. The unbranched part of the parasite is little more than a sac of reproductive organs, and these produce eggs and sperms, which unite to produce larvae. By this time the host is killed and, with the decay of its body, the larvae escape into the sea water ready for a brief period of free life before attacking another host. Almost every group of animals and most of the groups of plants have their parasitic representatives and every degree of parasitism and the accompanying degenerative changes are to be found. Of course, it is an open question whether parasitism causes degeneration or whether degenerating creatures take refuge in parasitism; but in either case the adaptive features of the situation are obvious. Commensalism. — If parasitism be defined as an association between two organisms in which one (the parasite) lives at the expense of and to the detriment of the other (the host), commensalism may be defined as an association in which the two organisms exist in close association without any positive detriment to either. In some cases the claim is made that the association is mutually beneficial, but as a rule the condition is relatively one-sided. An interesting example of commensalism is that of the sea cucum- ber and the little fish Fierasfer. This strange little animal inhabits THE BACKGROUND OF DARWINISM— ADAPTATIONS 199 the rectum of the sea cucumber and may be seen to He with only its head out. From this shelter it darts forth to capture its prey; which done, it returns to its shelter. Curiously enough the vent of the little fish is situated just back of its mouth so that its wastes may be voided when in its usual position. There can be no advantage to the sea cu- cumber in such an arrange- ment, though no particular ^,^p>^^ -idS#l?^- -- --^'-''^rvi'^B.' .• harm is done. Another case ^^^*K^ ; ^ : ^ T " "\' "" ^^^C "^ '-, of this sort is that of several sm^'-^ \ ' >....,„fic«;dU.iiri .-> . , ^'r^^ species of Remora which attach themselves by a large diskoid adaptation on top of Fig. 41. — Fierasfer acus, penetrating the the head to various fish such ^S""^ openings of holothurians, I natural size. {troni Boiiienger, ajtcr Emery.) as sharks, barracudas, etc. The sucking disk is a modified dorsal fin. The remora merely gains free transportation to more favorable feeding-grounds. When the desired food is sighted the passenger leaves its conveyance tempo- rarily, but returns by a sudden swift dash and resumes its hold. The shark gets nothing except perhaps the sense of companionship, and is also undoubtedly somewhat hindered in its locomotion. Some of the most remarkable cases of commensalism are found in connection with elaborate colonies of ants. In some cases two species of ants live together in the relationship of masters and slaves. The master species is unable to perform any of the ordinary duties of the colony, such as securing food, taking care of young, etc. In extreme cases the masters are only soldiers, specialized for fighting and maraud- ing, and cannot even feed themselves unaided. The slave species would be able to carry on to some extent if not captured, but thrives exceptionally well under the protection of the soldier species. There are among ants many varieties of commensal relationship less extreme than this, but this will serve as a typical case. Communal life. — Among the higher insects and higher vertebrates, especially among the ants and bees, we find a very elaborate social life. In ants, for example, the typical colony consists of a queen (the only fertile female in the colony), several males (mates of the queen). 200 READINGS IN EVOLUTION, GENETICS, AND EUGENICS • ordinary workers (sterile females of the first type), soldiers (sterile females of the second type), and sometimes officers (especially large and powerful sterile females that seem to direct the line of march in legionary ants). All of these casts are produced from the eggs of one female and are the result of various special diets permitted the larvae by the workers. Among bees, similarly, there is one queen, a number of drones (males), and the sterile female workers, who perform the functions of nursing the larvae, cleaning up the hive, collecting pollen and nectar, and making honey and wax. Detailed accounts of the lives of bees have been given by various authors, notably by Maeter- linck in his Life of the Bee. COLOR IN ANIMALS ''The phenomena of color in both animals and plants," says Metcalf,^ "are among the most remarkable and interesting in the whole realm of nature. It is not so much the way in which the color is produced, whether by pigments or by refraction, that interests us in this connection, as it is the uses to which colors are put. Let us first refer to the colors of animals. "According to the uses to which colors in animals are put, we may classify them, for purposes of description, as follows: "Indifferent coloration, not useful, so far as we can judge; Colors of direct physiological value; Protective coloration and resemblances; Aggressive coloration and resemblances; Alluring coloration and resemblances; Warning coloration; Immunity coloration; Mimetic coloration and resemblances; A. Protective B. Aggressive Signals and recognition marks; Confusing coloration; Sexual coloration." A few examples of these categories of animal coloration will serve to illustrate the ways in which they are believed to be adaptive and thus better to fit the organism for its struggle for existence. Protective resemblance. — Many animals that live at or near the surface of the sea are practically transparent. Fishes are commonly ^ M. M. Metcalf, Organic Evolution (191 1). THE BACKGROUND OF DARWINISM— ADAPTATIONS 201 dark-colored above and light-colored below, so that to the enemy above they blend with the dark bulk of the water and to the enemy below they are hidden by the fact that the shadow cast by their own bulk is sufficiently neutralized by the ventral light coloring to render them inconspicuous. A very large number of arboreal animals are green ; such as grass- hoppers, leaf hoppers, spiders, green lizards, parrots, etc. Prairie and desert animals are usually dull-colored like their surroundings. IVIany butterflies are brightly colored like the flowers upon which they feed. Many arctic animals are white like their snowy background. Some animals, like the chameleon and the flounder, change their colors so as to keep in harmony with changing backgrounds. There are many cases of protective resemblance, involving both form and color, between an organism and some particular feature of its environment. The walking-stick insect is long and slender and colored like a twig. Many caterpillars when disturbed stand out stiff and straight like leafless twigs. A species of sea-horse (a teleost fish) has its fins fringed out into structures that closely resemble the fronds of seaweed in which the animal lives. Many insects, belonging to several orders, have very striking resemblances to leaves. The case of Kallima (Fig. 42), the dead-leaf-butterfly is a classic example of this type of protective resemblance. The resemblance is so nearly perfect that, when one has mounted specimens of butterfly and leaf before him, he has to examine them closely to detect the fraud. The details of the leaf color, veins, petiole, marginal notches, even wormholes, so common in dead leaves, are reproduced in the butterfly's wings. Many tree- frogs have a leaf-shaped pattern bordered with black to resemble the shadow cast by a leaf. These are only a few scattering examples of an exceedingly prevalent type of adaptation. Aggressive coloration and resemblance. — There is a close simi- larity between this phenomenon and the one just dealt with, but instead of being used defensively, it is used offensively, in that it enables the predaceous animal to remain hidden from its prey. Tlius the polar bear and the arctic fox are white and therefore inconspicuous to seals and arctic birds, their prey. Perhaps the most striking instance of this type of coloration is that of the tiger, whose tawny coat and dark stripes resemble the reeds. and their vertical shadows in the jungle. Alluring coloration and resemblance. — 'Tn India," says Metcalf, ''there is a Mantis (insect) which in shape and color resembles an 202 READINGS IN EVOLUTION, GENETICS, AND EUGENICS orchid blossom. It deceives' butterflies and other insects, which it captures as they approach the seeming flower. In Java there is a spider which resembles a bit of bird excrement upon which butterflies are so apt to light. This resemblance enables it to capture the butterflies upon which it feeds." Fig. 42. — Kallima, the ''dead-leaf butterfly." (From Jordan and Kellogg.) Warning coloration. — Many animals that are for various reasons harmful or dangerous to other animals have strikingly distinct color patterns which have been interpreted by some authors as warning marks to keep off possible attackers. Bees, wasps, hornets, some poisonous snakes, many spiders, all of which have stings or fangs, are marked with bands of contrasting colors. Other animals that are THE BACKGROUND OF DARWINISM— ADAPTATIONS 203 nauseous if eaten, still others, like the skunk and his tribe, that pro- duce offensive odors, have well-defined markings that are classed as examples of warning coloration. Immunity coloration. — Professor Reighard has taken exception to the interpretation of conspicuous coloration as warning adaptations. His theory of "immunity coloration" furnishes an alternative interpre- tation which appears less open to criticism. According to this idea well-protected animals are relatively safe from attack and therefore may become conspicuous without endangering themselves. They are immune and therefore their conspicuous markings may be merely the result of color run riot without any check on the part of natural selection. Mimicry. — This is a special type of protective coloration in which an otherwise defenseless species has a striking resemblance to some well-protected species with warning coloration. Thus wasps and bees are "mimicked" by flies, beetles, and moths. They enhance the resemblance by similarities of behavior and habitat. Ants are pro- tected by the fact that they contain formic acid, which is distasteful to most animals (though some animals feed very largely on them). Consequently there are many ant mimics belonging to several orders of insects and to spiders. In some cases these ant mimics are inhabit- ants of ant colonies and succeed in passing themselves off as ants even among the ants themselves. The classic cases of mimicry, however, are those in which certain species of edible butterflies are said to mimic other, unrelated, nauseous species of butterflies. It is very difficult to distinguish the model from the mimic except by careful anatomical examination which, of course, could not be applied in nature. It should be said about mimicry, however, that it would work only in case the mimic occurs in much smaller numbers than the model, and that the two species occupy the same regions at the same time. Some critics have claimed that these conditions do not prevail in all cases. If their contention is valid, the usual expla- nations of mimicry need revision. Cases of aggressive mimicry are noted among certain predaceous animals, such as spiders which mimic flies and ants, and are therefore able to approach their prey more effectively. Signals or recognition marks. — The common cotton-tail rabbit raises its white tail when it runs. This is interpreted as a signal of danger to other rabbits. Some antelopes have a conspicuous white rump which is supposed to be a danger signal. Many distinct specific 204 READINGS IN EVOLUTION, GENETICS, AND EUGENICS markings such as the red heads of woodpeckers, distinct white or black bars on the wings of other birds, may serve for recognition purposes within the species. Confusing coloration. — Many butterflies and moths, and not a few birds have rather conspicuous markings when in flight, which may serve as specific recognition marks; but when they alight after a zigzag course through the air, they cover up the conspicuous markings and blend with the background in various ways. They are supposed to alight when in danger of capture and they apparently disappear, much to the confusion of the pursuer. Thus Kallima, the dead-leaf butterfly, is quite conspicuous from above while in flight, but when it alights, it cannot be distinguished from a dead leaf. Sexual coloration. — A great many groups of animals exhibit a pronounced sexual dimorphism in color and pattern. The most conspicuous instance of this is that of birds among which the female is usually protectively colored so as to be inconspicuous when on the nest or when sheltering the young, while the male of the same species is frequently conspicuously colored. Similar situations are found among butterflies and moths in which sometimes one sex and some- times the other is the more elaborately colored. Sexual coloration is also common among teleost fishes, lizards, spiders, and many other groups. Charles Darwin devised a special "sexual selection" theory to account for just this type of adaptation. One more kind of coloration that is not specifically dealt with by Metcalf is what has now come to be known as "camouflage. " Many animals when viewed out of their environment appear to be very conspicuous owing to the juxtaposition of patches of irregular colors. In their natural surroundings, however, they become practically invisible. Thus the nighthawk with its strongly contrasted patterns almost fades from view against the bark of a tree. For excellent illustrations of animal colorations the reader is referred to Professor Metcalf's book Organic Evolution, where he has gathered together in color plates many of the finest examples of the phenomena under discussion. General considerations. — Adaptations are characteristic of all living organisms and must be accounted for by any evolutionary theory that is to be acceptable. Any theory that claims to account for new species but does not account for adaptations is at best only a partial explanation. All of the phenomena which have been briefly men- tioned in this chapter, together with the more intricate phases of THE BACKGROUND OF DARWINISM— ADAPTATIONS 205 general adaptiveness involved in the idea of " the web of life/' are part of the background of Darwinism and were in the mind of Darwin when he thought out the great generalization called "natural selection." The "web of Hfe" idea has been admirably presented by Professor Thomson, Scotland's most skilful and prolific biological writer. The present writer feels that no student of evolution should miss the oppor- tunity of getting into the spirit of Darwinism with this distinguished author, and to make this desideratum easily attainable, the chapter is quoted unchanged as part of the general text and immediately follows this discussion. CHAPTER XV THE BACKGROUND OF DARWINISM— Continued THE WEB OF LIFE^ J. ARTHUR THOMSON Naturalists, in the true sense, who study the life of living creatures in nature, have always been distinguished by a keen perception of the interrelations of things. Whether we take Gilbert White as repre- senting the old school, or W. H. Hudson as representing the new, we get from their observations the same impression of nature as a vibrat- ing system, most surely and subtly interconnected. But it seems just to say that no naturalist, before or since, has come near Darwin in his realisation of the web of life, in his clear vision and picture of the vast system of linkages that penetrates throughout the animated world. Correlation of organisms as well as correlation of organs. — In thinking of a living body we are accustomed to the idea of the cor- relation of organs. It is of the very nature of an organism that there should be mutual dependence among its parts. The organs are all partners in the business of life, and if one member changes others also are affected. This is especially true of certain organs that have developed and evolved together, and are knit by close physiological bonds. We know in health how nerve and muscle, brain, and sense organs, heart and lungs, are closely bound together in the bundle of life. We know in disease that a change in one organ often affects another, and the fact remains though the nexus is sometimes myste- rious. The state of our liver may give colour to our whole intellectual firmament, and a slight ocular derangement may warp a wise man's philosophy. The far-reaching importance of a little organ like the thyroid gland beside the larynx is well known; our intellectual as well as our bodily health depends on its soundness. Now, just as there is a correlation of organs within the body, so there is a correlation of organisms in that system of things which we call Nature. In both cases we are here using the word " correlation " in its deeper sense — ^ From J. A. Thomson, Darwinism and Human Life (copyright 1909). Used by special permission of the publishers, Henry Holt & Company. 206 BACKGROUND OF DARWINISM— THE WEB OF LIFE 207 that the various parts are more than mutually dependent, that they are in some measure co-ordinated, making larger systems workable. What the metaphor of "the web of Ufe" suggests. — We may use the metaphor "web of life" in two ways. On the one hand, Nature has a woven pattern which science seeks to read, each science following the threads of a particular colour. There is a warp and woof in this web, which to the zoologist usually appear as "hunger" and "love." There is a changing pattern in the web, becoming more complex as the ages pass; and this is evolution. But the essential idea of a web is that of interlinking and ramifying. We can never tell where a thread will lead to. If one be pulled out, many are loosened. This is true of Nature through and through. The phrase "web of life" suggests another picture — the web of a spider — often an intricate system, with part delicately bound to part so that the whole system is made one. "The quivering fly entangled in a corner betrays itself throughout the web; often it is felt rather than seen by the lurking spinner. So in the substantial fabric of the world part is bound to part. In wind and weather, or in the business of our life, we are daily made aware of results whose first conditions are very remote; and chains of influence, not difficult to demonstrate, link man to beast, and flower to insect. The more we know of our surroundings the more we realise that nature is a vast system of link- ages, that isolation is impossible." Dependence of living creatures on their surroundings. — We do not know what life in principle is, but we may describe living as action and reaction between organisms and their environment. This is the fundamental relation — the dependence of living creatures on appro- priate surroundings, and the primary illustrations of linkages must be found here. The living creatures are real, just in the same sense as the surroundings are real; but it is plain that we cannot abstract the living creatures from their surroundings. When we try to do this they die — even in our thought of them, and our biology is only necrology. Huxley compared a living creature to a whirlpool in a river; it is always changing, yet always apparently the same; matter and energy stream in and stream out; the whirlpool has an individuality and a certain unity, yet it is wholly dependent upon the surrounding currents. One may push the whirlpool metaphor too far, so as to give a false sim- plicity to the facts, for when vital whirlpools began to be there also emerged what cannot be discerned in crystal or dewdrop — the will to live, a capacity of persistent experience, and the power of giving rise to 2o8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS other lives. To ignore this is to attempt a falsely simple natural history. But what Huxley's metaphor of the whirlpool does vividly express is the dependence of living creatures on their surroundings. We cannot understand either the whirlpool or the trout apart from the stream. When we think out this fundamental dependence upon surround- ings, we see, for instance, that all our supplies of energy, all our powers of every kind — with our own hands, or by the use of animals, or by means of machinery — are traceable to the sun. Or again, it is easy to show that our society depends fundamentally not on gold, but on iron. We depend for food on plants and animals, and through these animals on plants ultimately; the plants feed upon air, water, and salts, which, with the aid of the energy of the sunlight, they build up into complex organic compounds ; they cannot do this unless the sun shines through a screen of green pigment called chlorophyll; there cannot be chloro- phyll without iron; therefore our whole social framework is founded on iron. Nutritive chains. — Plants feed on their inanimate environment in a direct way that is impossible to animals, so we pass insensibly from dependence on surroundings to those nutritive chains which bind living creatures together in long series often quaintly suggestive of ''The House That Jack Built" and similar old rhymes. We have ceased to wonder at the circulation of the blood in our body; have we begun to wonder enough at the ceaseless circulation of matter in the system of nature ? As Heraclitus said, iravTa pel, all things are in flux. "The rain falls; the springs are fed; the streams are filled and flow to the sea; the mist rises from the deep and the clouds are formed, which break again on the mountain-side. The plant captures air, water, and salts, and, with the sun's aid, builds them up by vital alchemy into the bread of life, incorporating this into itself. The animal eats the plant and a new incarnation begins. A II flesh is grass. The animal becomes part of another animal, and the reincarnation continues." The silver cord of the bundle of life is loosed, and earth returns to earth. The microbes of decay break down the dead, and there is a return to air and water and salts. We may be sure that nothing real is ever lost; we are sure that all things flow. Penelope-like, Nature is continually unravelling her web and making a fresh start. Nexus between mud and clear thinking. — To keep a famous inland fish-pond from giving out, some boxes of mud and manure were placed at the sides. Bacteria — the minions of all putrefaction — BACKGROUND OF DARWINISM— THE WEB OF LIFE 209 worked in the mud and manure, making food for minute Infusorians which multiply so rapidly that there may be a million from one in a week's time. A cataract of Infusorians overflowed from box to pond, and the water-fleas and other small fry gathered at the foot of the fall and multiplied exceedingly. Thus the fishes were fed, and, as fish- flesh is said to be good for the brain, we can trace a nexus from mud to clear thinking. What was in the mud became part of the Infusoria n, which became part of the Crustacean, which became part of the fish, which became part of the man. And it is thus that the world goes round. Correlation between catches of mackerel and amount of spring sunlight. — A curious and most interesting correlation has been discovered by Dr. E. J. Allen between catches of mackerel and the amount of sunlight. The more sunshine in May, the more mackerel at Billingsgate. How does this work out ? Mr. G. E. Bullen shows that ''for the years 1903-1907 there appears to be a correlation between the number of mackerel taken during May, and the amount of Copepod plankton, upon which the mackerel feed, taken in the neighborhood of the fishing grounds during the same month." Mr. W. J. Dakin shows that the food of Copepods consists largely of the vegetable organisms of the plankton, such as diatoms, and of Infusoria-like organisms called Peridinidae. But the production of this microscopic plankton, the "stock" of the "seasoup," depends partly on the composition of the sea-water, partly on the tempera- ture, and partly on the amount of light available. There seems to be no correlation between the surface temperature and the abundance of mackerel, but Dr. Allen has shown a correspondence between sunshine and the catches. Thus we see that, if all flesh is grass, then in the same sense all fish is diatom. Nutritive chains in the deep sea. — If we pass from the sunlit open sea to the floor of the deep sea — that strange, dark, cold, silent, plantless world — we find carnivorous animal preying upon carnivorous animal through long series — fish feeds on fish, fish on Crustacean, Crustacean on worm, worm on still smaller fry, and all ultimately depend on the basal food-supply — the ceaseless shower of moribund atomies sinking from the surface waters many miles, it may be, over- head, like the snowflakes on a quiet winter day. Dependence of one organism on another for the continuance of the species. — Passing from "nutritive chains," we may select a few illustrations of the dependence of one creature upon another for the 2IO READINGS IX EVOLUTION, GENETICS, AND EUGENICS continuance of its kind. The crowning instances are to be found in in- terrelations between plants and animals which secure cross-fertilisation and the distribution of seeds. To both of these Darwin devoted much attention, and they were always favourite subjects with him. Everyone knows that flowering plants and flower-visiting insects have grown up throughout long ages together, in alternate influence and mutual perfecting. They are now fitted to one another as hand to glove. The insects visit the flowers for food ; in so doing they carry the fertiUsing golden dust from blossom to blossom, so that the possible seeds become real seeds. In 1793 a Berlin naturalist, Christian Konrad Sprengel, hke Darwin in his perception of the web of life, published a pioneer book entitled The Secret of Nature Discovered in the Structure and Fertili- zation of Flowers, in which he showed that most flowers have nectar which insects enjoy; that by the insects' visits polUnation is secured; that there is no detail of the flower without its meaning — the colour is a flag to attract the insect's eye, conspicuous spots are honey-guides to the explorers, there are arrangements for keeping the pollen dry and for dusting it on the insects, and so on. If Sprengel had only discovered the utility of the cross-fertiHsation, which Darwin proved experimentally, his work could hardly have been overlooked for nearly seventy years. In 1841 it came into Darwin's hands, and impressed him as being ''full of truth," although ''with some little nonsense." In Darwin's work Sprengel had his long-delayed reward. Darwin's instance of the connection between cats and clover. — One of Darwin's instances of the web of life — given in connection with the pollination of flowers — has become familiar all over the world. It should never become trite to us and it should never be regarded as more than a particularly clear illustration of a general fact. " Plants and animals, remote in the scale of nature, are bound together by a web of complex relations I have found, from experiments, that humble-bees are almost indispensable to the fertilisation of the heart' s- ease {Viola tricolor), for other bees do not visit this flower. I have also found that the visits of bees are necessary for the fertilisation of some kinds of clover — thus, 100 heads of red clover {Trifolium pratense) produced 27,000 seeds, but the same number of protected heads pro- duced not a single seed. Humble-bees alone visit red clover, as other bees cannot reach the nectar Hence we may infer as highly probable that, if the whole genus of humble-bees became extinct or very rare in England, the heart's-ease and red clover would become BACKGROUND OF DARWINISM— THE WEB OF LIFE 211 very rare, or wholly disappear." We know that the red clover imported to New Zealand did not bear fertile seeds until humble-bees were also imported. "The number of humble-bees in any district depends in a great measure on the number of field-mice, which destroy their combs and nests; and Colonel Newman, who has long attended to the habits of humble-bees, believes that more than two-thirds of them are thus destroyed all over England." Now the number of mice is largely dependent, as everyone knows, on the number of cats; and Colonel Newman says: ''Near villages and small towns I have found the nests of humble-bees more numerous than elsewhere, which I attribute to the number of cats that destroy the mice." Thus we may say, with Darwin, that next year's crop of purple clover is influenced by the number of humble-bees in the district, which varies with the number of field-mice; that is to say, with the abundance of cats! Scattering of seeds. — It is a fascinating chapter of natural history which tells us how cross-pollination is effected — here by a bee and there by a butterfly, occasionally by a long-billed humming-bird beautifully poised before the flower with almost invisibly rapid vibra- tions of its wings, and occasionally by a slowly moving snail of epicure appetite. But not less important is the part played by animals in the scattering of seeds, and here again Darwin gives us the classic case of fourscore seeds germinating out of a ball of mud from a bird's foot. From one instance you may learn all, and see that much of Darwin's work has been an eloquent commentary on that memorable saying about the sparrow that falls to the ground. Such a simple event literally sends a throb through surrounding nature; we can follow its effects a few steps, just as we follow for a few yards the ripples made when we throw a stone into a still lake; in either case can we doubt that the spreading influences are real, though they pass beyond our ken? Interrelations between fresh-water mussels and fishes. — As a striking illustration of the inter-linking of different forms of life, we may take the case of the fresh-water mussels and their larvae. The fertilised eggs develop in the outer gill-plate of the mother-mussel, and minute bivalve larvae, called Glochidia, are formed. The mussel keeps these within the cradle until a fresh-water fish — such as the minnow — comes into the vicinity, and then she sets them free. In a way that we do not understand, the simple constitution of the larvae is tuned to respond to the presence of minnows and the like, and with snapping valves they manage to fix themselves to their host. After a short 212 READINGS IN EVOLUTION, GENETICS, AND EUGENICS period of temporary parasitism, at the end of which there is a meta- morphosis, they drop off from the fish into the mud, often far from their birth-place. This is curious enough, but the idea of Unkages becomes incandescent in the mind when we note that, just as the fresh- water mussel has young temporarily parasitic on fishes, so a fresh- water fish, the bitterUng (Rhodeus amarus), has its young temporarily parasitic in the gills of the mussel. Life-histories of parasites. — When we pass to parasites in a stricter sense we find the most extraordinary interconnections, the most widely separated animals often sharing a parasite between them. Liver-rot, which has repeatedly killed a mil Hon sheep in a year in Britam alone, is due to a parasite which passes from sheep to water, from water to water-snail, from water-snail to grass, from grass to sheep. The tapeworm of the cat has its bladder-worm stage in the mouse, the sturdie-worm of the sheep's bram has its tape-worm stage in the dog, and similar relations hold for hundreds of species. The troublesome threadworm of human blood {Filaria sanguinis hominis) is transferred from man to man by the mosquito, and the guinea-worm which was probably the fiery serpent that vexed the Israelites in the desert, which passes into man in drinking-water, spends its youth in a minute water-flea, called by the giant's name of Cyclops. The importance cf tse-tse flies in transmitting the minute animals which cause sleeping-sickness and allied diseases is known to all. We have spoken of the connection between cats and clover, and there is a not less striking connection between cats and plague. For it seems to have been shown in India that the more cats the fewer rats, and the fewer rats the fewer rat-fleas, which are the agents in passing the plague- germs to man. Far-reaching influence of certain animals; earthworms. — We realise the idea of the web of life in another way when we consider the far-reaching influence of particular kinds of activity, the best instance being the work of earthworms. In 1777 Gilbert White got at the very root of the matter. "The most insignificant insects and reptiles are of much more consequence and have more influence in the economy of nature than the incurious are aware of Earthworms, though in appearance a small and despicable link in the chain of nature, yet, if lost, would make a lamentable chasm Worms seem to be the great promoters of vegetation, which would proceed but lamely with- out them, by boring, perforating, and loosening the soil, and rendering it pervious to rains and the fibres of plants; by drawing straws and BACKGROUND OF DARWINISM— THE WEB OF LIFE 213 Stalks of leaves and twigs into it; and, most of all, by throwing up such infinite numbers of lumps of earth called worm-casts, which, being their excrement, is a fine manure for grain and grass. Worms prob- ably provide new soil for hills and slopes where the rain washes the earth away; and they affect slopes probably to avoid being flooded. .... The earth without worms would soon become cokl, hard- bound, and void of fermentation, and consequently sterile These hints we think proper to throw out, in order to set the inquisitive and discerning at work. A good monograph of worms would afford much entertainment and information at the same time, and would open a large and new field in natural history." The monograph that Gilbert White wished for in 1777 was pub- lished by Darwin in i88r, the year before he died — '' the completion," he said, "of a short paper read before the Geological Society more than forty years ago." With his characteristic thoroughness and patience he worked out the part that earthworms have played in the history of the earth, and proved that they deserve to be called the most useful animals. By their burrowing they loosen the earth, making way for the plant rootlets and the raindrops; by bruising the soil in their gizzards, they reduce the particles to more useful, powdery form; by burying the surface with castings brought up from beneath, they have been for untold ages ploughers before the plough, and by burying leaves they have made a great part of the vegetable mould over the whole earth. In illustration of the last point, we may notice that we recently found thirteen midribs of the leaves of the rowan, or mountain ash, radiating round one hole like the spokes of a wheel; the withering leaflets had been carried down, and two were sticking up at the mouth of the burrow; that meant 91 leaflets to one hole. Darwin showed that there often are 50,000 (and there may be 500,000) earthworms in an acre; that they often pass ten tons of soil per acre per annum through their bodies; and that they often cover the surface at the rate of three inches in fifteen years. Though our British worms only pass out about 20 oz. of earth in a year, the weights thrown up in a year on two separate square yards which Darwin watched were respectively 6 .75 lb. and 8 .387 lb., which correspond to 14^ and 18 tons per acre per annum. We follow the work further and it becomes evident that the con- stant exposure of the soil bacteria on the surface is bound to be important, on the one hand, in allowing them to be scattered by wind and rain on the other in exposing them to the beneficent action of the 214 READINGS IN EVOLUTION, GENETICS, AND EUGENICS sunlight — which is the most universal, effective, and economical of all germicides. In Yorubaland, on the West Coast of Africa, Mr. Alvan Millson calculated that about 62,233 tons of subsoil are brought every year to the surface of each square mile, and that every particle of earth, to the depth of two feet, is brought to the surface once in twenty-seven years. It need hardly be added that the district is fertile and healthy. Earthworms play their part in the disintegration of rocks, letting the solvent humus-acids of the soil down to the buried surface. Their castings on the hill-slopes are carried down by wind and rain and go to swell the alluvium of the distant valleys or the wasted treasures of the sea. The well-known parallel ledges along the slopes of grass-clad hills are partly due to earthworm castings caught on sheep- tracks, and thus we begin to connect the earthworms not only with our wheat- supply but wdth our scenery. Well may we say, with Darwin: ''It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organised creatures." Those who wish to understand Darwin- ism should always begin with Darwin's last book — The Formation of Vegetable Mould through the Action of Worms (1881). It illus- trates the web of life, the idea of which is essential to an understanding of the struggle for existence and natural selection. But it also illus- trates what Darwin had learned from Lyell — that great results may be brought about by accumulation of infinitesimal items. As Professor A. Milnes Marshall said: "The lesson to be derived from Darwin's life and work cannot be better expressed than as the cumulative im- portance of infinitely little things. ^^ Termites, or white ants. — Henry Drummond, in his Tropical Africa, tried to make out a case for the agricultural importance of termites, or white ants. It is well known that these old-fashioned insects have a pruning action in the forest, destroying dead wood with great rapidity. Houses and furniture, fences and boxes, as well as forest- trees, fall under their jaws. In some places, "if a man lay down to sleep with a wooden leg, it would be a heap of sawdust in the morning." But what of the termites' agricultural importance ? The point is that they keep the soil circulating by constructing earthen tunnels up the sides of trees and posts and by making huge obelisk-like ant-hills, or termitaries. "The earth-tubes crumble to dust, which is scattered by the wind; the rams lash the forests and soils with fury, and wash off the loosened grains to swell the alluvium of a distant BACKGROUND OF DARWINISM— THE WEB OF LIFE 215 valley." It must be noted, however, that Drummond did not prove his case with sufficient precision, and there is, as Eschcrich points out in his beautiful study of termites, this difficulty, that, while the cast- ings of earthworms are soft and loose, the earth-tubes and construc- tions of termites are stony. Escherich does, however, admit that the termites have some agricultural importance, and he points out that there are other serv- ices to be put to the credit side of their account. They prune off wood that has begun to go; they destroy rotting things, including the bodies of small animals; they make for cleanliness and health. In some low-lying tracts, as Silvestri has shown, there are dry stretches, "termite islands," which have been gradually built up from the broken-down remains of termitaries. Nor should it be forgotten that the white ants are often used as food. On the other hand, Escherich does not hesitate to rank them as among the great hindrances to the spread of civilisation. They insidiously devour everything wooden, from the telegraph-post to the wooden butt of the gun hanging against the wall, from books in the library to corks in the cellar. There does not seem sufficiently precise information in regard to the living plants that they attack, and no safe general statement can be made except that their appetite is large and catholic. With a centre in earthworms, what a variety of interests must be included within the radius of their life and work! — centipedes, birds, moles, seedlings, man. The same is true of termites, and two further illustrations may be given. Observers have reported about thirty different species of termites with the habit of feeding on fungi gro\\Ti within the termitary on specially constructed mazy beds. The habit is interesting in many ways; for instance, because the fungi afford a supply of nitrogenous material which is scarce in the ordinary diet of wood, and also because a similar habit occurs in the quite unrelated true ants. Finally, the web is illustrated by the numerous boarders, mostly beetles, that are found in the termitaries — not hostile intruders or parasites, but guests which are fed and cared for apparently for the sake of a palatable exudation with a pleasant, narcotising effect on the termites. With a centre in termites, what a variety of interests must we not include within the radius of their hfe and work I — fungi and trees, beetles and birds, lizards and anteaters, and man more than any. The hand of life upon the earth.— The hand of life has been working upon the earth for untold ages. T^ke plants, for instance. The seaweeds lessen the force of the waves, the lichens eat into the 2l6 READINGS IN EVOLUTION, GENETICS, AND EUGENICS rocks, the mosses form huge sponges on the moors which keep the streams flowing in days of drought. Many Httle plants are forever smoothing away the wrinkleson the earth's — their mother's — face, and they adorn her with jewels. Others that have formed coal have enriched her with ages of entrapped sunlight. The grass — which began to appear in Tertiary ages — ^protects the earth like a garment; the forests affect rainfall and temper climate, besides sheltering multitudes of living things, to many of whom every blow of the axe is a death- knell. No plant, from bacterium to oak-tree, lives or dies to itself, or is without its influence upon the earth. So among animals there are destructive borers and burrowers and conservative agents, such as the coral-polyps and the chalk-forming Foraminifera. Practical importance of a realisation of the web of life. — What has Darwinism to do with human Hfe ? The answer at this stage in our inquiry is clear: we must respect the web of life if we wish to master Nature. She must be humoured, not bullied. Emerson included in his vision of a perfected earth the absence of spiders, but the absence of spiders — which snare so many injurious insects — would mean the absence of much else, man probably included. In a northern county in Scotland the proprietors were justly annoyed at the injuries inflicted on young trees by squirrels, and they formed a squirrel club, setting a price on the beautiful rodent's head. Perhaps a wiser course would have been to begin by inquiring what disturbance of the balance of nature had allowed the squirrels to multiply so disastrously. But, after a period of squirrel-slaughter and some jubilation thereat, a cloud began to rise in the sky. The wood-pigeons were multiplying worse than ever, and the farmers, at least, said with no uncertain voice that they preferred the squirrels. An imperfect recognition of the web of life had left out of account the notable fact that squirrels destroy large numbers of young wood-pigeons. One of the hopeful symptoms of the last few years is the reawaken- ing of an interest in woods and forests. Everyone knows how terribly these have been wasted, and how the disastrous results have affected rainfall and irrigation, climate and crops, and even the character of the people. Here what was once a pleasant stream is now like a gravelly road, and there the fertile plains are flooded; here the wind is sweeping away the soil, and there both beauty and health have departed. The birds which the woods once sheltered are driven elsewhere, and the insect-pests are rife among the crops. For ''the cheapest and most effective insecticides are birds." BACKGROUND OF DARWINISM— THE WEB OF LIFE 217 The recognition of consequences — often far-reaching — grows with us as we work with the idea of the web of hfe, as we see in proper perspective the criminahty of those who are ruthless. President Roosevelt has declared his abomination of "the land-skinner" — ''the individual whose idea of developing the country is to cut every stick of timber off it, and then leave a barren desert for the home-maker who comes in after him. That man is a curse, and not a blessing to the country. The prop of the country must be the man who intends so to run his business that it will be profitable to his children after him." Every right-thinking man, and especially those who have grasped the idea of the web of life, will say with Roosevelt, "I am against the land- skinner every time." It may be said that man must exterminate a good deal if he is to go on peaceably with his business, and it will be admitted that there has never been a strong enthusiasm, humanitarian or otherwise, against the elimination of rattlesnakes, and such like. The natural- ist's answer is that every crusade should be carefully considered on its own merits, and that every careless and hasty destruction of life is to be condemned. Even in regard to snakes killing may be carried too far. Some creatures are, as it were, on the fringes of the web, while others occupy a position where many threads meet. It is scientifically and aesthetically deplorable that birds like the great auk and mammals like the quagga should have been exterminated, but it is practically much more deplorable that we have lost so many hawks and weasels and other members of that pertinacious army whose guerilla warfare keeps hundreds of more humdrum creatures up to the scratch, and keeps "vermin" from becoming a plague. Moreover, it is extremely difficult to tell what may be the consequences of exterminating any creature — remote as it may seem from the beaten track of human affairs. One of the obvious lessons of Darwinism is that we should be slow to call any change unimportant. Everything counts, or may count. A so-called unimportant animal is destroyed and no imme- diate ill effects are seen. But who can tell ? Very pertinent, for instance, is the question: What about the parasites that used to complete their life-history in romantic routine in this extinguished animal ? Have we extinguished the parasite also ? Or is it waiting, with a whip of scorpions, to chastise mankind for their ignorance of Darwinism ? The practical importance of recognising the web of life has been proved by the heavy penalties which man has often had to pay for 2i8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS disturbing the balance of nature, careless of results and ruthless of beauty, for not admitting that if we would master Nature we must first understand her. How much has Australia had to pay for the introduction of rabbits in i860, or America for sparrows ? Sometimes the introduction has been unconscious, and man has only to blame himself for letting the intruder take hold, as in the case of the Phyl- loxera in France, or of the Colorado Beetle in Ireland. "Ignorance of nature," Mr. A. H. S. Lucas says, "is costly. By disturbing the balance of nature, man has introduced foes into his own household." Speaking of Australia, he says: "How much is needed for the eradi- cation of Bathurst Burr, Prickly Pear, Water-hyacinth, Bramble and Sweetbriar, Codlin Moth, Waxy Scale, Pear Slug, and Red Spider, owing to carelessness or lack of knowledge in early days?" An obvious moral is that we should be careful in our introduc- tions of new organisms — -man included — 'into new surroundings. The primary consequences may be predictable, but the secondary and the tertiary consequences — who is sufficient for these things ? We have records of the unconscious introduction of rats into Jamaica, where they became a pest. To destroy them mongooses were imported, and the rats were soon checked. But the m^ongooses, having finished the rats, began to eat up the poultry and young birds of various kinds. As this went on the injurious insects and ticks, that the birds used to eat, began to gain the ascendant. A recent report — which requires confirmation — says that the increase of ticks is making life a burden to the mongooses. Thus a balance will be again arrived at. There is no doubt of that, but how much is often unnecessarily lost by the way! CHAPTER XVI NATURAL SELECTION CHARLES DARWIN Introductory Note. — This entire chapter is made up of carefully chosen passages from Darwin's Origin of Species. So much has falsely been callerl "Darwinism" that it is well for the reader to have a statement of Darwin's views in his own words. Every student of evokition should read the whole of the Origin of Species. It is all so good that one finds it diflticult to leave out anything. The following excerpts will, we believe, give the gist of natural selection. We present first certain of the ideas that underlie or are postulates of the theory; then the theory itself is presented; the theory of sexual selection inter- polated; and then follow examples of the way in which adaptations are accounted for by natural selection. Darwin's own statement of the most serious difficulties and objections to the theory, and his answers to these, bring this chapter to a close. FOUNDATION STONES OF NATURAL SELECTION darwin's own estimate as to the role of natural selection in evolution No one ought to feel surprised at much remaining as yet unex- plained in regard to the origin of species and varieties, if he make due allowance for our profound ignorance in regard to the mutual relations of the many beings which live around us. Who can explain why one species ranges widely and is very numerous, and why another aUied species has a narrow range and is rare ? Yet these relations are of the highest importance, for they determine the present welfare and, as I believe, the future success and modification of every inhabitant of this world. Still less do we know of the mutual relations of the innumer- able inhabitants of the world during the many past geological epochs in its history. Although much remains obscure, and will long remain obscure, I can entertain no doubt, after the most deliberate study and dispassionate judgment of which I am capable, that the view which most naturalists until recently entertained, and which I for- merly entertained — namely, that each species has been independently created — is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. Furthermore, I am con- vinced that Natural Selection has been the most important, l)ut not the exclusive, means of modification. 219 2 20 RE.\DINGS IN EVOLUTION, GENETICS, AND EUGENICS effects of habit and of the use or disuse of parts; correlated variation; inheritance Changed habits produce an inherited effect, as in the period of the flowering of plants when transported from one cHmate to another. With animals the increased use or disuse of parts has had a more marked influence; thus I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild-duck; and this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parents. The great and inherited development of the udders in cows and goats in countries where they are habitually milked, in comparison with these organs in other countries, is probably another instance of the effects of use. Not one of our domestic animals can be named which has not in some country drooping ears; and the view, which has been suggested that the drooping is due to disuse of the muscles of the ear, from the animals being seldom much alarmed, seems probable. Many laws regulate variation, some few of which can be dimly seen, and will hereafter be briefly discussed. I will here only allude to what may be called correlated variation. Important changes in the embryo or larva will probably entail changes in the mature animal. In monstrosities, the correlations between quite distinct parts are very curious; and many instances are given in Isidore Geoffroy St. Hilaire's great work on this subject. Breeders believe that long limbs are almost always accompanied by an elongated head. Some instances of correlation are quite whimsical: thus cats which are entirely white and have blue eyes are generally deaf; but it has been lately stated by Mr. Tait that this is confined to the males. Color and constitutional peculiarities go together, of which many remarkable cases could be given amongst animals and plants. From facts collected by Heu- singer, it appears that white sheep and pigs are injured by certain plants, whilst dark-colored individuals escape: Professor Wyman has recently communicated to me a good illustration of this fact; on ask- ing some farmers in Virginia how it was that all their pigs were black, they informed him that the pigs ate the paint-root (Lachnanthes) , which colored their bones pink, and which caused the hoofs of all but the black varieties to drop off; and one of the" crackers" (i.e., Virginia squatters) added, ''we select the black members of a litter for raising, as they alone have a good chance of living." Hairless dogs have imperfect teeth; long-haired and coarse-haired animals are apt to NATURAL SELECTION 221 have, as is asserted, long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet. Hence if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of correlation. darwin's idea of the causes responsible for the origin of domestic races To sum up on the origin of our domestic races of animals and plants. Changed conditions of life are of the highest importance in causing variability, both by acting directly on the organization, and indirectly by affecting the reproductive system. It is not probable that variability is an inherent and necessary contingent, under all circumstances. The greater or less force of inheritance and reversion determine whether variations shall endure. Variability is governed by many unknown laws, of which correlated growth is probably the most important. Something, but how much we do not know, may be attributed to the definite action of the conditions of life. Some, per- haps a great, effect may be attributed to the increased use or disuse of parts. The final result is thus rendered infinitely complex. In some cases the intercrossing of aboriginally distinct species appears to have played an important part in the origin of our breeds. When several breeds have once been formed in any country, their occasional intercrossing, with the aid of selection, has, no doubt, largely aided in the formation of new sub-breeds; but the importance of crossing has been much exaggerated, both in regard to animals and to those plants which are propagated by seed. With plants which are temporarily propagated by cuttings, buds, etc., the importance of crossing is immense; for the cultivator may here disregard the extreme variability both of hybrids and of mongrels, and the sterility of hybrids; but plants not propagated by seed are of little importance to us, for their endurance is only temporary. Over all these causes of Change, the accumulative action of Selection, whether apphed methodically and quickly, or unconsciously and slowly but more efficiently, seems to have been the predominant Power. darwin's idea of the origin of varieties, species, and genera in nature Finally, varieties cannot be distinguished from species — except, first, by the discovery of intermediate linking forms; and, secondly, by a certain indefinite amount of difference between them; for two 222 READINGS IN EVOLUTION, GENETICS, AND EUGENICS forms, if differing very little, are generally ranked as varieties, not- withstanding that they cannot be closely connected; but the amount of difference considered necessary to give to any two forms the rank of species cannot be defined. In genera having more than the average number of species in any country, the species of these genera have more than the average number of varieties. In large genera the species are apt to be closely, but unequally, allied together, forming little clusters round other species. Species very closely allied to other species apparently have restricted ranges. In all these respects the species of large genera present a strong analogy with varieties. And we can clearly understand these analogies, if species once existed as varieties, and thus originated; whereas, these analogies are utterly inexplicable if species are independent creations. We have, also, seen that it is the most flourishing or dominant species of the larger genera within each class which on an average yield the greatest number of varieties; and varieties, as we shall hereafter see, tend to become converted into new and distinct species. Thus the larger genera tend to become larger; and throughout nature the forms of life which are now dominant tend to become still more domi- nant by leaving many modified and dominant descendants. But by steps hereafter to be explained, the larger genera also tend to break up into smaller genera. And thus, the forms of life throughout the uni- verse become divided into groups subordinate to groups. THE TERM "struggle FOR EXISTENCE" USED IN A LARGE SENSE I should premise that I use this term in a large and metaphorical sense including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny. Two canine animals, in a time of dearth, may be truly said to struggle with each other which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the drought, though more properly it should be said to be dependent on the moisture. A plant which annually produces a thousand seeds, of which only one of an average comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. The mistletoe is dependent on the apple and a few other trees, but can only in a far-fetched sense be said to struggle with these trees, for, if too many of these parasites grow on the same tree, it languishes and dies. But several seedling mistletoes, growing close together on the same branch, NATURAL SELECTION 223 may more truly be said to struggle with each other. As the mistletoe is disseminated by birds, its existence depends on them; and it may metaphorically be said to struggle with other fruit-bearing i)lants, in tempting the birds to devour and thus disseminate its seeds. In these several senses, which pass into each other, I use for con\en- ience' sake the general term of Struggle for Existence. GEOMETRICAL RATIO OF INCREASE A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruc- tion during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for exist- ence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life. It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage. Although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them. NATURAL selection; OR THE SURVIVAL OF THE FITTEST How will the struggle for existence, briefly discussed in the last chapter, act in regard to variation ? Can the principle of selection, which we have seen is so potent in the hands of man, appl}' under nature ? I think we shall see that it can act most efficiently. Let the endless number of slight variations and individual differences occurring in our domestic productions, and, in a lesser degree, in those under nature, be borne in mind; as well as the strength of the hereditary tendency. Under domestication, it may be truly said that the whole organization becomes in some degree plastic. But the variability, which we almost universally meet w^ith in our domestic productions, is not directly produced, as Hooker and Asa Gray have well remarked, by man; he can neither originate varieties, nor prevent their occur- rence; he can only preserve and accumulate such as do occur. Unin- tentionally he exposes organic beings to new and changing conditions 224 READINGS IN EVOLUTION, GENETICS, AND EUGENICS of life, and variability ensues; but similar changes of conditions might and do occur under nature. Let it also be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life; and consequently what infinitely varied diversities of structure might be of use to each being under changing conditions of life. Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should occur in the course of many successive generations ? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind ? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favorable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection, or the Survival of the Fittest. Variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions. Several writers have misapprehended or objected to the term Natural Selection. Some have even imagined that natural selection induces variability, whereas it implies only the preservation of such variations as arise and are beneficial to the being under its conditions of life. No one objects to agriculturists speaking of the potent effects of man's selection; and in this case the individual differences given by nature, which man for some object selects, must of necessity first occur. Others have objected that the term selection implies conscious choice in the animals which become modified ; and it has even been urged that, as plants have no voUtion, natural selection is not applic- able to them! In the literal sense of the word, no doubt, natural selection is a false term; but who ever objected to chemists speaking of the elective affinities of the various elements? — and yet an acid cannot strictly be said to elect the base with which it in preference combines. It has been said that I speak of natural selection as an active power or Deity; but who objects to an author speaking of the attraction of gravity as ruling the movements of the planets ? Every- one knows what is meant and is implied by such metaphorical expres- NATURAL SELECTION 225 sions; and they are almost necessary for brevity. So again it is difficult to avoid personifying the word Nature; but I mean by Nature only the aggregate action and product of many natural laws, and by laws the sequence of events as ascertained by us. With a little famili- arity such superficial objections will be forgotten. We shall best understand the probable course of natural selection by taking the case of a country undergoing some slight physical change, for instance, of climate. The proportional numbers of its inhabitants will almost immediately undergo a change, and some species will prob- ably become extinct. We may conclude, from what we have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical pro- portions of the inhabitants, independently of the change of climate itself, would seriously affect the others. If the country were open on its borders, new forms would certainly immigrate, and this would likewise seriously disturb the relations of some of the former inhabi- tants. Let it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be. But in the case of an island, or of a country partly surrounded by barriers, into which new and better adapted forms could not freely enter, we should then have places in the economy of nature which would assuredly be better filled up, if some of the original inhabitants were in some manner modified; for, had the area been open to immigration, these same places would have been seized on by intruders. In such cases, slight modifications, which in any way favored the individuals of any species by better adapting them to their altered conditions, would tend to be preserved; and natural selection would have free scope for the work of improvement. We have good reason to believe, as shown in the first chapter, that changes in the conditions of life give a tendency to increased variability and in the foregoing cases the conditions have changed, and this would manifestly be favorable to natural selection, by affording a better chance of the occurrence of profitable variations. Unless such occur, natural selection can do nothing. Under the term of "variations," it must never be forgotten that mere individual differences are included. As man can produce a great result with his domestic animals and plants by adding up in any given direction individual differences, so could natural selection, but far more easily from having incomparably longer time for action. Nor do I believe that any great physical change, as of climate, or any unusual degree of isolation to check immigration. 2 26 READINGS IN EVOLUTION, GENETICS, AND EUGENICS is necessary in order that new and unoccupied places should be left, for natural selection to fill up by improving some of the varying inhabitants. For as all the inhabitants of each country are struggling together with nicely balanced forces, extremely slight modifications in the structure or habits of one species would often give it an advantage over others; and still further modifications of the same kind would often still further increase the advantage, as long as the species con- tinued under the same conditions of life and profited by similar means of subsistence and defense. No country can be named in which all the native inhabitants are now so perfectly adapted to each other and to the physical conditions under which they live, that none of them could be still better adapted or improved; for in all countries, the natives have been so far conquered by naturalized productions, that they have allowed some foreigners to take firm possession of the land. And as foreigners have thus in every country beaten some of the natives, we may safely conclude that the natives might have been modified with advantage, so as to have better resisted the intruders. As man can produce, and certainly has produced, a great result by his methodical and unconscious means of selection, what may not natural selection effect? Man can act only on external and visible characters: Nature, if I may be allowed to personify the natural pres- ervation or survival of the fittest, cares nothing for appearances, except in so far as they are useful to any being. She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. Man selects only for his own good : Nature only for that of the being which she tends. Every selected character is fully exercised by her, as is implied by the fact of their selection. Man keeps the natives of many climates in the same country; he seldom exercises each selected character in some peculiar and fitting manner; he feeds a long- and a short-beaked pigeon on the same food; he does not exercise a long-backed or long-legged quadruped in any peculiar manner; he exposes sheep with long and short wool to the same climate. He does not allow the most vigorous males to struggle for the females. He does not rigidly destroy all inferior animals, but protects during each varying season, as far as lies in his power, all his productions. He often begins his selection by some half-monstrous form; or at least by some modification prominent enough to catch the eye or to be plainly useful to him. Under nature, the slightest differ- ences of structure or constitution may well turn the nicely balanced scale in the struggle for life, and so be preserved. How fleeting are the NATURAL SELF.CTION 227 wishes and efforts of man! how short his time! and consequently how poor will be his results, compared with those accumulated by Nature during whole geological periods! Can we wonder, then, that Nature's productions should be far" truer "in character than man's productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship ? It may metaphorically be said that natural selection is daily and hourly scrutinizing, throughout the world, the slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working whenever and wherever oppor- tunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the lapse of ages, and then so imperfect is our view into long-past geological ages, that we see only that the forms of life are now different from what they formerly were. In order that any great amount of modification should be effected in a species, a variety when once formed must again, perhaps after a long interval of time, vary or present individual differences of the same favorable nature as before; and these must be again preserved, and so onwards step by step. Seeing that individual differences of the same kind perpetually recur, this can hardly be considered as an unwarrantable assumption. But whether it is true, we can judge only by seeing how far the hypothesis accords with and explains the general phenomena of nature. On the other hand, the ordinary belief that the amount of possible variation is a strictly limited quantity is like- wise a simple assumption. Although natural selection can act only through and for the good of each being, yet characters and structures, which we are apt to con- sider as of very trifling importance, may thus be acted on. When we see leaf-eating insects green, and bark-feeders mottled-gray; the alpine ptarmigan white in winter, the red-grouse the color of heather, we must believe that these tints are of service to these birds and insects in preserving them from danger. Grouse, if not destroyed at some period of their lives, would increase in countless numbers; they are known to suffer largely from birds of prey; and hawks are guided by eyesight to their prey — so much so, that on parts of the Continent persons are warned not to keep white pigeons, as being the most liable to destruction. Hence natural selection might be effective in giving 22S READINGS IN EVOLUTION, GENETICS, AND EUGENICS the proper color to each kind of grouse, and in keeping that color, when once acquired, true and constant. Nor ought we to think that the occasional destruction of an animal of any particular color would produce little effect: we should remember how essential it is in a flock of white sheep to destroy a lamb with the faintest trace of black. We have seen how the color of the hogs, which feed on the '^paint-root'* in Virginia, determines whether they shall live or die. In plants, the down on the fruit and the color of the flesh are considered by botanists as characters of the most trifling importance: yet we hear from an excellent horticulturist. Downing, that in the United States smooth- skinned fruits suffer far more from a beetle, a Curculio, than those with down; that purple plums suffer far more from a certain disease than yellow plums; whereas another disease attacks yellow-fleshed peaches far more than those with other colored flesh. If, with all the aids of art, these slight differences make a great difference in cultivating the several varieties, assuredly, in a state of nature, where the trees would have to struggle with other trees and with a host of enemies, such dif- ferences would effectually settle which variety, whether a smooth or downy, a yellow- or purple-fleshed fruit, should succeed. In looking at many small points of difference between species, which, as far as our ignorance permits us to judge, seem quite unim- portant, we must not forget that climate, food, etc., have no doubt produced some direct effect. It is also necessary to bear in mind that, owing to the law of correlation, when one part varies, and the varia- tions are accumulated through natural selection, other modifications, often of the most unexpected nature, will ensue. As we see that those variations which, under domestication, appear at any particular period of life, tend to reappear in the offspring at the same period; for instance, in the shape, size, and flavor of the seeds of the many varieties of our culinary and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silk-worm ; in the eggs of poultry, and in the color of the down of their chickens; in the horns of our sheep and cattle when nearly adult; so in a state of nature natural selection will be enabled to act on and modify organic beings at any age, by the accumulation of variations profitable at that age, and by their inheritance at a corresponding age. If it profit a plant to have its seeds more and more widely disseminated by the wind, I can see no greater difficulty in this being effected through natural selection, than in the cotton-planter increasing and improving by selection the down in the pods on his cotton-trees. Natural NATURAL SELECTION 229 selection may modify and adapt the larva of an insect to a score of contingencies, wholly different from those which concern the mature insect; and these modifications may affect, through correlation, the structure of the adult. So, conversely, modifications in the adult may affect the structure of the larva; but in all cases natural selection will ensure that they shall not be injurious: for if they were so, the species would become extinct. Natural selection will modify the structure of the young in relation to the parent, and of the parent in relation to the young. In social animals it will adapt the structure of each individual for the benefit of the whole community, if the community profits by the selected change. What natural selection cannot do, is to modify the structure of one species, without giving it any advantage, for the good of another species; and though statements to this effect may be found in works of natural history, I cannot find one case which will bear investigation. A structure used only once in an animal's life, if of high importance to it, might be modified to any extent by natural selection ; for instance the great jaws possessed by certain insects, used exclusively for open- ing the cocoon — or the hard tip to the beak of unhatched birds, used for breaking the egg. It has been asserted, that of the best short- beaked tumbler-pigeons a greater number perish in the egg than are able to get out of it; so that fanciers assist in the act of hatching. Now if nature had to make the beak of a full-grown pigeon very short for the bird's own advantage, the process of modification would be very slow, and there would be simultaneously the most rigorous selection of all the young birds within the egg, which had the most powerful and hardest beaks, for all with weak beaks would inevitably perish; or, more delicate and more easily broken shells might be selected, the thickness of the shell being known to vary like every other structure. It may be well here to remark that with all beings there must be much fortuitous destruction, which can have little or no influence on the course of natural selection. For instance a vast number of eggs or seeds are annually devoured, and these could be modified through natural selection only if they varied in some manner which protected them from their enemies. Yet many of these eggs or seeds would perhaps, if not destroyed, have yielded individuals better adapted to their conditions of life than any of those which happened to survive. So again a vast number of mature animals and plants, whether or not they be the best adapted to their conditions, must be annually 230 READINGS IX EVOLUTION, GENETICS, AND EUGENICS destroyed by accidental causes, which would not be in the least degree mitigated by certain changes of structure or constitution which w^ould in other ways be beneficial to the species. But let the destruction of the adults be ever so heavy, if the number which can exist in any district be not wholly kept down by such causes, or again let the destruction of eggs or seeds be so great that only a hundredth or a thousandth part are developed, yet of those which do survive, the best adapted individuals, supposing that there is any variability in a favorable direction, will tend to propagate their kind in larger numbers than the less well adapted. If the numbers be wholly kept down by the causes just indicated, as will often have been the case, natural selection will be powerless in certain beneficial directions; but this is no valid objection to its efficiency at other times and in other ways; for we are far from having any reason to suppose that many species ever undergo modification and improvement at the same time in the same area. SEXUAL SELECTION Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to that sex, so no doubt it will be under nature. Thus it is rendered possible for the two sexes to be modified through natural selection in relation to different habits of life, as is sometimes the case; or for one sex to be modified in relation to the other sex, as commonly occurs. This leads me to say a few words on what I have called Sexual Selection. This form of selection depends, not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the indi- viduals of one sex, generally the males, for the possession of the other sex. The result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection. Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny. But in many cases, victory depends not so much on general vigor, as on having special weapons, confined to the male sex. A hornless stag or spurless cock would have a poor chance of leaving numerous offspring. Sexual selection, by always allowing the victor to breed, might surely give indomitable courage, length to the spur, and strength to the wing to strike in the spurred leg, in nearly the same manner as does the brutal cock-fighter by the careful selection of his best cocks. How low in the scale of nature the law of battle descends, I know not; male alligators have been described as fighting, bellowing, and whirl- NATURAL SELECTION 231 ing around, like Indians in a war-dance, for the possession of the females; male salmons have been observed fighting all day long; male stag-beetles sometimes bear wounds from the huge mandibles of other males; the males of certain hymenopterous insects have been fre- quently seen by that inimitable observer M. Fabre, fighting for a particular female who sits by, an apparently unconcerned beholder of the struggle, and then retires with the conqueror. The war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons. The males of carnivorous animals are already well armed; though to them and to others, special means of defense may be given through means of sexual selection, as the mane of the lion, and the hooked jaw to the male salmon; for the shield may be as important for victory as the sword or spear. Amongst birds, the contest is often of a more peaceful character. All those who have attended to the subject believe that there is the severest rivalry between the males of many species to attract, by singing, the females. The rock-thrush of Guiana, birds of paradise, and some others, congregate; and successive males display with the most elaborate care, and show off in the best manner, their gorgeous plumage; they likewise perform strange antics before the females, which, standing by as spectators, at last choose the most attractive partner. Those who have closely attended to birds in confinement well know that they often take individual preferences and dislikes: thus Sir R. Heron has described how a pied peacock was eminently attractive to all his hen birds. I cannot here enter on the necessary details; but if man can in a short time give beauty and an elegant carriage to his bantams, according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked etTect. Some well-known laws, with respect to the plumage of male and female birds, in comparison with the plumage of the young, can paflly be explained through the action of sexual selection on variations occurring at different ages, and transmitted to the males alone or to both sexes at corresponding ages; but I have not space here to enti?r on this subject. Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but ditler in structure, color, or ornament, such dift'erences have been mainly caused by sexual 232 READINGS IN EVOLUTION, GENETICS, AND EUGENICS selection: that is, by individual males having had, in successive gen- erations, some slight advantage over other males, in their weapons, means of defence, or charms, which they have transmitted to their male offspring alone. Yet, I would not wish to attribute all sexual differences to this agency: for we see in our domestic animals peculi- arities arising and becoming attached to the male sex, which appar- ently have not been augmented through selection by man. The tuft of hair on the breast of the wild turkey-cock cannot be of any use, and it is doubtful whether it can be ornamental in the eyes of the female bird; indeed, had the tuft appeared under domestication, it would have been called a monstrosity. ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE SURVIVAL OF THE FITTEST In order to make it clear how, as I believe, natural selection acts, I must beg permission to give one or two imaginary illustrations. Let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness; and let us suppose that the fleetest prey, a deer for instance, had from any change in the country increased in numbers, or that other prey had decreased in numbers, during that season of the year when the wolf was hardest pressed for food. Under such circumstances the swiftest and slimmest wolves would have the best chance of surviving and so be preserved or selected, provided always that they retained strength to master their prey at this or some other period of the year, when they were compelled to prey on other animals. I can see no more reason to doubt that this would be the result, than that man should be able to improve the fleetness of his greyhounds by careful and methodical selection, or by that kind of unconscious selection which follows from each man trying to keep the best dogs without any thought of modifying the breed. I may add, that, according to Mr. Pierce, there are two varieties of the wolf inhabiting the Catskill Mountains, in the United States, one with a light greyhound-like form, which pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd's flocks. It should be observed that, in the above illustration, I speak of the slimmest individual wolves, and not of any single strongly marked variation having been preserved. In former editions of this work I sometimes spoke as if this latter alternative had frequently occurred. I saw the great importance of individual differences, and this led me fully to discuss the results of unconscious selection by man, which NATURAL SELECTION 233 depends on the preservation of all the more or less valuable individuals, and on the destruction of the worst. I saw, also, that the preservation in a state of nature of any occasional deviation of structure, such as a monstrosity, would be a rare event; and that, if at first preserved, it would generally be lost by subsequent intercrossing with ordinary individuals. Nevertheless, until reading an able and valuable article in the North British Review (1867), I did not appreciate how rarely single variations, whether slight or strongly marked, could be per- petuated. The author takes the case of a pair of animals, producing during their lifetime two hundred offspring, of which, from various causes of destruction, only two on an average survive to pro-create their kind. This is rather an extreme estimate for most of the higher animals, but by no means so for many of the lower organisms. He then shows that if a single individual were born, which varied in some manner, giving it twice as good a chance of life as that of the other individuals, yet the chances would be strongly against its survival. Supposing it to survive and to breed, and that half its young inherited the favorable variation; still, as the Reviewer goes on to show, the young would have only a slightly better chance of surviving and breed- ing; and this chance would go on decreasing in the succeeding genera- tions. The justice of these remarks cannot, I think, be disputed. If, for instance, a bird of some kind could procure its food more easily by having its beak curved, and if one were born with its beak strongly curved, and which consequently flourished, nevertheless there would be a very poor chance of this one individual perpetuating its kind to the exclusion of the common form; but there can hardly be a doubt, judging by what we see taking place under domestication, that this result would follow from the preservation during many generations of a large number of individuals with more or less strongly curved beaks, and from the destruction of a still larger number with the straightest beaks. SUMMARY OF CHAPTER ON NATURAL SELECTION If under changing conditions of life organic beings present indi- vidual differences in almost every part of their structure, and this cannot be disputed; if there be, owing to their geometrical rale of increase, a severe struggle for life at some age, season, or year, and this certainly cannot be disputed; then, considering the infinite com- plexity of the relations of all organic beings to each other and to their conditions of life, causing an infinite diversity in structure, constitu- tion, and habits, to be advantageous to them, it would be a most 234 READINGS IN EVOLUTION, GENETICS, AND EUGENICS extraordinary fact if no variations had ever occurred useful to each being's own welfare, in the same manner as so many variations have occurred useful to man. But if variations useful to any organic being ever do occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance, these will tend to produce offspring similarly characterized. This principle of preservation, or the survival of the fittest, I have called Natural Selection. It leads to the im- provement of each creature in relation to its organic and inorganic conditions of life; and consequently, in most cases, to what must be regarded as an advance in organization. Nevertheless, low and simple forms will long endure if well fitted for their simple conditions of life. Natural selection, on the principle of qualities being inherited at corresponding ages, can modify the egg, seed, or young, as easily as the adult. Amongst many animals, sexual selection will have given its aid to ordinary selection, by assuring to the most vigorous and best adapted males the greatest number of offspring. Sexual selection will also give characters useful to the males alone, in their struggles or rivalry with other males; and these characters will be transmitted to one sex or to both sexes, according to the form of inheritance which prevails. Whether natural selection has really thus acted in adapting the various forms of life to their several conditions and stations, must be judged by the general tenor and balance of evidence given in the follow- ing chapters. But we have already seen how it entails extinction; and how largely extinction has acted in the world's history, geology plainly declares. Natural selection, also, leads to divergence of character; for the more organic beings diverge in structure, habits, and constitu- tion, by so much the more can a large number be supported on the area, of which we see proof by looking to the inhabitants of any small spot, and to the productions naturalized in foreign lands. There- fore, during the modification of the descendants of any one species, and during the incessant struggle of all species to increase in number, the more diversified the descendants become, the better will be their chance of success in the battle for life. Thus the small differences dis- tinguishing varieties of the same species, steadily tend to increase, till they equal the greater differences between species of the same genus, or even of distinct genera. We have seen that it is the common, the widely-diffused and widely ranging species, belonging to the larger genera within each NATURAL SELECTION 2.^5 class, which vary most; and these tend to transmit to their modified offspring that superiority which now makes them dominant in their own countries. Natural selection, as has just been remarked, leads to divergence of character and to much extinction of the less improved and intermediate forms of life. On these principles, the nature of the affinities, and the generally well-defined distinctions between the innumerable organic beings in each class throughout the world, may be explained. It is a truly wonderful fact — the wonder of which we are apt to overlook from familiarity — that all animals and all plants throughout all time and space should be related to each other in groups subordinate to groups, in the manner which we everywhere behold — namely, varieties of the same species rqost closely related, species of the same genus less closely and unequally related, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes and classes. The several subordinate groups in any class cannot be ranked in a single file, but seem clustered round points, and these round other points, and so on in almost endless cycles. If species had been independently created, no explanation would have been possible of this kind of classification; but it is explained through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram. The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during former years may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have at all times overmastered other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was young, budding twigs; and this connection of the former and present buds by ramifying branches may well represent the classifica- tion of all extinct and living species in groups subordinate to groups. Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear the other branches; so with the species which lived during long-past geological periods, very few have left living and modified descendants. 236 READINGS IN EVOLUTION, GENETICS, AND EUGENICS From the first growth of the tree, many a Hmb and branch has decayed and dropped off; and these fallen branches of various sizes may repre- sent those whole orders, families, and genera which have now no living representatives, and which are known to us only in a fossil state. As we here and there see a thin straggling branch springing from a fork low down in a tree, and which by some chance has been favored and is still alive on its summit, so we occasionally see an animal like the Ornithorh}mchus or Lepidosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications. DIFFICULTIES AND OBJECTIONS TO NATURAL SELECTION AS SEEN BY DARWIN Long before the reader has arrived at this part of my work, a crowd of difficulties will have occurred to him. Some of them are so serious that to this day I can hardly reflect on them without being in some degree staggered; but, to the best of my judgment, the greater number are only apparent, and those that are real are not, I think, fatal to the theory. These difficulties and objections may be classed under the follow- ing heads: First, why, if species have descended from other species by fine gradations, do we not everywhere see innumerable, transitional forms? Why is not all nature in confusion, instead of the species being, as we see them, well defined ? Secondly, is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the modifica- tion of some other animal with widely different habits and structure ? Can we believe that natural selection could produce, on the one hand, an organ of trifling importance, such as the tail of a giraffe, which serves as a fly-flapper, and, on the other hand, an organ so wonderful as the eye ? Thirdly, can instincts be acquired and modified through natural selection ? What shall we say to the instinct which leads the bee to make cells, and which has practically anticipated the discoveries of profound mathematicians ? NATURAL SELECTION 237 Fourthly, how can we account for species, when crossed, being sterile and producing sterile offspring, whereas, when varieties are crossed, their fertility is unimpaired ? ANSWER TO THE FIRST DIFFICULTY On the Absence or Rarity of Transitional Varieties, — As natural selection acts solely by the preservation of profitable modifications, each new form will tend in a fully stocked country to take the place of, and finally to exterminate, its own less improved parent-form and other less-favored forms with which it comes into competition. Thus extinction and natural selection go hand in hand. Hence, if we look at each species as descended from some unknown form, both the parent and all the transitional varieties will generally have been exterminated by the very process of the formation and perfection of the new form. But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth? It will be more convenient to discuss this question in the chapter on the Imperfection of the Geological Record; and I will here only state that I beheve the answer mainly lies in the record being incomparably less perfect than is generally supposed. The crust of the earth is a vast museum; but the natural collections have been imperfectly made, and only at long intervals of time. ANSWER TO THE SECOND DIFFICULTY: ORGANS OF EXTREME PERFECTION AND COMPLICATION To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree. When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any ani- mal under changing conditions of life, then the difficulty of believing 238 READINGS IN EVOLUTION, GENETICS, AND EUGENICS that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but I may remark that, as some of the lowest organisms, in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggre- gated and developed into nerves, endowed with this special sensi- bility. In searching for the gradations through which an organ in any species has been perfected, we ought to look exclusively to its lineal progenitors; but this is scarcely ever possible, and we are forced to look to other species and genera of the same group, that is to the collateral descendants from the same parent-form, in order to see what gradations are possible, and for the chance of some gradations hav- ing been transmitted in an unaltered or little altered condition. But the state of the same organ in distinct classes may incidentally throw light on the steps by which it has been perfected. The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-Cells and covered by translucent skin, but without any lens or other refractive body. We may, however, according to M. Jourdain, descend even a step lower and find aggre- gates, of pigment-cells, apparently serving as organs of vision, without any nerves, and resting merely on sarcodic tissue. Eyes of the above simple nature are not capable of distinct vision, and serve only to dis- tinguish light from darkness. In certain star-fishes, small depressions in the layer of pigment which surrounds the nerve are filled, as de- scribed by the author just quoted, with transparent gelatinous matter, projecting with a convex surface, like the cornea in the higher animals. He suggests that this serves not to form an image, but only to con- centrate the luminous rays and render their perception more easy. In this concentration of the rays we gain the first and by far the most important step towards the formation of a true, picture-forming eye; for we have only to place the naked extremity of the optic nerve, which in some of the lower animals lies deeply buried in the body, and in some near the surface, at the right distance from the concentrating apparatus, and an image will be formed on it. In the great class of the Articulata, we may start from an optic nerve simply coated with pigment, the latter sometimes forming a sort NATURAL SELECTION 239 of pupil, but destitute of a lens or other optical contrivance. With insects it is now known that the numerous facets on the cornea of their great compound eyes form true lenses, and that the cones include curiously modified nervous filaments. But these organs in the Articulata are so much diversified that Muller formerly made three main classes with seven subdivisions, besides a fourth main class of aggregated simple eyes. When we reflect on these facts, here given much too briefly, with respect to the wide, diversified, and graduated range of structure in the eyes of the lower animals; and when we bear in mind how small the number of all living forms must be in comparison with those which have become extinct, the difliculty ceases to be very great in believing that natural selection may have converted the simple apparatus of an optic nerve, coated with pigment and invested by transparent mem- brane, into an optical instrument as perfect as is possessed by any member of the Articulate Class. He who will go thus far, ought not to hesitate to go one step fur- ther, if he finds on finishing this volume that large bodies of facts, otherwise inexplicable, can be explained by the theory of modification through natural selection; he ought to admit that a structure even as perfect as an eagle's eye might thus be formed, although in this case he does not know the transitional states. It has been objected that in order to modify the eye and still preserve it as a perfect instrument, many changes would have to be effected simultaneously, which, it is assumed, could not be done through natural selection; but as I have attempted to show in my work on the variation of domestic animals, it is not necessary to suppose that the modifications were all simulta- neous, if they were extremely slight and gradual. Different kinds of modification would, also, serve for the same general purpose: as Mr. Wallace has remarked, "if a lens has too short or too long a focus, it may be amended either by an alteration of curvature, or an alteration of density; if the curvature be irregular, and the rays do not converge to a point, then any increased regularity of curvature will be an improvement. So the contraction of the iris and the muscular movements of the eye are neither of them essential to vision, but only improvements which might have been added and perfected at any stage of the construction of the instrument.'' Within the highest division of the animal kingdon, namely, the Vertebrata, we can start from an eye so simple, that it consists, as in the lancelet, of a little 240 READINGS IN EVOLUTION, GENETICS, AND EUGENICS ck of transparent skin, furnished with a nerve and Hned with pig- ment, but destitute of any other apparatus. In fishes and reptiles, as Owen has remarked, " the range of gradations of dioptric structures is very great. " It is a significant fact that even in man, according to the high authority of Virchow, the beautiful crystalline lens is formed in the embryo by an accumulation of epidermic cells, lying in a sack- like fold of the skin; and the vitreous body is formed from embryonic sub-cutaneous tissue. To arrive, however, at a just conclusion regarding the formation of the eye, with all its marvellous yet not absolutely perfect characters, it is indispensable that the reason should conquer the imagination; but I have felt the difficulty far too keen to be surprised at others hesitating to extend the principle of natural selection to so startling a length. It is scarcely possible to avoid comparing the eye with a telescope. We know that this instrument has been perfected by the long- continued efforts of the highest human intellects; and we naturally infer that the eye has been formed by a somewhat analogous process. But may not this inference be presumptuous ? Have we any right to assume that the Creator works by intellectual powers like those of man ? If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with spaces filled with fluid, and with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thick- nesses, placed at different distances from each other, and with the sur- faces of each layer slowly changing in form. Further we must suppose that there is a power, represented by natural selection or the survival of the fittest, always intently watching each slight alteration in the transparent layers; and carefully preserving each which, under varied circumstances, in any way or in any degree, tends to produce a dis- tincter image. We must suppose each new state of the instrument to be multiplied by the million; each to be preserved until a better one is produced, and then the old ones to be all destroyed. In living bodies, variation will cause the slight alterations, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man ? NATURAL SELECTION 241 Darwin's summary of his answer to the third difficulty, that of accounting FOR THE acquisition AND MODIFICATION OF INSTINCTS through natural SELECTION I have endeavored in this chapter briefly to show that the mental quahties of our domestic animals vary, and that the variations are inherited. Still more briefly I have attempted to show that instincts vary slightly in a state of nature. No one will dispute that instincts are of the highest importance to each animal. Therefore there is no real difficulty, under changing conditions of life, in natural selection accumulating to any extent slight modifications of instinct which are in any way useful. In many cases habit or use and disuse have prob- ably come into play. I do not pretend that the facts given in this chapter strengthen in any great degree my theory; but none of the cases of difficulty, to the best of my judgment, annihilate it. On the other hand, the fact that instincts are not always absolutely perfect and are liable to mistakes: that no instinct can be shown to have been produced for the good of other animals, though animals take advantage of the instincts of others; that the canon in natural history, of '^Natura non facit saltum," is applicable to instincts as well as to cor- poreal structure, and is plainly explicable on the foregoing views, but is otherwise inexplicable, all tend to corroborate the theory of natural selection. This theory is also strengthened by some few other facts in regard to instincts; as by that common case of closely allied, but distinct, species, when inhabiting distant parts of the world and living under considerably different conditions of life, yet often retaining nearly the same instincts. For instance, we can understand, on the principle of inheritance, how it is that the thrush of tropical South America lines its nest with mud, in the same pecuHar manner as does our British thrush; how it is that the Hornbills of Africa and India have the same extraordinary instinct of plastering up and imprisoning the females in a hole in a tree, with only a small hole left in the plaster through which the males feed them and their young when hatched; how it is that the male wrens (Troglodytes) of North America build "cock- nests," to roost in, like the males of our Kitty-wrens, a habit wholly unlike that of any other known bird. Finally, it may not be a logical deduction, but to my imagination it is far more satisfactory to look at such instincts as the young cuckoo ejecting its foster-brothers, ants making slaves, the larvae of ichneumonidae feeding within the live bodies of caterpillars, not as specially endowed or created 242 READINGS IN EVOLUTION, GENETICS, AND EUGENICS instincts, but as small consequences of one general law leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die. darwin's summary of his answer to the difficulty as to the inability of NATURAL selection TO ACCOUNT FOR THE FACT THAT SPECIES WHEN CROSSED ARE STERILE OR PRODUCE STERILE OFFSPRING, WHEREAS WHEN VARIETIES ARE CROSSED THEIR FERTILITY IS UNIMPAIRED First crosses between forms, sufficiently distinct to be ranked as species, and their hybrids, are very generally, but not universally sterile. The sterility is of all degrees, and is often so slight that the most careful experimentalists have arrived at diametrically opposite conclusions in ranking forms by this test. The sterility is innately variable in individuals of the same species, and is eminently suscept- ible to the action of favorable and unfavorable conditions. The degree of sterility does not strictly follow systematic affinity, but is governed by several curious and complex laws. It is generally different, and sometimes widely different in reciprocal crosses between the same two species. It is not always equal in degree in a first cross and in the hybrids produced from this cross. In the same manner as in grafting trees, the capacity in one species or variety to take on another, is incidental on differences, generally of an unknown nature, in their vegetative systems, so in crossing, the greater or less facility of one species to unite with another is incidental on unknown differences in their reproductive systems. There is no more reason to think that species have been specially endowed with various degrees of sterility to prevent their crossing and blending in nature, than to think that trees have been specially endowed with various and somewhat analogous degrees of difficulty in being grafted together in order to prevent their inarching in our forests. The sterility of first crosses and of their hybrid progeny has not been acquired through natural selection. In the case of first crosses it seems to depend on several circumstances; in some instances in chief part on the early death of the embryo. In the case of hybrids, it apparently depends on their whole organization having been dis- turbed by being compounded from two distinct forms; the sterility being closely allied to that which so frequently affects pure species, when exposed to new and unnatural conditions of life. He who will explain these latter cases will be able to explain the sterility of hybrids. This view is strongly supported by a parallelism of another k,ind: namely, that, firstly, slight changes in the conditions of life add to the NATUR.\L SELECTION 243 vigor and fertility of all organic beings; and secondly, that the cross- ing of forms, which have been exposed to slightly different conditions of life or which have varied, favors the size, vigor, and fertility of their offspring. The facts given on the sterility of the illegitimate unions of dimorphic and trimorphic plants and of their illegitimate progeny, perhaps render it probable that some unknown bond in all cases con- nects the degree of fertility of first unions with that of their offspring. The consideration of these facts on dimorphism, as well as of the results of reciprocal crosses, clearly leads to the conclusion that the primary cause of the sterility of crossed species is confined to differences in their sexual elements. But why, in the case of distinct species, the sexual elements should so generally have become more or less modified, lead- ing to their mutual infertility, we do not know; but it seems to stand in some close relation to species having been exposed for long periods of time to nearly uniform conditions of life. It is not surprising that the difficulty in crossing any two species, and the sterility of their hybrid offspring, should in most cases corre- spond, even if due to distinct causes: for both depend on the amount of difference between the species which are crossed. Nor is it sur- prising that the facility of effecting a first cross, and the fertility of the hybrids thus produced, and the capacity of being grafted together — though this latter capacity evidently depends on widely different cir- cumstances — should all run, to a certain extent, parallel with the systematic affinity of the forms subjected to experiment; for system- atic affinity includes resemblances of all kinds. First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their mongrel offspring, are very generally, but not, as is so often stated, invariably fertile. Nor is this almost universal and perfect fertiUty surprising, when it is remembered how liable we are to argue in a circle with respect to varieties in a state of nature; and when we remember that the greater number of varieties have been produced under domestication by the selection of mere external differences, and that they have not been long exposed to uniform conditions of life. It should also be espe- cially kept in mind that long-continued domestication tends to elimi- nate sterility, and is therefore little likely to induce this same quality. Independently of the question of fertility, in all other respects there is the closest general resemblance between hybrids and mongrels, in their variability, in their power of absorbing each other by repeated crosses, and in their inheritance of characters from both parent-forms. 244 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Finally, then, although we are as ignorant of the precise cause of the sterility of first crosses and of hybrids as we are why animals and plants removed from their natural conditions become sterile, yet the facts given in this chapter do not seem to me opposed to the belief that species aboriginally existed as varieties. CHAPTER XVII CRITIQUE OF DARWINISM [The last chapter dealt with the central ideas of Darwin as told by himself. Some of the chief objections to the theory were also presented as Darwin saw them, and his own answers to these objections were given. These four objections are not by any means all that Darwin foresaw, for he presented in another chapter a discussion of "Miscel- laneous Objections to the Theory of Natural Selection." Before entering upon a general criticism of Darwinism, it would be advanta- geous to have before us a brief and pointed summary of Darwin's theory — natural selection — now known technically as Darwinism. The writer knows of no better short statement of the true content of Darwinism than the following summary by Professor Vernon L. Kellogg. — Ed.] SUMMARY OF DARWIN'S NATURAL-SELECTION THEORY' VERNON L. KELLOGG Darwinism may be defined as a certain rational, causo-mechanical (hence, non-teleologic) explanation of the origin of new species. The Darwinian explanation rests on certain observed facts, and certain inductions from these facts. The observed facts are: (i) the increase by multiplication in geometrical ratio of the individuals in every species, whatever the kind of reproduction which may be peculiar to each species, whether this be simple division, sporulation, budding, parthenogenesis, conjugation and subsequent division, or amphimixis (sexual reproduction); (2) the always apparent slight (to greater) variation in form and function existing among all individuals even though of the same generation or brood; and (3) the transmission, with these inevitable slight variations, by the parent to its offspring of a form and physiology essentially like the parental. The inferred (also partly observed) facts are: (i) a lack of room and food for all these new individuals produced by geometrical multiplication and consequently a competition (active or passive) among those individuals having any ecologic relations to one another, as, for example, among ^ From V. L. Kellogg, Danvinism To-Day (copyright 1907). Used by per- roission of the publishers, Henry Holt & Company. 245 246 READINGS IN EVOLUTION, GENETICS, AND EUGENICS those occupying the same locaHty, or needmg the same food, or needing each other as food; (2) the probable success in this competition of those individuals whose slight differences (variations) are of such a nature as to give them an advantage over their confreres, which results in saving their life, at least until they have produced offspring; and (3) the fact that these "saved" individuals will, by virtue of the already referred to action of heredity, hand down to the offspring their advantageous condition of structure and physiology (at least, as the ''mode" or most abundantly represented condition, among the offspring). The competition among individuals and kinds (species) of organ- isms may fairly be called a struggle. This is obvious when it is active, as in actual personal battling for a piece of food or in attempts to capture prey or to escape capture, and less obvious when it is passive, as in the endurance of stress of weather, hunger, thirst, and untoward conditions of any kind. The struggle is, or may be, for each individual threefold in nature: (i) an active struggle or competition with other individuals of its own kind for space in the habitat, sufficient share of the food, and opportunity to produce offspring in the way peculiar and common to its species; (2) an active or passive struggle or compe- tition with the individuals of other species, which may need the same space and food as itself, or may need it or its eggs or young for food; and (3) an active (or more usually passive) struggle with the physico- chemical external conditions of the world it lives in, as varying temperature and humidity, storms and floods, and natural catas- trophes of all sorts. For any individual or group of individuals any of these forms of struggle may be temporarily ameliorated, as is (i) the intra-specific struggle among the thousands of honey-bee individuals living together altruistically, in one hive, or (2) the inter-specific struggle, when two species live together symbiotically as the hermit crab Eupa gurus and the sea-anemone Podocoryne, or (3) the struggle against untoward natural conditions as in special times or places of highly favourable climate, etc. Or for any individual or group of individuals all forms of the struggle may be coincidently active and severe. The resultant of these existing conditions is, accord- ing to Darwin and his followers, an inevitable natural selection of individuals and of species. Thousands must die where one or ten may live to maturity (i.e., to the time of producing young). Which ten of the thousand shall live depends on the slight but sufficient advantage possessed by ten individuals in the complex struggle for CRITIQUE OF DARWINISM 247 existence due to the fortuitous possession of fortunate congenital differences (variations). The nine hundred and ninety with unfortu- nate congenital variations are extinguished in the struggle and with them the opportunity for the perpetuation (by transmission to the offspring) of their particular variations. There are thus left ten to reproduce their advantageous variations. The offspring of the ten of course will vary in their turn, but will vary around the new and already proved advantageous parental condition: among the thou- sand, say, offspring of the original saved ten the same limitations of space and food will again work to the killing off before maturity of nine hundred and ninety, leaving the ten best equipped to reproduce. This repeated and intensive selection leads to a slow but steady and certain modification through the successive generations of the form and functions of the species; a modification always toward adapta- tion, toward fitness, toward a moulding of the body and its behaviour to safe conformity with external conditions. The exquisite adapta- tion of the parts and functions of the animal and plant as we see it every day to our infinite admiration and wonder has all come to exist through the purely mechanical, inevitable weeding out and selecting by Nature (by the environmental determining of what may and what may not live) through uncounted generations in unreckonable time. This is Darwin's causo-mechanical theory to explain the transforma- tion of species and the infinite variety of adaptive modification. A rigorous automatic Natural Selection is the essential idea in Darwin- ism, at least in Darwinism as it is held by the present-day followers of Darwin. OBJECTIONS TO DARWINISM I. Darwin in a letter to his friend Hooker (January 11, 1844) expresses his contempt of Lamarck's ideas in the following words: ''Heaven defend me from Lamarck's nonsense of a 'tendency to pro- gression,' 'adaptations from the slow willing of animals,' etc Lamarck's work appeared to me to be extremely poor; I got not a fact or idea from it." In spite of these views Darwin's Origin of Species is interlarded with Lamarckian explanations. Whenever the author feels the short- comings of the selection factor he lapses into an explanation involving the idea that the effects of use and disuse of organs are inherited. Followers of Darwin, especially Weismann, felt this to be the chief defect in the fabric of Darwinism and bent their efforts chiefly toward purging Darwinism of all taint of Lamarckism. 248 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 2. Darwin insisted upon the idea that minute fluctuating varia- tions, which we now know are to a large extent non-heritable, were the principal, if not the sole, materials for natural selection to work upon. He knew of a considerable number of "sports" or "saltatory variations" (now called mutations), but considered these too infre- quent to furnish the necessary basis for selection. We now know that mutations may be as small as fluctuating variations or as large as "sports" and that they are of much more frequent occurrence than Darwin supposed. 3. Darwin 'considered all variations as heritable. He did not distinguish between somatic variations and germinal variations. In fact, as we learn from a study of his pangenesis theory, he considered all variations as in the first instance somatic, and subsequently transferred by means of gemmules to the germ cells. Every somatic variation, whether induced by use, disuse, in response to environ- mental stimulus, or through mere spontaneous variability, was sup- posed to be able to give off gemmules into the blood stream that would carry to the germ cells the physical basis of the varying charac- ter. The pangenesis mechanism is now known to have no basis in fact. 4. The natural-selection theory is based upon a mistaken concep- tion of the methods of artificial selection. Darwin believed, without having any proof for this belief, that the way in which domestic varieties had been so profoundly modified at the hands of man was by the conscious or unconscious selection of slight fluctuating varia- tions in favorable or desired directions, and that this resulted in the cumulative improvement or enhancement of the desired characters over a long series of generations. Darwin supposes that the radically changed conditions of domestication hasten and stimulate variability, thus offering a better opportunity for selection. Transferring this idea to nature, he thinks that changed natural conditions stimulate variability, just as does domestication, and that this is seized upon by natural selection to make for adaptation to the new environment and the resultant origin of new species. Our modern experimental studies have shown that somatic modifications due to environmental changes are not hereditary, and that all of the recent domestic varieties whose origin has been observed have been the result of suddenly appearing germinal variations or mutations, that arrive fully formed and cannot be improved by selec- tion, except that they usually need to be selected out or isolated in CRITIQUE OF DARWINISM 249 order to prevent swamping out through intercrossing with the parent-type. 5. Objection has frequently been made to Darwin's idea of the purely fortuitous or chance character of variations. According to this view variations occur in all structures and in all directions at haphazard, so that there would be the widest possible opportunity for a given adaptive variation to occur just when the circumstances would demand. It now appears that variations do not occur in all directions in random fashion, but that they tend to follow certain definite paths of change; in other words, variations are, to a consider- able extent at least, orthogenetic. If variations really tend to follow certain definite lines, owing to purely internal causes, natural selection would be unnecessary, at least until orthogenesis went too far for the good of the species, or far enough to be of real importance in the struggle for* existence. 6. The difficulty of explaining how natural selection could make use of the initial stages of adaptive structures is obvious. It is incon- ceivable that the first, almost imperceptible variation in a favorable direction could be of selective value, so as to effect the survival of the individual or the relative number of its offspring. What would be the advantage of the first few hairs of a mammal or the first steps toward feathers in a bird when these creatures were beginning to diverge from their reptilian ancestors? This objection is, of course, based on the fluctuating-variation idea. If the mutation idea were substituted, the difficulty would, to a great extent, clear up; for a mutation might be of sufficient importance in one generation to have selective value from the very first. 7. Natural selection is said to be incapable of explaining the origin of coadaptive and highly complex adaptations whose effectiveness depends upon the perfection of their adjustments to one another. For example, we may refer to some of the perfected adaptations described in chapter xiv. In the case of the electric organs of certain fish, the -Darwinian assumption would be that the first step in the direction of an electric organ would be a very small one, and that it was built up Httle by little by means of natural selection. But, say the critics, the electric organ would be of no value until it became powerful enough to impart an effective shock to the intruder, and this would not be possible if the character began in a small way. The whole phenome- non of protective resemblance is open to the same tyi^e of criticism. As a specific example of this we may cite the case of the dried-leaf 250 READINGS IN EVOLUTION, GENETICS, AND EUGENICS butterfly, Kallima, previously described (pp. 201, 202). In its present condition this animal has a strikingly detailed resemblance to a dried leaf, which is therefore doubtless of some value. But of what value would be the first tiny change in the direction of resemblance ? Until its resemblance became close enough actually to deceive the enemies of butterflies, the critics claim, there would be no chance for selection to act. 8. It is frequently objected that a vast number of characters of organs are useless or non-adaptive and, as such, could not have arisen through the instrumentality of natural selection. If these useless characters, which are sometimes quite large and prominent, are independent of natural selection, why do we need natural selection to explain adaptive characters ? It is also claimed that a vast number of specific peculiarities are useless and therefore could not have helped in the differentiation of species. It should be said in defense that Darwin realized this difficulty quite as clearly as do his critics and was greatly puzzled by it. His idea of correlated variability, however, helps to answer it, for it may well be that many of these apparently useless characters are correlated, or linked in inheritance, with charac- ters of supreme selective value such as general hardiness or great fecundity. Darwin also points out that we are not in a position at* present to pronounce judgment on the value of many structures or functions that have been adjudged non-adaptive. 9. Certain characters in organisms, past and present, have been interpreted as overspecializations, organs that have evolved beyond the range of usefulness or that are more elaborate than is demanded for survival under the conditions of life. The case of the extinct Irish elk is often cited as an example of overspecialization. This group of animals went to extremes in the development of size and elaboration of horns far beyond the range of usefulness, so that it is said to have brought about the extinction of the race. Natural selection, which is supposed to have brought the horns up to the point of adaptive perfection, should have kept them within the bounds of usefulness. Again, the enormously overgrown and overspecialized dinosaurs of long ago are thought of as having followed their lines of evolution far beyond the point of greatest effectiveness and adaptability. 10. The rudimentation of structures, which is such a common phenomenon in nature, is said to meet with no adequate explanation on a selection basis. The case of the whale's vestigial hind limbs is a case in point. Darwin's explanation would be that under aquatic conditions the first whale ancestors would be handicapped by hind CRITIQUE OF DARWINISM 251 legs and that any decrease in their size, which would be enhanced by disuse, would be of advantage. This might seem reasonable during the main period of hmb reduction, but, after the limb is reduced to a subcutaneous rudiment, there could be Uttle advantage in carrying the rudimentation still farther. Some whales have the hind Hmbs much more profoundly reduced than others, although they are all thoroughly out of the way and involve no hindrance in swimming. Any number of similar cases of the same kind might be cited. Darwin had no explanation to offer except a resort to Lamarckism; but Weismann, the ablest neo-Darwinian, offered the theory of panmixia to cover this objection, a theory which is mentioned in chapter i and will be discussed later. 11. It is objected that, unless favorable variations occur in a large number of individuals at the same time, the character would be swamped out by intercrossing with individuals not possessing the favorable variation. The probability that such a swamping-out would occur was shown mathematically by various critics. By way of answer to this objection there arose a number of ''isolation theo- ries," according to which favorably varying individuals would be protected from back-crossing with the non-varying individuals. We might also point out that the Mendelian laws of dominance and segregation would serve to prevent loss of any new favorable character. 12. It is objected that natural selection might explain the ''sur- vival, but not the arrival, of the fittest." But Darwin met this perfectly when he said: ''Some have even imagined that natural selection induces variability, whereas it implies only the preservation of such variations as arise and are beneficial to the being under its conditions of life." 13. Criticism has been directed against natural selection because of the fact that some of thje supporters of Darwinism, notably Weis- mann, have made the claim that natural selection is the sole cause of evolution. This idea of the Allmacht or all-sufhciency of natural selection was not Darwin's, as is clear from the following statement: "I am convinced that natural selection has been the most important, but not the exclusive means of modification." 14. It is objected that many, if not most, of the fluctuating varia- tions with which Darwinism deals are purely quantitative or plus- and-minus variations; whereas the differences between species are quahtative. This is a serious objection and difficult to meet, yet a fair defense has been formulated by leading neo-Darwinians. 252 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 15. There is a growing skepticism on the part of biologists as to the extreme fierceness of the struggle for existence and of the conse- quent rigor of selection. It may be answered that no very obvious fierceness is implied in the theory. So long as overproduction and a shortage of space and food exists the struggle for existence is inevitable. 16. Special objections are offered to the subsidiary theory of sexual selection. It is said that the type of sexual selection involving active rivalry and battling for mates needs no special theory, inasmuch, as this is a mere phase of the struggle for the maintenance of the full life, including the chance to leave offspring. It is against the other side of sexual selection, which involves passivity on the part of the male and active choice on the part of the female of the more beautiful or otherwise attractive male, that objection is raised. It is claimed that such choice implies too high aesthetic powers in animals of relatively poor vision and mentality. Experiments have been per- formed with moths, in which the male and female coloration is strikingly different, in order to determine whether females actually do exercise any choice of mates that is based on considerations of appearance. The result proved conclusively that color patterns have no value in mating, but that the female is passive and mates with the first male to present himself, while the male finds the female through his exquisitely effective sense of smell. We know now, however, that secondary sexual characters are intimately bound up in a physiological way with the functioning of the sex glands and are therefore doubtless to be interpreted as mere non-adaptive correlative variations that need no special evolutionary explanation. DEFENSE OF DARWINISM In presenting these sixteen objections, we have in most cases indicated the lines upon which the objections have been met, if they have been met. Not all of these objections are considered serious at the present time, for some are based upon lack of a full knowledge of what Darwin actually wrote; others are largely academic in character and fail to stand up under actual test; still others have been more or less adequately met by subsidiary or supporting theories which have been advanced by various neo-Darwinians. Most of the special objections raised in this chapter have received the attention of various able Darwinians, and the student of evolution would doubtless be interested in the expert and fair-minded defense CRITIQUE OF DARWlxMSM ■56 of Darwinism at the hands of Professor V. L. Kellogg as it appears in his book Darwinism To-Day. A much briefer and considerably more general defense is that of J. L. Tayler, which is as follows: GENERAL DEFENSE OF DARWINISM^ J. L. TAYLER To realise how far the theory of selection is capable of explaining the facts of organic evolution, it is necessary to bear in mind the postulates in which the theory is founded. 1. It is obvious that natural selection can only act by preserv- ing or eliminating the complete organism. Selection must therefore be organismal. This Darwin and other selectionists have clearly recognised. 2. As the whole organism must survive, if the favourable variation or variations are to be preserved, it follows that certain minor un- favourable variations may also be preserved if they happen to exist in an individual which survives on account of its major favourable variations. And since no individual is completely adapted to its environment, it follows that there must be always a variable amount of residual unfavourable variability in every organism. 3. This residual unfavourable variability may be of considerable utility under changed conditions. 4. Complementary specialisation of parts, as Spencer has shown, is favourable to successful competition, and as it is the whole organism that is selected or eliminated, it follows that any weakness of one specialised part, since it would disturb the balance of all, would be detrimental. The more complex the organism, the more specialised the structures, the more dependent one part will be on the others for its existence, hence a complementary specialising tendency will be favoured by selection, and therefore all struggles of one part of an organism with another will be reduced to a minimum. It is clear that there must be some underlying criterion which determines whether any given organism shall be selected or not, and that criterion must be the net result of its adaptability to its environ- ment. One organism may conceivably survive, by its possession of a large number of small favourable variations, while another may survive in virtue of a single valuable one, but in each case it would be ^ From J. L. Tayler, "The Scope of Natural Selection," Naturd Scieme, 1899. 254 READINGS IN EVOLUTION, GENETICS, AND EUGENICS the whole value of that organism which determined its survival. This fact is continually disregarded by opponents of the neo-Darwinian position, yet this selection of the organism as a whole is the fundamental postulate from which the theory of selection starts. Thus it is not uncom.mom to read criticisms bearing on the early development of some organ, in which the inadequacy of selection is supposed to be proved by the writer demonstrating, or believing he has demonstrated, the fact that the particular variation in question must have been too small to be by itself of selection value. In many cases the particular variation would, no doubt, if taken alone be, as the objector asserts, too unimportant to be selected, but as it is the whole organism that is selected, it is not logical to make an artificial separation and study the development of one organ or structure irrespective of the other organs with which it is in nature associated. Every organ in its evolution must he considered in relation to the whole of the particular orga^iism in which that particular stage of development of that organ is found. Starting, therefore, with this fact that the net value of adaptability of the whole organism to its environment must be the basis which determines selection or elimination, it will follow that certain lines of development will result from the application of this criterion. In a series of organisms placed under new conditions, elimination will proceed along lines essential to bring about a proper adjustment to the new conditions. If the offspring of these adjusted organisms merely repeated in their generation the characters of the exterminated as well as of the surviving organisms, that temporary adjustment would be permanent as long as the conditions were unchanged. But since the offspring are produced only by the surviving organisms, selection is continually raised to higher and higher planes of adapta- tion, and, therefore, as long as conditions remain constant, the tendency of selection must be, as Darwin clearly saw, cumulative. He did not, however, apparently see that from this cumulative tendency definite variability must arise out of indefinite. Selection in direct relation to climatic conditions is, therefore, of very minor importance, while selection among the members of a species and all forms of inter-organismal selection is of infinitely more importance, since it is this interaction, produced by the offspring in different degrees inheriting the advantages of both parents (both of whom have survived on account of certain advantages), that leads to the cumulative development and never-ending struggle for survival. CRITIQUE OF DARWINISM 255 Darwin came very near to this conception of definite variability when he pointed out that "if a country were changing, the altered conditions would tend to cause variation, not but what I believe most beings vary at all times enough for selection to act on." Extermination would expose the remainder to the "mutual action of a different set of inhabitants, which I believe to be more important to the life of each being than mere climate," and as "the same spot will support more life if occupied by very diverse forms," it is evident that selection will favour very great diversity of structure. Bearing in mind this cumulative action of selection it will follow that under constant or relatively constant conditions the struggle for successful living will become more and more selective in character, even if the actual number of inhabitants remain more or less the same as when the struggle first commenced. The selection of variations will thus tend to pass through certain more or less ill-defined, but nevertheless, real stages. In proportion as the struggle becomes intense, either from the number or from the increasing adaptability of the organisms, or both, certain major essential adaptations, which were necessary for the climatic and other more or less comparativelv simple conditions, will be supplemented by minor auxiliary variations which in the earlier stages would not have appeared. And still later, as more and more rigorous conditions of life were imposed, the advan- tage would tend to rest with those organisms which possessed highly co-ordinated adaptations, since this would entail more rapid respon- siveness to environment. As evolution advances from the unspecialised to the specialised, and higher and higher forms of life come into being, with increasing complexity and specialisation of parts entailing an increasingly delicate adjustment of those parts to each other's needs, the relation of each part to the whole organism becomes of more and more importance, and it follows that selection must become more and more generalised in its action. No single variation could be of service to any of the higher forms of life unless it was in more or less complete harmony with the whole tendency of the individual. The adjustment of parts and their mutual interdependence make it essential for adaptation that the relation of parts be preserved; consequently, correlated minute favourable variations will tend to be more and more selected as evolution passes from the unspecialised to the specialised forms of life. This response of the whole organism should be still more delicate in those forms of life that are continually subjecting themselves to 256 READINGS IN EVOLUTION, GENETICS, AND EUGENICS changed conditions; hence this delicacy of adjustment is far more necessary in the higher forms of animal hfe than in more stationary plant organisms, and in the developing nervous systems of animals we have just the central adjusting system that is required for these condi- tions. With evolution of type there will thus be an increasingly definite tendency given to organic, especially the animal, forms of life, if the acting principle of evolution has been selectional. Selection is, therefore, able to account for the steadily progressive tendency of life as a whole without calling to its aid any unknown and doubtful perfecting principle. To summarise: Natural selection, acting on the whole organism, tends to produce more and more definite tendencies in all surviving forms of life, which tendencies are progressive and continuous in char- acter. Variable conditions, by partially altering the line of selection, induce a temporary indefiniteness. And lastly, the process of selec- tion being itself able to be the indirect, though not the direct, cause of those favourable variations, which it subsequently selects from, is able to dispense with any subsidiary factors, provided it has a certain number of elementary properties of life which afford sufficient material to work with. EXPERIMENTAL SUPPORT OF THE EFFECTIVENESS OF NATURAL SELECTION Weldon's experiments with the shore-crabs of Plymouth Sound. — These experiments seem to show that under changed environmental conditions natural selection acts upon minute fluctuating variations of linear or quantitative type so as to produce an alteration in the species; exactly as Darwinism would hold. A large breakwater was so placed near the mouth of Plymouth Sound that the rate of flow of the river water was greatly slowed down in certain regions. This allowed an increased settling of the fine china-clay sediment that is carried by the river, and the changed condition caused the death of numerous crabs of the species Carcinus maenas. The question arose as to whether the survivors and those that had perished showed any consistent differences on the basis of which selection could be operat- ing. Careful measurements of hundreds of individuals showed that the mean breadth of frontum is slightly less in the survivors than in the perished. Measurements were repeated in two subsequent years and it was found that there was a progressive narrowing of the frontum. As an experimental check upon these conclusions Weldon CRITIQUE OF DARWINISM 257 placed a number of crabs in a large aquarium, in which china-clay was kept partly in suspension, and found that about half of them died. Again the survivors were compared statistically with the perished and the same relation was found to hold : that the survivors had a mean frontal breadth distinctly narrower than that of the perished. W'el- don concludes that his experiments ''have demonstrated two facts about these crabs; the first that their mean frontal breadth is dimin- ishing year by year at a measurable rate, which is more rapid in males than in females; the second is that this diminution in frontal breadth occurs in the presence of a material, namely, fine mud, which is increasing in amount, and which can be shown experimentally to destroy broad-fronted crabs at a greater rate than crabs with narrower frontal margins .... and I see no escape from the conclusion that we have here a case of Natural Selection acting with great rapidity, because of the rapidity with which the conditions of life are changing." Cesnola's experiments with Mantis. — ^To test the selective value of color markings Cesnola fixed specimens of the brown and green Mantis religiosa on plants, some of which were against harmonious, others against disharmonious backgrounds. The result was that most of those which were inconspicuous because of a harmonious back- ground escaped, while most of the others were eaten up by birds. Poulton's and Sanders' experiments with butterfly pupae. — Numerous pupae of various colors were placed under conditions favor- ing protective coloration and others under opposite conditions. The conclusion was that protective coloration is a real survival factor, and one that operates so as to give the protectively colored individual a decided advantage in the struggle for existence. Davenport's experiments with chickens. — A number of chickens, some black, some white, and some barred or checkered in color, were allowed to wander free in the fields. Hawks killed most of the whites and many of the blacks, but spared, to a large extent, the less con- spicuous checkered and barred types which are harder to detect against a mixed background. All of these experiments merely tend to show that discriminate survival actually occurs, but only the experiment of Weldon has a bearing on the possibility that mere quantitative changes of small dimensions might under certain conditions be of selective value. We badly need more experimental evidence of this sort and until this is forthcoming we shall have to admit that there is very little 258 READINGS IN EVOLUTION, GENETICS, AND EUGENICS experimental evidence in favor of the type of natural selection that Darwin stood for. THE PRESENT STATUS OF NATURAL SELECTION It has come to be rather generally believed that the natural selection that Darwin himself believed in stands almost unscathed as one very important causal factor. In fact it is the only explanation ever offered for adaptation that even approaches adequacy. As an explanation of the origin of new types or new species it falls far short of adequacy, and I think Darwin evidently realized this, a?lthough he was unfortunate enough to entitle his book Origin of Species. As an explanation of the origin and perfection of adaptation natural selection has only one rival, the far less satisfactory Lamarckian theory of the inheritance of acquired characters. There is a strong tendency among geneticists to conclude that the modern germ-plasm hypothe- sis, with the aid of mutations and the mechanism of Mendelian inherit- ance, furnishes all the necessary explanation of the causes of evolution. There is, however, marked dissent to this extreme position. In his critique of De Vries's rather extreme position that the mutation theory needs no aid from natural selection, Weismann shows in most able fashion the inadequacy of mutations to account for adaptation, and, in contrast, how well natural selection accounts for them. In a very recent paper Professor C. C. Nutting attempts to show that natural selection is still an important factor in evolution and quite in harmony with both the mutation theory and Mendelism. We perhaps can close the present chapter no more fittingly than by quoting Professor Nutting's paper. THE RELATION OF MENDELISM AND THE MUTATION THEORY TO NATURAL SELECTION^ C. C. NUTTING Two marked tendencies are evident in the history of any important theory after its publication. First. The followers of the discoverer carry the theory too far and attempt too universal an application. This is manifestly true of Wallace and Weismann who out-Darwined Darwin in their claims for natural selection; of the followers of Mendel, such as Morgan and '' From an address given before the Genetics branch of the American Associa- tion for the Advancement of Science, December, 1920; Science^ N.S., Vol. LIII. CRITIQUE OF DARWINISM 259 Pearl; and of many mutationists who make much greater claims for that theory than does De Vries himself. Second. Each generation of biologists is so occupied with its own work and contemporary theories that it makes no real effort to understand preceding theories. This second tendency seems to me most marked in the attitude of present workers along genetic lines towards natural selection. They reveal an apparent lack of understanding of what Darwin really meant and of what he claimed; and when criticising that theory they are often engaged in the classic, but unprofitable, exercise of ''fighting windmills." In view of these facts I hope you will pardon me if I present in as few words as possible just what I believe to be the main factors which Darwin presented as resulting, in their actions and reactions, in natural selection. These factors are three in number: First. Heredity, by which the progeny tend to resemble their parents more than they do other individuals of the same species. Second. Individual variation, by which the progeny tend to depart from the parental type and sometimes from the specific tvpe. Third. Geometrical ratio of increase, by which each species tends to produce more individuals than can survive. Each of these factors is practically axiomatic, so little is it open to argument. No one doubts the fact of heredity, whether pangenesis, Weis- mannism or Mendelism be the correct expression of the mechanism involved. These do not affect the fact of heredity nor invalidate it as a factor in natural selection. No one doubts the fact of variation; whether it is the '' individual variation" of Darwin, the "fluctuating variety" or the "mutation" of De Vries. All that is necessary for Darwin's purpose is that there be heritable variations. That there are such things all parties agree and it matters httle what you call them. They are adequate to act as a factor in the Darwinian scheme. No one doubts the fact of geometrical ratio of increase. It is a proposition easily capable of mathematical demonstration, and that is sufficient for Darwin's purpose. These three factors, then, are not debatable as facts, whatever their mechanism or causes. A moment's reflection will show that geometrical ratio of increase is a quantitative factor, giving an abundance lof individuals from 26o READINGS IN EVOLUTION, GENETICS, AND EUGENICS which to select ; that individual variation is a qualitative factor, giving the differences which make a selection possible; and that heredity is a conservative factor, holding fast those characters which better fit the organism to its environment. Now it seems to me that there is no possible outcome of the necessary action and interaction of these three factors that would not be a selection of some sort. Darwin thought it comparable in a large way to the selection by which the stock-breeder improves his herd, and therefore called it ''natural selection," carefully guarding the phrase from misinterpretation from the teleological angle as well as from a too close parallelism between artificial and natural selection. And I believe no one has suggested a more acceptable term for the process of selection resulting from the interplay of natural laws. Three outstanding theories have been advanced since the publica- tion of the Origin, each involving an advance in our knowledge of the mechanism of heredity on the one hand and the origin of varia- tions on the other. Weismann's theory of the continuity and stability of the germ plasm was of immense importance in its discussion of the mechanism of heredity, and his amphimixis gave a plausible explanation of the origin of variations. His results were almost universally regarded as confirming and greatly extending the scope of natural selection. Mendel's theory regarding the purity of the gametes, their segre- gation in the sex cells, and the whole complex Mendelian mechanism so admirably described by Morgan; all of these, fascinating and important as they are, deal with the mechanism rather than the fact of heredity. In my opinion their acceptance or rejection does not affect the status of natural selection as a theory of organic evolution. But it is the theory of mutation that has furnished most of the ammunition for the opponents of natural selection; and this in spite of the fact that De Vries, the originator of the mutation theory, expresses himself with great clarity as follows: " My work claims to be in full accord with the principles laid down by Darwin and to give a thorough and sharp analysis to some of the ideas of variability, inheritance, selection, and mutation which were necessarily vague in his time." In 1904, when these words were published, there did seem to be a sharp distinction between the ideas of Darwin and those of De Vries. The former believed that natural selection acted upon many small variations and accumulated them until the differences were sufficient CRlTIQtlf: OV bARWINISAt 261 to constitute new species; while De Vries claimed that new species were formed by the sudden appearance by mutations of forms specifi- cally distinct from the parents. That mutants were new species! It seems evident that Darwin did not regard " saltatory evolution " as the common method, while De Vries did. Darwin believed that individual, usually small, variations fur- nished the material on which selection acts; while De Vries thought that mutants, usually large variations, furnished the material. Both, however, believed thoroughly that natural selection was a vera causa of evolution. But things have changed greatly since 1904. The work of Morgan, Castle, Jennings and a host of others has shown that many mutations are so small, from a phenotypic standpoint, that they are quantitatively no greater than the individual variations of Darwin; and that they are' heritable in the Mendelian way. Castle produced a perfectly graded series of hooded rats which exhibits almost ideally the steps by which a new form might be produced by natural selection. He says: "If artificial selection can, in the brief span of a man's lifetime, mould a character steadily in a particular direction, why may not natural selection in unlimited time also cause progressive evolution in directions useful to the organism ?" Jennings says: ''Sufficiently thorough study shows that minute heritable v-aria- tions — so minute as to represent practically continuous gradations — occur in many organisms: some reproducing from a single parent, others by biparental reproduction It is not established that heritable changes must be sudden large steps; while these may occur, minute heritable changes are more frequent. Evolution according to the typical Darwinian scheme, through the occurrence of many small variations and their guidance by natural selection, is perfectly con- sistent with what experimental and paleontological studies show us; to me it appears more consistent with the data than does any other theory." Many believers in mutation have been needlessly befuddled by the diverse meanings of "variations" as used by Darwin and De Vries. Darwin included in his "individual variations" both the ^'fluctuating varieties" and the "mutations" of De Vries. Pheno- typically they cannot even now be distinguished. De \'rics himself candidly admits that this was Darwin's attitude, thus proving himself 262 READINGS IN EVOLUTION, GENETICS, AND EUGENICS more clear-sighted than many of his followers. All that Darwin needed for his purpose was proof of variations that are heritable, and these are found in mutations, be they large or small. Just as Mendelism has to do with the mechanism and not the fact of heredity, so the mutation theory deals with the nature and not the fact of variations. Neither, in my opinion, has any implication that is antagonistic to the theory of natural selection. The statement has been made that natural .selection "originates nothing" because it does not explain the origin of variations. I must confess scant patience with this point of view. As well say that the sculptor does not make the statue because he does not manufacture the marble or his chisel; or that the worker in mosaic originates nothing because he does not make the bits of stone which he assembles in his design! The material corresponding to the bits of stone" in the mosaic is furnished by heredity and variation, and its quantity by geometrical ratio of increase. Natural selection acts in selecting and putting together this material in the formation of new species. Thus, in a true sense, it seems evident that something new has appeared — something that is, but was not. Another favorite figure, introduced I believe by De Vries, is "Natural selection acts only as a sieve" determining which forms shall be retained and which shall be discarded. This also seems to me to fall short of a complete statement of the truth. If the material subjected to the sifting process be regarded as changing with each generation by the addition of variations, or mutations if you prefer, some of which are favorable to a nicer adjustment of the species to its environment, the figure would be more nearly correct. To make it complete, however, the mesh of the sieve must change from generation to generation so that a quantitative variation which would be preserved in one generation would be discarded in a later one. But in this case natural selection would do more than a sieve could do. It would combine a number of favorable variations in the production of something new, a new species! In conclusion it seems to me that we are justified in maintaining that Mendelism and the mutation theory, while forming the basis of the most brilliant and important advances in biological knowledge of the last half-century, have neither weakened nor supplanted the Darwinian conception of the "Origin of species by means of Natural Selection." CHAPTER XVIII OTHER THEORIES OF SPECIES-FORAHNG H. H. N. THEORIES AUXILIARY TO NATURAL SELECTION The post-Darwinian causo-mechanical theories fall quite naturally into two categories: those that were devised by Darwinians to bolster up natural selection and to free it of some of its most obvious objec- tions, while retaining the essential features of the principle; and those that were meant to be substitutes for and therefore quite opposed to natural selection. The former theories have been classed as auxiliary, and the latter as alternative theories to natural selection. The several theories of Weismann will be dealt with first as the most important of the purely auxihary theories. ''Panmixia" is designed to explain, without recourse to Lamarckism and in harmon\' v/ith natural selection, the degeneration or atrophy of organs which seemed to be inadequately explained by Darwin. "Germinal Selec- tion" is supposed to explain the initial stages of adaptations and allied phenomena, and thus to aid natural selection at one of its weakest points. weismann's theory of panmixia The following statement of "panmixia, " as given by S. Herbert, is concise and to the point: "Cessation of selection as a cause of atrophy was first proposed by Romanes. Later on, Weismann, whilst examining the validity of the principle of use-inheritance, adopted the same idea, called by him 'panmixia,' in order to account for the dwindling and disappearance of useless organs without having recourse to the Lamarckian factors. If natural selection leads to the mating of select types, so that those below a certain standard are prevented from propagating, it follows that, with the cessation of selection, a general crossing of all lyi)es, including the inferior ones, must take place, and thus lower the average quality of the whole stock. Weismann explained in this manner, for instance, the prevalence of short-sightedness among civilized people. The individuals with defective eyesight not being weeded out in modern society, the sharpness of the eyesight of the population sinks gradually. The same would apply to the deterioration of the teeth 263 264 READINGS IN EVOLUTION, GENETICS, AND EUGENICS of man, of the breast-gland of modern women, etc. The fact that degeneration generally progresses so slowly, often taking thousands and thousands of years, seemed to him a sufficient proof of the inade- quacy of the Lamarck ian explanation. For if the effect of disuse were transmitted in accumulating ratio in the successive generations, a useless organ ought to disappear much more quickly. "Weismann originally attributed a great effect to panmixia, and considered that nearly 90 per cent, of the reduction of rudimentary organs was due to it; the remainder, up to the complete loss of the organs, being accounted for by reversed selection. Romanes was much more modest in his estimate, and only allowed about 10 to 20 per cent, to this cause; while Lloyd Morgan gave only 5 per cent, reduction of the original size. The final reduction of the organ to zero is still not accounted for by any of these theories. Calling to aid a failure of the force of heredity, as Romanes did, can hardly be con- sidered a solution of the problem. First of all, the force of heredity does not explain anything in the case. It only restates the problem. We want to know what the force of heredity is. Secondly, if the force of heredity does fail, we should have to explain why it wanes in some cases and not in others. For the reduction and elimination of rudimen- tary organs occurs apparently in the most irregular, haphazard manner. ''But can panmixia really reduce an organ ? Plate, in agreement with Spencer, Eimer, and others, denies any such possibility. An organ in a given condition of its existence varies around a mean or average, the plus and minus variations generally being equally fre- quent. It follows, therefore, that if all the existing variations are crossed in propagation, the organ remains stationary. Selection only improves the organ by cutting off the minus variations; the absence of selection would simply leave the organ where it was before the selection. At most it could only sink a very Httle below the aver- age. That this is so is seen in organs which are not under the sway of selection at all. There are numberless such indifferent species charac- ters, which ought gradually to dwindle and disappear, yet they remain fairly constant, though continually exposed to the swamping effect of panmixia. Panmixia may explain the functional degeneration of an organ, but cannot explain its actual rudimentation. "Weismann himself in later times abandoned panmixia as a suffi- cient means of explanation, and resorted to a new theory — that of germinal selection."^ ^ From S. Herbert, First Principles of Evolution (19 13). OTHER THEORIES OF SPECIES-FOR^UXG 265 weismann's theory of germinal selection This theory was intended to rehaljihtate the selection principle which had lost a great deal of prestige because of the serious character of the objections that had been raised against it, most of which have been stated in the last chapter. The theory is believed by its author to overcome all objections and doubts and to clear away all difficulties. "Its strength," says Plate, "shall avail in four directions. First, it shall explain how not only degeneration (physiological) but rudimenta- tion (morphological) occurs in panmixia; second, why exactly those variations needed for the development of a certain adaptation appear at the right time; third, how correlation of adaptation comes to exist; and fourth, how variations are able to develop orthogenetically along a definite line without depending on the necessity of a personal selec- tion raising them step by step." The essential feature of germinal selection, as the name implies, is a transfer of the struggle for existence to the germ cell. The germ cell is assumed to be a greatly reduced and simplified sample of the characters of the whole organism. Each independently variable part of the organism is supposed to be represented in the germ cell by a minute physiological unit, unique in composition and capable of reproducing the part in question in a new organism. These hereditary units are called "determinants." Thus there is a different kind of determinant for each muscle of the body, for each bone, or for each independently functioning blood vessel; but, since all red blood cor- puscles are alike, there would be only one determinant for all of them. These determinants have to grow, and in cell division, to divide so as to furnish to daughter germ cells all of the necessary determinants for a whole individual. In their process of growth and multiplication, which goes on very rapidly at certain periods in the germ-cell cycle, these determinants are in competition among themselves for the available food supply. Some may be more favorably placed than others or may be more active chemically than others. There will thus arise a struggle within the germ for a chance to grow and reproduce their kind, which, for these determinants, might be as bitter as would be the struggle in nature among the whole organisms that arc in com- petition for a place in the world. A determinant favored, perhaps accidentally or possibly because of inherent activity, by a good food supply would wax stronger and grow faster and would, logically, pro- duce a larger and more effective part when that particular germ cell developed into an adult. Other germ cells that would be the offspring 266 READINGS IN EVOLUTION, GENETICS, AND EUGENICS of this germ cell would continue the struggle among determinants, and it would be expected that the strong determinant would continue to gain further advantage until the structure it represents reached its maximum efficiency. Similarly, a determinant that was for some reason deprived of its fair share of nutriment at any time would be weakened and would produce in cell division weakened daughter determinants. These in turn, unless especially favored, would wage a losing fight and continue to grow smaller and weaker. Each indi- vidual that might develop from such germ cells would have the charac- ters whose determinant had been weakened in a reduced and progressively degenerating condition. Finally, certain determinants might starve entirely, and the part for which they stood would dis- appear entirely from the ontogeny of the individual arising from these germ cells. In this way Weismann tried to explain the gradual dwindling and the final elimination of useless organs. So also he would explain definitely directed or orthogenetic variations, because germinal selec- tion, once started in a given direction, continues automatically till the goal of adaptiveness is reached. The most potent objections to the theory of germinal selection are as follows: 1. There should be, according to this theory, certain pronounced tendencies in variability in definite directions, whereas fluctuating variations nearly always distribute themselves evenly about the mean or mode, and the same specific mean or mode is stationary in succes- sive generations. 2. The theory implies too rapid and too general modification of parts and therefore does not accord with the fact that species are decidedly constant, except for occasional mutations, over long periods of time. To meet this objection Weismann proposes a new self- correcting mechanism that checks too rapid a development of char- acters. 3. The over- or undernourishment of determinants might con- ceivably induce size changes in characters already present, but could hardly be responsible for the origin of qualitatively different characters. 4. Actual experiments in over- and underfeeding of animals have been carried on by certain experimenters in order to test out the theory of germinal selection. In the experiments of Kellogg, for example, involving feeding silkworm larvae only one-eighth of the normal amount of food, the only result was that the mature individuals were OTHER THEORIES OF SPECIES-FOR.\HXG 267 dwarfed in size. The relative sizes of the parts were unaltered, show- ing that there had been no real struggle among the determinants; for, on the theory of germinal selection, only the stronger determinants would have survived and the weaker ones would have been starved out. Partial individuals, moreover, lacking certain organs and over- developed in others, would have been produced instead of individuals merely smaller in all parts. These are the specific objections to the theory, but more important than all of these is the general objection that follows: "Thus Weismann," says Morgan,^ ''has piled up one hypothesis on another as though he could save the integrity of the theory of natural selection by adding new speculative matter to it. The most unfortunate feature is that the new speculation is skilfully removed from the field of verification, and invisible germs whose sole functions are those which Weismann's imagination bestows on them, are brought forward as though they could supply the deficiencies of Darwin's theory. This is, indeed, the old method of the philosophizers of nature. An imaginary system has been invented which attempts to explain all difificulties, and if it fails, then new inventions are to be thought of. Thus we see where the theory of selection of fluctuating germs has led one of the most widely known disciples of the Darwin- ian theory. ''The worst feature of the situation is not so much that Weismann has advanced new hypotheses unsupported by experimental evidence, but that the speculation is of such a kind that it is, from its very nature, unverifiable, and therefore useless. Weismann is mistaken when he assumes that many zoologists object to his methods because they are largely speculative. The real reason is that the speculation is so often of a kind that cannot be tested by observation and experiment. " It seems almost impossible that the same Professor Morgan, who wrote the foregoing paragraphs in 1903, should now be the leading exponent of a theory of the mechanics of hereditary transmission which depends upon hereditary units almost identical with Weis- mann's "determinants," for the "genes" or "factors" of Morgan are minute corporeal bodies in the germ cells which determine the charac- ters of the adult individual. The difference is, however, that the "genes" of Morgan are experi- mentally demonstrable and have behind them a vast amount of real evidence for their existence. ^ From T. H. Morgan, Evolution arid Adaptation, 268 READINGS IN EVOLUTION, GENETICS, AND EUGENICS In this chapter the writer has purposely avoided entering into the more elaborate intricacies of the Weismannian theories of develop- ment and heredity. The theories have been so generally discredited and play so small a part in modern biological thought that it seems useless to burden the reader's mind with needless complexities. Certain other phases of Weismann's work, especially his ideas of the germ plasm, its separateness and its continuity, are more appro- priately studied in connection with genetics than at the present time. ROUX'S THEORY OF IXTRASELECTIOX OR THE BATTLE OF THE PARTS In point of time this theory antedates Weismann's theories, since it was proposed in 1881. In some respects it is a more acceptable theory than germinal selection, but in others quite unacceptable. The theory is designed primarily to explain the origin of the "fine and deHcate inner adaptations" of organisms, which do not come in con- tact with the external environment and therefore could not be directly affected by it. The idea is that there is a sort of struggle among the tissues for a chance to develop in the direction of functional perfection. Certain contacts, stresses, and pressures of part on part aid or hinder the development of parts. Thus, where muscular pressure on bone is greatest or weight borne by bone is greatest there will the most bony tissue be laid down in the form of lamellae. The result is that any given bone improves its structure by resistance to strain and pressure, which is a case of improvement with use. We may then inquire how such a change in the individual could affect the evolution of the race. The only reply involves the adoption of a distinctly Lamarckian con- cept, and this at present is quite unacceptable. COINCIDENT SELECTION OR ORG.ANIC SELECTION This theory has been masked under various guises. In addition to the two titles given above, it has appeared under the names "onto- genetic selection" and "orthoplasy." The main idea, according to Herbert, is that "the individually acquired characters, though not transmitted to the offspring, serve to tide the successive generations over the critical period until germinal (inborn) variations of the same kind appear which are inheritable. Ontogenetic (individually acquired) adaptations and natural selection work together towards the same end. "This hypothesis would help to account for two related difficult points in the theory of natural selection. Firstly, it would explain the possibihty of the slow accumulation of germinal variations in their OTHEk THEORIES OF SPECTES-FORMIXG 269 first stages before they attain selective value; seconclly, it would make correlated adaptations feasible by supplying ontogenetic (individually acquired) modifications, until the material for the api)ropriate germi- nal adaptations arose. "It has been objected to this theory that, since the individually acquired modifications possess the main selective value in these instances, there is no reason why the corresponding germinal variations should be fostered at all. The individuals with the right, but slight, congenital variations would have no advantage over their fellows who show no such coincident variations. Nor is there any ground to assume that individuals with the greatest amount of plastic modifica- tion in a given direction will tend to exhibit similar innate variations to a greater degree than those individuals not possessing this plas- ticity."^ ISOLATION THEORIES One of the objections to natural selection was that a favorable variation appearing in one or a few individuals would be lost because the individuals possessing it would interbreed with those not possessing it, which presumably would be much more numerous. If there were any kind of agency whose effect would be a partial or complete inhibi- tion of intercrossing, the favorable character would have a chance to survive. Several related theories have arisen that deal with possible isola- tive or segregative agencies that might serve to prevent promiscuous intercrossing, and these have received the names geographic isolation, climatic isolation, reproductive isolation, and physiologic isolation. Geographic isolation. — Moritz Wagner was the founder of this theory. He was a very extensive traveler and had a vast knowledge of the details of the geographic distribution of animals. He believed that isolation was absolutely essential in the differentiation of species. He at first thought of his theory as auxiliary to natural selection, but later, strongly impressed by the facts he had collected, he concluded that isolation was an independent and alternative explanation of species-forming. The underlying idea is one that has already received attention in chapter vii, under ''Evidences from Geographic Distri- bution." Any successful species tends to spread in all directions until checked by barriers. Some few members of a species under favorable conditions may surmount the barrier and become isolated. The result ^ From S. Herbert, First Principles oj Evolution (19 13). 270 READINGS IN EVOLUTION, GENETICS, AND EUGENICS will be that, if they differ in any definite way from the main body of the species, a new elementary species will at once gain a foothold and will evolve independently of the parent-species. If a certain area of land is cut off from a continent so as to form a continental island-, the members of each species that have become isolated will evolve independ- ently of the main body of the species and will have their own peculiar lines of variation preserved from back-crossing with the parent-species. Professor David Starr Jordan,^ the leading proponent of the theory of geographic isolation in America says : "It is now nearly forty years since Moritz Wagner (1868) first made it clear that geographic isolation {rdumlicke Sojuierung) was a factor or condition in the formation of every species, race, or tribe of animal or plant we know on the face of the earth. This conclusion is accepted as almost self-evident by every competent student of species or of the geographical distribution of species. But to those who approach the subject of evolution from some other side the principles set forth by Wagner seem less clear. They have never been confuted, scarcely ever attacked, so far as the present writer remem- bers, but in the literature of evolution of the present day they have been almost universally ignored. Nowadays much of our discussion turns on the question of whether or not minute favorable variations would enable their possessors little by little to gain on the parent stock, so that a new race would be established side by side with the old, or on whether a wide fluctuation or mutation would give rise to a new species which would hold its own in competition with the parent. In theory, either of these conditions might exist. In fact, both of them are virtually unknown. In nature a closely related distinct species is not often quite side by side with the old. It is simply next to it, geo- graphically or geologically speaking, and the degree of distinction almost always bears a relation to the importance or the permanence of the barrier separating the supposed new stock from the parent stock. "A flood of light may be thrown on the theoretical problem of the origin of species by the study of the probable, actual origin of species with which we are familiar or of which the actual history or the actual ramifications may in some degree be traced. *'In regions broken by few barriers, migration and interbreeding being allowed, we find widely distributed species, homogeneous in their character, the members showing individual fluctuation and climatic ^ Science, N.S., Vol. XXII (1905). OTHER THEORIES OF SPECIES-FORMING 27 1 effects, but remaining uniform in most regards, all representatives slowly changing together in the process of adaptation by natural selection. In regions broken by barriers which isolate groups of indi- viduals we find a great number of related species, though in most cases the same region contains a smaller number of genera or families. In other words, the new species will be formed conditioned on isolation, though these same barriers may shut out altogether forms of life which would invade the open district. '' Given any species in any region, the nearest related species is not likely to be found in the same region nor in a remote region, but in a neighboring district separated from the first by a barrier of some sort. ''Doubtless wide fluctuations or mutations in every species are more common than we suppose. With free access to the mass of the species, these are lost through interbreeding. Isolate them as in a garden or an enclosure or on an island, and these may be con- tinued and intensified to form new species or races. Any horticul- turist will illustrate this. ''In all these and in similar cases we may confidently affirm: The adaptive characters a species may present are due to natural selection or are developed in connection with the demands of competition. The characters, non-adaptive, which chiefly distinguish species do not result from natural selection, but from some form of geographical isolation and the segregation of individuals resulting from it." J. T. Gulick, another exponent of the efficacy of geographic isola- tion in species-forming, has offered in evidence of his views facts about the distribution of Hawaiian land snails. In the island of Oahu, for example, the volcanic ridges have been eroded out into a series of isolated valleys in the bottoms of which grows abundant vegetation, while on the highlands there is little but barren rock. The climatic conditions of all the numerous valleys are the same, but, remarkably enough, each variety of snail is confined not only to one island, but to a definite valley on an island. The degree of difference, moreover, between varieties is in proportion to the distance that separates them. Gulick claimed that he was able to estimate the degree of divergence between the snails of any two valleys by measuring the number of miles that lay between them. Gulick's findings have been extensively corroborated by recent explorations on the snails of other oceanic islands by Crampton. An interesting type of isolation that hardly can be termed geo- graphic, yet is essentially equivalent to the latter in its efl"ects, is found 272 READINGS IN EVOLUTION, GENETICS, AND EUGENICS in connection with the extensive group of Hce (Mallophaga) that Hve their whole hves buried among the feathers of birds or the hair of mammals. These animals cannot fly and are quite effectively isolated for life upon a particular bird. They do, however, during the intimate period of nesting, pass from parent to offspring, so that they may be said to be isolated upon definite genetic lines. In the case, especially, of birds like the eagle, a bird of long life and monogamous habits, the parasite becomes as isolated as might be a race on a small island. The result is that sometimes the lice of a single bird and its offspring are of quite a distinct variety, which has become fixed by inbreeding until a high degree of uniformity has been attained. Such an isolated variety may be almost as distinct as a true species. Obviously in this case, as in others, isolation must have had a real effect upon species- forming quite apart from natural selection, except in so far as the unfit variants have not survived. The writer's impression is that isolation as a factor in evolution has been undervalued by the majority of writers on the subject. It is a highly important and essential factor in the establishment of species. If natural selection may be said to be the prime factor in producing adaptations, isolation may be said to be the prime factor in species differentiation, guided only within moderate limits by natural selection. Biologic isolation. — The effects of this type of isolation are not nearly so well established as are those of geographic isolation. Accord- ing to this theory, differences in the rate of development or earliness or lateness of the breeding season would serve to prevent certain varieties from intercrossing. Only those individuals which were sexually active simultaneously would mate, and individuals with different breeding times and seasons would be isolated from one another and would likely maintain the variations that arose in the isolated stocks. The main weakness of this phase of isolation is, however, that we have so little actual evidence that it is operative in nature. Reproductive isolation. — A much more real type of isolation than the last named is involved in reproduction. Several conditions may arise of entirely distinct sorts that will tend to inhibit mating at ran- dom. The first agency has been called "assortative mating" and impUes a sort of race feeling involving either a special attraction of like for like, based on similarity of odors, colors, etc., or an antipathy toward opposites or unlikes. The inhibition to general mating may Involve a mere mechanical lack of fit in certain organs necessary for OTHER THEORIES OF SPECIES-FORMING 273 successful mating. Such conditions are readily observable between closely allied species. Again, the prevention of intercrossing may result from the appearance of a lowered interfertility between the variant individuals and those of the parent-stock. If individuals varying in the same direction were even slightly more fertile inter se than those varying in different directions there would be a progressive tendency in a series of generations for the varying individuals to diverge more and more markedly, and ultimately to become practi- cally sterile except with members of their own group. That environmental changes do frequently affect the fertility of animals is seen when wild animals are kept in confinement. Rela- tively few wild animals breed in captivity. Such a lowering of fer- tility as the result of environmental changes might restrict crossing between unlike forms, while permitting it among the like ones. Summary on isolation theories. — There is a great divergence of opinion as to the importance of isolation as a causal factor in species- forming. Some writers, such as D. S. Jordan and V. L. Kellogg, con- sider isolation an indispensable, and therefore primary, factor; others, especially geneticists, almost ignore it as an effective factor. Still others, like the present writer, take a middle ground and conclude that isolation, especially geographic isolation, has helped greatly in the segregation and establishment of well-defined groups such as species or varieties, the latter developing into the former after prolonged isolation and the addition of new variations. Isolation theories, how- ever, have no light to shed upon the difficult problem of adaptation, and it is here that isolation is auxiliary to natural selection. THEORIES ALTERNATIVE TO NATURAL SELECTION The three theories that have been offered by their authors as sub- stitutes for natural selection are: 1. Theory of the inheritance of acquired characters commonly called Lamarckism: This theory has been outlined in the chapter on the history of evolution (pp. 19 ff.). It will again be dealt with in con- siderable detail in chapter xxii. For the present, then, we may pass by this theory without further comment. 2. The orthogenesis theories: These theories have already been presented in sufficient detail for our purposes in chapter ii (pp. t,^ ff.). 3. The mutation theory of Hugo De Vries: This theory has been dealt with in chapter ii, and will be discussed in further detail in chapter xxiv. 274 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 4. The tetrakinetic theory of H. F. Oshorn: This is a recent restate- ment in energistic terms, of the causo-mechanical basis of evolution. It is placed in the next chapter, but cannot fully be understood until the subject of genetics has been presented. It is almost impossible satisfactorily to pursue a further study of the causal factors of evolution without encroaching upon the field that is now called genetics, and so we shall pass without further explana- tions to a consideration of this field of experimental and analytical evolution. CHAPTER XIX A NEW COMPOSITE CAUSO-MECHANICAL THEORY OF EVOLUTION (THE TETR AKINETIC THEORY)^ HENRY FAIRFIELD OSBORN THE ENERGY CONCEPT OF LIFE While we owe to matter and form the revelation of the existence of the great law of evolution, we must reverse our thought in search for causes and take steps toward an energy conception of the origin of life and an energy conception of the nature of heredity. So far as the creative power of energy is concerned, we are on sure ground: in physics energy controls matter and form; in physiology function controls the organ; in animal mechanics motion controls and, in a sense, creates the form of muscles and bones. In every instance some kind of energy or work precedes some kind of form, rendering it probable that energy also precedes and controls the evolution of life. The total disparity between invisible energy and visible form is the second point which strikes us as in favor of such a conception, because the most phenomenal thing about the heredity-germ is its microscopic size as contrasted with the titanic beings which may rise out of it. The electric energy transmitted through a small copper wire is yet capable of moving a long and heavy train of cars. The discovery by Becquerel and Curie of radiant energy and of the proper- ties of radium the energy per unit of mass is enormously greater than the energy quanta which we were accustomed to associate with units of mass; whereas, in most man-made machines with metallic wheels and levers, and in certain parts of the animal machine constructed of muscle and bone, the work done is proportionate to the size and form. The slow dissipation or degradation of energy in radium has been shown by Curie to be concomitant with the giving off of an enormous amount of heat, while Rutherford and Strutt declare that in a very minute amount of active radium the energy of degradation would entirely dominate and mask all other cosmic modes of transformation ^ From H. F. Osborn, TJie Origin and Evolution of Life (copyright 19 16). Used by special permission of the publishers, Charles Scribner's Sons. 275 276 READINGS IN EVOLUTION, GENETICS, AND EUGENICS of energy; for example, it far outweighs that arising from the gravita- tional energy which is an ample supply for our cosmic system, the explanation being that the minutest energy elements of which radium is composed are moving at incredible velocities, approaching often the velocity of light, i.e., 180,000 miles per second. The energy of radium differs from the supposed energy of life in being constantly dissipated and degraded; its apparently unlimited power is being lost and scattered. We may imagine that the energy which lies in the life-germ of heredity is very great per unit of mass of the matter which contains it, but that the life-germ energy, unlike that of radium, is in process of accumulation, construction, conservation, rather than of dissipation and destruction. Following the time (1620) when Francis Bacon divined that heat consists of a kind of motion or brisk agitation of the particles of matter, it has step by step been demonstrated that the energy of heat, of light, of electricity, the electric energy of chemical configurations, the energy of gravitation, are all utilized in living as well as in lifeless substances. Moreover, no form of energy has thus far been discovered in living substances which is peculiar to them and not derived from the inorganic world. In a broad sense all these manifestations of energy are subject to Newton's dynamical laws which were formulated in connection with the motions of the heavenly bodies, but are found to apply equally to all motions great or little. These three fundamental laws are as follows : I I Corpus omne perseverare in statu suo Every body perseveres in its state of quiescendi v^el movendi uniformiter in rest, or of uniform motion in a right directum, nisi quatenus illud a viribus line, unless it is compelled to change impressis cogitur statum suum mutare. that state by forces impressed thereon. II II Mutationem motus proportionalem The alteration of motion is ever esse vi motrici impressae, et fieri secun- proportional to the motive force im- dum lineam rectam qua vis ilia im- pressed; and is made in the direction of primitur. the right line in which that force is impressed. III III Actioni contrariam semper et aequa- To every action there is always lem esse reactionem: sive corporum opposed an equal reaction: or the duorum actione? in se mutuo semper mutual actions of two bodies upon each esse aequales et in partes contrarias other are always equal, and directed to dirigi. contrary parts. THE TETRAKINETIC THEORY 277 Newton's third law of the equaUty of action and reaction is the foundation of the modern doctrine of energy, not only in the Newto- nian sense but in the most general sense. Newton divined the prin- ciple of the conservation of energy in mechanics; Rumford (1798) maintained the universality of the laws of energy; Joule (1843) established the particular principle of the conservation of energy, namely the exact equivalence between the amount of heat produced and the amount of mechanical energy destroyed; and Helmholtz, in his great memoir tjber die Erhaltung der Kraft, extended this system of conservation of energy throughout the whole range of natural phenomena. A familiar instance of the so-called transformation of energy is where the sudden arrest of a cool but rapidly moving body produces heat. This was developed as ih.Q first law of thermodynamics. At the same time there arose the distinction between potential energy, which is stored away in some latent form or manner so that it can be drawn upon for work — such energy being exemplified me- chanically by the bent spring, chemically by gunpowder, and elec- trically by a Leyden jar — and kinetic energy, the active energy of motion and of heat. While all active mechanical energy or work may be converted into an equivalent amount of heat, the opposite process of turning heat into work involves more or less loss, dissipation, or degradation of energy. This is known as the second law of thermodynamics and is the outgrowth of a principle discovered by Sadi Carnot (1824) and developed by Kelvin (1852, 1853). The far-reaching conception of cyclic processes in energy enunciated in Kelvin's principle of the dissipation of available energy puts a diminishing limit upon the amount of heat energy available for mechanical purposes. The avail- able kinetic energy of motion and of heat which we can turn into work or mechanical effect is possessed by any system of two or more bodies in virtue of the relative rates of motion of their parts, velocity being essentially relative. These two great dynamical principles that the energy of motion can be converted into an equivalent amount of heat, and that a certain amount of heat can be converted into a more limited amount of power were discovered through observations on the motions of larger masses of matter, but they are believed to apply equally to such motions as are involved in the smallest electrically charged atoms (ions) of the chemical elements and the particles flying off in radiant energy as phosphorescence. Such movements of infinitesimal particles undcrhc 278 READINGS IN EVOLUTION, GENETICS, AND EUGENICS all the physicochemical laws of action and reaction which have been observed to occur within living things. In all physicochemical processes within and without the organism by which energy is cap- tured, stored, transformed, or released the actions and reactions are equal, as expressed in Newton's third law. Actions and reactions refer chiefly to what is going on between the parts of the organism in chemical or physical contact, and are subject to the two dynamical principles referred to above. Inter- actions, on the other hand, refer to what is going on between material parts which are connected with each other by other parts, and cannot be analyzed at all by the two great dynamical principles alone without a knowledge of the structure which connects the interacting parts. For example, in interaction between distant bodies the cause may be very feeble, yet the potential or stored energy which may be liberated at a distant point may be tremendous. Action and reaction are chiefly simultaneous, whereas interaction connects actions and reac- tions which are not simultaneous; to use a simple illustration: when one pulls at the reins the horse feels it a little later than the moment at which the reins are pulled — there is interaction between the hand and the horse's mouth, the reins being the interacting part. An interacting nerve-impulse starting from a microscopic cell in the brain may give rise to a powerful muscular action and reaction at some distant point. An interacting enzyme, hormone, or other chemical messenger circulating in the blood may profoundly modify the growth of a great organism. Out of these physicochemical principles has arisen the conception of a living organism as composed of an incessant series of actions and reactions operating under the dynamical laws which govern the transfer and transformation of energy. The central theory which is developed in our speculation on the Origin of Life is that every physicochemical action and reaction concerned in the transformation, conservation, and dissipation of energy, produces also, either as a direct result or as a hy-product a physicochemical agent of interaction which permeases and affects the organism as a whole or affects only some special part. Through such interaction the organism is made a unit and acts as one, because the activities of all its parts are correlated. This idea may be expressed in the following simplified scheme of the functions or physiology of the organism : THE TETRAKINETIC THEORY 279 Action AND Reaction . Interaction . Action AND Reaction Functions of the Functions of the Functions of the Capture, Storage, Coordination, Balance, Capture, Storage, and Release of Cooperation, Compensation, and Release of Energy Acceleration, Retardation, Energy of Actions and Reactions Since it is known that 7na}iy actions and reactions of the organ- ism — such as those of general and locaUzed growth, of nutrition, of respiration — are coordinated with other actions and reactions through interaction, it is but a step to extend the principle and suppose that all actions and reactions are similarly coordinated; and that while there was an evolution of action and reaction there was also a cor- responding evolution of interaction, for without this the organism would not evolve harmoniously. Evidence for such universality of the interaction principle has been accumulating rapidly of late, especially in experimental medicine and in experimental biology. It is a further step in our theory to suppose that the directing power of heredity which regulates the initial and all the subsequent steps of development in action and reaction, gives the orders, hastens development at one point, retards it at another, is an elaboration of the principle of interaction. In lowly organisms like the monads these interactions are very simple; in higher organisms like man these interactions are elaborated through physicochemical and other agents, some of which have already been discovered although doubtless many more await discovery. Thus we conceive of the origin and development of the organism as a con- comitant evolution of the action, reaction, and interaction of energy. Actions and reactions are borrowed from the inorganic world, and elaborated through the production of the new organic chemical compounds; it is the peculiar evolution and elaboration of the physi- cal principle of interaction which distinguishes the living organism. Thus the evolution of life may be rewritten in terms of invi sibe energy, as it has long since been written in terms of visible form. All visible tissues, organs, and structures are seen to be the more or less simple or elaborate agents of the different modes of energy. One after another special groups of tissues and organs are created and co- ordinated — organs for the capture of energy from the morganic environ- ment and from the Hfe environment, organs for the storage of energy, organs for the transformation of energy from the potential state into 28o READINGS IN EVOLUTION, GENETICS, AND EUGENICS the states of motion and heat. Other agents of control are evolved to bring about a harmonious balance between the various organs and tissues in which energy is released, hastened or accelerated, slowed down or retarded, or actually arrested or inhibited. In the simplest organisms energy may be captured while the organism as a whole is in a state of rest; but at an early stage of life special organs of locomotion are evolved by which energy is sought out, and organs of prehension by which it may be seized. Along with these motor organs are developed organs of ofense and defense of many kinds, by means of which stored energy is protected from cap- ture or invasion by other organisms. Finally, there is the most mysterious and comprehensive process of all, by which all these manifold modes of energy are reproduced in another organism. THE FOUR COMPLEXES OF ENERGY The theoretic evolution of the four complexes is somewhat as follows: 1. In the order of time the Inorganic Environment comes first; energy and matter are first seen in the sun, in the earth, in the air, and in the water — each a very wonderful complex of energies in itself. They form, nevertheless, an entirely orderly system, held together by gravitation, moving under Newton's laws of motion, subject to the more newly discovered laws of thermodynamics. In this complex we observe actions and reactions, the sum of the taking in and giving out of energy, the conservation of energy. We also observe inter- actions wherein the energy released at certain points may be greater than the energy received, which is merely a stimulus for the beginning of the local energy transformations. This energy is distributed among the eighty or more chemical elements of the sun and other stars. These elements are combined in plants into complex substances, gener- ally with a storage of energy. Such substances are disintegrated into simple substances in animals, generally with a release of energy. All these processes are termed by us physicochemical. 2. With life something new appears in the universe, namely, a union of the internal and external adjustment of energy which we appropriately call an Organism. In the course of the evolution of life every law and property in the physicochemical world is turned to advantage; every chemical element is assembled in which inorganic properties may serve organic functions. There is an immediate or gradual separation of the organism into two complexes of energy, THE TETRAKINETIC THEORY 281 namely, first, the energy complex of the organism, which is perishable with the term of life of the individual, and second, the germ or heredity substance, which is perpetual. 3. The idea that the germ is an energy complex is an as yet un- proved hypothesis; it has not been demonstrated. The Heredity-Germ in some respects bears a likeness to latent or potential interacting energy, while in other respects it is entirely unique. The supposed germ energy is not only cumulative but is in a sense imperishable, self- perpetuating, and continuous during the whole period of the evolution of life upon the earth, a conception which we owe chiefly to the law of the continuity of the germ-plasm formulated by Weismann. Some of the observed phenomena of the germ in Heredity are chiefly analogous to those of interaction in the Organism, namely, directive of a series of actions and reactions, but in general we know no complete physical or inorganic analogy to the phenomena of heredity; they are unique in nature. 4. With the multiplication and diversification of individual or- ganisms there enters a new factor in the environment, namely, the energy complex of the Life Environment. Thus there are combined certainly three and, possibly, four com- plexes of energy, of which each has its own actions, reactions, and interactions. The evolution of life proceeds by sustaining these actions, reactions, and interactions and constantly building up new ones : at the same time the potentiality of reproducing these actions, re- actions, and interactions in the course of the development of each new organism is gradually being accumulated and perpetuated in the germ. From the very beginning every individual organism is competing with other organisms of its own kind and of other kinds, and the law of the survival of the" fittest is operating between the forms and func- tions of organisms as a whole and between their separate actions, reactions, and interactions. This, as Weismann pointed out, while apparently a selection of the individual organism itself, is actually a selection of the heredity-germ complex, of its potentialities, powers, and predispositions. Thus Selectioii is not a form of energy, nor a part of the energy complex; it is an arbiter between different com- plexes and forms of energy; it antedates the origin of life just as adaptation or fitness antedates the origin of life, as remarked by Henderson. Thus we arrive at a conception of the relations of organisms to each other and to their environment as of an enormous and always 282 READINGS IN EVOLUTION, GENETICS, AND EUGENICS increasing complexity, sustained through the interchange of energy. Darwin's principle of the survival or elimination of various forms of living energy is, in fact, adumbrated in the survival or elimination of various forms of lifeless energy as witnessed among the stars and planets. In other words, Darwin's principle operates as one of the causes of evolution in making the lifeless and living worlds what they now appear to be, but not as one of the energies of evolution. Selec- tion merely determines which one of a combination of energies shall survive and which shall perish. The complex of four interrelated sets of physicochemical energies which I have previously set forth as the most fundamental biologic scheme or principle of development may now be restated as follows: In each organism the phenomena of life represent the action, reaction, and interaction of four complexes of physicochemical energy, namely, those of (j) the Inorganic Environment, (2) the developing Organism {protoplasm and hody-chromatin) , (3) the germ or Heredity -Chromatin, {4) the Life Environment. Upon the resultant actions, reactions^ and interactions of potential and kinetic energy in each organism Selection is constantly operating wherever there is competition with the correspond- ing actions, reactions, and interactions of other organisms. This principle I shall put forth in different aspects as the central thought of these lectures, stating at the outset and often recurring to the admission that it involves several unknown principles and especially the largely hypothetical question whether there is a relation between the action, reaction, and interaction of the internal energies of the germ or heredity-chromatin with the external energies of the inorganic environment, of the developing organism, and of its life environment. In other words, while this is a principle which largely governs the Organism, it remains to be discovered whether it also governs the causes of the Evolution of the Germ. As observed in the preface we are studying not one but four simultaneous evolutions. Each of these evolutions appears to be almost infinite in itself as soon as we can examine it in detail, but of the four that of the germ or heredity-chromatin so far surpasses all the others in complexity that it appears to us infinite. The physicochemical relations between these four evolutions, including the activities of the single and of the multiplying organisms of the Life Environment, may be expressed in diagrammatic form as follows : THE TETRAKINETIC THEORY 283 Organism A Under Newtoti's Laws of Motion and Modern Thermodynamics Actions, Reactions, and Interactions of the 1 . InorganicEnvironmetit: physicochemical en- ergies of space, of the sun, earth, air, and water. 2. Organism,: physicochemical en- ergies of the devel- oping individual in the tissues, cells, protoplasm, and cell- chromatin. 3. Heredity-Germ: physicochemical en- ergies of the heredity- chro matin included in the reproductive cells and tissues. 4. Life Environment: physicochemical en- ergies of other or- ganisms. Under Darwin^ s Law of Natural Selection Survival of the fittest: competition, selec- tion, and elimination of the energies and forms. Organisms B-Z Under Newton's Laws of Motion and Modern Thermodynamics Actions, Reactions, and Interactions of the 1. InorganicEnvironmcnt: physicochemical en- ergies of space, of the sun, earth, air, and water. 2. Organism: physicochemical en- ergies of the devel- oping individual in the tissues, cells, protoplasm, and ccU- chromatin. 3. IIcredity-Germ: physicochemical en- ergies of the heredity- chromatin included in the reproductive cells and tissues. 4. Life Environment: physicochemical en- ergies of other or- ganisms. If a single name is demanded for this conception of evolution it might be termed the tetrakinetic theory in reference to the four sets of internal and external energies which play upon and within every individual and every race. In respect to form it is a tetraplastic theory in the sense that every living plant and animal form is plas- tically moulded by four sets of energies. The derivation of this conception of life and of the possible causes of evolution from the laws which have been developed out of the Newtonian system, and from those of the other great Cambridge philosopher, Charles Darwin, are clearly shown in the above diagram. PART IV GENETICS CHAPTER XX THE SCOPE AND METHODS OF GENETICS H. H. N. DEFINITIONS " Genetics is the science which seeks to account for the resem- blances and the differences which are exhibited among organisms related by descent." — Babcock and Clausen. "Genetics may be defined as the science which deals with the coming into being of organisms. It does not refer, however, to the first creation of organic beings, but rather to the present every-day creation of new individuals or new races. It refers particularly to the part that parent organisms have in bringing new organisms into being and to the influence which parents exert on the characteristics of their offspring. In this sense it is nearly equivalent to the term heredity.'' — W. E. Castle. ''Heredity may be defined as organic resemblance based on de- scent." — W. E. Castle. ''Heredity is commonly defined as the tendency of offspring to develop characters like those of the parents." — Babcock and Clausen. THE SCOPE AND METHODS OF GENETICS Genetics is the study of evolution from a new point of view. The great evolutionists of the past were devotees of the inductive method in science which consists of collecting data and devising theories to explain the data. None of the older evolutionists attempted to put their theories to experimental tests. Thus their theories, though in some respects well founded, have never reached that stage of scientific proof which involves the use of the experimental method. The new method in evolution is that of experiment under controlled conditions. If new characters arise before the eyes of the investigator in a known stock of animals or plants and the factors responsible for the change are known and are capable of control, it may be said that man has actually taken a hand in evolution. If new characters arise in a known stock, but from an unknown cause, the course of the new character in inheritance may be controlled and some knowledge of the 287 288 READINGS IN EVOLUTION, GENETICS, AND EUGENICS mechanism of heredity may be obtained by an analysis of its modes of heredity. It is this new experimental and analytic method of study- ing evolution that we have come to designate as genetics. Three principal methods of attack upon the problems of genetics have been successful in advancing our knowledge. a) Experimental breeding. — This method was first systematized by Mendel and consists of breeding together two individuals possessing certain more or less contrasting characters and determining the ratios in which the parental characters reappear in the offspring. This method has been extremely fruitful and in connection with the second method, that of cytology, has made clear much that was obscure to Darwin and his followers. b) Cytology. — This second method involves the microscopic study of the germ cells during the most critical periods of their cycle. It seems very probable that we can now view under the microscope the actual heredity machine and see how it works. c) The statistical method. — ^It is usually conceded that Sir Francis Galton was the first to use the method of statistics in the study of heredity. By means of correlation tables he was able to compare large groups of parents with large groups of offspring with respect to any unit character, and to state the degree of heredity in defin- ite mathematical terms. The modern science of biometry is used extensively at the present time for determining the degree of vari- ability of characters which vary only slightly or irregularly and the exact degree of correlation that exists between different hereditary characters. All three of these methods of attacking the problems of genetics have been fruitful in results and all are essential to an adequate under- standing of the workings of evolution. The subject-matter of genetics consists of: {a) a knowledge of the principles of ontogeny, the development of the individual from the germ-cell stage to the adult stage; {h) a knowledge of the behavior of the germ cells from one generation to the next, involving the so-called "origin" of germ cells, maturation and fertilization of germ cells, and the exact behavior of the chromosomes during the entire germ-cell cycle; (c) a knowledge of variation, including a determination of what distinct kinds of variation occur, where in ontogeny variations are initiated, the causes of variation, etc. ; {d) what kinds of variations are inherited and according to what laws — the whole subject of Men- delian heredity; {e) the determination of sex and the relation of 'sex THE SCOPE AND METHODS OF GENETICS 289 to heredity; (/) theories as to the mechanism that brings about the observed regularity in heredity, including theories of linkage, cross- overs, and other phases of neo-Mendelian heredity. THE IMPORTANCE OF THE CELL THEORY IN GENETICS All organisms are composed of minute units called cells. A cell is the smallest particle of living matter capable of growing and multi- plying. The only continuity between two successive generations is a cellular continuity, involving a continuous series of cell divisions. All cells are the offspring by division of previous cells. We distinguish two main categories of cells: body or somatic cells and germ cells. In germinal reproduction, the only kind of reproduction possible in the more highly differential animals, the material continuity between parent and offspring is through the germ cell. A parental germ cell produces an offspring. The germ cell therefore is called the hereditary bridge. The only way, then, in which a parent can transmit his or her characteristics to an offspring is through the germ cell. Every hereditary character, whether old or new, must have its differential cause in the germ cell. The germ cells are therefore called the bearers of the heritage, and in the next chapter Professor M. F. Guyer gives an account of the cellular basis of variation and heredity. CHAPTER XXI THE BEARERS OF THE HERITAGE AN ACCOUNT OF THE CELLULAR BASIS OF HEREDITY^ MICHAEL F. GUYER Structure of the cell. — Before we can understand certain necessary- details of the physical mechanism of inheritance we must inquire a little further into the finer structure of the cell and into the nature of cell-division. A typical cell, as it would appear after treatment with various stains which bring out the different parts more dis- tinctly, is shown in Fig. 43. Typical, not that any particular kind of living cell resembles it very closely in appearance, but because it shows in a diagrammatic way the essential parts of a cell. In the diagram, there are two well-marked regions : a central nucleus and a peripheral cell-body or cytoplasm. Other structures are pictured, but only a few of them need command our attention at present. At one side of the nucleus one observes a small dot or granule surrounded by a denser area of cytoplasm. This body is called the centrosome. The nucleus in this instance is bounded by a well-marked nuclear membrane and within it are several substances. What appear to be threads of a faintly staining material, the linin, traverse it in every direction and form an apparent network. The parts on which we wish particularly to rivet our attention are the densely stained substances scattered along or imbedded in the strands of this network in irregular granules and patches. This substance is called chromatin. It takes its name from the fact that it shows great affinity for certain stains and becomes intensely colored by them. This deeply colored portion of the cell, the chromatin, is by most biologists regarded as of great importance from the standpoint of heredity. One or more larger masses of chromatin or chromatin-like material, known as chromatin nucleoli, are often present, and not infrequently a small spheroidal body, differing in its staining reactions from the chromatin-nucleolus and sometimes called the true nucleolus, exists. Cell-division. — In the simplest type of cell-division the nucleus first constricts in the middle, and finally the two halves separate. * From M. F. Guyer, Being Well Born (copyright 19 16). Used by special permission of the publishers, The Bobbs-Merrill Company. 290 THE BEARERS OF THE HERITAGE 291 This separation is followed by a similar constriction and final division of the entire cell-body, which results in the production of two new cells. This form of cell-division is known as s'wiple or direct dimsion. Such a simple division, while found in higher animals, is less frequent and apparently much less significant than another type of division which involves profound changes and rearrangements of the nuclear contents. The latter is termed mitotic or indirect cell-division. Fig. 44 illustrates some of the stages which are passed through in indirect cell-division. The centrosome which lies passively at the side of the nucleus in the typical cell (Fig. 44, a) awakens to activity, divides and the two components come to lie at the ends of a fibrous spindle. In Centrosome. ■Nucleus. Chromatin. - — Triie nucleolus (plasmosome). Chromatin »— nucleolus. Linin — network. 1 _-j> P last ids. V Vacuole. Aletaf'lasnt (passne buiies). Fig. 43. — Diagram of a cell, showing various parts. {From Guyer.) the meantime, the interior of the nucleus is undergoing a transforma- tion. The granules and patches of chromatin begin to flow together along the nuclear network and become more and more crowded until they take on the appearance of one or more long deepl\'- stained threads wound back and forth in a loose skein in the nucleus (Fig. 44, b). If we examine this thread closely, in some forms it may be seen to consist of a series of deeply-stained chromatin granules packed closely together, intermingled with the substance of the original nuclear network. As the preparations for division go on the coil in the nucleus breaks up into a number of segments which are designated as chromosomes (Fig. 44, c). The nuclear membrane disappears. The chromosomes 292 READINGS IN EVOLUTION, GENETICS, AND EUGENICS and the spindle-fibers ultimately come to lie at the equator of the spindle as shown in Fig. 44, d. Each chromosome splits lengthwise to form two daughter chromosomes which then diverge to pass to the mss Fig. 44. — Diagram showing representative stages in mitotic or indirect cell- di\dsion. a, resting cell with reticular nucleus and single centrosome; h, the two new centrosomes formed by division of the old one are separating and the nucleus is in the spireme stage; c, the nuclear wall has disappeared, the spireme has broken up into six separate chromosomes, and the spindle is forming between the two centrosomes; J, equatorial plate stage in which the chromosomes occupy the equator of the spindle; e,f, each chromosome splits lengthwise and the daughter chromosomes thus formed approach their respective poles; g, reconstruction of the new nuclei and division of the cell body; h, cell division completed. {From Qiiyer.) poles of the spindle (Fig. 44, e and/). Thus each end of the spindle comes ultimately to be occupied by a set of chromosomes. Moreover, each set is a duplicate of the other, because the substance of any individual chromosome in one group has its counterpart in the other. THE BEARERS OF THE HERITAGE 293 In fact this whole complicated system of indirect division is regarded by most biologists as a mechanism for bringing about the precise halving of the chromosomes. The chromosomes of each group at the poles finally fuse and two new nuclei, each similar to the original one, are constructed (Fig. 44, g and h). In the meantime a division of the cell-body is in progress which, when completed, results in the formation of two complete new cells. As all living matter, if given suitable food, can convert it into living matter of its own kind, there is no difficulty in conceiving how the new cell or the chromatin material finally attains to the same bulk that was characteristic of the parent cell. In the case of the chro- matin, indeed, it seems that there is at times a precocious doubling of the ordinary amount of material before the actual division occurs. Chromosomes constant in number and appearance. — With some minor exceptions, to be noted later, which increase rather than detract from the significance of the facts, the chromosomes are always the same in number and appearance in all individuals of a given species of plants or animals. That is, every species has a fixed number which regularly recurs in all of its cell-divisions. Thus the ordinary cells of the rat, when preparing to divide, each display sixteen chromo- somes, the frog or the mouse, twenty-four, the lily twenty-four and the maw- worm of the horse only four. The chromosomes of dififerent kinds of animals or plants may differ very much in appearance. In some they are spherical, in others rod-like, filamentous or perhaps of other forms. In some organisms the chromosomes of the same nucleus may differ from one another in size, shape, and proportions, but if such differences appear at one division they appear at others, thus showing that in such cases the differences are constant from one generation to the next. Significance of the chromosomes. — The question naturally arises as to what is the significance of the chromosomes. Why is the accur- ate adjustment which we have noted for their division necessary ? The very existence of an elaborate mechanism so admirably adapted to their precise halving, predisposes one toward the belief that the chromosomes have an important function which necessitates the retention of their individuality and their equal division. Many biolo- gists accept this along with other evidences as indicating that in chromatin we have a substance which is not the same throughout, that 294 READINGS IN EVOLUTION, GENETICS, AND EUGENICS different regions of the same chromosome have different physiological values. When the cell prepares for divisions, the granules, as we have seen, arrange themselves serially into a definite number of strands which we have termed chromosomes. Judging from all available evidence, the granules are self-propagating units; this is, they can grow and reproduce themselves. So that what really happens in mito- sis in the splitting of the chromosomes is a precise halving of the series of individual granules of which each chromosome is constituted, or in other words each granule has reproduced itself. Thus each of the two daughter cells presumably gets a sample of every kind of chromosomal particle, hence, the two cells are qualitatively alike. To use a homely illustration we may picture the individual chromosomes to ourselves as so many separate trains of freight cars, each car of which is loaded with different merchandise. Now, if every one of the trains could split along its entire length and the resulting halves each grow into a train similar to the original, so that instead of one there would exist two identical trains, we should have a phenomenon analogous to that of a dividmg chromosome. Cleavage of the egg. — It is through a series of such divisions as these that the zygote or fertilized egg-cell builds up the tissues and organs of the new organism. The process is technically spoken of as cleavage. Cleavage generally begins very shortly after fertilization. The fertile egg-cell divides into two, the resulting cells divide again and thus the process continues, with an ever-increasing number of cells. Chief processes operative in building the body. — Although of much interest, space will not permit of a discussion in detail of the building up of the special organs and tissues of the body. It must suffice merely to mention the four chief processes which are operative. These are, (i) infoldings and outfoldings of the various cell com- plexes; (2) multiplication of the component cells; (3) special changes {histological diferentiation) in groups of cells; and (4) occasionally resorption of certain areas of parts. The origin of the new germ-cells. — On account of the unusual importance from the standpoint of inheritance, which attaches to the germ-cells, a final word must be said about their origin in the embryo. While the evidence is conflicting in some cases, in others it has been well established that the germ-cells are set apart very early from the cells which are to differentiate into the ordinary body tissues. Fig. 45, A, shows a section through the eight-celled stage of Miastor, a fly. THE BEARERS OF THE HERITAGE 295 in which a single large, primordial germ-cell (/>. g. c.) has already been set apart at one end of the developing embryo. The nuclei of the rest of the embryo still lie in a continuous protoplasmic mass which has not yet divided up into separate cells. The densely stainerl nuclei at the opposite end of the section are the remnants of nurse-cells which originally nourished the tgg. Fig. 45, B, is a longitudinal section o'6 g Fig. 45. — A, germ-cell (p.g.c.) set apart in the eight-celled stage of cleavage in Miastor americana. {After Hegner.) The walls of the remaining seven somatic cells have not yet formed, though the resting or the dividing {M p) nuclei may be seen; c R, chromatin fragments cast off from the somatic cells; B, section length- wise of a later embryo of Miastor; the primordial egg-cells (odgj) are conspicuous. {From Guyer, after Hegner.) through a later stage in the development of Miastor; the primitive germ-cells (oog) are plainly visible. Still other striking examples might be cited. Even in vertebrates the germ-cells may often be detected at a very early period. Significance of the early setting apart of the germ-cells. —It is of great importance for the reader to grasp the significance of this early setting apart of the germ-cells because so much in our future hinges on this fact. The truth of the statement made in a previous chapter that the body of an individual and the reproductive substance 296 READINGS IN EVOLUTION, GENETICS, AND EUGENICS in that body are not identical now becomes obvious. For in such cases as those just cited one sees the germinal substance which is to carry on the race set aside at an early period in a given individual; it takes no part in the formation of that individual's body, but remains a slumbering mass of potentialities which must bide its time to awaken into expression in a subsequent generation. Thus an egg does not develop into a body which in turn makes new germ-cells, but body and germ-cells are established at the same time, the body harboring and nourishing the germ-cells, but not generating them. The same must be true also in many cases where the earliest history of the germ-cells cannot be visibly followed, because in any event, in all higher animals, they appear long before the embryo is mature and must therefore be descendants of the original egg-cell and not of the functioning tissues of the mature individual. This need not necessarily mean that the germ-cells have remained wholly unmodified or that they continue uninfluenced by the conditions which prevail in the body, especially in the nutritive blood and lymph stream, although as a matter of fact most biologists are extremely skeptical as to the probability that influences from the body beyond such general indefinite effects as might result from under-nutrition or from poisons carried in the blood, modify the intrinsic nature of the germinal substances to any measur- able extent. Germinal continuity. — The germ-cells are collectively termed the germinal protoplasm and it is obvious that as long as any race continues to exist, although successive individuals die, some germinal protoplasm is handed on from generation to generation without interruption. This is known as the theory of germinal continuity. When the organ- ism is ready to reproduce its kind the germ-cells awaken to activity, usually undergoing a period of multiplication to form more germ-cells before finally passing through a process of what is known as matura- tion^ which makes them ready for fertilization. The maturation process proper, which consists typically of two rapidly succeeding divisions, is preceded by a marked growth in size of the individual cells. Individuality of chromosomes. — Before we can understand fully the significance of the changes which go on during maturation we shall have to know more about the conditions which prevail among the chromosomes of cells. As already noted each kind of animal or plant has its own characteristic number and types of chromosomes when these appear for division by mitosis. In many organisms the chromo- somes are so nearly of one size as to make it difficult or impossible to THE BEARERS OF THE HERITAGE 297 be sure of the identity of each individual chromosome, but on the other hand, there are some organisms known in which the chromo- somes of a single nucleus are not of the same size and form (Fig. 46). These latter cases enable us to determine some very significant facts. Where such differences of shape and proportion occur they are constant in each succeeding division so that similar chromosomes may be iden- tified each time. Moreover, in all ordinary mitotic divisions where the conditions are accurately known, these chromosomes of ditTcrent types are found to be present as pairs of similar elements; that is, there are two of each form or size. Pairs of similar chromosomes in the nucleus because one chromo- some comes from each parent— When we recall that the original fertilized egg from which the individual develops is really formed by the union of two gametes, ovum and spermatozoon, and that each B Fig. 46. — A, chromosomes of the mosquito (Culex). (After Stevens.) B, chromosomes of the fruit fly (Drosophila). (After Metz.) Both of these forms have an unusually small number of chromosomes. (From Guyer.) gamete, being a true cell, must carry its own set of chromosomes, the significance of the pairs of similar chromosomes becomes evident; one of each kind has probably been contributed by each gamete. This means that the zygote or fertile ovum contains double the number of chromosomes possessed by either gamete, and that, moreover, each tissue-cell of the new individual will contain this dual number. For, as we have seen, the number of chromosomes is, with possibly a few exceptions, constant in the tissue-cells and early germ-cells in suc- cessive generations of individuals. For this to be true it is obvious that in some way the nuclei of the conjugating gametes have come to contain only half the usual number. Technically the tissue-cells are said to contain the diploid number of chromosomes, the gametes the reduced or haploid number. In maturation the number of chromosomes is reduced by one- half. — This halving, or as it is known, reduction in the number of chromosomes is the essential feature of the process of maturation. It 298 READINGS IN EVOLUTION, GENETICS, AND EUGENICS is accomplished by a modification in the mitotic division in which instead of each chromosome spHtting lengthwise, as in ordinary mito- sis, the chromosomes unite in pairs (Fig. 47, b), a process known tech- nically as synapsis, and then apparently one member of each pair passes entire into one new daughter cell, the other member going to the other daughter cell (Fig. 47, c). In the pairing preliminary to this reduction division, leaving out of account certain special cases to be considered Fig. 47. — Diagram to illustrate spermatogenesis, a, showing the diploid number of chromosomes (six is arbitrarily chosen) as they occur in divisions of ordinary cells and spermatogonia; h, the pairing (synapsis) of corresponding mates in the primary spermatocyte preparatory to reduction; c, each secondary spermatocyte receives three, the haploid number of chromosomes; d, division of the secondary spermatocytes to form e, spermatids, which transform into /, sper- matozoa. {From Guyer.) later, according to the best evidence at our command the union always takes place between two chromosomes which match each other in size and appearance. Since one of these is believed to be of maternal and the other of paternal origin, the ensuing division separates correspond- ing mates and insures that each gamete gets one of each kind of chro- mosome although it appears to be a matter of mere chance whether or not a given cell gets the paternal or the maternal representative of that kind. THE BEARERS OF THE HERITAGE 299 Maturation of the sperm-cell.— In the maturation of the male gamete the germ-cell, now known as a spermalogoniion, increases greatly in size to become a primary spermatocyte. In each primary spermatocyte the pairing of the chromosomes already alluded to occurs as indicated in Fig. 47, where six is taken arbitrarily to indicate the ordinary or diploid number of chromosomes, and three the reduced or haploid number. The division of the primary spermatocyte gives rise to two secondary spermatocytes {c), the i)aired chromosomes separating in such a way that a member of each pair goes to each Fig. 48. — Diagram to illustrate oogenesis, a, showing the diploid number of chromosomes (six is arbitrarily chosen) as they occur in ordinary cells and in oogonia; 6, the pairing of corresponding mates preparatory to reduction; r. d, the reduction division, giving off the first polar body; e, egg preparing to give off the second polar body, first polar body ready for division; /, second polar body ready for division; g, second polar body given off, division of first polar body completed. The egg nucleus, now known as the female pronucleus, and each polar body contain the reduced or haploid number of chromosomes. {From Guycr.) secondary spermatocyte. Each secondary spermatocyte {d) soon divides again into two spermatids (e), but in this second division the chromosomes each split lengthwise as in an ordinary division so that there is no further reduction. In some forms the reduction division occurs in the secondary spermatocytes instead of the primar\'. Kach spermatid transforms into a mature spermatozoon (/). The sper- matozoa of most animals are of linear form, each with a head, a middle-piece and a long vibratile tail which is used for locomotion. The head consists for the most part of the transformed nucleus and is consequently the part which bears the chromosomes. 300 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Maturation of the egg-cell. — As regards the behavior of the chromosomes the maturation of the ovum parallels that of the sperm- cell. There are not so many primordial germ-cells formed and only one out of four of the ultimate cells becomes a functional egg. As in maturation of the sperm-cell there is a growth period in which oogonia enlarge to become primary oocytes (Fig. 48, b). In each primary Spermatogenesis Oogenesis i gofiia \^'^ MulltpliCQlion Period . t \ J>/Jm9rv.. fAt^ 1 ipermocpc i/te V n^ »/ ' CroMth period Pairing of Chromosomes ©•' '% \ } Re<^ucing division Secanaory ( ^ J per mot o- ' " ei/ttj Sperm f <' Saerm eRozoa Obgonio .-■■ s Primori^ C80/l^^ Seconc/ari/ odcytt fpyufTi and ftrsl ^ polar bo&jf) /loture ofufoy , anc/ ptior bt^itj. ©nature oyum\ full number of Chromoiomti restored Z If gate or Jeriiliztd ovum Fig. 49. — Diagram showing the parallel between maturation of the sperm- cell and maturation of the ovum. {From Guyer.) oocyte as in the primary spermatocyte the chromosomes pair and two rapidly succeeding divisions follow in one of which the typical numeri- cal reduction in the chromosomes occurs. A peculiarity in the maturation of the ovum is that there is a very unequal division in the cytoplasm in cell-division so that three of the resulting cells usually termed polar bodies are very small and appear like minute buds on the side of the fourth or egg-cell proper. THE BEARERS OF THE HERITAGE 301 The scheme of this formation of the polar bodies is indicated in Fig. 48. In Fig. 48, b the chromosomes are seen paired and ready for the first division; that is, for the formation of the first polar body. Figs. 48, c, d show the giving off of this body. Note that while only a small proportion of the cytoplasm passes into this tiny cell, its chro- matin content is as great as that of the ovum. A second polar body (Fig. 48, e) is formed by the egg, but in this case each chromosome spHts lengthwise, as in ordinary mitosis, and there is no further numeri- cal reduction. In the meantime, typically, a third polar bo(h- is formed by division of the first. (Stages e, /, g.) Parallel between the maturation of sperm- and egg-cell. — This rather complex procedure of the germ-cells will be rendered more intelHgible through a careful study of Figs. 47, 48, and 49, which indicates the parallel conditions in spermatogenesis and oogenesis. The view now generally held regarding the polar bodies is that they are really abortive eggs. They later disappear, taking no part in embryo-formation. It can readily be seen how such an unequal division is advantageous to the large cell, for it receives all of the rich store of food material that would be distributed among the four cells if all were of equal size. This increased amount of food is a favorable provision for the forthcoming offspring whose nourishment is thus more thoroughly insured. On the other hand, all of the sperm-cells develop int3 complete active forms, which, as aforesaid, usually become very much elongated and develop a motile organ of some kind. In such cells an accumula- tion of food to any large extent would hinder rather than help them, because it would seriously interfere with their activity. Fertilization. — In fertilization (Fig. 50) the spermatozoon pene- trates the wall of the ovum and after undergoing considerable altera- tion its nucleus fuses with the nucleus of the egg. In some forms only the head (nucleus) and middle-piece enter, the tail being cut off by a so-called fertilization membrane which forms at the surface of the egg and effectually blocks the entrance of other spermatozoa. Thus normally only one spermatozoon unites with an egg. In some forms while several may enter the egg only one becomes functional. As soon as the nucleus of the spermatozoon, now known as the ffuile pronucleus, reaches the interior of the egg, it enlarges and becomes simi- lar in appearance to the female pronucleus. It swings around in such a way (Fig. 50, b) that the middle piece, now transformed into a centro- some, lies between it and the female pronucleus. The two pronuclei 302 READINGS IN EVOLUTION, GENETICS, AND EUGENICS {c, d, e) , each containing the reduced number of chromosomes, approach, the centrosome divides, the nuclear walls disappear, the typical division spindle forms, and the chromosomes of paternal and maternal origin respectively come to lie side by side at the equator of the spindle ready for the first division or cleavage (/, g). It will be noted that the Fig. 50. — Diagram to illustrate fertilization; $, male pronucleus; ?, female pronucleus; observe that the chromosomes of maternal and paternal origin respectiv^ely do not fuse. {From Giiyer.) individual chromosomes do not intermingle their substance at this time, but each apparently retains its own individuality. There is considerable evidence which indicates that throughout life the chro- mosomes contributed by the male parent remain distinct from those of the female parent. Inasmuch as each germ-cell, after maturation, contains only half the characteristic number of chromosomes, the original number is restored in fertilization. Significance of the behavior of chromosomes. — The question confronts us as to what is the significance of this elaborate system THE BEARERS OF THE HERITAGE 303 which keeps the chromosomes of constant size, shape and number; which partitions them so accurately in ordinary cell-division; and which provides for a reduction of their numbers by half in the germ-cell while yet securing that each mature gamete gets one of each kind of chromosome. Most biologists look on these facts i.s indicating that the chromosomes are specifically concerned in inheritance. In the first place it is recognized that as regards the definable characters which separate individuals of the same species, offspring may inherit equally from either parent. And it is a very significant fact that while the ovum and spermatozoon are very unequal in size themselves, the chromosomes of .he two germ-cells are of th^ same size and number. This parity in chromosomal contributi )n points clearly to the means by which an equal number of character deter- miners might be conveyed from each parent. Moreover it is mainly the nucleus of the sperm-cell in some organisms which enters the egg, hence the determiners from the male line must exist wholly or largely somewhere in the nucleus. And the bulk of the nucleus in the sper- matozoon consists of the chromosomes or their products. A single set of chromosomes derived from one parent only is sufficient for the production of a complete organism. — That a single or haploid set of chromosomes as seen in the gametes is sufficient contribution of chromatin for the production of a complete organism is proved by the fact that the unfertilized eggs of various animals (many echinoderms, worms, mollusks, and even the frog) may be artificially stimulated to development without uniting at all with a spermatozoon. The resulting individual is normal in every respect except that instead of the usual diploid number it has only the single or haploid number of chromosomes. Its inheritance of course is wholly of maternal origin. The converse experiment in echinoderms in which a nucleus of male origin (that is, a spermatozoon) has been introduced into an egg from which the original nucleus has been removed shows that the single set of chromosomes carried by the male gamete is also sufficient to cooperate with the egg-cytoplasm in developing a complete individual. The duality of the body and the singleness of the germ. — Since every maternal chromosome in the ordinary cell has an equivalent mate derived from the male parent, it follows therefore, supposing the chromosomes do have the significance in inheritance attributed to them, that as regards the measurable inheritable differences between two individuals, the ordinary organism produced through the union 304 REx\DINGS IN EVOLUTION, GENETICS, AND EUGENICS of the two germ-cells is, potentially at least, dual in nature. On the other hand through the process of reduction the gametes are provided with only a single set of such representatives. This duality of the body and singleness of the mature germ is one of the most striking facts that come to light in embryology. How well the facts lit in with the behavior of certain hereditary characters will be seen later in our dis- cussions of Mendelism. The cytoplasm not negligible in inheritance. — Just what part is played by the cytoplasm in inheritance is not clear, but it is probably by no means a negligible one. The cytoplasm of a given organism is just as distinctive of the species or of the individual of which it forms a part as are the chromosomes. It is well established that neither nucleus nor cytoplasm can fully function or even exist long without the other, and neither can alone produce the other. They undoubtedly must cooperate in building up the new individual, and the cytoplasm of the new individual is predominantly of maternal origin. It is obvious that all of the more fundamental characters which make up an organism, such, for instance, as make it an animal of a certain order or family, as a human being or a dog or a horse, are common to both parents, and there is no way of measuring how much of this fundamental constitution comes from either parent, since only closely related forms will interbreed. In some forms, moreover, the broader fundamental features of embryogeny are already established before the entrance of the spermatozoon. It is probable therefore that instead of asserting that the entire quota of characters which go to make up a complete individual are inherited from each parent equally, we are justified only in maintaining that this equality is restricted to those measurable differences which veneer or top off, as it were, the individual. We may infer that in the development of the new being the chromosomes of the egg together with those derived from the male work jointly on or with the other germinal contents which are mostly cytoplasmic materials of maternal origin. The chromosomes possibly responsible for the distinctiveness of given characters. — It seems probable that in the establishment of certain basic features of the organism the cooperation of the cytoplasm with chromatin of either maternal or paternal origin might accomplish the same end, but that certain distinctive touches are added or come cumulatively into expression through influences carried, predomi- nantly at least, in the chromatin from one as against the other parent. These last distinctive characters of the plant or animal constitute the THE BEARERS OF THE HERITAGE 305 individual differences of such organisms. In this connection it is a significant fact that in young hybrids between two distinct species the early stages of development, especially as regards symmetry and regional specifications, are exclusively or predominantly maternal in character, but the male influence becomes more and more apparent as development progresses until the final degree of intermcdiacy is attained. From the evidence at hand this much seems sure, that the paternal and maternal chromosomes respectively carry substances, l)e they ferments, nutritive materials or what not, that are instrumental in giving the final parity of personal characters which we observe to be equally heritable from either line of ancestry. It is clear that most of the characters of an adult organism cannot be merely the outcome of any unitary substance of the germ. Each is the product of many cooperating factors and for the final outcome any one cooperant is probably just as important in its way as any other. The individual characters which we juggle to and fro in our breeding experiments seem apexed, as it were, on more fundamental features of organic chemical constitution, polarity, regional differentiation, and physiological balance, but since such individual characters parallel so closely the visible segregations and associations which go on among the chromo- somes of the germ-cells it would seem that they, at least, are repre- sented in the chromosomes by distinctive cooperants which give the final touch of specificity to those hereditary characters which can be shifted about as units of inheritance. Sex and heredity. — Whatever the origin of fertilization may have been in the world of life, or whatever its earliest significance, the important fact remains that to-day it is unquestionably of very great significance in relation to the phenomena of heredity. For in all higher animals, at least, offspring may possess some of the character- istics originally present in either of two lines of ancestry, and this commingling of such possessions is possible only through sexual repro- duction. As has already been seen, in the pairing of chromosomes previous to reduction, the corresponding members of a pair always come together so that in the final segregation each gamete is sure to have one of each kind although whether a given chromosome of the haploid set is of maternal or paternal origin seems to be merely a matter of chance. Thus, for instance, if we arbitrarily represent the chromosomes of a given individual by ABC, abc, and regard A,B and C as of paternal and a, h, and c as of maternal origin, then in synapsis 3o6 READINGS IN EVOLUTION, GENETICS, AND EUGENICS only yl and a can pair together, B and b^ and C and c, but each pair operates independently of the other so that in the ensuing reduction division either member of a pair may get into a cell with either member of the other pairs. That is, the line up for division at a given reduc- tion might be any one of the following, 7 ^ T.^ T> • This would yield the folio wins; eight kinds of aoc aoC aBC aBc gametes, ABC, abc, ABc, abC, Abe, aBC, AbC, aBc, each bearing one of each kind of chromosome required to cover the entire field of characters necessary to a complete organism. And since each sex would be equally likely to have these eight types of gametes and any one of the eight in one individual might meet any one of the eight of the other, the possible number of combinations in the production of a new individual from such germ-cells would be 8X8, or 64. With the larger numbers of chromosomes which exist in most animals it is readily seen that the number of possible combinations becomes very great. Thus any individual of a species with twenty chromosomes — and many animals, including man, have more — would have ten pairs at the reduction period and could therefore form (2)^°, or 1,024 different gametes in each sex. And since any one of these in one sex would have an equal chance of meeting with any one in the oppo- site sex, the total number of possible different zygotes that might be produced would be (1,024)^, or 1,048,576. Sex, therefore, through recombinations of ancestral materials, undoubtedly means, among other things, the production of great diversity in offspring. CHAPTER XXII VARIATION^ ERNEST BROWN BABCOCK AND ROY ELWOOD CLAUSEN Organic differences, their nature and causes, have furnished abundant material for speculative enquiry since time immemorial. The great significance of the fact of organic individuality was not fully grasped until Lamarck founded his theory of evolution which postu- lated the progressive, imperceptible change of one species into another. It remained for Darwin to scrutinize all phases of organic life, past and present, wild and domesticated, in his search for a guiding ])rin- ciple which should explain the course of evolution. Darwin's hypothe- sis of natural selection assumes variability without enquiring into its causes, but this does not mean that Darwin was not concerned with the problem of causes. In both his Origin of Species and Variation in Animals and Plants under Domestication the causes of variability are often referred to and he suggested among others, the kind and amount of food, climatic changes and hybridization. Our respect for the great naturalist's keen perception deepens when we realize that very little has been added as yet to our knowledge of the causes of variation. The universality of variation. — Individuality is common to all organisms. No two trees, no two leaves, no two cells in a leaf are identical in every respect. Individuals sometimes appear exactly alike but even identical twins will be found to differ in some features. The shepherd knows his sheep individually and the orchardist his trees. Were there no differences in individuals there would be no changes in species and there could be no improvement of cultivated plants. ''Variation is at once the hope and despair of the breeder," ib.c hope because without it no improvement would be possible, the desixiir because very often, when improvement has been made, variation results in a tendency to fall below the standard previously reached. In the sugar beet, for example, a high percentage of sugar has been maintained by continually testing and selecting the ''mother" beets for the next crop of seed. However, this necessity for continual ^ From E. R. Babcock and R. E. Clausen, Genetics in Rehition to Ai^rieuHurr (copyright 1918). Used by special permission of the publishers, The McClraw- Hill Book Company. 307 3o8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS selection does not exist in respect to all important field crops although they are subject to the general law of variation. That this must be so is clear when we realize that many natural species as well as culti- vated varieties of plants are really mixtures of sub-species, varieties, or races and that upon being isolated these distinct forms reproduce their own particular type. This is most easily demonstrated in plants normally self-fertilized, yet in all naturally cross-fertilized plants and in higher animals this same endless diversity among individuals is even more marked. The variation concept. — As we have implied in the above remarks the term variation may be used in very different senses in referring to different phenomena. Thus variation within a species or va'riety means that the group in question is heterogeneous. Among indi- viduals variation may consist of differences between members of the same generation or between parents and offspring. Even when thus restricted, however, the term is apt to prove ambiguous. Hence it is necessary to give some thought to the sources, nature and causes of these individual differences in order that we may use clear-cut expres- sions which shall always convey to one another a concept of the same particular sort of organic difference. Classification of variations. — i. Heritahility. Character differ- ences either represent something specific in the germ or they are merely the effect of external stimuli upon the individual soma. In the first case they are inherited, although they will not reappear necessarily in all later generations or in all the progeny. In the second case they will not be inherited. This is a fundamental dis- tinction and may well serve as our primary basis of classification. According to heritahility variations are either germinal or somatic. Under germinal variations we recognize two sub-classes, combinations and mutations. Purely somatic variations will be referred to here- after as modifications. Modifications are non-heritable differences between the individuals of a race caused by the unequal influence of different environmental factors. Such variations frequently approximate continuity and, when studied statistically, display the normal variabiHty curve, which will be explained in a subsequent chapter. [See chap, xxv.] Combinations are heritable differences between the individuals of a race or between the offspring of a pair of parents caused by segrega- tion and recombination of hereditary units. They also frequently display the normal variability curve. VARIATION 309 Mutations are herital^le differences between parents and offspring which do not depend upon segregation and recombination. These three categories, as Baur has shown, are not to be recognized and separated merely according to appearances. The cause of any individual differences can usually be established only by careful breeding experiments; but by this means the separation of the three categories is always possible as the boundaries between them are quite sharp. Modifications are somatic effects of environmental differences and should not be confused with germinal changes which are some- times induced by natural or artificial means and which result in the production of mutations. Within this first category must be included all place-effects in plants and somatic environmental effects in ani- mals. Modifications comprise a large portion of what are commonly spoken of as fluctuations due to environment, but all cases of fluctua- ting variation are not modifications inasmuch as variations due to combinations frequently display the normal variability curve also. Modifications are not heritable. The second category, variation by combination of hereditary units is often confused with modification, as already stated, because of the fact that variations caused by segregation and recombination when studied statistically often dis- play the normal variability curve. This is especially apt to be the case in quantitative characters (those of size or weight) and segrega- tion and recombination may be the cause of gradation in color inten- sity. In autogamous (self-fertilized) organisms hybridization between races is sufficiently rare to be negligible in this connection, i. e., in such species the fluctuating variations are caused by the environment. But in allogamous organisms (those in which two individuals are necessary to accomplish sexual reproduction) fluctuating variations may be caused either by the environment, by segregation and recombination of factors, or by both causes acting together. We shall take up the third category, mutations, in a later chapter. For the present it is sufficient to remember that mutations are no doubt the least frequent of the three classes, that easily distinguishable mutations are comparatively rare, but that there may also occur true mutations of such moderate extent, as compared with the population, that their existence would only be detected by breeding tests, since their progeny would exhibit a different range of fluctuation from that of the population. 2. Nature. We may next enquire into the nature of variation as it affects the organism. Upon this basis we may distinguish between 3IO READINGS IN EVOLUTION, GENETICS, AND EUGENICS four classes: morphological, physiological, psychological and eco- logical. Morphological variations are differences in size and form. In general morphological variations have more significance for the biologist than for the agriculturist. However, in many products of the farm, size and conformation are of decided importance. Two sub-classes under morphological variations are meristic and homeotic variations. Meristic variations are diif erences in number of repeated parts such as the petals in a flower, the leaflets in a compound leaf or number of phalanges. Homeotic variations are differences caused by the replacement of one part by another, as the production of an antenna in place of an eye in an insect. Physiological variations are differences in quality and performance. Examples of qualitative variations are difference in degree of hardness of bone, flavor of meat, richness of milk, difference in normal color, resistance to drouth, frost or alkali. Variations in performance con- stitute the most important group for the producer. Differences in performance are sometimes, though not necessarily, associated with certain details of structure. Psychological variations are differences in mental traits. That mental and nervous conditions have very definite effects upon physical conditions is well known, but the problem of distinguishing between purposeful action and automatic response, between manifestations of reason and manifestations of instinct, is set for the students of animal behavior. While variations in mental characteristics have an impor- tant place in eugenics and merit the attention of livestock breeders, yet the inheritance of psychological characters must be more exten- sively investigated before the subject can be considered with profit in a fundamental study of genetics. Ecological variations are those differences between individuals that result from their fixed relation to the environment. These differences are especially noticeable in plants and are known as place-effects or place variations. This category includes some of the phenomena of variation in crop yield and hence is of immediate significance to agriculture. 3. According to differences between them there are two general classes of variations: first, the slight differences in every character which are always to be observed even among individuals of identical heredity; second, unusual, striking differences commonly known as sports. The first class are called normal, indefinite fluctuating or VARIATION 311 continuous variations and the second, abnormal, definite and discon- tinuous variations. It should be noted, however, that all discontin- uous variations are not necessarily definite or even distinguishable. Continuous variations when examined statistically are found to con- form to the law of statistical regularity. That is, if measured and plotted the graph will approximate the normal curve of variability. Continuous variations are either heritable (combinations) or non- heritable (modifications) and, as was stated above, the only certain method of determining the class in which "a given case may fall is the breeding test. Discontinuous variations are essentially discrete dif- ferences whether they be large or small. They are also either herit- able and there is no correlation between size and heritability. Thus the extremely large and small mustard plants, considered by them- selves, are discontinuous variations, but they are almost certainly due entirely to environmental difi'erences and seed fr m the small plant if grown under optimum conditions would produce plants of normal size. On the other hand, it is known that many minute differences in organisms are heritable. 4. According to direction variations are classed as orthogenetic and fortuitous. Orthogenetic variations are those differences found in individuals related by descent which form progressive series tending in a definite direction. Many remarkable illustrations are found among paleontological records of the evolution of animals. Occa- sional examples are found arnong short-lived or vegetatively propa- gated species. The remarkable series of variations of the Boston fern is a good example. Fortuitous variations are chance ditTerences occurring in all directions. 5. According to cause variations are either ectogcnctic, differences arising from conditions acting upon the organism from without; or autogenetic, differences resulting from strictly internal relations be- tween germ and s^ma. Variation and development. — Somatogenesis, in sexually produced multicellular organisms, includes the entire history o' cellular mulli- phcation and speciaHzation from the first cleavage of the fertilized (or parthenogenetic) egg to the completion of all adult features. From the standpoint of individual development it includes gamcto- genesis, for the production of sexual glands and of secondary sexual characters are merely phases of differentiation. Cell growth and cell function depend directly upon the activity of the living substance within the cell. The nature and degree of this activity depends upon 312 READINGS IX EVOLUTION, GENETICS, AND EUGENICS two sets of determining causes acting simultaneously. First, there are the specific hereditary determiners cr genetic factors, which react with the other elements of the protoplasm and, under favorable circumstances, condition normal development. Second, there are all the conditions external to the cell which stimulate or inhibit proto- plasmic activity. These "developmental stimuli" are chemical and physical changes wrought by energy from without the organism or caused by its own physiological activities. Chemical stimuli are exerted mainly through the" medium of the circulating liquid which surrounds each living cell. Normally this fluid contains the elements essential for maintenance of life as well as various waste products. It may also bear toxic substances that suppress or inhibit the cell functions and in higher animals it contains the secretions of the duct- less, sexual and other glands that profound^ affect development. Physical stimuli are exerted chiefly from without and upon the organ- ism as a whole. They include changes in temperature, light and density of medium, the effects of electric and radiant energy, force of gravity, etc. Obviously, so many interrelated causes acting simulta- neously, each being independently capable of inducing a change in the end product, may cause an infinite number of differences in substance and in degree of development. Variation and environment. — External stimuli affect the develop- ment of characters in three ways: (i) they modify the development of inherited characters; (2) they actually condition the production of characters whose hereditary determiners are present in the germ- plasm; (3) they may cause germinal variations which result in the appearance of new heritable characters. The following are illustra- tions of these effects with reference to particular environmental factors. I. Environment modifies development of inherited characters. — {a) Light and Function. Klebs reports the result of growing the Showy Sedum {Sedum spectahile) in white, red, and blue light. The diverse effects of the three kinds of light are clearly shown in Fig. 5 1 . Although the visible differences between the three plants were very pronounced the experiment was carried much farther. During 1905--6 observations were made on the numbers of stamens in the flowers of plants similarly propagated under white, red, and blue light and under variations, conditions of temperature, moisture,, and food. About 20,000 flowers were examined and six distinct types were found, according to the variation in number of stamens. These had the VARIATIOX 3^,i following average numbers of stamens: (i) 9.68, (2) 8.45, f.^) 6.54, (4) 5-05. (5) 947, C6) 7.33. Finally, Klebs subjected similar plants from white, red, and blue light to chemical analysis in order to secure further evidence of the physiological effects of light of different wave Fig. 51. — Sediini spedahile. The three shoots (taken from a sinfile plant) were planted in small pots on March 12, 1904, and placed in different greenhouses, /, in blue light; //, in mixed white light; ///, in red light. Photographed on September 30, 19 14. {From Bahcock and Clausen, after Klebs.) lengths. Table I shows the composition of the leaves in three plants like those shown in Fig. 51. They were in their respective greenhouses from June 6 to September 7. The percentages shown are per 100 g. of dry substance. In comparing these percentages it should be remembered thai the plant in white light produced 1324 flower buds and the plant in red light 405, while the plant in blue light produced none. This explains the higher percentage of ash, nitrogen and protein in the last. On the other hand, the amounts of starch and sugar found in the jdant from white Hght are decidedlv larp^er than the one from blue light. 314 READINGS IN EVOLUTION, GENETICS, AND EUGENICS In shortj according to Klebs, in comparison with normal white light, the production of organic substances, such as starch and sugar, is TABLE I Chemical Composition of Three Plants of Sedum Spectahile Grown in White, Red, and Blue Light Substance Ash Sugar Calcium malate Free nitrogen Starch Crude protein White 13.20 II .04 22 . 29 o. 16 5.82 5-33 Red 13. 20 15-40 18 .02 0-33 3.66 6.15 Blue 18.60 2.40 18.10 0-59 1 . 20 7.64 diminished under the influence of blue light as microchemical and macrochemical tests distinctly show. In consequence of this dimin- ished assimilation of carbon dioxide the rosettes become purely Fig. 52. — Above the diurnal peacock butterfly {Vanessa io), and below, forms produced by subjecting the pupae to unusual temperatures. {From Bahcock and Clausen, after Goldschmidt.) vegetative. In red light the carbon assimilation is greater than in blue light but less than in white. These experiments prove that the transformation of a plant "ripe to flower" into a vegetative one VARIATION 315 is possible on the one hand by an increase of temperature and of inorganic salts, and on the other hand by a decrease of carbon assimilation. b) Temperature and pigmentation. Many experiments in the rearing of moths and butterflies under controlled temperatures prove that degree of pigmentation is profoundly influenced l)y the tem{)cra- ture at which the pupae are kept. Some species exhibit seasonal dimorphism in the wild state. By taking pupae of the common European form of the swallowtail butterfly, Papilio machaon, and subjecting them to a temperature of 37° to 38° C, Standfuss obtainerl the characteristic summer form which occurs in Palestine. Again it has been shown by temperature experiments that many variations 28-VI 30-VII 15-IX Fig. 53. — Morphological cycle of head height in Hyalodaphnia. Roman numerals designate months. {From Babcock and Clausen, after Woltcreck.) found among insects in nature are merely aberrations due to tempera- ture effects. Goldschmidt by artificially controlled temperatures has produced a series of forms of the diurnal peacock butterfly, Vanessa io, which show the fading out of the "peacock eye" mark (see Fig. 52). c) Food and structure. Woltereck was able to prove that the form (hence the structure) of the fresh water crustacean, Hyalodaphnia, varies directly with the food supply. These minute animals produce many generations during a season and the successive generations from the same water exhibit a morphological cycle, the earlier and later generations having shorter heads and the generations produced from midsummer to autumn having longer ones. Fig. 53 is a reproduction of Woltereck's diagram of the morphological cycle in Hyalodaphnia showing variation in head and shell length as found on successive 3i6 READINGS IN EVOLUTION, GENETICS, AND EUGENICS dates from June 3 to January 3. By raising these animals under constant temperature conditions and varying the strength of the nutrient solution, Woltereck proved that the relative size of body parts varied with the food. In Fig. 54 the percentages of head height to shell in length are plotted as abscissas and the numbers of indi- viduals as ordinates. Animals from three strengths of nutrient media were measured, the curves of those from the weaker, the medium and the richer media being shown at mi, ma and m3 respectively. d) Moisture and plumage color. Beebe experimented with the pigeon, Scardafella inca. This species, as found in North and Central America, is very constant in color of plumage, but in the moist tropics 50 55 CO 65 70 ' 75 80 m2 85 90 95 ni3 Fig. 54. — Schematic curves of head height in Hyalodaphnia as grown in media of three different food values. {From Bahcock arid Clausen, after Woltereck.) the following darker colored forms occur: in Honduras, dialeucos; in Venezuela, ridgwayi; in Brazil, hrazilieiisls; and these differ in the amount of pigment in the feathers. By subjecting the birds of the northern type to an especially moist atmosphere, Beebe caused them to be so influenced that with each new moulting, whether natural or artificially induced, they always developed darker feathers. Thus a wild bird having pigment in 25.9 per cent of its area, would have after the second moulting under experimental conditions, 38 per cent and after the third, 41.6 per cent. Thus during the experiment the typical form assumed the appearance of the three other forms and finally developed plumage markings which have never been seen in nature. Fig. 55 shows the type form, inca, the three geographical variants, and the darkest artificially produced form. VARIATION' 317 2. Environment conditions development of inherited characters.— (a) Light and metabolism. In a general sense light conditions life in all normally green plants. It certainly conditions normal develop- ment in such plants. Potatoes sprouted in a dark room develop no chlorophyll in the stems and the rudimentary leaves are abortive. In many bulbous plants, however, the influence of moisture and heat are sufficient to induce leaf growth and even development of the iniiorescense, but it is all done at the expense of the food storcrl up in the bulbs. Fig. 55. — a, Typical wild pigeon, Scardafclla inca; b, the form dialcucos; r, hraziliensis; d, ridgwayi; e, inca after three moultings in a moist atmosphere. (After Beebe, from Babcock and Clausen.) b) Temperature and flower color. Baur reports an experiment with a red variety of the Chinese primrose, Primula sinensis rubra. If plants of this variety are raised by the usual method until about one week before time to bloom and then some of the plants are put in a warm room under partial shade (temperature from ^0° to 35° C.) and the remainder in a cool house (temperature from 15° to 20° C), when they bloom those in the warm temperature have pure white flowers while those in the cool temperature have the normal red color of the variety. Moreover, if plants are brought from the warm into the 3i8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS cool temperature the flowers which develop later on will be normal red in color. Thus it cannot be said that this primula inherits either red or white flowers. WTiat it really inherits is ability to react in certain ways under the influence of temperature. c) Food and fertility. It is well known that the kind of food supplied to the larvae of bees determines whether the females shall be fertile (queens) or infertile (workers). The striking differences in structure and instincts of the two classes of females are all conditioned by the food provided for the larvae. Each larva inherited the capacity to react in either way according to the stimulus received. d) Moisture and structure. Morgan reports a variety of the pomace fly, Drosophila ampelophila, with abnormal abdomen; "the normal black bands of the abdomen are broken and irregular or even entirely absent. In flies reared on moist food the abnormality is extreme; but even in the same culture the flies that continue to hatch become less and less abnormal as the culture becomes more dry and the food scarce, until finally the flies that emerge later cannot be told from normal flies. If the culture is kept well fed (and moist) the change does not occur, but if the flies are reared on dry food, they are normal from the beginning." 3. Environment may cause new heritable characters. — ^As yet there is a dearth of evidence which can be accepted as scientific proof that external stimuli actually cause germinal variations. At the same time there is an abundance of data which falls into the class of circum- stantial evidence in favor of such a doctrine. Moreover, there are a few cases in which new heritable characters have been artificially produced by carefully controlled external stimuli. Hence some germinal variations are apparently caused by known environmental conditions and we are justified in recognizing this third category of developmental differences due to environmental effects. Considerable evidence of permanent changes in both morphologi- cal and physiological characters has been secured from experiments with the culture of bacteria and yeast, in unusual culture media, in the presence of toxic solutions, or under extreme temperature condi- tions. The significant results of four investigators who worked independently, Hansen, Barber, Wolf, and Jordan, have been reviewed and discussed in regard to their bearing on genetic theory by Cole and Wright. The four investigators mentioned above used refined methods and three of them began by isolating a single organism from whose progeny they obtained distinct strains or biotypes which VARIATION 319 remained constant for hundreds of test-tube "generations/' Ii must be admitted that in most of these cases no specific inlluences can be named as the direct cause of the inherited variation. But there is no longer any doubt that permanent, discontinuous variations do occur spontaneously in these lowest organisms, and it is highly probable that certain incidental, external forces play an important part in inducing such variations. Direct experimental attack upon the germ cells themselves has been made with plants by a number of investigators, notably bv MacDougal, who injected very dilute solutions of potassium iodide, zinc sulphate, sugar, etc., directly into the ovaries of various plants immediately before fertilization. Consequently somatic changes have been produced which were inherited throughout several generations. Fig. 56. — 0, portion of leaf of Scrophularia showing branching lateral view; D, branching vein replaced by two laterals in leaf of seedling grown from seed produced by an injected ovary. Also note the difference in size and margin of leaves. {From Babcock and Clausen, after MacDougal.) By means of check experiments and observations it was found that these germinal variations were not caused by the wounding of the ovary and it is thought that they must have been induced in some way by the presence of the foreign chemical solution in the ovary. Fig. 56 shows a morphological change which appeared in a seedling of an unnamed species of Scrophularia as a result of ovarial injection. Hav- ing tested the species sufficiently to determine that it was a simple one, MacDougal treated several ovaries with potassium iodide, one part in 40,000 and secured seed. No other species of Scrophularia grew near the cultures. From this seed only three plants were raised. ''One formed a shoot fairly equivalent to the normal, finally producing flowers in which the anthocyans were of a noticeal^ly deep hue. The two remaining plantlets were characterized by a succulent aspect of the leaves and by a lighter and yellow color of the leaves and stems. 320 READINGS IN EVOLUTION, GENETICS, AND EUGENICS The flowers on one of the derivatives, as they may be called, were so completely lacking in color as to be a cream-white, this derivative being designated as alhida, while the other showed some marginal color and a rusty tinge and was designated as rufida Seeds of the original two derivatives were sowed in the greenhouse. But one plant of albida, the most extreme departure, survived, while four of rufida were secured." MacDougal compared these second generation seedlings with seedlings from the original stock of the species, noting differences in size and margin of leaves, length of petioles and number of marginal glands. He found that the differences shown by the first generation appeared again in the second generation. Striking as these results appear it must be admitted that it would be difficult, on account of the small numbers of individuals differing from the parent type, to prove satisfactorily to the biometrician that they were not mutations which would have occurred regardless of the ovarial treatment. What appear to be germinal variations in the tomato have been induced by intensive feeding. T. H. White tested the effect of dried blood, dissolved phosphate rock, sulphate of potash and iron filings all in excessive amounts, and (with the exception of the iron) in various combinations, on the Red Cherry tomato. The lack of data on control cultures of seedlings from the same parent as the experi- mental cultures makes it impossible to compare the actual amount of permanent variation produced. T. H. White states that measure- ments "show that the plants of the sixth generation growTi under the influence of the dried blood are one-third larger in height, length of leaf and size of fruit, than those of the second." The author con- cludes that ''there can be no doubt .... that, in the case of Red Cherry treated with dried blood, there is permanent variation to the third generation." If these results are corroborated by more care- fully planned and rigidly controlled experiments they will add the weight of scientific proof of a principle in plant breeding long since recognized on empirical grounds, to wit, that the introduction of wild plants into intensive cultivation induces variation. Furthermore, it suggests a possible means for rapid permanent improvement of wild forms with which hybridization may be impracticable. In experiments on lower animals, e.g., the protozoa, the same difficulty is met with as has been encountered in bacteria and yeasts, in that it is manifestly impossible to distinguish between somatic and germinal variations. Moreover, in most of these experiments, as with most of those on higher animals, the necessary conditions for rigid VARIATION 321 scientific analysis have been lacking. Either the same strain as was subjected to artificial conditions was not grown for comparison under natural conditions or else the conditions themselves were not suffi- ciently well controlled to permit of certain analysis. It is interesting to note that the pomace fly, Drosophila ampelophila, which has pro- duced more mutations so far as we know than any other organism, was subjected to the effects of ether on a grand scale and under controlled conditions by Morgan, but that not a single mutation was observed to result from this treatment. However, mutations have subsequently appeared again and again in cultures of '''wild" flies not only of this species but also of other species of Drosophila. Thus it appears that germinal variations frequently occur independently of external stimuli. It also seems that a tendency to produce mutations may be inherited. With animals the best known experiments on the artificial pro- duction of germinal variations are those of Tower who worked with the Colorado potato beetle, Leptinotarsa decemlineata, and related species. Like other arthropods these beetles are more directly under the influence of temperature changes at least than are warm-blooded animals. Tower first determined the period in ontogeny when ex- ternal stimuli will affect the germ cells. He found that in Leptino- tarsa the germ cells do not become susceptible to external stimuli until after the time in ontogeny when the color pattern of the individ- uals subjected to the stimuli can be influenced. He found that eggs were most susceptible just before and during maturation and this observation is in agreement with those of Fischer, Standfuss, Weis- mann and others who have conducted similar investigations. Tower concluded that certain individuals from the germ cells of a stimulated parent ''show intense heritable variations, whereas those not acted upon do not show these changes." Most of the inherited variations involve changes in the pigmentation of the body parts. In certain cases there was an actual change in the color pattern. It is to these results that Tower attaches the greatest significance inasmuch as most similar experiments have not succeeded in causing pattern changes. In spite of the elaborateness of Tower's methods consider- able skepticism exists regarding the validity of his conclusions, and this has not been lessened by the non-appearance of confirmatory data. In a recent paper he reports the production of very striking germinal modifications in L. decemlineata as a result of subjecting a morphologically homogeneous race to an extreme change in environ- 322 READINGS IN EVOLUTION, GENETICS, AND EUGENICS ment. However, it is still a question whether the material used may not be heterogeneous as regards the germinal factors that condition certain physiological characters. Stockard's investigations on the effect of alcohol on the progeny of guinea pigs have shown that the germ cells as well as the somatic tissues of the alcoholized animals are injured. On the whole it must be admitted that the experimental induction of heritable variations is still largely an unworked field. The complex conditions to be considered and consequent obstacles to be overcome are appreciated by no one more fully than by those who have at- tempted such investigations. For, as Tower has said: "It is evident that the problem of germinal change is one of difficulty, and involves more of indirect than of direct methods of investigation. There is little reason to expect that present biochemical methods can give a solution, but they may give valuable suggestions for further indirect investigation. It seems not improbable, however, that this problem, like so many others in biology, must await the solution of the larger question of what life is before it will be possible to express in exact terms the nature of germinal changes. Our present status, with several methods of production and much knowledge of the behavior of induced germinal changes available, is a basis from which great advances in knowledge and in operation may reasonably be expected." CHAPTER XXIII ARE ACQUIRED CHARACTERS (MODIFICATIONS) HEREDITARY ? Introductory Note. — In the previous chapter, under the heading "Classifica- tion of Variations," the authors pointed out that germinal variations are hereditary, and somatic variations (modifications) are not hereditary. That germinal varia- tions are hereditary and may be produced in a number of different ways was made clear in the last chapter, but the statement that somatic modifications are never in the least hereditary is equivalent to a total denial of the doctrine of the "Inheritance of Acquired Characters," the so-called Lamarckian theory', which was briefly presented in chapter ii. This is not a closed question and the final answer has been given neither in the negative nor in the affirmative. The problem is of utmost import for evolu- tionists and for all who are interested in race improvement. So important is it to view this question fairly that we shall quote extensively from several of the leading students of the problem. MISUNDERSTANDINGS AS TO THE QUESTION AT ISSUE^ J. .\RTHUR THOMSON The precise question is this: Can a structural change in the body induced by some change in use or disuse, or by a change in surrounding influence, affect the germ-cells in such a specific or representative way that the offspring will through its inheritance exhibit, even in a slight degree, the modification which the parent acquired ? Before we pass to discuss the evidence pro and con it will be useful to notice some frequently recurring misunderstandings, the persistence of which would make further argument futile. Misunderstanding I. — How can there be progressive evolution if acquired characters are not transmitted ? — Those who have not thought clearly on the subject often shake their heads sagely and remark that they "do not see how evolution could have been possible at all unless what is acquired by one generation is handed on to the next." To this we have simply to answer (i) that our first business is to find out the facts of the case, careless whether it makes our interpretation of the history of life more or less difhcult, and (2) that in the supply of germinal variations, whose transmissibility is unquestioned, there is ample raw material for evolution. We know a Httle about the abundant ' From J. A. Thomson, Heredity (copyright 1907). Used by special permis- sion of the publisher, John Murray, London. 323 324 READINGS IN EVOLUTION, GENETICS, AND EUGENICS crop of variations at present supplied; there is no reason to believe that it was less abundant in the past. Misunderstanding II. — Interpretations are not fads. — There are many adaptive characters in plants and animals which may be super- ficially interpreted as due to the direct result of use and disuse or of environmental influence. The Lamarckians have so interpreted them, and the Lamarckian way of looking at adaptations has become habitual to many uncritical minds. They see on modern flowers the footprints of insects which have visited them for untold ages; they speak of the dwindling of the whale's hind-limbs through disuse, of the hardening of the ancestral horses' hoofs as they left the marshes and ran oh harder ground; they picture the giraffe by persistent effort lengthening out its neck a few millimetres every century, as the acacia raised its leaves higher and higher off the ground; and they say that animate nature is so full of evidences of the inheritance of acquired characters that no further argument is needed. But all this is a begging of the question. It is easy to find struc- tural features which may he interpreted as entailed acquired characters, if acquired characters can be entailed. Obviously, however, we must deal with what we can prove to be modifications, or with what we can plausibly regard as modifications because we find their analogues in actual process of being effected to-day. It is easy to say that the blackness of the negro's skin was produced by the tropical sun, and that it is now part of his natural inheritance. It is easy to say this, but absolutely futile. Let us first catch our modifications. The Golden Rod {Solidago virgaurea) growing on the Alps is pre- cocious in its flowering when compared with representatives of the same species growing in the lowlands. Hoffmann found that Alpine forms transplanted to Giessen remained precocious, therefore the acquired precocity had become heritable. But there is no evidence that the precocity was acquired; it may have been the outcome of the selection of germinal variations. The African Wart-hog {Phacochoerus) has the peculiar habit of kneeling down on its fore-limbs as it routs with its huge tusks in the ground and pushes itself forward with its hind-limbs. It has strong horny callosities protecting the surfaces on which it kneels, and these are seen even in the embryos. This seems to some naturalists to be a satisfactory proof of the inheritance of an acquired character. It is to others simply an instance of an adaptive peculiarity of germinal origin wrought out by natural selection. ARE ACQUIRED CHARACTI-RS HKKKI)rr.\R\? 325 Misunderstanding Ill.—Begging the question by darting with what is not proved to be a modification. — There is no relevancy in citing cases where an abnormal bodily peculiarity re-appears generation after generation, unless it be shown that the peculiarity is a modifica- tion, and not an inborn variation whose transmissibility is admitted by all. Short-sightedness may recur in a family-series generation after generation, but there is no evidence to prove that the original short-sightedness was a modification. In all probability, short- sightedness is in its origin a germinal variation, like so many other bodily idiosyncrasies. In regard to some diseases, such as rheumatism, it is often said dogmatically by those who know little about the matter that the original affection in the ancestor was brought about by some definite external influence — such as a cold drive or a damp bed; but it seems practically certain that in all such cases we have to do with an inborn predisposition, to the expression of which the cold drive or the damp bed were merely the liberating stimulus, comparable to the pulling of the trigger in a loaded gun. The liberating stimulus is, of course, of great importance, both in the case of the gun's discharge and the organism's disease, but it only goes a little way towards a satisfactory interpretation in either case. Not that we can explain the origin of rheumatism or shortsightedness or any such thing — there is no expla- nation in calling them germinal variations that cropped up; but we are almost certain that they never are modifications or acquired characters. Herbert Spencer twits those who are sceptical as to the trans- mission of acquired modifications with assigning the most flimsy reasons for rejecting a conclusion they are averse to; but when Spencer cites the prevalence of short-sightedness among the ''notoriously studious" Germans, the inheritance of a musical talent, and the inheri- tance of a liabiHty to consumption, as evidence of the inheritance of modifications, we are reminded of the pot calling the kettle black. Over and over again in the prolific literature of this discussion the syllogism is advanced, either in regard to gout or something analogous — Gout is a modification of the body, an acquired character; Gout is transmissible; Modifications are sometimes transmissible. It may be formally a good argument, but there is every reason to deny the major premise. There is no proof that the gouty habit had an exogenous origin — that it was, to begin with, for instance, the direct result of high living; though it is generally admitted that 326 READINGS IN EVOLUTION, GENETICS, AND EUGENICS excesses in eating or drinking may give a stimulus to its expression. ''The conclusion that I have arrived at," says Prof. D. J. Hamilton, "is that the gouty habit of body has arisen as a variation, and as such is hereditarily transmissible, and that excess of diet and alcohol merely renders the habit of body apparent." It may also be pointed out that gout and rheumatism and the like are rather processes of metabolism than structural modifications, though the latter may ensue. After pointing out the irrelevancy of citing cases of the hereditary recurrence of polydactylism, haerriophilia, colour-blindness in man, or the absence of horns in cattle or of tails in cats, as instances of the transmission of acquired characters. Prof. Ernst Ziegler says: "Only that can be regarded as ' acquired ' which is produced in the course of the individual life, during and after the period of development, exclu- sively under the influence of external conditions; the term is in no wise applicable to peculiarities which, as one says, arise of themselves from a predisposition already present in the germ." Misunderstanding IV. — Mistaking the reappearance of a modifica- tion for transmission of a modification. — It is of little service to cite cases where a particular modification reappears generation after gen- eration unless it be shown that the change recurs as part of the inheri- tance, and not simply because the external conditions which evoked it in the first generation still persisted to evoke it in those that followed. Reappearance is not synonymous with inheritance. Misunderstanding V. — Mistaking re-infection for transmission. — A particular form of the fourth misunderstanding has to do with facts so special that it may be conveniently treated of separately. It has to do with microbic diseases. It is admitted that a parent infected with tubercle-bacillus or with the microbe of syphilis may have off- spring also infected. But such cases are irrelevant in the discussion. Infection, whether before or after birth, has nothing to do with inheri- tance. As Dr. Ogilvie says, "Wherever the transmission of infectious disease from parent to offspring has been adduced to support the doctrine of the inheritance of acquired characters, it has been done in utter misconception of its meaning and scope." Medical men have sometimes condescended to make a subtle distinction between "hereditary" and "congenital" syphiHs — the latter manifested at birth, the former some time afterwards! It seems strange that they have failed to recognise that there is no reason to use the word "hereditary" at all in this connection. What occurs is an infection, and it is theoretically immaterial at what stage the infection occurs. A microbe cannot be part of an inheritance. ARE ACQUIRED CHARACTERS IIEREDITARV? •y f Misunderstanding Yl.~Trausmission in unicellular s is not to the point.— It is not to the point to cite cases where unicellular organisms, such as bacteria or monads, have been profoundly and heritably modi- lied by artificial culture, so that, for instance, the descendants of a virulent microbe have been made to lose their evil potency. It is irrelevant because in regard to unicellular organisms we cannot draw the distinction between body and germinal matter, apart from which the concept of modifications is of no value. In artificial culture the whole character of the unicellular organism — its particular metabolism — is altered; it multiplies by dividing into two or more parts, which naturally retain the altered constitution. But this is worlds away from the supposed case of an alteration in the structure of the little toe so affecting the germ-cells that the offspring inherit a corre.sponding deformation. Professor L. Errera (1899) reported an experiment with a simi)le but multicellular mould {Aspergillus niger), which adapted itself to a medium more concentrated than the normal. The second generation of the mould was more adapted than the first, and the adaptation to the concentrated medium was not wholly lost after rearing in the nor- mal medium again. This looks like evidence of the inheritance of the acquired adaptive quality which was brought about as a direct modifi- cation. But the case does not really help us, since the distinction between soma and germ-plasm is not more than incipient in the mould in question. And even if the distinction were more marked, it would only show that the germ-plasm is capable of being affected along with the body, by a deeply saturating influence, which nobody has ever denied. Misunderstanding VII. — Changes in the germ-cells along with changes in the body are not relevant. — Another misunderstanding is due to a failure to appreciate the distinction between a change of the repro- ductive cells along with the body, and a change in the reproductive cells conditioned by and representative of a particular change in bodily structure. The supporters of the hypothesis that modifications may be transmitted point to the tragic cases where some })oisoning of the parent's system, by alcohol, opium, or some toxin, is followed by some deterioration in the offspring. There is no doubt as to the fact; the question is as to the correct interpretation. I. In some cases it may be that the whole system of the jxirent is poisoned— reproductive cells as well as body; the effect may be as direct On the germ-cells as on the nerve-cells. These, therefore, are not cases on which to test the transmissibility of an acquired character— i.e., 328 READINGS IN EVOLUTION, GENETICS, AND EUGENICS of a particular somatic modification. If a local poisoning had a structural effect on some particular organ, and if that structural effect was reproduced in any degree in the offspring, the case would be relevant; but when the whole organism is soaked in a poison the case is irrelevant. If it could be said that the sunshine, which brings about sun-burning in the skin, soaks through the organism even to its repro- ductive cells and specifically affects them, in a manner analogous to the saturating poison, we should have a physiological basis for expect- ing the inheritance of sun-burning. But we cannot make this assump- tion. We have no warrant for believing that the modification of a part re-echoes in a definite specific way through the organism until even the penetralia of the germ-cells reverberate. 2. A parent organism is poisoned, and there are structural results of that poisoning. The offspring are born poisoned, and show similar structural peculiarities. This may be due to the fact that the germ- cells were poisoned along with the parental body; but it may also be due, in the case of a mother, to a poisoning of the embryo before birth, in a manner comparable to a pre-natal infection. 3. In some cases — e.g., of alcoholism in successive generations — there may be poisoning of the germ-cells along with the body, there may be poisoning of the embryo before birth, and of the infant after; but it may also be that what is really inherited is a specific degeneracy of nature, an innate deficiency of control, perhaps, which led the parent to alcoholism, and which may find the same or some other expression in the child. Cases are known in which the children of a dipsomaniac father and a quite normal mother have exhibited a tendency to alcoholism, insanity, and the like. In this case the possibility of poisoning the unborn child is eliminated, but there remain three possibilities of interpretation — that there was specific poisoning of the paternal germ- cells; that what was inherited was the constitutional weakness which expressed itself as alcoholism in the father; and that there were detri- mental influences in the early nutrition, environment, education — "nurture," in short — of the offspring. But while we have admitted a good deal, we have not admitted the transmissibihty of a particular structural modification brought about in the parental body as a result of the toxin. Misunderstanding VIII. — Failure to distinguish between the possible inheritance of a particular modification and the possible inheri- tance of indirect results of that modification, or of changes correlated with ARE ACQUIRED CHARACTERS HEREDITARY? 329 it.— At first sight this seems hair-splittin^^ ])ut it is a crucial jioint. Through his vigorous exercise the blacksmith develops a muscular arm worthy of admiration; the shoemaker acquires skeletal and muscular peculiarities less admirable. There are many permanent and profound modifications associated with particular occupations. Are we to believe, it is asked, that the occupation of the parents has no influence on the offspring? Are we to believe, it is asked, that the children of soldier, sailor, tinker, tailor, are in no way affected by the parental functions ? It would be interesting to have precise data in regard to this, l)ut it is generally admitted that when parents have healthful occupations their offspring are likely to be more vigorous. The matter is comi)li- cated by the difficulty of estimating how much is due to good nurture before and after birth. It is not unlikely, too, that some profound parental modifications may influence the general constitution, may even affect the germ-cells, and may thus have results in the offspring. But unless the offspring show peculiarities in the same direction as the original modifications, we have no data bearing precisely on the ques- tion at issue. A belief in the inheritance of modifications was perhaps expressed in the old proverb, "The fathers have eaten sour grapes, and the chfldren's teeth are set on edge" — a proverb which Ezekiel with such solemnity said was not any more to be used in Israel. Now if " setting on edge" was a structural modification, and if the children's teeth were "set on edge" as their fathers' had been before them, there would be a presumption in favour of the transmission of this acquired character, though it would be stiff necessary to inquire carefully whether the children had not been in the vineyard too. But if, as Romanes said, the children were born with wry necks, we should have to deal with the inheritance of an indirect result of the parent's vagaries of appetite, and not with any direct representation in inheritance of the particular modification produced in the paternal dentition. Misunderstanding IX. — Appealing to data from not more than two generations. — It has often been pointed out that animals transported to a new country or environment may exhibit some modification apparently the result of the novel influence, and that their offspring in the same environment may exhibit the same modification /;/ a greater degree. Thus sheep may show a change in the character and length of their fleece, and their progeny may show the same change more markedly. 330 READINGS IN EVOLUTION, GENETICS, AND EUGENICS But it is perfectly clear that if the evidence does not go beyond this, nothing is proved that affects the question at issue. It was to be expected that the offspring should show the modification in a more marked degree than their parents did, since the offspring were sub- jected to the modifying influences from birth, whereas their parents were influenced only from the date of their importation. What would be welcome is evidence that the third generation is more markedly modified than the second; then there would be data worth considering. Only then would it be necessary to consider Weismann's somewhat subtle discussion as to the influence of climate. THE INHERITANCE OR NON-INHERITANCE OF ACQUIRED CHARACTERS^ EDWIN GRANT CONKLIN Few questions in biology have been discussed so fully and so fruitlessly as this. It is a problem of the greatest interest not only to students of biology but also to sociologists, educators and philan- thropists and yet it is still to a certain extent an unsolved problem. Opinions of Lamarck and Darwin. — It is well known that Lamarck taught that characters due to desire or need, use or disuse, and to changed environment or conditions of life were inherited and thus brought about progressive evolution. Long ago desire or need was repudiated as a factor of evolution. Lowell satirized it in his Biglow Papers in these words: "Some filosifers think that a fakkilty's granted The minnit it's felt to be thoroughly wanted. That the fears of a monkey whose holt chanced to fail Drawed the vertibry out to a prehensile tail." Darwin wrote to Hooker, ''Heaven forfend me from Lamarck's non- sense of adaptation from the slow willing of animals"; but although he repudiated this feature of Lamarckism he held that characters due to use or disuse and to changed conditions of life might be inherited and he proposed his hypothesis of pangenesis in order to explain the process of the transmission of such characters to the germ cells. Weismann's theories. — Weismann introduced a new era in biology by denying the inheritance of all kinds of acquired characters, and by challenging the world to produce evidence that would stand a * From E. G. Conklin, Heredity and Environment (copyright 1919). Used by special permission of the publishers, The Princeton University Press. ARE ACQUIRED CHARACTERS HERKDITARV? 331 rigorous analysis. But Weismann's greatest service lay in his con- structive theories rather than in destructive criticism; he forever disposed of theories of pangenesis and the like by showing that the germ cells are not built up by contributions from the body and that characters are not transmitted from generation to generation; but on the other hand that there is transmitted a germ plasm which is relatively independent of the body and which is relatively very stable in organization. This epoch-making theory of Weismann's has natu- rally undergone some changes, as the result of new discoveries. It is no longer believed that the germ plasm is really independent of the body, nor that it is absolutely stable, as Weismann at one time held. There is no doubt that the germ cells and the germ plasm are physiologically related to other cells and to other plasms, and similarly there is no doubt that the germ plasm although very stable can and does change its constitution under some rare conditions. But in the main the germ plasm theory is accepted by the great majority of biologists to-day, and recent work in genetics and cytology has brought many confirmations of this theory. Distinctions between hereditary and acquired characters. — As long as it was believed that the developed characters of an organism could be transmitted as such to its descendants it was customary to speak of developed characters as hereditary or acquired and to talk of the inheritance or non-inheritance of acquired characters. This dis- tinction is not a logical one for all developed characters are invariably the result of the responses of the germinal organization to environ- mental stimuli; and of course no developed character can be purely hereditary or purely environmental. But when a given character arises in many individuals of the same genotype under different environmental conditions it is probable that heredity, which is the constant factor in this case, is also the determining factor for that character. On the other hand if a character develops in response to peculiar stimuli and does not appear in other individuals of the same genotype in which such stimuli are lacking it is said to be an environ- mental or acquired character. In line, inherited characters are those whose distinctive or differential causes are in the germ cells, while acquired characters are those whose differential causes are environ- mental. Statement of problem.— Briefly stated the question of the inheri- tance of acquired characters is this: Can the differential cause of a character be shifted from the environment to the germ i)lasm ? Can 332 READINGS IX EVOLUTION, GENETICS, AND EUGENICS peculiarities of the environment which influence the development of somatic characters so affect the germ cells that they will produce these somatic characters in the absence of the peculiar environment ? Can the characteristics of a developed organism enter into its germ cells and be born again in the next generation ? Considering the fact that germ cells are cells and contain no adult characteristics, it seems very improbable that any peculiarity of environment whether of nutri- tion, use, disuse or injury, which brings about certain peculiarities of developed characters in the adult, could so change the structure of the germ cells as to cause them to produce this same character in subse- quent generations in the absence of its extrinsic cause. How, for example, could defective nutrition, which leads to the production of rickets, affect the germ cells, which contain no bones, so as to produce rickets in subsequent generations, although well nourished ? Or how can over-exertion, leading to hypertrophy of the heart, so affect the germ cells that they, in turn, would produce hypertrophied hearts in the absence of over-exertion, seeing that germ cells have no hearts ? Or how could the loss or injury of eyes or teeth or legs lead to the absence or weakened development of these organs in future generations, seeing that inheritance must be through germ cells which possess none of these structures ? Lack of evidence for inheritance of acquired characters. — But, apart from these general objections to the doctrine of the inheritance of acquired characters, there are many special difficulties. There is no conclusive and satisfactory evidence in favor of such inheritance. Almost all the evidence adduced serves to show only that characters are acquired, not that they are inherited. It is a matter of common observation that mutilations are not inherited; wooden legs do not run in families, although wooden heads do. The evidence for the inheritance of peculiarities due to use or disuse is wholly inconclusive; for example, did the giraffe get his long neck because he browsed on trees, or does he browse on trees because he has by inheritance a long neck ? Did attempts to fly lead to the development of wings in birds, or do birds fly because heredity has given them wings.? Did Hfe in caves make cave animals blind, or did blind animals resort to caves because the struggle for existence there was less severe for them ? The evidence is in favor of the second of each of these alternatives rather than of the first. There stiU remains the question of the inheritance of certain characters due to environment, though here also the most clear-cut ARE ACQUIRED CHARACTERS HEREDITARY? 333 evidence is against this proposition. That unusual conditions of food, temperature, moisture, etc., may affect the germ cells so as to produce general and indefinite variations in offspring is probable, but this is a very different thing from the inheritance of acquired characters. The germ cells being a part of the parental organism may be modified by such changes in the environment as affect the body as a whole, they may be well nourished or starved, they may be modified by changed conditions of gravity, salinity, pressure, temperature, etc., and these modifications of the germ cells probably lead to certain general modi- fications of the adult, which may be larger or smaller, stronger or weaker, according as the germ is well or poorly nourished, but it is incredible that the environment which produces rickets, or hyper- trophied heart, or loss of sight in one generation should modify the germ cells in such a peculiar and definite way that they should give rise in the next generation to these particular peculiarities, in the absence of the extrinsic cause which first produced them. The inheritance of acquired characters is incredible, because the egg is a cell and not an adult organism; and in this case there is no suffi- cient evidence that the thing which is incredible really does happen. No inherited influence of stock on graft. — If specific changes of environment produced specific changes in heredity we should expect to find that where different plants or animals are grafted together each would modify more or less the hereditary constitution of the other. But this does not occur. Everybody knows that when a branch of a particular kind of fruit tree is grafted upon a tree of a different variety the quality of the fruit borne by that branch is not altered by its close union with the new stock. The same is true of all forms of animal grafts. Harrison cut in two young tadpoles of two species of frog, Rana syhatica and Rana palustris, and spliced the anterior half of one to the posterior half of the other. These frogs and their tadpoles differ in color as well as in other respects, R. syhatica being more deeply pigmented than R. palustris. In the grafted tadpoles each half pre- served its own peculiarities even up to the adult condition. A still more striking case of the persistence of heredity in spite of environmental changes is found in experiments in which the ovaries are removed from one variety of animal and transplanted to another variety. Guthrie made such transplation in the case of fowls and concluded that there was some influence of the foster mother upon the transplanted ovary, but Davenport, who repeated his experiments, was unable to confirm his results. Finally Castle and Phillips furnished 334 READINGS IN EVOLUTION, GENETICS, AND EUGENICS the most conclusive demonstration that the hereditary character- istics of the transplanted ova are in no wise changed by the foster mother. They removed the ovary from a pure black guinea-pig and put it in the place of the ovary of a pure white animal. After recover- ing from the operation this white female with the "black" ovary was bred to a pure white male. Three litters of offspring from these parents were all pure black. Although both parents were pure white all the offspring of the Fi generation were black because they came from ''black'' eggs and black is dominant over white. The fact that these "black" eggs developed in the body of a white female did not in the least change their hereditary constitution. Dominants and recessives remain pure. — A still more intimate union takes place when the dominant and recessive characters come together in any zygote. These characters, or rather the factors which determine them, may be intimately associated in every cell of the organism throughout an entire generation and yet we may get a clean separation of these characters in the next generation; in many cases neither the dominant nor the recessive character has been at all modi- fied by its most intimate association with the other. Climatic effects not inherited. — A striking instance of the purely temporary effect of the environment and of the long persistence of hereditarv constitution amidst new environmental conditions, which have greatly changed the appearance of the developed organisms, is found in the case of alpine plants. Nageli says that such plants, which have preserved the characters of high mountain plants since the ice age, lose these characters perfectly during their first summer in the lowlands. Summary. — If acquired characters were really inherited we should expect to find many positive evidences of this instead of a few sporadic and doubtful cases. In particular why do we not find in plant or animal grafting that the influence of the stock changes the hereditary potencies of the graft ? Why do we not find that transplanted ovaries show the influence of the foster mother as Guthrie supposed — a thing which has been disproved by Castle ? Why do dominant and recessive characters remain pure, even after their intimate union in a hybrid, so that pure dominants and pure recessives may be obtained in subse- quent generations from this mixture ? Why does every child have to learn anew what his parents learned so laboriously before him ? Even the strongest defenders of the inheritance of acquired characters are constrained to admit that it occurs only sporadically and excep- tionally. ARE ACQUIRED CHARACTERS HEREDITARY? 335 Neo-Lamarckism.— Many modifications of the Lamarckian hypothesis of the inheritance of acquired characters have been pro- posed in recent years. Foremost among those are the '"mneme" theory of Semon and the " centro-epigenesis " theory of Rignano. To Semon as to many other biologists the apparent resemblance between memory and heredity has seemed significant, and this furnishes the basis of his theory. Semon holds that every condition of life, every functional activity of an organism leaves a permanent record of itself in what he calls an "engramme." If these conditions or activities are long continued their engrammes are heaped up and affect heredity. Semon does not ask if "acquired characters" are inherited, but rather "Are the hereditary potencies of the germ cells altered by stimuli acting on the parental body?" This is a very different thing from the inheritance of a particular acquired character, and there is some evidence that such stimuli may in rare instances produce changes in the hereditary constitution of the germ plasm though these evidences are by no means conclusive. Temporary effects of environment; "induction." — On the other hand certain changes may be produced in germ cells or embryos which last for only a generation or two and then disappear. It is well known that plants grown in poor soil are smaller and produce smaller seeds than those grown in good soil, and De Vries, Bauer and Harris find that such seeds produce smaller plants having smaller seeds than do seed of normal size. This is an after effect of poor nutrition which changes the amount of food material in the seeds and through this the size of the plant which develops from the seed, but it does not change the hereditary constitution. Woltereck found that in Daphnia there is an after effect of cold lasting for one or two generations, and this he calls "induction," when the effect lasts for one generation, or "pre- induction " when it lasts for two or three generations. Whitney found that rotifers poisoned with alcohol were weaker in resistance to copper salts and were less fertile than others, and when brought back to normal conditions the first generation was weak but the second was normal. On the other hand Stockard finds that the injurious effects of alcohol on guinea pigs persist through two or more generations. In man alcohol may have an "induction" effect on offspring, but fortu- nately it does not seem to alter hereditary constitution. Probably of a similar character are Sumner's results; he found that mice raised in the cold have shorter tails than those raised in higher temperatures and this modified character appears in the next generation. If this is an after effect or "induction" it should disappear in the following generations. 336 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Kammerer found that salamanders with black and yellow spots when reared on yellow soil gradually lose their black color, becoming more \ellow, and their young continue to grow more yellow until finally almost all black may disappear. The offspring of such sala- manders are said to be more yellow than normal; but this work has been called in question and needs confirmation. Even if confirmed the result may be an after effect or "induction" which would soon disappear under usual conditions, and there is no evidence that it is really inherited. Such cases are not instances of true inheritance; they do not signify a change in the hereditary constitution but an influence on the germ cells of a nutritive or chemical sort comparable with what takes place when fat stains are fed to animals; the eggs of such animals are stained, and the young which develop from such eggs are also stained, though the germinal constitution rem ins unchanged. The very fact that the changed condition is reversible and that it disappears within a short time is evidence that it is not really inherited. In conclusion: (i) Developed characters, whether "acquired" or not, are never transmitted by heredity, and the hereditary constitu- tion of the germ is not changed by changes in such characters. (2) Possibly environmental stimuli acting upon germ cells at an early stage in their development may rarely cause changes in hereditary constitution, but changes produced in somatic cells do not cause corresponding changes in the hereditary constitution of the germ cells. (3) Germ cells like somatic cells may undergo modifications which are not hereditary; if starved they may produce stunted individuals and this effect may last for two or three generations; they may be stained with fat stains and the generation to which they give rise be similarly stained; they may be poisoned with alcohol or modified by tempera- ture and such influence be carried over to the next generation without becoming hereditary. All such cases are known as "induction" and many instances of the supposed inheritance of acquired characters come under this category. (4) Environment may profoundly modify individual development but it does not generally modify heredity. THE OTHEl^ SIDE TO THE QUESTION [It will have been noted that the chief objection to the idea of the possibility of acquired characters being inherited comes to us as a heritage of the rather extreme Weismannian concept of the "germ ARE ACQUIRED CHARACTERS HEREDITARY? 337 plasm." According to this view as brought out Ijy Professor Guyer (p. 296), there is an unbroken continuity from generation to generation of the germ plasm. Germ cells are thought of as remaining entirely undifferentiated for any somatic function and as therefore capable of starting at the beginning to develop a new individual. The germ cell is supposed to be "set apart at an early period in a given individual; it takes no part in the formation of the individual's body, but remains a slumbering mass of potentialities which must bide its time to awaken into expression in a subsequent generation." Physiologists object to this idea that the germ cells are so dis- tinctly different from body cells and that they are so insulated, as it were, from the soma as to be immune to any changes that may affect the latter. Two kinds of data are offered in opposition to this con- cept. A few observers, notably Professor C. M. Child, have described cases in which somatic cells, that already had become differentiated as primitive muscle cells, lost their differentiation and returned to a germinal condition. If this kind of thing were general, and it is probably not, germ cells might conceivably be produced from func- tioning soma cells and might therefore furnish a mechanism for the transmission of the effects of use and disuse. It should be empha- sized, however, that, among animals at least, there is extremely little evidence in support of the idea that differentiated body cells give rise to germ cells. Among plants, however, a different situation prevails. In the Begonia, for example, any part of a plant if cut off is capable of pro- ducing a whole new plant. Even a purely vegetative organ like a leaf, if cut off and partially buried in soil, will bud off a new plant which will produce flowers with perfectly typical germ cells. We have to admit, in this case, either that leaf tissues contain undifferentiated germ cells or that somatic tissues give rise to germ cells. The first alterna- tive is in harmony with the germ-plasm hypothesis, the second is the preferred view of the opponents of this hypothesis. Among animals, as for example annelid worms, it is quite common to find the germ cells aggregated in a few segments (^f the body. If a part of the body in which there are no recognizable germ cells be cut off, it will, under proper conditions, regenerate the lost jxirts and become a complete worm with functional germ cells. The same alternative explanations that were offered for the Begonia case apply equally well here. Numerous other cases of the same sort are well known to all zoologists. To the advocate of the " germ-j^lasm " theory 338 READINGS IN EVOLUTION, GENETICS, AND EUGENICS they offer no difficulties because he can always fall back upon the statement that there is, among the lower forms at least, reserve germ plasm equally distributed over the whole body ready to differentiate into definite germ cells when needed. This type of appeal is abhor- rent to the physiologist, and with some justification, for it really begs the question by assuming that any cell that is capable of form- ing germ cells belongs to the more or less sacred lineage of germ plasm. If we confine the application of the germ-plasm idea to the higher animals, such as vertebrates and insects, we w^ould obviate these chief objections, and the present writer would take the view that it is only among the upper ranges of highly specialized animals that the con- tinuity of the germ -plasm concept holds solidly. Another chief objection to the germ-plasm system has to do with the supposed insulation or apartness of the germ plasm. Physiolo- gists have found that there is an extremely intimate correlation in function between practically all parts of a living organism. Many of the structures, such as the rudimentary pituitary body, the thyroids, the adrenal body, and various other bodies whose function was long unknown, have now been shown to exercise a profound effect on the development of the whole body. Since practically all tissues are known to affect at least some other tissues, is it likely, the physiologist asks, that none of the other tissues affect the germinal tissues ? The organism is to be viewed, it is said, not as a collection of independently functioning parts, but as a single coherent unit. On this view no tissue can be thought of as beyond the influence of organic changes. The classic argument of the Weismannians was that we can con- ceive of no mechanism by means of which somatic changes can be carried back into the germ cells, and therefore there is no such mechanism. Now the fallacy of this argument is obvious; even if we could con- ceive of no suitable mechanism for this purpose, this does not preclude the existence of such a mechanism. Moreover, according to Professor Guyer, just such a mechanism actually exists, as will be brought out in the following quotation from one of his recent publications. — Ed.] A POSSIBLE MECHANISM EOR THE TRANSMISSION OF ACQUIRED CHARACTERS' MICHAEL F. GUYER Some selectionists glibly assert that new characters arise as the result of spontaneous changes in the germ. What is meant by this ? ^ From M. F. Guyer, "Immune Sera and Certain Biological Problems," American Naturalist^ Vol. LV (192 1). ARE ACQUIRED CHARACTERS HEREDITARY? 339 Just what is a spontaneous change ? No one has ever succeeded in telling us. And we may suspect, though perhaps it is heresy to do so, that it is a well-sounding phrase that is the equivalent of the three words, "I don't know." Unwilling to admit of the modifying inlluence of external agencies on the germ, such theorists resort to the fiction of a spontaneous change. Coleridge somewhere has said, "What's gray with age becomes religion." We have toyed so long with this idea of germinal continuity and the invulnerability of the germ, that it has become for some of us wellnigh sacrosanct. Living matter is living matter wherever it may be found, but when it happens to be in the germ-cells, verily, ''this corruptible has put on incorruption and this mortal immortality"! Now, no one to-day, qualified by his knowledge of embryology and genetics to the right of an opinion, would, I think, deny that the new organism is in the main the expression of what was in the germ- line, rather than of what it got directly from the body of its parents, but does this fact necessarily carry with it the implication that the germ is insusceptible to modification from without ? Is not the serum of organisms with blood or lymph an excellent medium through which external influences may operate upon it ? Is it not more reasonable to postulate the origination of germinal changes through some such mechanism as this than to attribute it to mysterious ''spontaneous changes" ? With such thoughts in mind I and my research associate. Dr. E. A. Smith, set about making various tests. Without attempting to tell you of our as yet unsuccessful attempts to secure cytolysins which will operate in the developmental stages of such periodically renewed structures as feathers, or to weary you with the history of our various other failures — of which there are an abundance — I wish to speak briefly about certain antenatal effects we secured in rabbits by means of fowl-serum sensitized against rabbit crystalline lens, and of the fact that such induced defects may become heritable. The crystalline lens of the rabbit was selected as antigen, and fowls as the source of the antibodies. The lenses of newly killed rabbits were pulped thoroughly in a mortar and diluted with normal saline solution. About four cubic centimeters of this emulsion was then injected intraperitoneally or intravenously into each of several fowls. Four or five weekly treatments with such lens-emulsions were given. Then a week or ten days after the last injection the blood-serum of one or more of the fowls was used for injection into pregnant rabbits. The rabbits had been so bred as to have the young advanced to about 340 READINGS IN EVOLUTION, GENETICS, AND EUGENICS the tenth day of pregnancy, since from the tenth to the thirteenth day seems to be a particularly important period in the development of the lens. It is then growing rapidly and becomes surrounded by a rich vascular network that later disappears. From four to seven cubic centimeters of the sensitized fowl-serum were injected intravenously into the pregnant rabbits at intervals of two .or three days for from ten days to two weeks. Several rabbits died from the treatment and many young were killed in utero Of sixty-one surviving young from mothers thus treated, four had one or both eyes conspicuously defect- ive and five others had eyes which were clearly abnormal. It is possible that still others wxre more or less affected, since we judged only by obvious, visible effects. We found later in some of the descendants of these individuals that rabbits which passed for normal during their earher months subsequently manifested traces of defects in their lenses or in other parts of the eye. The commonest abnormality seen in both the original subjects and in their descendants was partial or complete opacity of the lens, usually accompanied by reduction in size. Other defects were cleft iris, persistent hyaloid artery, bluish or silvery color instead of the characteristic red of the albino eye, microphthalmia and even almost complete disappearance of the eyeball. Taking into account the method of enbryological development, however — the relation of lens, optic cup, and choroid fissure — the defects are probably all attributable to the early injury of the lens. In some cases, both among originals and descendants, an eye microphth.ilmic at birth may undergo fur- ther degeneration such as collapse of the ball and what appears to ' e a resorption as if some solvent wxre operating upon it. The eyes of the mothers apparently remained unaffected. This is probaLly due to the fact that the lens tissue of the adult rabbit is largely avascular and therefore did not come into contact with the injected anti- bodies. That the changes in the eyes of the fetuses resulted from the action of lens antibodies is indicated by the fact that in not one of the forty- eight controls obtained from mothers which had been treated with unsensitized fowl-serum or with fowl-serum sensitized to rabbit tissue other than lens, was there evidence of eye-defects, and L may add, that among the hundred or more young obtained later from mothers which were being experimented upon with various types of sera or protein extracts, for other purposes, not a single case of eye-defect has appeared. ARE ACQUIRED CHARACTERS HEREDITARY? 341 As already stated, once the anomaly is secured it may Ijc trans- mitted to subsequent generations through breeding. So far we have succeeded in passing it to the eighth generation without any other than the original treatment. The imperfection, indeed, tends to become worse in succeeding generations and also to occur in a proportionately greater number of young. Though not analyzed completely as to its exact mode of inheritance, it has in general, the characteristics of a Mendelian recessive. Like such anomalies as brachydactyly or Poly- dactyly in man, the transmission is not infrequently of an irregular, unilateral type, sometimes only the right, at others only the left eye showing the defect. In the later generations, probably in some measure as the result of selective breeding, there is an increasing num- ber of young which have both eyes affected. To determine whether the reappearance of the defect was due merely to the passing on of antibodies or kindred substances from the blood stream of the mother, or to true inheritance, we mated defective- eyed males to normal females from strains of rabbits unrelated to our defective-eyed stock. The first generations produced in this way were invariably normal-eyed, but when females of this generation were mated to defective-eyed males again, we secured defective-eyed young after the manner of an extracted Mendelian recessive. It is obvious that in such cases the abnormality could only have been conveyed through the germ-cells of the male, and that it is, therefore, an example of true inheritance. Subsequent matings have shown that these young transmit the eye-anomalies as effectively as do individuals of the original lines. A new strain of defective-eyed young, estab- lished about the time our original paper went to press, is also flourish- ing and, as regards transmission of the defect, seems to differ in no way from the earlier stock. But now, let us inquire as to where all this leads. Without enter- ing into a discussion of just what, serologically, is taking place in the body or in the germ of fetuses borne by the lens-treated mothers, the point I wish to emphasize is that a certain specific effect has been pro- duced; and, what is of greater moment, once the condition is estab- lished it may be not merely transmitted, but inherited. Whether the lens of the uterine young is first changed and then in turn induces a change in the lens-producing antecedents in the germ-cells of these young, or whether the specific antibody simultaneously affects the eyes and the germ-cells of the young is not clear. In any event it is evident that there is some constitutional identity between the 342 READINGS IN EVOLUTION, GENETICS, AND EUGENICS substance of the mature organ in question and the material ante- cedents of such an organ as it exists in the germ. Biologically considered, the most significant fact is that specific antibodies can induce specific modifications in the germ-cell. Whether these antibodies are transmitted from the mother's blood or engen- dered in that of the young would seem to be of secondary importance. It stands to reason that antibodies originated in an animal's own blood will modify germinal factors if corresponding antibodies introduced from without can accomplish this. The whole question as to how important such a fact may be in contributing to an understanding of the causes of the germinal changes in organisms in general, which lead to variation and evolution, hinges on the question of whether changes in an animal's tissue will induce the formation of antibodies or kindred active substances in its own body. We have steadily accumulating evidence that such reactions do occur. In our own laboratory, for example, after many attempts we have succeeded in securing a defective-eyed young rabbit from a mother of normal stock by injecting her repeatedly with pulped rabbit lens before and during pregnancy. Since the young rabbit in question has both eyes badly affected there can be no question that a rabbit can build antibodies against rabbit-tissue which are as effective as those engendered in a foreign species such as the fowl. We have likewise found it relatively easy to secure spermatoxins by directly injecting rabbits, both male and female, with rabbit spermatozoa. Moreover, a given male will develop antibodies against his own spermatozoa if he is injected intravenously with the latter. We are also securing evidence that serologic reactions induced in the fetus through operations on the mother are not mere passive trans- missions, but may become actively participated in by the tissues of the fetus. For example, female rabbits sensitized with typhoid vaccine followed by living typhoid germs may transmit to their young and even to their grand descendants the ability to agglutinate typhoid bacilli in serum diluted from 60 to 160 times. From the standpoint of heredity we have no reason so far for maintaining that this is anything but placental transmission, though we are going to practice immunization generation after generation for a number of generations to determine if a truly hereditary immunity will be established. How- ever, facts have come to light which show that there is more concerned in the operation than a mere transfer of antibodies from mother to ARE ACQUIRED CHARACTERS HEREDITARY? 343 fetus. For instance, the blood of young shortly after birth may show a higher titer than that of the mother. Again, after two or three months of development the young of certain of the sensitized mothers have shown a rather sudden rise in titer, much above that of the mothers. In such cases it would seem that some mechanism in the young rabbit itself is constructing antibodies which supplement those passively derived from the mother. Possibly in the process of develop- ment some organ important in such reactions just came into function- ing. If this is true further experiments may throw some light on the perplexing question of the source or sources of the antibodies in an animal. After a few weeks, in such cases, the titer drops back again. In still another set of experiments we found that young from a sen- sitized mother, when nursed by a normal untreated mother, retained a fairly high titer for several months and even showed the rise of titer mentioned. On the other hand, young of an untreated mother when nursed by a sensitized mother acquired a fairly high titer from the milk of the foster mother but lost it rapidly after weaning time. Thus there are evidently constitutional factors operative in the young which have acquired their immunity through the placenta which are absent in the young whose antibodies were conveyed through food. That changes in the blood serum may be caused by changed con- ditions in the tissues is further attested by many facts. For example, in pregnancy, the newly forming placenta may set free cells or cell- products which, sometimes at least, cause changes in the blood-serum of the mother, though the exact nature of these changes is in dispute. Romer, using the complement-fixation technique, found that the serum of adult human beings may possess antibodies for their own lens proteins. Bradley and Sansum, employing anaphylactic reactions, found that guinea-pigs injected with guinea-pig tissue-proteins (liver, heart, muscle, testicle, kidney) develop immunity reactions. Again during the late war, the type of toxic action to which anaphylactic shock conforms was found to exist after extensive injury of the soft tissues. It resulted apparently from the absorption of poisonous substances of tissue origin into the circulation. In fact, various cells and tissues when injured liberate such poisons, and even blood in clot- ting is known to acquire a transient toxicity of this type. With facts such as these before us, is it not a rational hypothesis to assume that changes in various parts of a body may on occasion influence the representatives of such parts in the germ-cells borne by that body? This appears all the more probable when we recall the 344 READINGS IN EVOLUtlON, GENETICS, AND EUGENICS facts learned from a study of precipitins and of anaphylaxis that each species of animal has a thread of fundamental similarity underlying the proteins of all its tissues. There is no reason to suppose that germi- nal tissue forms an exception. The further fact that homologous tissues, though existing in different species of animals, possess similar chemical characteristics, shows that to get an effect there need not be absolute chemical identity between the substance of such a tissue as the lens and the germinal constituents of which it is the expres- sion. And if this is true for lens, why not for other tissues ? The blood-serum of any organism with blood thus affords a means of conveying the effects of changes in a parental organ to the germ- cell which contains the antecedent of such an organ. As long as there is little change in the somatic element its germinal correlative would presumably remain constant, but any alternations of the soma which give rise to the formation of anti-bodies or other active agents, par- ticularly if long continued, might induce changes in the germ. Such a h}^othesis would seem to be plausible at least in accounting for degenerative changes such as the deterioration of eyes in such forms as the mole, or in fact, in the formation of vestigial organs in general. On the other hand, there is no reason to infer that changes induced in the blood-serum may not also be instrumental in leading to pro- gressive as well as regressive evolution. If we may have germinally destructive constituents engendered in the blood there is no valid reason for supposing that we may not also have constructive ones. When we learn more about what initiates and promotes growth in a part through exercise, or what causes hypertrophy of an organ, we may likewise find how corresponding germinal antecedents of that part may be enhanced. Until such time we shall probably remain in the dark regarding the mechanism of progressive germinal changes. As already indicated, in the hormones and chalones we have a wonderful series of secretions normally circulating in the blood and maintaining general physiological equilibrium. That reciprocal stimulations of various organs occur by this means is a well-established fact. Hyper- trophy or atrophy of an endocrine gland may produce pronounced effects in the furthermost reaches of the body. Again we may inquire, is it reasonable to suppose that the germinal tissues will be inviolate to all this ebb and flow of chemical influence ? Should we not expect specific reactions or selections here no less surely than in other tissues ? Destruction of the pars buccalis of the hypophysis in the frog-tadpole will cause profound alteration in other endocrine organs such as the ARE ACQUIRED CHARACTERS HEREDITARY? 345 adrenals and thyroids, will retard the growth rate, render the entire organism albinous, and produce in the individual pigment cells a con- dition of sustained contraction. Shall we conclude that such a far- reachin'T influence as this, particularly in a developing organism, will pass the germ-cells by unscathed ? Similarly, growth in man is known to be controlled by a pituitary secretion that is carried by the blood to the various organs. The normal development of secondary sexual characters is determined by products from the testes or ovaries, and the activities of the generative organs themselves are intimately associated with the functioning of the adrenal and other glands. The periods of ovulation are inhibited by secretions from the corpus luteum; lactation is incited by products of the corpus luteum, the involuting uterus and the placenta; the car- bohydrate metabolism in the liver and even in the most distant muscles is profoundly influenced by substances formed in the pan- creas; the pancreas, liver, and intestinal glands are set to secreting through the stimulus of a product fornied in the duodenal and jejunal mucosae. And still others of such remarkable interrelations can be cited. Truly one may pronounce that social complex of reciprocating individuals termed cells w^hich make up an organism, ''members one of another." And with all of these co-operative activities of the various parts of the body it is inconceivable to me, at least, that the germ-cells, bathed in the same fluid, nourished with the same food, stand wholly apart. May we not surmise then that as regards inheritance and evolu- tion, Lamarck was not wholly in error when he stressed the importance of use and disuse of a part, or of modifications due to environmental change, in altering the course of the hereditary stream, particularly if we conceive of these influences as being prolonged, possibly over many generations ? Have we not in the serological mechanism of the body of animals an adequate means for the incitement of the germinal changes which underly certain aspects of evolution ? CHAPTER XXIV THE MUTATION THEORY [It will be recalled that Darwin, although depending upon the ever-present fluctuating variations as the material for natural selection to work upon, recognized the occasional occurrence of ''sports" or "saltatory variations." These, however, seemed to him to be so rare in nature as to offer no adequate basis for selection. During the latter part of the nineteenth century several investigators, feeling the inadequacy of fluctuating variations to produce qualitatively new characters, decided to make a more careful examination of animals and plants in nature in order to discover whether saltatory variations might not be of more frequent occurrence than Darwin had supposed. In England William Bateson collected a large number of instances of a type of variation which he called discontinuous in contradistinc- tion to the continuous type which we have been calling fluctuations. Such variations, instead of being in a closely graded series with the typical variations of a species, were frequently quite sharply different from the majority. Although no experiments were conducted in order to test the hereditability of these "discontinuous variations," it is probable that some of them were "mutations" in the sense of De Vries. At about the same time Hugo De Vries in Holland, probably as the result of his rediscovery of Mendel's work and his confirmation of th latter's laws of heredity, became convinced that new species arise not by the accumulation, through natural selection, of minute fluc- tuating variations, but by the sudden appearance in one generation of fully formed new elementary species. He began a systematic research for species of plants in nature that were giving rise to new species. Many species were examined in their natural surroundings and were then brought into the experimental garden for more careful observation, but for a long time the search for a species throwing off new elementary species was unsuccessful. Finally, however, in a field near Hilversum, in the vicinity of Amsterdam, he found what seemed to him to be just the kind of plant he had been looking for in the evening, primrose {Oenothera lamarckiana). 346 THE MUTATION THEORY 347 'Tamarck's evening-primrose" (Fig. 57), says De Vries, "is a stately plant, with a stout stem, attaining often a height of 1.6 meters and more. When not crowded the main stem is surrounded by a large Fig. 57. — Oenothera lamarckiana, the original type used by De\'rics in his experiments. This is the stock from Hilvcrsum, from which arose in successixc generations a series of mutants. {From De ]'ries.) 348 READINGS IN EVOLUTION, GENETICS, AND EUGENICS circle of smaller branches, growing upwards from its base so as to form a dense brush The flowers are large and bright yellow attract- ing immediate attention even from a distance. They open toward evening, as the name indicates, and are pollinated by bumble-bees and moths." On account of the classic character of De Vries's mutants of Oenothera lamarckiana we shall follow his own detailed description of the more significant of these. — Ed.] NEW SPECIES (iVIUTANTS) OF OENOTHERA^ HUGO DE VRIES This striking species (Oenothera lamarckiana) was found in a locality near Hilversum, in the vicinity of Amsterdam, where it grew in some thousands of individuals. Ordinarily biennial, it produces roset.es in the first, and stems in the second year. Both the stems and the rosettes were at once seen to be highly variable, and soon distinct varieties could be distinguished among them. The first discovery of this locality was made in 1886. Afterwards I visited it many times, often weekly or even daily during the first few years, and always at least once a year up to the present time. This stately plant showed the long-sought pecuHarity of producing a number of new species every year. Some of them were observed directly on the field, either as stems or as rosettes. The latter could be transplanted into my garden for further observation, and the stems yielded seeds to be sown under like control. Others were too weak to live a sufiiciently long time in the field. They were discovered by sowing seed from indifferent plants of the wild locality in the garden. A third and last method of getting still more new species from the original strain was the repetition of the sowing process, by saving and sowing the seed which ripened on the introduced plants. These various methods have led to the discovery of over a dozen new types never previously observed or described. Leaving the physiological side of the relations of these new forms for the next lecture, it would be profitable to give a short description of the several novelties. To this end they may be combined under five different heads, according to their systematic value. The first head includes those which are evidently to be considered as varieties, ^ From H. De Vries, Species and Varieties (copyright 1904). Used by special permission of the publishers, The Open Court Publishing Company. THE MUTATION THEORY 349 in the narrower sense of the word, as previously given. The second and third heads indicate the real progressive elementary species, first those which are as strong as the parent-species, and secondly a group of weaker types, apparently not destined to be successful. Under the fourth head I shall include some inconstant forms, and under the last head those that are organically incomplete. Of varieties with a negative attribute, or real retrograde varieties, I have found three, all of them in a flowering condition in the field. I have given them the names of laevifolia, brevistylis and nannella. The laevifolia, or smooth-leaved variety, was one of the very first deviating types found in the original field. This was in the summer of 1887, seventeen years ago. It formed a little group of plants grow- ing at some distance from the main body, in the same field. I found some rosettes and some flowering stems and sowed some seed in the fall. The variety has been quite constant in the field, neither increas- ing in number of individual plants nor changing its place, though now closely surrounded by other lamarckianas. In my garden it has proved to be constant from seed, never reverting to the original lamarckiana, provided intercrossing was excluded. It is chiefly distinguished from Lamarck's evening-primrose by its smooth leaves, as the name indicates. The leaves of the original form show numerous sinuosities in their blades, not at the edge, but anywhere between the veins. The blade shows numbers of convexi- ties on either surface, the whole surface being undulated in this manner; it lacks also the brightness of the ordinary evening-primrose or Oenothera biennis. These undulations are lacking or at least very rare on the leaves of the new laevifolia. Ordinarily they are wholly wanting, but at times single leaves with slight manifestations of this character may make their appearance. They warn us that the capacity for such sinuosities is not wholly lost, but only lies dormant in the new variety. It is reduced to a latent state, exactly as are the apparently lost characters of so many ordinary horticultural varieties. Lacking the undulations, the laevifolia-leaves are smooth and bright. They are a little narrower and more slender than those of the lamarckiana. The convexities and concavities of leaves are a useful character in dry seasons, but during wet summers, such as those of the last few years, they must be considered as very harmful, as they retain some of the water which falls on the plants, prolonging the action of the water on the leaves. This is considered by some writers 350 READINGS IN EVOLUTION, GENETICS, AND EUGENICS to be of some utility after slight showers, but was observed to be a source of weakness during wet weather in my garden, preventing the leaves from drying. Whether the laevifolia would do better under such circumstances, I have, however, omitted to test. The flowers of the laevifolia are also in a slight degree different from those of lamarckiana. The yellow color is paler and the petals are smoother. Later, in the fall, on the weaker side branches these differences increase. The laevifolia petals become smaller and are devoid of the emargination at the apex, becoming ovate instead of obcordate. This shape is often the most easily recognized and most striking mark of the variety. In respect to the reproductive organs, the fertility and abundance of good seed, the laevifolia is by no means inferior or superior to the original species. 0. brevistylis, or the short-styled evening-primrose, is the most curious of all my new forms. It has very short styles, which bring the stigmas only up to the throat of the calyx-tube, instead of upwards of the anthers. The stigmas themselves are of another shape, more flattened and not cylindrical. The pollen falls from the anthers abundantly on them, and germinates in the ordinary manner. The ovary which in lamarckiana and in all other new forms is wholly underneath the calyx-tube, is here only partially so. This tube is inserted at some distances under its summit. The insertion divides the ovary into two parts : an upper and a lower one. The upper part is much reduced in breadth and somewhat attenuated, simulating a prolongation of the base of the style. The lower part is also reduced, but in another manner. At the time of flowering it is like the ovary of lamarckiana, neither smaller nor larger. But it is only reached by very few pollen- tubes, and is therefore always very incompletely fertilized. It does not fall off after the fading away of the flower, as unfertilized ovaries usually do; neither does it grow out, nor assume the upright position of normal capsules. It is checked in its develop- ment, and at the time of ripening it is nearly of the same length as in the beginning. Many of them contain no good seeds at all; from others I have succeeded in saving only a hundred seeds from thousands of capsules. These seeds, if purely polhnated, and with the exclusion of the visits of insects, reproduce the variety entirely and without any reversion to the lamarckiana type. Correlated with the detailed structures is the form of the flower- buds. They lack the high stigma placed above the anthers, which in THE MUTATION THEORY 351 the lamarcklana, by the vigorous growth of the style, extends the calyx and renders the flower-bud thinner and more slender. Those of the brevistylis are therefore broader and more swollen. It is quite easy to distinguish the individuals by this striking character alone, although it differs from the parent in other particulars. The leaves of the 0. brevistylis are more rounded at the tip, but the difference is only pronounced at times, slightly in the adult rosettes, but more clearly on the growing summits of the stems and branches. By this character the plants may be discerned among the others some weeks before the flowers begin to show themselves. But the character by which the plants may be most easily recog- nized from a distance in the field is the failure of the fruits. They were found nearly every year in varying, but always small numbers. Leaving the short-styled primrose, we come now to the last of our group of retrograde varieties. This is the 0. nannella, or the dwarf, and is a most attractive little plant. It is very short of stature, reaching often a height of only 20-30 cm., or less than one-fourth of that of the parent. It commences flowering at a height of 10-15 ^^^-j whfle the parent-form often measures nearly a meter at this stage of its development. Being so very dwarfed the large flowers are all the more striking. They are hardly inferior to those of the lamarckiana, and agree with them in structure. When they fade away the spike is rapidly lengthened, and often becomes much longer than the lower or vegetative part of the stem. The dwarfs are one of the most common mutations in my garden, and were observed in the native locality and also grown from seeds saved there. Once produced they are absolutely constant. I have tried many thousands of seeds from various dwarf mutants, and never observed any trace of reversion to the lamarckiana type. I ha\'e also cultivated them in successive generations with the same result. In a former lecture we have seen that contrary to the general run of horticultural belief, varieties are as constant as the best species, if kept free from hybrid admixtures. This is a general rule, and the ex- ceptions, or cases of atavism, are extremely rare. In this respect it is of great interest to observe that this constancy is not an acquired quality, but is to be considered as innate, because it is already fully developed at the very moment when the original mutation takes place. From its first leaves to the rosette period, and through this to the lengthening of the stem, the dwarfs are easily distinguished from any other of their congeners. The most remarkable feature is the shape 352 READINGS IN EVOLUTION, GENETICS, AND EUGENICS THE MUTATION THEORY 353 of the leaves. They are broader and shorter, and especially at the base they are broadened in such a way as to become apparently sessile. The stalk is very brittle, and any rough treatment may cause the leaves to break off. The young seedlings are recognizable by the shape of the first two or three leaves, and when more of them are produced, the rosettes Ijecome dense and strikingly dilTerent from others. Later leaves are more nearly like the parent-type, Init ihe petioles remain short. The bases of the blades are frequently almost cordate, the laminae themselves varying from oblong-ovate to ovate in outline. The stems are often quite unb ranched, or branched only at the base of the spike. Strong secondary stems are a striking attribute of the lamarckiana parent, but they are lacking, or almost so in the dwarfs. The stem is straight and short, and this, combined with the large crown of bright flowers, makes the dwarfs eminently suitable for bed or border plants. Unfortunately they are very sensitive, especially to wet weather. Oenothera gigas and 0. rubrinervis, or the giant, and the red-veined evening primroses, are the names given to two robust and stout species, which seem to be equal in vigor to the parent-plant, while diverging from it in striking characters. Both are true elementary species, differentiated from lamarckiana in nearly all their organs and qualities, but not showing any preponderating character of a retrograde nature. Their differences may be compared with those of the elemen- tary species of other genera, as for instance, of Draba, or of violets, as will be seen by their description. The giant evening-primrose, though not taller in stature than 0. lamarckiana, deserves its name because it is so much stouter in all respects. The stems are robust, often with twice the diameter of lamarckiana throughout. The internodes are shorter, and the leaves more numerous, covering the stems with a denser foliage. This shortness of internodes extends itself to the spike, and for this reason the flowers and fruits grow closer together than on the parent-plant. Hence the crown of bright flowers, opening each evening, is more dense and more strikingly brilliant, so much the more so as the individual flowers are markedly larger than those of the parents. In connection with these characters, the flower-buds are seen to be much stouter than those of lamarckiana. The fruits attain only half the normal size, but are broader and contain fewer, but larger seeds. 354 READINGS IN EVOLUTION, GENETICS, AND EUGENICS The ruhrinervis is in many respects a counterpart to the gigas, but its stature is more slender. The spikes and flowers are those of the lamarckiana, but the bracts are narrower. Red veins and red streaks on the fruits afford a striking differentiating mark, though they are not absolutely lacking in the parent-species. A red hue may be seen on the calyx, and even the yellow color of the petals is somewhat deepened in the same way. Young plants are often marked by the pale red tinge of the mid-veins, but in adult rosettes, or from lack of sunshine, this hue is often very faint. The leaves are narrow, and a curious feature of this species is the great brittleness of the leaves and stems, especially on annual indi- viduals, for example, on those that make their stem and flowers in the first year. High turgidity and weak development of the mechani- cal and supporting tissues are the anatomical cause of this deficiency, the bast-fibres showing thinner walls than those of the parent-type under the microscope. Young stems of ruhrinervis may be broken off by a sharp stroke, and show a smooth rupture across all the tissues, while those of lamarckiana are very tough and strong. Both the giant and the red- veined species are easily recognized in the rosette-stage. The very young seedlings of the latter are not clearly differentiated from the lamarckiana, and often a dozen leaves are required before the difference may be seen. Under ordinary circumstances the young plants must reach an age of about two months before it is possible to discern their characters, or at least before these characters have become reliable enough to enable us to judge of each individual without doubt. But the divergencies rapidly become greater. The leaves of O. gigas are broader, of a deeper green, the blade more sharply set off against the stalk, the whole rosettes becoming stout and crowded with leaves. Those of 0. ruhri- nervis on the contrary are thin, of a paler green and with a silvery white surface; the blades are elliptic, often being only 2 cm. or less in width. They are acute at the apex and gradually narrowed into the petiole. It is quite evident that such pale narrow leaves must produce smaller quantities of organic food than the darker green and broad organs of the gigas. Perhaps this fact is accountable partly, at least, for the more robust growth of the giant in the second year. Perhaps also some relation exists between this difference in chemical activity and the tendency to become annual or biennial. The gigas, as a rule, produces far more, and the ruhrinervis far less biennial plants, than the lamarckiana. Annual culture for the one is as unreliable as THE MUTATION THEORY 355 biennial culture for the other. Rubrinerms may be annual in appar- ently all specimens, in sunny seasons, which would allow a large part of the gigas to remain in the state of rosettes during the entire first summer. It would be very interesting to obtain a fuller insight into the relation of the length of life to other qualities, but as yet the facts can only be detailed as they stand. Both of these stout species have been found quite constant from the very first moment of their appearance. I have cultivated them from seed in large numbers, and they have never reverted to the lamarckiana. From this they have inherited the mutability or the capacity of producing in their turn new mutants. But they seem to have done so incompletely, changing in the direction of more absolute constancy. This was especially observed in the case of rtibrinervis, which is not of such rare occurrence as O. gigds, and which it has been possible to study in large numbers of individuals. So for instance, "the red- veins" have never produced any dwarfs, notwithstanding they are produced very often by the parent-type. And in crossing experiments the red-veins gave proof of the absence of a mutative capacity for their production. [Besides the mutants just described there occurred two weak forms that could survive only if reared under protection and would have failed to survive in nature. Here we have a place for the action of natural selection, but operating with mutations instead of with fluctuating variations. These two mutants are " the whitish and the oblong-leaved evening-primroses or the Oenothera albida and oblonga.'^ All of the mutants so far mentioned are constant forms that breed true to type. Certain other types were either incapable of being bred or else were decidedly inconstant. Oenothera lata had only pistillate flowers and therefore could not be fertilized by pollen of the same mutant. Oenothera scintillous and O. elliptica are fertile to their own pollen, but produce progeny only partly like the parent, the rest reverting to the original type Oenothera lamarckiana. — Ed.] SUMMARY OF DE VRIES'S MUTATION THEORY' THOMAS HUNT MORGAN We may now proceed to examine the evidence from which De Vries has been led to the general conclusions given in the preceding pages. De Vries found at Hilversum, near Amsterdam, a locality ^ T. H. Morgan, Evolution and Adaptation (1903). Used by special permission of the publishers, The Macmillan Company. 356 READINGS IN EVOLUTION, GENETICS, AND EUGENICS where a number of plants of the evening primrose, Oenothera lamarck- iana, grow in large numbers. This plant is an American form that has been imported into Europe. It often escapes from cultivation, as is the case at Hilversum, where for ten years it had been growing wild. Its rapid increase in numbers in the course of a few years may be one of the causes that has led to the appearance of a mutation period. The escaped plants showed fluctuating variations in nearly all of their organs. They also had produced a number of abnormal forms. Some of the plants came to maturity in one year, others in two, or in rare cases, in three, years. A year after the first finding of these plants De Vries observed two well-characterized forms, which he at once recognized as new elemen- tary species. One of these was O. brevistylis, which occurred only as female plants. The other new species was a smooth-leafed form with a more beautiful foliage than O. lamarckiana. This is O. laevifolia. It was found that both of these new forms bred true from self- fertilized seeds. At first only a few specimens were found, each form in a particular part of the field, which looks as though each might have come from the seeds of a single plant. These two new forms, as well as the common 0. lamarckiana, were collected, and from these plants there have arisen the three groups of famiUes of elementary species that De Vries has studied. In his garden other new forms also arose from those that had been brought under cultivation. The largest group and the most important one is that from the original 0. lamarckiana form. The accompanying table shows the mutations that arose between 1887 and 1899 from these plants. The seeds were selected in each case from self-fertilized plants of the lamarckiana form, so that the new plants appearing in each horizontal line are the descendants in each generation of lamarckiana parents. It will be observed that the species, 0. oblonga, appeared again and again in considerable numbers, and the same is true for several of the other forms also. Only the two species, 0. gigas and 0. scintillans, appeared very rarely (Fig. 59). Thus De Vries had, in his seven generations, about fifty thousand plants, and about eight hundred of these were mutations. When the flowers of the new forms were artificially fertilized with pollen from the flowers of the same plant, or of the same kind of plant, they gave rise to forms like themselves, thus showing that they are true elemen- tary species. It is also a point of some interest to observe that all these forms differed from each other in a large number of particulars. THE MUTATION THEORY 357 Only one form, 0. scintillans, that appeared eight times, is not constant as are the other species. When self-fertihzed its seeds produce always three other forms, O. scintillans, 0. oblonga, and O. lamarckiana. It differs in this respect from all the other elementary species, which mutate not more than once in ten thousand individuals. .1 ,5 1 -•.- 21 1. 1, ,11 .9 37C0 11. 5 2.9 ? 1800 9 ,25 \^S 20 8000 49 ^4s^ 6. 15 Y 176 8 14: )00 60 75 1. 10000 s 3. ■ .1 - - r 5 p. , ,1^0 00 9 ■ Fig. 59. — Diagram showing in condensed form the genealogy of the Or;/<)/// tn CO CTl O •- CM cs in in ID '^ IX) lo' i ' ' I I I 0> r^ in fr) .— 00 CTl O .- CM in m lo ^ ID 00 in Fig. 62. — Polygon of variation for the total number of scutes in the nine bands of the armadillo (Dasypus novemcinctus) , as determined by the seriation of 508 individuals. Class range = 8 scutes. The solid line represents the observa- tional, and the broken line the theoretical, normal curve. The abscissae refer to the number of scutes, and the ordinates to the number of individuals. {From Newman.) they were arranged in a variation polygon as shown in Figure 62. On the abscissa are arranged groups including individuals between 517 and 524 scutes inclusive, those between 525 and 532, those between 533 and 540, on up to a group of those from 621 to 625. All of these except the last included a small class with a range of 8 scutes. This arranging in classes was essential, for without it there would have been BIOMETRY 367 113 classes and a very irregular and meaningless distribution. On the ordinate we find by tens the numbers of individuals in each class. It will be noted that the solid line is one connecting the points of inter- section between the class of scute numl)ers and the numljcr of indi- viduals in these classes. The dotted line represents an ideal fluctuat- ing variation curve, which is practically a mathematical curve of chance. The closeness of fit between the actual and the theoretical curve is very good. The mode is the class including individuals with a scute count of 557-64, and there is a fairly even balance of individuals in the plus and the minus directions. It seems fairly evident from examination of the curve that the individuals with 613 scutes and over are beyond the limits of the theoretical distril)ution. A further study of these exceptional individuals shows that they are mutations, in which a splitting up of single scutes into paired and twinned scutes has taken place to such an extent as greatly to increase the total num- ber of scutes. From the data used in constructing this variation polygon several significant constants may be obtained. The ''arithmetical mean" (average number of scutes in the entire 508 individuals) is 558.2. The ''median" or halfway point between the extremes is 558. The "mode" or most frequently occurring single type is 557 (the theoreti- cal value being 557.6). If we wished to compare a large group of parents with a large group of offspring, or if it were necessary to compare the armadillos of Texas with those of Mexico or Brazil, we could compare them as to mean, median, and mode, and also as to the shape of the polygon of variation. This would give us a very good idea as to whether or not the old species present in these three regions is tending to evolve in ditlerent directions under different conditions of life. Instead of having to depend on the visual comparison between the variation polygon of two or more different populations, we can reduce the facts about the distribution of the different types about the mean or mode to a simple arithmetical constant, called the ''standard deviation," which is usually given the symbol a. This constant is computed as follows: In this formula x represents the deviation of each class from the arithmetical mean; /, the number of individuals in each separate class; S, the sum of all the classes, and ;/, the total number of individuals. 368 READINGS IN EVOLUTION, GENETICS, AND EUGENICS By the use of this formula we have calculated the standard devia- tion ((j) of the individuals represented in Figure 62 to be 14.89 =±= 0.31 scutes. This means that the average deviation from the mean is about 14.89 scutes. The =*=o.3i scutes is called the "probable error" and means that the figure 14.89 is inaccurate to the extent of being 0.31 scutes too high or too low. The probable error is an essential feature of such computations, as, without it, we would not be able to rely on the signifi- cance of small differences. Suppose, for example, we should find that the armadillos of Brazil had a standard deviation of 15.43 ±0.44 scutes, we might conclude that the variability of the Brazilian individuals was 0.34 scutes greater than that of the Texas individuals. In view of the fact, however, that the probable error in one case is ±0.31 scutes and in the other ±0.44 scutes we would have to conclude that there was no significant difference. In actual practice it has been decided that unless the actual difference between two constants is about 4.6 times as great as the probable error, the difference is not significant. The method of determining the probable error of any calculated constant is difficult to understand, but easy to put into practice. For example, the formula for calculating the probable error of the standard deviation is as follows: 0.67450- E(T = V 271 where E is the probable error, and n the number of individuals. It will be seen that the probability of error diminishes steadily with the increase in number of individuals studied. With very large numbers the error due to what is known as ''random sampling" practically disappears. BIMODAL AND MULTIMODAL CURVES If we confine our biometrical studies to homogeneous populations, we get only fairly simple monomodal curves that resemble the normal curve of variation, which is a curve of chance; but when we study ordinary wild populations, we frequently find that we are dealing with a complex of several races, each of which has its own mode and stand- ard deviation. Bateson has given us a classic example of this type of phenomenon. In studying the length of pinchers in the common earwig {Forjiculata auricularia) , he found that he got a two-humped or bimodal curve as shown in Figure 63. It then became evident that there were two distinct varieties as figured above. Such studies have frequently revealed the heterogeneity of supposedly homogeneous BIOMETRY 369 populations. Opinion differs as to the significance of these Inuiings. The more optimistic evolutionists look upon such instances as that of Bateson's earwigs as visual demonstrations of a species actually split- ting up into two or more species. It seems quite Hkely that one of these types is a successful mutant type that has not fully segregated itself as a true species from the parent-type. Another view of the significance of bimodal curves is that the condition results from hybridi- zation and that the bimodality is the result of the segregation of dominant and recessive types. A / \ r~ T \ / \ \ / \ \^^ ^ \ \ ^T-n- 3 4 5 6 7 8 9 Fig. 63. — Bimodal polygon plotted from data on the earwig. Mean types (X|) indicated above corresponding modes. Numbers below the base line indicate length of pincers in millimeters. {From Bateson and Johannscn.) THE COEFFICIENT OF CORRELATION Only one more biometrical constant need be mentioned here: the ''coefficient of correlation." It is often necessary to discover the exact relation that exists between two sets of variables in order to discover whether they are totally independent or partially correlated with each other. For example, we have found that there is a close correlation between stature and head length in man, also between color of hair and color of eyes; but it is very important to be able to reduce the degree of correlation to a simple arithmetical constant. This is called the "coefficient of correlation" (commonly expressed as Yxy, where x is one variable and y the other). The formula for com- puting r^y is as follows: ^xy n \^3f^y 370 READINGS IN EVOLUTION, GENETICS, AND EUGENICS where d represents the actual deviation and 2 the sum, n the number of individuals; a the standard deviation. "Correlation tables" show graphically whether or not there is correlation. If, as in Figure 64, we want to find out what is the rela- tionship between total yield of oats and number of culms to the plant, we may make a table with subject classes arranged perpendicularly, and the relative classes, horizontally. If the individuals tend to group themselves about a diagonal ranging from upper left- to lower right- hand corners, the amount of correlation is quite marked. Complete correlation would be represented by a single line of points along this diagonal. No correlation would be shown by random distributing 2 3 4 5 6 7 0-1 3 3 1-2 28 19 3 50 2-3 18 66 20 1 1 106 3-4 1 42 58 7 1 109 4-5 7 59 11 3 80 5-6 26 14 2 42 &-7 4 3 7 7-8 1 1 2 8-9 1 1 50 134 167 38 10 1 400 Fig. 64. — Correlation table of 400 plants of Sixty-Day oats. Total yield of plant in grams, subject. Number of culms per plant, relative. 19 10. Coefficient of correlation = o. 712 ±0.017. {From Love and Leighty, 1914.) over the whole rectangle. Inverse correlation would tend to give a grouping about a diagonal ranging from the upper right- to lower left-hand corners. In the particular correlation table used for illustration, the coeffi- cient of correlation (vxy) turns out to be 0.712^0.017. Since com- plete correlation would be i, the degree of positive correlation is very high, as we might expect. The correlation table was used quite effectively by Galton, as we shall now show. STATISTICAL STUDY OF INHERITANCE^ EDWIN GRANT CONKLIN Francis Galton was one of the first who attempted to reduce the mass of conflicting observations on heredity and variation to some *From E. G. Conklin, Heredity and Environment (copyright 1920). Used by special permission of the publishers, The Princeton University Press. BIOMETRY 371 system and to establish certain principles as a result of statistical study. He was the real founder of the scientific study of inheritance; he studied characters singly and he introduced quantitative measures. Galton's researches, which were i)ublished in several volumes, con- sisted chiefly in a study of certain families with regard to several selected traits, viz., genius or marked intellectual capacity, artistic faculty, stature, eye color and disease. As a result of his very e.xten- sive studies two main principles appeared to be established : I. The Law of Ancestral Inheritance which he stated as follows: The two parents contribute between them on the average one-half of each inherited faculty, each of them contributing one-quarter of it. The four grandparents contribute between them one-quarter, or each of them one-sixteenth ; and so on, thesum of the series I + 4 + s + iV, being equal to i, as it should be. It is a property of this infinite series that each term is equal to the sum of all those that follow: thus i = |+|+iV . . . . , 4 = l+iV+ • • . . , and so on. The pre- potencies of particular ancestors in any given pedigree are eliminated by a law which deals only with average contributions, and the various prepotencies of sex with respect to different qualities are also presum- ably eliminated. The average contribution of each ancestor was thus stated defi- nitely, the contribution diminishing with the remoteness of the ances- tor. This Law of Ancestral Inheritance is represented graphically in Figure 65. Pearson has somewhat modified the figures given by Galton, holding that in horses and dogs the parents contribute ^, the grandparents J, the great-grandparents |, etc. Number of ancestors. — Theoretically the number of ancestors doubles in each ascending generation; there are two parents, four grandparents, eight great grandparents, etc. If this continued to be true indefinitely the number of ancestors in any ascending generation would be (2)", in which n represents the number of generations. There have been about 57 generations since the beginning of the Chris- tian Era, and if this rule held true indefinitely each of us would have had at the time of the birth of Christ a number of ancestors repre- sented by (2)" or about 120 quadrillions — a number far greater than the entire human population of the globe since that time. As a matter of fact, owing to the intermarriage of cousins of various degrees, the actual number of ancestors is much smaller than the theoretical number. For example, Plate says that the late Emperor of Germany had only 162 ancestors in the loth ascending generation, instead of 372 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 512, the theoretical number. Nevertheless this calculation will serve to show how widespread our ancestral lines are, and how nearly related are all people of the same race. Davenport concludes that no people of Enghsh descent are more distantly related than 30th cousins, while most people are much more closely related than that. If we allow three generations to a century, and calculate that the degree of cousinship is determined by the num- ber of generations less two, since first cousins appear only in the third generation, the first being that of the parents and the second that of the sons and daughters, we find that 30th cousins at the present time d' 9 cT ? S ? cT ? c5' ? d ? ^ is as follows. When two similar gametes unite to form a zygote it is called a homozygote; when the two pairing gametes are different the zygote is called a heterozygote. Using this terminology it is evident that the 3:1 ratio of the F2 generation is really a 1:2:1 ratio, as follows: i homozygote for the dominant character, 2 heterozygotes, and i homozygote for the recessive charac- ter. The 1:2:1 ratio therefore is the significant one and appears as a 3 : 1 ratio only because of dominance. In the experiment represented in Fig. 68 three tall individuals appear in the F2 generation. Superficially the individuals look alike, but it is realized that i differs from the other 2 in germinal constitu- tion, for I will produce only one kind of gamete, while the other 2 will produce two kinds. To indicate this situation Johannsen has introduced some appropriate terminology. Organisms which seem to be alike, regardless of their germinal constitution, are said to be phenotypically alike, or to belong to the same phenotype. On the other hand, organisms having identical germinal constitution are said to be genotypically alike, or to belong to the same genotype. From the standpoint of phenotypes only, Mendel's F2 generation shows the 3:1 ratio; but if genotypes are considered, it shows the 1:2:1 ratio. In other words, this group of forms contains two phenotypes but three genotypes. Referring again to Fig. 68 several things may be inferred. It can be seen what will happen in the F3 generation when the F2 individuals are inbred. The dominant homozygote will produce only dominant homozygotes in the F3 generation and will continue to produce them as long as it is inbred. The two heterozygotes will split up in the F3 generation in the same 1:2:1 ratio as did their hybrid parents of the Fi generation. The recessive homozygote will produce only recessive homozygotes as long as it is kept pure by being inbred. It is interesting to consider what will happen if a heterozygote form is crossed with a homozygous recessive. It should be obvious that one-half of the progeny would be pure recessives, while the other aalf would be heterozygotes, that is, there would be a 1:1 ratio. A similar result would be obtained by crossing a heterozygote with a dominant homozygote, although all the immediate progeny would show the dominant character. The real situation would be revealed, however, when this progeny was inbred, for one-half would be homo- zygous (pure breeders) and the other half would be heterozygous (hybrid breeders). MENDEL'S LAWS OF HEREDITY 391 Thus far we have considered only what is called the monohybrid ratio, that is, the ratio obtained from one pair of contrasting charac- ters, such as tallness and dwarfness. The next step is to consider the dihybrid ratio. Mendel also used contrasting seed characters, find- ing, for example, that smoothness in seeds is dominant to a wrinkled condition. Introducing this pair of contrasting characters into the situation we have been considering, the dihybrid ratio will be the result. Crossing a tall, smooth-seeded individual with a dwarf wrinkled-seeded individual it is evident that all of the Fi or first hybrid generation will be tall, smooth-seeded individuals, since both of these characters are dominant. In the F2 generation, however, the follow- ing ratio will appear: 9 tall smooth, 3 dwarf smooth, 3 tall wrinkled, I dwarf wrinkled; which is a 9:3:3:1 ratio. This is the dihybrid ratio, the explanation of which may be indicated in Fig. 69. The question may be raised why the characters for tallness and smoothness are not represented on the same chromosome. If they were, the result would be a simple monohybrid ratio, except that the tall indi- viduals would always be smooth-seeded as well, and dwarfs would be always wrinkled-seeded. The possibility of one chromosome carrying two different determiners will be considered later, but at present we shall assume that these determiners are on different chromosomes. Fig. 69 shows that we are dealing with two homozygotes, each pro- ducing only one kind of gamete, so that all the hybrid progeny will be similar, both genotypically and phenotypically, that is, with the same germinal constitution and the same appearance. By inbreeding these Fi individuals, it will be seen that four kinds of gametes are involved. Crossing these four kinds of gametes the resulting com- binations are indicated in Fig. 69. The result is four pheno types, as follows: Nos. I, 2, 3, 4, 5, 7, 9, 10, 13 are tall smooth individuals; Nos. II, 12, 15 are dwarf smooth; Nos. 6, 8, 14 are tall wrinkled; No. 16 is dwarf wrinkled. This is the 9:3:3:1 ratio. It will be noticed that Nos. i, 6, 11, 16 are homozygotes and there- fore will breed true; but the rest are heterozygotes, either for one pair of characters or for both, and these would split into various types upon further breeding. The next step is the trihybrid ratio. Mendel found yellow seeds dominant over green seeds, and if this pair of characters is included with those used above the trihybrid result can be observed. The experiment would cansist in crossing tall, smooth, yellow individuals with dwarf, wrinkled, green individuals; and it is obvious that the 392 RE.\DINGS IN EVOLUTION, GENETICS, AND EUGENICS hybrid progeny would all be tall, smooth, yellow, since these three characters are dominant. Inbreeding the hybrids gives the following result in the F2 generation: 27 tall smooth yellow, 9 tall smooth green. Dwarf Wrinkled Parent Gametes ®® ®® ®® ®.® ®® ® ® ®,„® ®® ®,® (w) (w T) {^D S)'(S ®'® ®® ®® ®.® ®»® ® ® ®® ®.® ® ® Fig. 69. — Diagram illustrating dihybrid ratio. Upper part shows how original parents were crossed to give Fj hybrid; lower part shows Fi hybrid producing four kinds of gametes; chance matings among these gametes, when Fi is inbred, result as indicated in the large set of squares and explains the 9:3:3:1 ratio in the F2 generation. {From Coulter atid Coulter.) 9 tall wrinkled yellow, 9 dwarf smooth yellow, 3 tall wrinkled green, 3 dwarf smooth green, 3 dwarf wrinkled yellow, i dwarf wrinkled green. The trihybrid ratio therefore is 27:9:9:9:3:3:3:1. This involves 64 individuals and 8 phenotypes. MENDEL'S LAWS OF HEREDITY 393 ILLUSTRATIONS OF SIMPLE MENDELIAN INHERITANCE IN BOTH ANIMALS AND PLANTS^ J. ARTHUR THOMSON How far has Mendel's experience been confirmed? — There has been confirmatory work by Correns (on peas, maize, and garden- stock), by Tschermak (on peas), by De Vries (on maize, etc.), by Bateson and his collaborators (on a large variety of organisms), by Darbishire (on mice), by Hurst (on rabbits), by Toyama (on silk- moths), by Davenport (on poultry), and so on. There are some difficulties and not a few discrepancies, but, as Bateson says, " the truth of the law enunciated by Mendel is now established for a large number of cases of most dissimilar characters." In experimenting with Lychnis, Atropa, and Datura, Bateson and Saunders found that the phenomena conformed with Mendel's law "with considerable accuracy, and no exceptions that do not appear to be merely fortuitous were discovered. In the case of Matthiola (garden-stock), the phenomena are much more complex. There are simple cases which follow Mendelian principles, but others of various kinds which apparently do not. The latter cases fall into fairly defin- ite groups, but their nature is obscure." In experiments with poultry, the phenomena of dominance and recession were detected; interbreeding of the hybrid offspring resulted in a mixed progeny, " some presenting the dominant, others the reces- sive character, in proportions following Mendel's Law with fair con- sistency, though in certain cases disturbing factors are to be suspected." The general result, so far, is that Mendel's law has received con- firmation in a number of very dissimilar cases. Dominant and recessive characters. — Let us first of all collect a number of instances of contrasted characters which behave in relation to one another as dominants and recessives. Dominant Recessive Pisum sativum Tallness Dwarfness Round seeds Wrinkled seeds Coloured seed-coats White seed-coats Yellow albumen in coty- Green albumen in coty- ledons ledons Purple flowers White flowers Sweet pea Tall ordinary form Dwarf or "cupid" vari- ety =" From J. Arthur Thomson, Heredity (copyright 1907). Used by special permission of the publisher, John Murray, London. 394 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Dominant Recessive Stocks Coloured WOiite Wheat and barley Beardless Bearded Later ripening Rivett Early ripening Polish wheat wheat Non-immune to "rust" Immune to "rust" Maize "Starch" seed "Sugar" seed Nettles (Urtica pilulijera and U. dodartii) Serrate leaf margin Entire leaf margin Mirabilis jolapa and M, rosea . Rose colour Other colours Mice Coloured coat Albino coat Normal "Waltzing" variety Rabbits Coloured coat Albino coat Angora fur Short fur Poultry "Rose" comb of Ham- High serrated "single" burghs and Wyandottes comb of Leghorns and Andalusians Cattle Hornlessness Horns Snails Bandless shell Banded shell Other instances in plants. — As is well known, there are two almost equally common forms of wild primrose: (A) thrum- types, with short styles and with anthers at the top of the corolla-tube; and (B) pin-types, with long styles and with anthers half way down the tube. The thrum-type is dominant over the pin-type. The original species of Chinese primrose {Primula sinensis) has a palmate leaf. About i860 a sport arose (from seed) which had a pinnate or "fern" leaf. The palmate form is dominant, and the fern leaf is recessive. The deformed "Snapdragon" variety of sweet pea behaves as a recessive to the normal type. The 2 -row barley has certain lateral flowers which are exclusively staminate; in 6-row barley all the flowers are staminate and pistillate, and all set seed. Mr. Biffen crossed these forms, and found that the more negative character was dominant. The offspring were 2-rowed. Maize. — When the common or starchy round-seeded maize is crossed with the wrinkled-seeded sugar-maize, the round starchy char- acter dominates. When an egg-cell of the wrinkled sugar-maize stock is fertilised by a pollen-cell of the round starchy stock, the result is a round seed with starchy endosperm. If this seed is sown, it becomes a plant which, on self-fertilisation, forms a cob with a mixture of round starchy and wrinkled sugary seeds in the ratio 3:1. The wrinkled seeds yield sugar- maize; the round seeds yield two "impure rounds" to one "pure round." Correns has observed a very inter- esting case in which two pairs of contrasted characters are imphcated. MENDEL'S LAWS OF HEREDITY 395 One variety, Zea .mays alba, which has smooth white seeds, was crossed with another variety, Zea mays coeruleodulcis, which has wrinkled blue seeds. The hybrids (Fj) had smooth blue seeds, one character of each parent being dominant, and one character of each parent being recessive. The hybrids were inbred, and the progeny (F2) showed four combinations — smooth blue, smooth white, wrinkled blue, and v/rinkled white (the dominant characters are italicised). In the next generation (F3), the wrinkled white, inbred, yielded wrinkled white — a case of extracted recessives, breeding true. The smooth whites and wrinkled blues, inbred, yielded partly forms like themselves and partly wrinkled white. The smooth blues, inbred, yielded the same combinations as in F3. A finer corroboration of Mendelian could hardly be wished. Nettles. — ^Correns crossed two ''species of stinging-nettle," Urtica pilulifera L. and U. dodartii L., which resemble one another except as regards leaf-margin, strongly dentate in the former, almost entire in the latter. The hybrid offspring (Fi) have all dentate leaves like the male or the female parent, as the case may be. The dentate character is absolutely dominant. The inbred (self-fertilised) hybrids produce offspring (F2) of two kinds, with dentate and with entire margins, on an average in the Mendelian proportion, 3:1. "Immunity to rust in wheat. — Some kinds of wheat are very susceptible to the fungoid disease known as ' rust ' ; others are immune. The quality of immunity to rust is recessive to the quality of predis- position to rust. "When an immune and a non-immune strain are crossed together the resulting hybrids are all susceptible to ' rust.' On self-fertilisation such hybrids produce seed from which appear dominant 'rusts' and recessive immune plants in the expected ratio of 3:1. From this simple experiment the phrase 'resistance to disease' has acquired a more precise significance, and the wide field of research here opened up in this connection promises results of the utmost practical as well as theoretical importance. To the question, ' Who can bring a clean thing out of an unclean ? ' we are beginning to find an answer, nor is the answer thie same as that once given by Job" (R. C. Punnett). Silkworms. — Toyama paired Siamese silkmoths with yellow or with white cocoons; the offspring produced only yellow cocoons. When the hybrids were inbred, the result was two sets, one producing white cocoons, the other producing yellow cocoons, and the proportion was Mendelian — 25.037 white and 74.96 yellow. The whites bred 396 READINGS IN EVOLUTION, GENETICS, AND EUGENICS true; the yellows when inbred showed themselves to be pure domi- nants or "yellows" and dominant-recessives — i.e., splitting up again into yellows and whites in the usual proportion. More intricate experiments confirmed this general result. It must be noted, however, that Coutagne has made much more elaborate experiments with different results, which in many cases can- not be interpreted on the Mendelian theory. Thus he found (i) that the hybrid forms were sometimes blends of the parents and different from both; (2) that in other cases the brood included some like one parent in a particular character, some Hke the other parent, and some intermediate; and (3) that in other cases the individuals showed no fusion of characters, but resembled one or other parent. It is likely that the discrepancy may be explained as due to considerable diversity of origin in the domesticated races of silkworm, so that, while they breed true when left to themselves, a disturbance of the usual routine leads to the liberation of latent characters. Lina lapponica. — Miss McCracken has made a fine study of the hereditary relations in this Californian beetle, which occurs in two types, spotted (dominant) and black (recessive). They are always crossing in natural conditions, but there are no intermediates, and it is easy by isolation to rear a ''pure" spotted race and a "pure" black race. When spotted forms are paired they may produce only spotted progeny — a case of extracted dominants. In other cases, however, they yield spotted and black forms (1,021 spotted, 345 black), i.e., in the Mendelian proportion of 3 : i — a case of dominant-recessives inbred. Snails. — Lang paired "pure" five-banded forms of the common or garden snail. Helix hortensis, with bandless forms from bandless colonies. The young of the first generation were all bandless, the banded character being recessive. When these were paired the off- spring were bandless and banded in the Mendelian ratio, 3:1. Fur- ther experiments confirmed this, not only as regards bands, but also as regards colour (yellow or red), size, and the form of the umbilicus. // may be said, therefore, that common snails {Helix hortensis and Helix nemoralis) illustrate Mendelian inheritance. Poultry. — Numerous breeding experiments with poultry have been made by Bateson, Bateson and Punnett, Hurst, Davenport, and others, many of which show Mendelian phenomena with great clear- ness, while others are strangely conflicting. One of the reasons for the complicated results is evidently to be found in the difficulty of securing thoroughly "pure" breeds, for many that breed true as long as they MENDEL'S LAWS OF HEREDITY 397 are inbred tend to liberate latent characters when the ordinary course of breeding is departed from. Hurst contrasts the following characters, which usually show them- selves dominants and recessives; but it has to be admitted that the dominance — always complete for some characters — is for others fre- quently, or even always incomplete — i.e., showing traces of the corre- sponding recessives. Dominant Characters Recessive Characters Rose comb Leaf comb, single comb White plumage Black plumage, buff plumage Extra toes Normal toes Feathered shanks Bare shanks Crested head Uncrested head Brown eggs White eggs Broodiness Non-broodiness Davenport's copiously illustrated work is also of great interest. He shows in case after case that the character dominant in the first hybrids is more or less influenced by the recessive character. Polish fowls with a large hernia of the brain on the top of the head were paired with Minorcas with normal heads. The hybrids showed no hernia, but most of them showed a frontal prominence. When the hybrids were inbred the hernia occurred in 23.5 per cent — a close approximation to the theoretical 25 per cent. Single-combed black Minorcas were crossed with white-crested black Polish fowls with a very small bifid comb. The hybrids had combs single in front, split behind. When the hybrids were inbred there resulted in a total of loi offspring, 29.7 per cent with single combs (like Minorcas), 46.5 per cent with Y-shaped combs, and 23.8 per cent with no combs or only papillae (like the Polish forms). Here, again, the result is in a general way Mendelian, but the Y-like comb is a complication. Pigeons. — R. Staples-Browne crossed a web-footed pigeon (an occasional discontinuous variation) with a normal form, and got six normal young. In other words, the web-foot character is recessive to the normal foot character. The hybrids were inbred, and in one case produced nine with normal feet and three with webbed-feet — a Mendehan splitting-up. But from another pair of hybrids seventeen normal offspring resulted. Thus, the illustration of Mendelian inheritance is inconclusive. Besides the numbers were too small. We have noticed elsewhere that crossing different breeds of pigeons often results in forms which more or less resemble the reputed original 398 READINGS IN EVOLUTION, GENETICS, AND EUGENICS ancestor, the wild rock dove; in other words, reversions occur. Often, however, the results seem quite anomalous, which is probably due to the number of latent characters which different races of pigeons appear to carry. Mice. — Mendelian phenomena have been carefully studied in mice. Thus, when a grey mouse is paired with an albino, the hybrid offspring are always grey. When these are inbred, they yield greys and albinos, approximately in the proportion of 3:1. Thus Cuenot obtained 198 grey, and 72 albinos. Darbishire has obtained many results which harmonise well with Mendelian theory, while others require some ingenuity if they are to be fitted in with this interpretation. As a good case we may cite one where the inbreeding of pigmented mice — derived from crossing pig- mented and albino individuals — yielded 159 pigmented young and 55 albinos (53.5 being the theoretical anticipation). When similar hybrids were paired with pure albinos, they yielded 69 pigmented and 69 albino forms, precisely as the theory would lead us to expect: D R \. D(R) X D(R) I D+2 D(R) + i R X 2 R D(R) R Cuenot crossed an albino AG (with latent grey) with an albino AB (with latent black), and obtained albinos (AGAB). He crossed a black mouse CB with an albino AY (with latent yellow), and obtained yellow mice (CBAY). He then paired AGAB (albino) with CBAY (yellow) and obtained 151 young — 81 albinos, 34 yellow, 20 black, 16 grey; the theoretical anticipation being — 76 albinos, 38 yellow, 19 black, 19 grey. This is an exceedingly striking and convincing case. Waltzing mice. — The mice of this interesting Japanese breed have among other peculiarities the habit of waltzing round in circles. When waltzing mice are crossed with normal mice, their abnormal quality behaves as a recessive. MEN'DEL'S LAWS OF HEREDITY 399 Guinea-pigs. — If a black guinea-pig of pure race be crossed with a white one the offspring will be all white, and if these are mated with each other the recessive white character reappears on the average in one in four of their offspring. These whites mated with each other produce only white offspring, while the black are as usual of two kinds, pure blacks and impure blacks. Similarly, as Professor Castle has shown, a rough coat is dominant over a smooth coat, and a short coat over a long coat. Rabbits. — Hurst paired white Angora rabbits (with pink eyes and silky hair) with "Belgian hare" rabbits (with pigmented skin, dark eyes, and short yellow fur). The hybrids were pigmented like the " Belgian hares," but the fur was grey like that of the wild rabbit. These hybrids were inbred, and 14 distinct types resulted — an apparent ''epidemic of variation" to which Mendel's theory has supplied the clue, for four pairs of contrasted characters are involved in the hybrid inbreeding — namely, short hair versus long hair, pigmented coat versus albinos, grey versus black coat, uniform versus marked coat (Dutch marking latent in the albinos), and the 14 distinct types illustrate the possible combinations. As regards short hair versus long hair, Hurst found that when the short-coated hybrids were inbred they produced short-haired forms like the Belgian hare grandparent, and long-haired forms Hke the Angora grandparent. Out of 70 which reached the age of two months or more, 53 were short-haired and 17 long-haired — a close approxi- mation to the Mendelian anticipation, 52.5 : 17.5. Similarly, as regards pigmented coat versus albino, the hybrids, when inbred, yielded 132 pigmented and 39 albino forms — a close approximation to the Mendelian expectation, 129 : 43; and so on. Cats. — There are some interesting results as to colour (Doncaster). Thus, ''pure" orange ? crossed by "pure" black 6 gives tortoiseshell females and yellow males, but black crossed by orange gives black males or females, tortoiseshell females, and orange males. It seems that orange usually dominates over black in males, while in females the orange (for some unknown reason) is less dominant and tortoise- shell results. Male tortoiseshell cats are very rare. In this case the results are complicated by some peculiarity wrapped up with "sex." When a male tortoiseshell is paired with a female tortoiseshell the kittens are tortoiseshell, orange, and black — which is what Mendelian theory would lead us to expect. Man. — Evidence of Mendelian phenomena in man is as yet very scanty. It appears that the condition known as brachydactylism, 4oo READINGS IN EVOLUTION, GENETICS, AND EUGENICS where the fingers are all thumbs with two joints instead of three, is dominant over the normal. In five generations chronicled by Fara- bee about half of the offspring were of the abnormal type, though the marriages were apparently always with unrelated normal individuals. Moreover, no normal member of the lineage is known to have trans- mitted the abnormality. Another good case has been recently dis- cussed by Drinkwater. Of great interest also is Mr. Nettleship's account of the descend- ants of one Jean Nougaret (born 1637), who was afflicted with "night- blindness" — a condition apparently due to loss of visual purple. It seems to behave like a unit character. There are records of over 2,000 individuals; and the night-blindness is dominant over normal eye- sight. The notable point is that during two and a half centuries no normal member of the lineage who married another normal, whether related or not, ever transmitted the disease. Human eye-colour affords another illustration. It is largely determined by the presence or absence of two distinct layers of pig- ment. In the true blue eye only one of these pigmentary layers is visibly present, the posterior purple pigment of the choroid, which, being reflected through the fibrous structure of the iris, produces the blue colour. In the absence or partial absence of this pigment the eye appears to be "pink," as in albinos. In the ordinary brown eye two layers of pigment are present, for in addition to the posterior purple layer there is also an anterior brown layer, in front of the iris. Major C. C. Hurst found that the eye with two layers of visible pigment (duplex) is dominant and the eye with one layer of visible pigment (simplex) recessive. Or, putting it in another way, the presence of the brown front layer is dominant to its absence. Practically the same con- clusion was reached independently by Professor and Mrs. Davenport. The Davenports and Major Hurst have also brought forward some evidence illustrating in typical Caucasians the dominance of dark to fair skins, their segregation in the same family, and the apparent purity of the extracted fair individuals. Hurst also gives evidence that "fiery red" hair behaves as a recessive to brown, and that the musical sense or temperament is also recessive. It seems as if an individual is non-musical owing to the presence of an inhibitory factor preventing the expression of musical temperament which is poten- tially present in everyone (Hurst, 19 12). It would be interesting to have precise information as to the pro- geny of Eurasians who intermarry, for here the original hybrids result from the mixture of two very distinct races. CHAPTER XXVIII THE PHYSICAL BASIS OF MENDELISM^ ERNEST B. BABCOCK AND ROY E. CLAUSEN Recent investigations in heredity have focused attention upon the chromosome mechanism as the physical basis for the segregation and recombination of the units of Mendehan inheritance. The importance of cytological phenomena to students of genetics is admirably summed up by E. B. Wilson in the brief statement that "heredity is a conse- quence of the genetic continuity of cells by division, and the germ cells form the vehicle of transmission from one generation to another." It is appropriate, therefore, to introduce the subject of Mendelism with a formal and brief treatment of the chromosome mechanism and its mode of operation, on the one hand, in the building up of the body from the single cell with which the individual begins its existence, and, on the other hand, in the production of germ cells when the individual reaches the reproductive period of its life cycle. It is the purpose of this chapter merely to deal with the fundamental facts of cytology which are necessary to an understanding of the connection between cell behavior and Mendelian phenomena. Details unessential to such an understanding, however well established cytologically, will not be dealt with in this treatment to the end that the cardinal points may be presented as simply and as clearly as possible. The chromosomes. — ^With few exceptions the number of chromo- somes in the cells of any individual is constant and characteristic of the species to which the individual belongs. Thus it is characteristic of Drosophila ampelophila that the cells contain eight chromosomes. In maize the cells contain twenty chromosomes, in wheat sixteen, and in man forty-eight, and so on through the entire plant and animal kingdoms. Not only is the number of chromosomes in a particular species constant, but the chromosomes themselves possess a definite indi- viduality. Man and tobacco have cells with the same number of chromosomes. It is needless to point out that these chromosomes, ^ From E. B. Babcock and R. E. Clausen, Genetics in Relation to Agriculture (copyright 1918). Used by special permission of the publishers, The McGraw- Hill Book Company. 401 402 READINGS IN EVOLUTION, GENETICS, AND EUGENICS however, are qualitatively very different. Similarly within the species the chromosomes are not all alike; on the contrary, especially in certain forms, they exhibit very marked differences in size and shape. This is peculiarly well illustrated in Drosophila as shown in Fig. 70. Here it is possible to recognize in the female two large pairs of curved chromosomes very similar in size and shape. There is also a very small pair of chromosomes, and finally there is a pair of straight ones about two-thirds as long as the large curved chromosomes. In the male the same relations hold except that instead of the pair of straight chromo- somes there is a pair consisting of one straight and one somewhat larger hooked chromosome. The significance of this difference in chromosome content in the sexes will be pointed out in a consideration FEMALE HALE Fig. 70. — Diagram showing the characteristic pairing, size relations, and shapes of the chromosomes of Drosophila ampelophila. In the male an X and a Y chromosome correspond to the X pair of the female. On the basis of X 100 the length of each long autosome 159, of each small autosome 12, and of P' 112, of the long arm of F 71, and of the short arm of F 41. {From Bahcock and Clausen, after Bridges.) of the inheritance of sex. The pair of straight chromosomes we call the sex or X-chromosomes, the unequal mate of the X-chromosome in the male of this species is called the Y-chromosome. The other chromosomes are called autosomes when it is desired to distinguish them as a class from the sex chromosomes. Drosophila is not unique in possessing chromosomes of such characteristic shapes and sizes; but more and more as cytology advances it is becoming possible to dis- tinguish chromosomes, and to recognize them a,t every cell division. Moreover; the characteristic paired relations which exist among the chromosomes of Drosophila are of general significance. When mature germ cells are formed in an individual, reduction divisions occur by means of which the chromosome number is reduced in the germ cells to one-half that characteristic of the body cells. Thus the THE PHYSICAL BASIS OF MENDELISM 403 germ cells of Drosophila contain four chromosomes as the result of a reduction which takes place in such a manner that each germ cell con- tains one member of each pair of chromosomes. As a consequence, the germ cell of Drosophila contains two large curved autosomes, representing the two pairs of these chromosomes, one small autosome, and one X- or one Y-chromosome. The same thing is true for other species of plants and animals — in the reduction divisions the chromosomes are distributed in such a manner that each germ cell receives one member of each pair of chromosomes. It follows from this that in general a definite number of pairs of chromosomes is characteristic of the body cells of individuals of a given species, and, taking the chromosomes by pairs, one member of each pair is derived from one parent and the other from the other parent. From the standpoint of interpretation the chromosomes are aggre- gates of chromatin material which in itself is definitely and highly organized. Our conceptions of this feature of cell organization are based on appearances of the cytological preparations from certain of the more favorable plants and animals and further interpreted by investigations on heredity. Accordingly the entire chromatin con- tent of the nucleus is regarded as made up of a definite number of indi- vidual chromatin elements called chromomeres. The number of chromomeres in a cell of any species must run into the thousands. A certain definite group of these elements make up each chromosome, and at every cell division this chromosome is reformed from the same group of chromomeres, but the chromosome is definitely organized with respect to the position or locus occupied by each chromomere. At certain stages in the history of chromosomes, they are simply lines of chromomeres, very much like single strings of beads with each bead corresponding to a chromomere. Now it appears probable that all the chromomeres in a chromosome are different, as though our string of beads had no duplicates throughout its length. Moreover, each chromomere has a definite place or locus in the particular chromosome in which it belongs and it is always found at that particular locus. The chromomeres of this discussion are identified with the factors of Mendelian heredity, and how closely this conception of the nature of chromatin and its complex organization corresponds to the modern view of Mendelian phenomena will be pointed out as each new phase of Mendelism is taken up. Somatic cell division. — The phenomena of cell division (called mitosis) are represented in outline in Fig. 71, for a species having four 404 READINGS IN EVOLUTION, GENETICS, AND EUGENICS chromosomes in its body cell. Bearing in mind the description which has just been given of the organization of the chromatin material we may follow the steps involved in mitosis as they are outlined in this figure. In the " resting " cell at A the chromatin is scattered through- out the nucleus in clumps or knots loosely strung together to form an irregular network. As the cell prepares for division the chromatin elements appear in more definite form until at B the chromomeres have Fig. 71. — Diagram of mitosis in a species having four chromosomes in its cells. ^, the "resting" cell; 5, formation of the spireme thread; C, longitudinal division of the spireme thread and transverse segmentation into four chromosomes; D, separation of the daughter chromosomes formed by longitudinal splitting of spireme thread; E, beginnings of nuclear reconstruction and division of the cell body; F, cell division complete and daughter nuclei in the "resting" stage. {From Babcock and Clausen.) arranged themselves in a single row in a long continuous spireme- thread. This spireme-thread may be considered to be made up of the four chromosomes united end to end with the chromomeres arranged in a linear series. As mitosis progresses to the next stage represented at C, each chromomere of the spireme-thread divides into two, so that a double spireme-thread results from the longitudinal splitting of the original thread. Both parts of the thread are quantitatively and quali- tatively equal, for, by the splitting of all the chromomeres both of the THE PHYSICAL BASIS OF MENDELISM 405 threads come to possess all of the individual elements of the original spireme thread. Following the splitting of the chromomeres and the formation of a double spireme, the spireme-thread contracts and seg- ments transversely forming four double chromosomes, the number characteristic of the cells of this individual. This is the stage shown at C where also is shown the origin of the spindle, a part of the mechan- ism in mitosis. The chromosomes now still further contract until they assume their characteristic shapes and sizes. They next appear in an equatorial position on the spindle as shown at D, where the two pairs of double chromosomes, one larger and one smaller, are dia- grammed and the nucleolus, the large black body of the previous steps, is shown cast out and degenerating. The daughter chromosomes of each pair now separate from each other until at E they have moved nearly to the opposite poles of the spindles and are beginning to fray out and seemingly to lose their identity. At this stage actual division of the cell body has begun. Finally at F, the chromosomes have com- pletely lost all appearance of their identity, the chromatin material is distributed thruout the nucleus as in the original cell shown at A , and the nucleolus has been reformed in each nucleus. Division of the cell-body has resulted in two daughter cells, each of which, so far as chromomeres are concerned, contains exactly the same chromatin elements as the original cell. There are many variations in this process particularly in the order of occurrence of the steps, but these variations in nowise modify the essential fact of mitosis which is that the chromatin material of the cell is converted into a thread which splits thruout its entire length into two halves so that the daughter nuclei receive exactly equivalent portions of chromatin material. This precise division of the chro- matin is brought about by a division of each chromomere so that not only do the daughter nuclei receive equivalent portions of chromatin but these portions are also equivalent qualitatively to the entire chromatin content of the mother cell. By this method then each of the cells of the body finally comes to possess not only the whole num- ber of chromosomes contributed by the two parents, but also the entire set of chromatin elements which it received from them. The extreme care with which the cell mechanism partitions the chromatin material in each successive cell division is in itself eloquent testimony of the fundamental importance of this material. The production of germ cells. — In the production of germ cells a different set of phenomena occur which result in a reduction of this 4o6 READINGS IN EVOLUTION, GENETICS, AND EUGENICS number of chromosomes to one-half that characteristic of the somatic cells. Preceding the actual reduction division the chromatin passes through a complex series of steps which may be included under the term synapsis. (This term is sometimes applied in a specific sense to the pairing of homologous chromosomes and sometimes to the con- traction of the chromatin threads in the conjugation stage.) The essential steps in the prereduction process are shown in outline in Fig. 72. At y4 is diagrammed a " resting " nucleus at the completion of Fig. 72. — The reduction division as represented for a species whose diploid number is four. A, "resting" nucleus of a primary germ cell; B, formation of paired threads of chromomeres; C, conjugation of homologous chromosomes (synapsis) ; D, loosening of the synaptic knots; E, condensation of the chromosomes and disappearance of the nuclear membrane; F, homologous chromosomes about to pass to opposite poles, thus giving each secondary germ cell a member of each pair and one-half the somatic number. {From Babcock and Clausen.) the multiplication divisions in the germ plasm. As a result of the exact type of mitosis which has been outlined above it contains the full num- ber of chromosomes characteristic of the species. The chromatin of the nucleus next becomes organized into threads of chromomeres which pair as shown at B. In this diagram the paired threads are taken to represent homologous chromosomes, and the opposite chro- momeres of the two chromosomes. The paired threads contract and THE PHYSICAL BASIS OF MENDELISM 407 fuse along their entire length giving the figure diagrammed at C in which the two loops represent two pairs of homologous chromosomes in the conjugation stage, the essential step in synapsis. Following this stage the two contracted loops of chromatin split lengthwise and unravel in somewhat the manner shown in D. These filaments con- tract again forming the intertwined pairs of chromosomes shown at E, and the nuclear membrane thereupon begins to disappear. Further contraction and the formation of a spindle results in the reduction figure at F, the significant feature of which is the fact that each of the daughter nuclei resulting from this division receives only two chromo- somes instead of the four which the original cell at A contained. Since the original cell contained one pair of larger and one pair of smaller chromosomes, the daughter cells which are formed each receive one larger and one smaller chromosome. Cytological investigation is not yet in agreement as to the inter- pretation of synapsis especially as to the manner in which the phe- nomena therein concerned are connected with preceding mitotic divi- sions. Considering certain cytological investigations and the results of research in heredity together, it appears that the threads which pair in stage B represent pairs of chromosomes with homologous chromo- meres occupying corresponding positions along their entire length. Likewise the contraction stage at C is taken to represent a conjugation of the members of pairs of chromosomes which later again separate. Other cytological evidence indicates that in some forms the conjuga- tion of pairs of homologous chromosomes is brought about in another way. However, the essential fact is the same in either case. In the reduction figure the members of each pair of chromosomes are dis- tributed to the opposite poles of the spindle so that the daughter nuclei received only one member of each pair. The significance of synapsis lies in the conjugation of homologous chromosomes. In the mitoses which have preceded this particular division, the chromosomes were each time conceived to be reformed from the identical group of chromomeres which they contained origi- nally". In synapsis, however, as shown at B there is a certain amount of intertwining of the paired threads and in the unraveling of the chromosomes after the contraction stage there is likewise a twisting of the filaments about each other. The indications are, therefore, that in synapsis there is a possibility of interchange of chromatin material between the members of a pair of homologous chromosomes. In all cases, however, in order to uphold our conception of the definite 4o8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS pair of chromosomes. {From Babcock and Clausen, after Muller.) organization of the chromosomes with respect to the chromomeres which they contain, this interchange of material must involve exactly equivalent portions of the two chromosomes. The chromosomes of the reduction division shown at F may not, therefore, be identi- cal with the four originally present in A , but may represent various combinations of portions of both members of a particular pair of chromosomes. The re- sults of such interchange between Fig. 73.— Diagram of chromatin inter- members of homologous pairs of change between homologous members of a chromosomes is shown in Fig. 73 . At the left is shown a pair of chromosomes, one in outline, the other in full black. In the middle the steps in chromatin interchange are diagrammed and finally at the right this interchange results in a pair of chromosomes each of which is made up of parts of both members of the original pair of chromosomes. Various combinations may result depending on the points at which interchange takes place, but in every case the exchange involves corresponding portions of the two chromosomes. Independent distribution of chromosomes. — In Fig. 74 are illus- trated diagrammatically the chromosomes of Drosophila, with particu- lar reference to their size and form relations and to their character- istic pairing in the cell. One member of each of these pairs of chro- mosomes was contributed by the female parent and one member by the male parent. In the reduction divisions these chromosomes are separated so that each germ cell contains one member of each pair of chromosomes. The simplest condition which could obtain is that of independent distribution in each pair of chromosomes such that the particular member of one pair which went to a given pole of the reduc- tion spindle would have no influence on the distribution of the mem- bers of any other pair. Such independent distribution of chromo- somes appears to be actually the type followed in reduction. As a consequence the germ cells contain various combinations of chromo- somes with respect to their original parental derivation. In Fig. 74 the types of combinations of maternal and paternal chromosomes and their mode of derivation in Drosophila are shown diagrammatically. Two germ cells, one from the female with the chromosomes in outline, THE PHYSICAL BASIS OF MENDELISM 409 and the other from the male with the chromosomes in full black, unite to form the female zygote shown in the middle of the figure. The combinations of maternal and paternal chromosomes which result in the production of germ cells in such an individual are shown diagram- FiG. 74. — Diagram showing consequences of independent segregation of chromosomes in Drosophila ampelophila. {From Babcock and Clausen.) matically in the lower portion of the figure. There are eight different ways in which the chromosomes may be grouped in the reduction figures and on the basis of chance any one of these types is as likely to occur as any other. As a result there are sixteen possible combi- nations of chromosomes in the germ cells with respect to the original 4IO READINGS IN EVOLUTION, GENETICS, AND EUGENICS derivation of the chromosomes, whether from the female or from the male parent. This of course represents only the total number of possible combinations of entire chromosomes. By exchange of chromatin material between homologous chromosomes resulting in the formation of combination-chromosomes the number of actual com- binations is greatly increased. The number of chromosome combinations resulting from inde- pendent distribution is that number possible when each pair of chro- mosomes is considered separately, and every combination has an equal chance of occurrence. With a form having but two pairs of chromo- somes there would be only four possible combinations, three pairs would give eight, four pairs sixteen, and in general the number of possible combinations is given by the expression 2" in which n is the number of pairs of chromosomes in the individual in question. In tobacco which has 24 pairs of chromosomes the number of possible combinations in the germ cells reaches the enormous total of 16,777,- 216. This means that in the formation of zygotes in a self-fertilized tobacco plant the actual parental combinations, i.e., combinations identical with those of the germ cells which united to form the indi- vidual in question, occur only twice in over sixteen million times, and this proportion is still further lessened when the interchange of chro- matin material between homologous chromosomes is taken into account. The condition of independent distribution although simple in itself results in a rapid increase in complexity with the increase in the number of pairs of chromosomes involved. Chromosomes and sex in Drosophila. — The relation between inheritance and the chromosome mechanism is perhaps most simply displayed in the inheritance of sex in those animal forms in which the sexes occur in approximately equal proportions. Thus in Drosophila as indicated in Fig. 75 there are three pairs of autosomes which are alike in both the male and the female. The remaining pair of chromo- somes, however, differ, for Ihe female possesses two X-chromosomes whereas in the male a single X-chromosome is paired with a Y-chromo- some and these differences are characteristic of all normal males and females of this species. The bearing of these differences on the inheritance of sex is shown diagrammatically in Fig. 75. Beginning with the parents, the diploid number is shown in the circles represent- ing the female and the male. In the female the three pairs of autosomes are outlined and the X-chromosomes only are drawn in black to indicate that they are the ones primarily concerned in the determination of sex. Similarly in THE PHYSICAL BASTS OF MENDELISM 411 the male the three pairs of autosomes which are exactly like those in the female are outlined, but the X-chromosome and the Y-chromosome are drawn in black. The reduction division in the female results in a Fig. 75. — Diagram to show chromosome relations in the inheritance of sex in Drosophila ampelophila. {From Bahcock and Clausen.) separation of the members of each pair of chromosomes, so that every secondary germ cell (or egg) contains two large curved autosomes, a small autosome, and an X-chromosome. Consequently as far as chromosome content goes the eggs are all exactly alike. In the male, 412 READINGS IN EVOLUTION, GENETICS, AND EUGENICS however, the separation of the members of the chromosome pairs results in sperms half of which contain an X-chromosome and half a Y-chromosome in addition to the three autosomes. The reduction division in the male insures an equality in numbers for the two kinds of sperm cells and the chances that either kind of sperm will fertilize an egg-cell are equal. By this arrangement the numerical equality of the sexes is maintained. When, later, the egg cells of the female are fertilized by the sperm cells of the male, as shown in the lower portion of the figure, half of them being fertilized by sperm cells which contain an X-chromosome will give females, and half uniting with sperm cells which contain Y-chromosomes will produce males. The inheritance of sex in Drosophila provides a beautiful illustration of the parallel behavior of the chromosome mechanism and a somatic difference, in this case, sex. To recapitulate, the essential phenomena of cell behavior which fur- nish the mechanism for the distribution of hereditary factors are these : 1 . Every species is characterized by a definitely organized group of chromosomes. The chromosomes occur in pairs, in each of which one member is derived from each parent. In ordinary somatic mitosis the distribution of chromatin is such that each daughter cell receives a full complement of chromosomes which are equivalent qualitatively to those of the mother cell. 2. In germ cell formation the homologous chromosomes conjugate during synapsis, then separate, and pass into a division figure in which entire homologous chromosomes are opposed to each other. The resulting reduction division gives daughter cells with half the number of chromosomes characteristic of the species, the half number being made up of one member of each pair of chromosomes. During synap- sis there is an opportunity for the members of a pair of chromosomes to exchange chromatin material. When such interchange takes place equivalent portions of chromosomes both qualitatively and quantita- tively are involved. In the reduction division segregation within one pair of chromosomes is entirely independent of that of any other pair so that the combinations of parental chromosomes in the germ cells represent all those to be expected on the basis of chance distribution. The student should constantly endeavor to harmonize this con- ception of the distributing mechanism of the chromatin material with the Mendelian interpretations of hereditary phemomena which will be presented in what follows, to the end that he may obtain a clear and definite idea of the interrelations between the known facts of heredity and cell behavior. CHAPTER XXIX NEO-MENDELISM IN PLANTS^ JOHN M. COULTER AND MERLE C. COULTER Thus far we have been considering Mendel's law in its simple form and have enlarged but little upon Mendel's original statement. The value of the law is apparent. Upon its republication in 1900 it was taken up by biologists and numerous breeders set to work to test it. As a consequence data for and against it began to accumulate. As might be expected, there was much apparent evidence against the law, but as geneticists developed a better conception of the mechanism the contradictory evidence was explained away. Almost every type of inheritance has now been explained according to Mendel's law. Some of the explanations are very complicated and cannot be included in this presentation. A few of the more important cases, however, will be presented. I. PRESENCE AND ABSENCE HYPOTHESIS This may be regarded as a new method of Mendelian thought. It was first suggested by Correns, but later was worked out in detail by other geneticists, especially Hurst, Bateson, Shull, and East. It is merely a modification of the mechanism involved. For example, in the case of a hybrid obtained by crossing tall and dwarf parents the result had been explained as due to the fact that one chromosome bears a determiner for tallness and the other one of the pair carries the deter- miner for dwarfness. In other words, each one of a pair of allelo- morphs is represented by a determiner, two determiners thus being present. Dwarfness in this case would be the result of the interaction of that determiner and its environment during the development of the body; and the same for tallness. When both were present , however, the conception of the situation was as follows. The determiner for dwarfness, setting up its usual series of reactions, early became para- lyzed by the determiner for tallness or its products. This result was called the dominance of the character for tallness. It was as if the determiner for tallness completely prevented the activity of the deter- miner for dwarfness. This conception was apparently borne out » From Coulter and Coulter, Plant Genetics (The University of Chicago Press, copyright 191 8). 413 414 READINGS IN EVOLUTION, GENETICS, AND EUGENICS by the facts and was the explanation of the mechanism generally accepted. According to the presence and absence hypothesis, however, the situation is looked at from an entirely different point of view. Tall- ness is the result of a determiner, but dwarfness is merely the result of the absence of the determiner for tallness. The dominant character is produced by an inheritable determiner, but the recessive character appears only when the dominant determiner is lacking. This con- ception has some evident advantages and may modify the previous Mendehan diagram, as shown in Fig. 76. This appears to be a simpler mechanism to account for the phenomenon c'alled dominance. In the case of the dwarf form there is a normal course of development; in the case of the tall parent or hybrid, however, an additional determiner Dwarf Parent Gametes Fig. 76. — Diagram showing how the original scheme must be modified to satisfy the presence and absence hypothesis. {From Coulter afid Coulter.) stimulates cell growth, or cell division, or both. It is a simpler and more useful conception, so long as it fits the facts. Some investigators, however, claim that it cannot be applied to all the situations that have been discovered. This hypothesis introduces some additional terminology suggested by Bateson. In our illustration the tall parent has two determiners for tallness and therefore Bateson calls it duplex, having a double dose. For the same reason the Fi individuals, having only one determiner for tallness, he calls simplex. According to the same terminology the dwarf parent is nulliplex with respect to its character of tallness. Additional advantages of the presence and absence hypothesis will appear in connection with a consideration of blending inheritance and of cumulative factors in inheritance. Attention, however, should be called to the fact that those who accept the presence and absence neo-:mendelism in plants 415 hypothesis do not use the form of notation thus far used in explaining Mendehan inheritance. Assume that T is used to express the deter- miner for tallness, its same letter (/) is used to express the absence. For example, instead of using D for dwarfness, / is used for '* lack of tallness" (Fig. 77). It is a matter of convenience to have a symbol to represent the recessive, the absence of something that is present in another individual. In summary, the essential difference between the presence and absence hypothesis and that of dominant and recessive is that in the former case the recessive determiner has no existence at all, while in the latter case it exists, but is in a latent condition when associated with the dominant. Dwarf Parent Gameti es Fig. 77. — Diagram showing how presence and absence scheme is actually used, with small letter representing ''absence." {From Coulter and Coulter.) II. BLENDS This type of inheritance when first discovered was thought to be in direct conflict with Mendel's law. It is a case in which dominance seems to fail, for the two alternative characters both express them- selves and the result is an average between them. It is easy to explain this situation in accordance with the presence and absence hypothesis without any violation of Mendel's law. The classic example of blending inheritance was presented by Correns in breeding work upon Mirabilis Jala pa, the common four- o'clock. Correns crossed red and white varieties, and all the hybrid progeny had rose pink flowers. This was a color blend, distinctly intermediate between the colors of the two parents. The Fi genera- tion, therefore, seemed to contradict Mendel's law in that one color character was not completely dominant over the other. The real situa- tion, however, appeared in the F2 generation obtained by inbreeding 4i6 READINGS IN EVOLUTION, GENETICS, AND EUGENICS individuals of the Fi generation which showed the blend. By inbreeding the pink hybrids Correns obtained the perfect 1:2:1 ratio, that is, I red like one grandparent, 2 pink like the hybrid parent, and I white like the other grandparent. Segregation was evidently taking place, the only unusual thing being the appearance of the Fi indi- viduals, and that was explained immediately as failure of dominance (see Fig. 78). The question this introduces, therefore, is that of a mechanism which could account for such a result. The easiest explanation offered is that the red parent was a homozygote for redness (double dose) and the hybrid a heterozygote (single dose) ; the inference is that ® ® Red Parent Gamete White Parent (R)pinkQ Eggs Sperms ®..® 0P.® ®P.0 Fig. 78. — Diagram illustrating blending inheritance, discovered by Correns in Mirabilis Jalapa. {From Coulter and Coulter.) a single dose produces pink while a double dose produces red. A theoretical explanation of this occasional difference in the result of double and single doses is as follows. Imagine that the body cells of a plant have a certain capacity for expressing hereditary characters. In such a case, just as a given quantity of solvent can dissolve only a given amount of solute, so the body cells can express hereditary charac- ters only to a definite limited extent. In the four-o'clock a single dose of redness may be thought of as half saturating the body cells, while a double dose completely saturates them. In cases showing complete dominance, however, a single dose completely saturates the cells and a double dose can do nothing more. This analogy assists in visualizing on the one hand the necessary mechanism of blends (apparent failure NEO-MENDELISM IN PLANTS 417 of dominance) and on the other hand that for cases of complete domi- nance. Another example of simple blending inheritance is the case of Adzuki beans, described by Blakeslee. In this bean the mottling of the seed coat is dominant to the lack of mottling. In the hybrid condition, however, the mottling is lighter than in the pure or homo- zygous condition. Heterozygous plants, therefore, can be easily dis- tinguished from homozygous plants, so that the 1:2:1 ratio is evident on externa] inspection rather than the usual 3 : i ratio. III. THE FACTOR HYPOTHESIS Mendel concluded that each plant character depends upon a single determiner. Inheritance, however, has proved to be a much more complex phenomenon than indicated by Mendel's peas. Ratios have appeared that were puzzling, and geneticists were forced to the conclu- sion that there may be a compound determiner for a single character. This conception is called the factor hypothesis, and the growing com- plexity of genetics has developed in connection with this hypothesis. With the consideration of factors instead of determiners one passes from elementary to advanced genetics. Previously we have used the word determiner, implying Mendel's idea that a single determiner is responsible for the development of a plant character, and this has been true of the examples of inheritance previously considered. It is understood now, however, that a character is frequently deter- mined by the interaction of two or more separately heritable factors, and hence the factor hypothesis. The distinction between factors and determiners should be made clear. In case only one factor is involved in determining a character, there is no distinction between factor and determiner; and in such a case the term factor should not be used. I. Complementary factors. — This is the simplest expression of the factor hypothesis and it may be illustrated by some of East's work. Crossing red-grained and white-grained corn he obtained all red in the F2 generation. This would suggest that the F2 generation would show 3 red to i white; but it showed 9 reds to 7 whites, which did not suggest Mendelian inheritance. It is in accord with Mendel's law, however, if we consider that two complementary factors are necessary to produce the red character, and that each of these factors is inherited separately. Such a situation would give a dihybrid ratio, as indicated in Fig. 79. It will be seen that out of 16 progeny 9 will be red, for they alone contain the complementary factors; the other 7 will be white. 4i8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS The situation is thus explained by the dihybrid ratio, but although only one character is involved that character depends upon two com- plementary factors. Another situation is worth noting. No. 6 of the diagram is white because it contains only one of the necessary factors; No. ii is white Fig. 79. — Diagram illustrating behavior of complementary factors in cross between red-grained and white-grained corn. R and C must both be present to produce red-grained corn. {From Coulter and Coulter.) for the same reason, but its germinal constitution is just the opposite. What would happen if these two are crossed? There is only one possibility, since each is a homozygote producing only one kind of gamete. The result would be red, and thus a cross between two whites would produce only reds. What would happen from crossing Nos. 6 and 15, the former being a homozygote and the latter a heterozygote ? NEO-MENDELISM IX PLANTS 419 It is obvious that the resulting progeny would be one-half white and one-half red, although both parents are white. The same result would be secured in crossing Nos. 11 and 14. A cross between Nos. 14 and 15, both of which are heterozygotes, would result in 3 whites and i red, the ordinary 3:1 ratio. These illustrations show how diilerently the same phenotype may behave in inheritance. In each case two whites were crossed, that is, the same phenotypes, but three different ratios were obtained because the genotypes were different. The striking feature of this situation is that one can cross two whites and get a red. This gives an insight into the so-called phenome- non of reversion. For example, in the course of numerous breeding experiments Bateson obtained two strains of white sweet peas, each of which when normally "selfed'' bred true to the white color; but when these two were artificially crossed all the progeny had purple flowers, like the wild Sicilian ancestors of all cultivated varieties of the sweet pea. This appeared to be a typical case of reversion. Fur- ther breeding, however, showed that this was just such a case of com- plementary factors as we have been considering. One of Bateson's white strains had one of the factors for purple and the other strain had the other factor. Complementary factors have been defined and the method of their Inheritance described, but is there any mechanism to explain the situation? A suggestion may be obtained from plant chemistry. The most prominent group of pigments in plants is the group of antho- cyanins, which are produced as follows. Plants contain compounds called chromogens, which are colorless themselves but which produce pigments when acted upon by certain oxidizing enzymes or oxidases. This is a sufficient mechanism for the behavior of complementary factors. If one of East's white strains of corn contained a chromogen capable of producing red but lacked the necessary oxidase it would remain colorless. If the other white strain contained the oxidase but no chromogen it would remain colorless. In crossing them, however, chromogen and oxidase would be brought together and a red-grained hybrid would be the result. Inbreeding such red-grained individuals of course would give red and white progeny in a ratio of 9:7, as explainedin connection with East's corn. This seems to be the explana- tion of the behavior of complementary factors in many cases of color inheritance. Where other characters are involved the mechanism must be some- what difTerent. In some cases the two factors mav be the enzvme 420 READINGS IN EVOLUTION, GENETICS, AND EUGENICS and the compound the enzyme attacks, as in the oxidase and chromo- gen situation just described. On the other hand, we might be deahng with two chemical compounds that are inert when occurring separately but active when brought together, active in such a way as to produce a distinctly new character. Also two active substances might neutral- ize one another when brought together in a hybrid, and the failure in their activity might result either in a new character or the failure of some parental character to develop. Such are some of the possible mechanisms to explain the behavior of complementary factors. Hybridizing, therefore, is much like mixing chemicals in a test tube. We know that very wide crosses cannot be made successfully; but the surprising thing is that certain very close crosses are constantly unsuccessful, even though both parents may cross freely with closely related types. We obtain a glimpse of the possibility of such appar- ently inconsistent behavior when we consider the chemical possibilities suggested by the behavior of complementary factors. The origin of complementary factors is an interesting field of speculation. Did they originate together or separately? A natural inference would be that they originated together, for neither would be of any use without the other. It should be remembered, however, that the idea of use as explaining the occurrence of everything in a plant is being abandoned; one must think rather of a plant as a com- plex physico-chemical laboratory. No one claims that all chemical reactions are useful; they are simply inevitable ; and plant characters are the result of chemical reactions and physical necessities. Even though we assume the simultaneous origin of two complementary factors, they would have to be put on separate chromosomes, for the factors are separately inherited. The other alternative is to suppose that these factors originated independently in the history of a plant. In this case, of course, the first one to be produced would remain functionless until finally its complement came into existence. This might be an explanation of what are called latent characters. Also they might have not only originated independently but in different varieties or species. In this case if natural hybridizing should bring them together the result would be the appearance of a new character, and this may have been a very important factor in the origin of species. This may serve as an introduction to the factor hypothesis, with complementary factors as an illustration, simply because it is the simplest situation. There are many other kinds of factors recognized, NEO-MENDELISM IN PLANTS 421 but we shall not attempt to list all of the proposed t\pes. A simple illustration of the better known types is as follows: a) A complementary factor is added to a dissimilar factor to pro- duce a particular character. b) An inhibitory factor prevents the action of some other factor. c) A supplementary factor is added to a dissimilar factor with the result that the character is modified in some way. d) A cumulative factor, when added to another similar factor, affects the degree of development of the character. Some examples of these types will make them clear, those for complementary factors having been given previously. Pure Red Parent Gamete Gamete White Parent with Red Inhibitor Fig. 80. — Diagram illustrating behavior of inhibitory factor. and Coulter.) {From Coulter 2. Inhibitory factors. — Recalling East's experiment with red- grained corn it will be remembered that when both factors for red were present the grain was red, but when either factor was absent the grain was white. Later he crossed these strains with a new white strain, and the result was surprising. The pure red strain produced gametes carrying both the red factors, and it would be expected that whatever such a gamete mated with would result in red progeny; but when this pure red was crossed with the new strain of white the pro- geny were all white, although the hybrids certainly contained both factors for red. The explanation which first occurred to East, and which later experiments confirmed, was that the new white strain con- tained an inhibitory factor, which prevented the development of red even though both the complementary factors for red were present. 422 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Fig. 80 illustrates the situation and shows why all the individuals of the Fi generation are white. It is interesting to note further the possibilities of white and red in the F2 generation. They would be ^ — ^ Red R^ White White Fig. 81. — Diagram showing some possible combinations in F2 when Fi of Figure 80 is inbred. Individual on left end of upper set red-grained, because R and C both present and / absent; other individuals in upper set white, because lacking C or i? or both; individuals in lower set with inhibitory factor and there- fore white, whatever other combinations of factors they may contain. {From Coulter and Coulter.) numerous, since we are dealing with trihybrid ratios (see Fig. 81). This does not exhaust the possibilities, for the cases given were honiozygotes, each producing a single kind of gamete. There remains for consideration the heterozygote situation (see Fig. 82). The possible mechanism of the inhibitory factor is as follows. We have assumed that red is produced only when the enzyme is present to oxidize the chromogen. Enzymes are very sensi- tive; their activities may be affected or com- pletely checked by various agents. Suppose that I of the diagram be such an agent and the neces- sary mechanism is apparent. When / is present R is paralyzed, so that it cannot oxidize C. 3. Supplementary factors. — A supplementary factor is one that is added to a dissimilar factor, with the result that a character is modified in some way. © © Fig. 82. — (From Coulter and Coulter.) NEO-MENDELISM IX PLANTS 423 In his work upon red-grained races of corn East found occasionally a few purple grains. His conception of the situation is as follows. The pure red plant contains two complementary factors, one (C) a chromogen, and the other (R) an enzyme, which when brought together produced the red color. The purple grains, however, must be explained by the presence of still another factor (P), the resulting situation being represented in Fig. S^. Of course when C is absent no pigment whatsoever can be produced. As a consequence we will assume that the presence of C is constant, and that F and R are vari- ables. For a similar reason we will assume that the absence of / is constant. The figure shows three possibilities, from which the follow- ing conclusions may be drawn: (i) when P and R are both present the result is purple grains; (2) red appears only in the absence of P; (3) P although present will not develop any color in the absence of R. Purple ® ® ® ® © © © © Red White Fig. 83. — Diagram illustrating action of supplementary factor. Coulter and Coulter.) (From This is a typical case of a supplementary factor, that is, one which is added to a dissimilar factor, with the result that the color character is modified. The mechanism of this situation will make clearer the behavior of the supplementary factor. If C is the chromogen and R the enzyme, what is P? The suggested answer can be obtained from plant chemistry. It is found that the purple pigment is produced by the same substance as the red, but represents a higher state of oxidation. The conclusion is obvious. C is oxidized by 7? up to a certain point, where red is produced; but if P is also present it repre- sents an additional enzyme, which attacks the red pigment and oxidizes it still further into purple. P is incapable of attacking the original chromogen, but when R carries the attack to a certain point, P can function and carry the oxidation further. As a consequence P without R gives white grains, while R gives red grains only in the absence of P. 424 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 4. Cumulative factors. — These will be considered under the next heading, ''Inheritance of quantitative characters." In addition to the four types of factors given, the literature of genetics also contains discussions on intensifying factors, diluting factors, distribution factors, etc. These, however, do not introduce any new mechanisms. 5. Inheritance of quantitative characters. — This phase of the factor hypothesis, if true, is of fundamental importance not only to genetics but to general biology. It is based upon the conception of cumulative factors, and as it is presented it will be realized that it throws light not only upon numerous breeding experiments but also upon variation in general, which means evolution also. A cumulative factor was defined as one which, when added to another similar factor, affects the degree of development of the character. It will be recalled that Correns crossed red and white strains of Mirabilis and obtained pink hybrids. The suggested explanation of this result was that a single dose of the red determiner gives pink while a double dose gives red. When Correns inbred these pink hybrids, he obtained the result presented in Fig. 78, that is, i red, 2 pink, I white. This result is obvious and the mechanism is plain. With this diagram in mind we shall consider some of the experi- ments of Nilsson-Ehle at the Swedish Experiment Station. He crossed two strains of wheat with red and white kernels. The Fi individuals had light red kernels, which of course suggests a repetition of the situation shown by Mirabilis in the experiment of Correns. The F2 generation, however, showed a very different result. The reds and whites appeared in the ratio of 15:1; but in addition to this, among the 15 reds there could be distinguished varying degrees of redness. Nilsson-Ehle suspected that 15:1 meant a dihybrid ratio, 16 individuals being necessary to give the ratio, so that he constructed the tentative scheme shown in Fig. 84. This shows a regular dihybrid ratio, except that the two factors involved are similar. Applying the single dose and double dose con- ception, as used in the case of Corren's pink Mirabilis, we reach the following conclusions: No. i only has four doses and therefore it only is deep red; Nos. 2, 3, 5, 9 have three doses and are somewhat lighter red; Nos. 4, 6, 7, 10, 11, 13 have two doses and are still lighter red; Nos. 8, 12, 14, 15 have one dose and are very light red; while No. 16 alone has no dose and is the only pure white. This accounts for the 15:1 ratio, and the different shades of red. This is entirely in NEO-MENDELISM IN PLANTS 425 accord with the conceptions that have been presented, and only two assumptions are necessary: (i) that dominance is absent, and two doses have twice the effect of one: (2) that the independent similar factors are cumulative in their operation, and are paired with their absence in the hybrid. This was Nilsson-Khle's conception, and of Fig. 84. — Diagram illustrating Nilsson-Ehle's explanation of 15:1 ratio obtained in F2 generation from cross between red-grained and white-grained wheat. {From Coulter atid Coulter.) course he tested it by further experimental work, the results consist- ently confirming the conception. Since it is important to fix this conception clearly in mind, another type of diagram may represent the facts even more clearly. The proportion of individuals showing the various degrees of redness in the F2 is graphically recorded in Fig. 85, each dot representing one dose of the factors in question. 426 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Continuing these investigations, Nilsson-Ehle next discovered a new strain of red-grained wheat, which, when crossed with the pure white strain, yielded Fj hybrids of intermediate intensity of red as before. The F2 generation, however, showed a different situation. Reds and whites were obtained in the proportion of 63 : i ; the 63 reds as before falhng naturally into different groups on the basis of degree of redness. Applying the same conception as before Nilsson-Ehle # # Pure Red Grades of Pink White Fig. 85. — Another method of visualizing Nilsson-Ehle 's 15:1 ratio (see Fig. 84). {From Coulter and Coulter.) discovered that in this case he was dealing with a trihybrid situation. Without constructing the usual Mendel ian diagram, which would have to be extensive enough for 64 individuals, the situation as it appeared in the F2 generation may be represented by Fig. 86. If the graph is surmounted by a curve we recognize the regular ''probability curve," exactly the kind of curve biometricians use to represent the fluctuating individuals about a specific type. This conception of cumulative factors, therefore, has far-reaching significance. For a long time biologists have recognized individual NEO-MENDELISM IX PLANTS 427 variation within the species. Darwin depended upon it as the basis of his theory of natural selection as the origin of species; in fact, ever Pure Red liilt.'riiH'(li.iU' Grades While Fig. 86. — Diagram illustrating Nilsson-Ehle's 63:1 ratio. {From Coulter and Coidter.) since Darwin's Origin of Species, individual variation has been funda- mental in our conceptions. To account for this universally recog- nized phenomenon, Darwin proposed his transportation hypothesis as 428 READINGS IN EVOLUTION, GENETICS, AND EUGENICS a possible explanation, which, as will be recalled, did not long sur- vive. Weismann offered in explanation his germinal selection, which was soon discarded because it was beyond the possibility of experi- mental testing. Aside from these two attempts to explain individual variation no other comprehensive scheme had been presented. Biolo- gists had simply recognized the fact of individual variation without any conception of the mechanism. They knew that individual varia- tion existed but had even stopped asking why it existed. The importance of this new theory, therefore, is obvious. It is an ingenious explanation of the inheritance of quantitative characters and of the existence of individual variations. Furthermore, the theory has not been developed through meditation, but has its basis in scientific experiments. It is imaginative to a certain extent, of course, as is every other valuable theory, but unlike most such theories it has a substantial foundation, namely, Mendel's law. CHAPTER XXX NEO-MENDELIAN HEREDITY IN ANIMALS H. H. Newman Immediately after the announcement by De Vries in 1900 of the rediscovery of Mendel's paper, zoologists in Europe and in America began experiments in animal breeding with the idea of discovering to what extent Mendel's laws were applicable. It was soon found that the principles of unit characters, dominance, segregation, mono- hybrid, dihybrid, and trihybrid ratios were of practically universal application. A number of instances of Mendelian heredity in animals have already been presented in the preceding chapter and no more simple Mendelian cases need be described. For a considerable period the animal-breeders proceeded no farther in their analysis of the mechanism of heredity than Mendel had done so many years before. In time, however, new facts came to light that needed further analysis, and the older Mendelism was superseded by neo-Mendelism. This new phase in the study of heredity is in the forefront of interest today. Neo-Mendelian heredity in plants has already been discussed. It remains for us to present the data on some phases of neo-Mendelism in animals. ILLUSTRATIONS OF THE FACTOR HYPOTHESIS THE FACTORIAL ANALYSIS OF COLOR IN MICE Miss Durham, after extensive breeding experiments with numer- ous strains of differently colored mice, has been able to show that the appearance of a particular color in an individual mouse is depend- ent upon the presence or absence of several independently inherited factors, evidently represented by genes in as many different chromo- somes. It seems possible to classify these factors as follows: .5 = black pigment, which masks chocolate pigment b = absence of B, which gives chocolate / = intensity factor i = absence of intensity, or dilution factor C =a complementary color factor acting with F P=a complementary pigment factor acting with C If either C or P are absent, albino mice result no matter what other color factors may be present. 429 430 READINGS IN EVOLUTION, GENETICS, AND EUGENICS The factorial make-up of the various mice in Miss Durham's experiments would, then, be represented as follows: ^/CP = black BiCP =blue (dilute black) hICP = chocolate (absence of black) hiCP = silver fawn (dilute chocolate) The following experiments indicate the mode of heredity on the factorial basis: 1. P Black (^/CP)X Silver-fawn {biCP) Fx ICO per cent Black (BICP-biCP) Fj Black (BICP) Blue (BiCP) Chocolate (blCP) Silver-fawn ibiCP) 9331 2. P Blue (BiCP)XChoco\site (blCP) Fi 100 per cent Black (BiCP-blCP) F2 Black {BICP) Blue (BiCP) Chocolate (bICP) Silver-fawn (biCP) 9 3 3 I 3 . P Blue (BiCP) X Silver-fawn (biCP) Fx 100 per cent Blue (BiCP-biCP) F, Blue (BiCP) Silver-fawn {biCP) 3 I DIFFERENT KINDS OF ALBINOS Any one of the color types mentioned, if lacking in the factor C, will be an albino, though carrying the other factors for color. For example, there may be a Black-albino (BIcP), a Blue-albino (BicP), a Chocolate-albino (bIcP), or a Silver-fawn-albino {bicP). That color factors are present in albinos may be shown by the following experiment. An albino had appeared in a Black stock and was crossed with a Silver-fawn, thus: 4. P Silver-fawn (^>^'CP)X Albino extracted from Black {BIcP) Fi 100 per cent Black (biCP-BIcP) Black Blue Chocolate Albino-Black Silver-fawn (BICP) (BiCP) (bICP) (BIcP) (biCP) 27 9 9 9 3 Albino-Blue Albino-Chocolate Albino-Silver-fawn (BicP) (bIcP) (biCP) NEO-MEXDELIAN HEREDITY IX AXIMALS 431 The ratios given are the theoretical ratios for a trihybrid Mendel- ian experiment, and the actual results have closely approximated these. As a matter of fact, sixteen albinos appeared, and it is not possible, except by breeding, to tell one kind from another. Breeding each with, for example. Silver-fawn would readily reveal the differences; for the Fi generation would all be of the color that is masked by the lack of C in these albinos. In the language of Johanssen there is only one albino phenotype, but there are four albino genotypes. Similarly in experiments (i) and (4), which have just been described, the indi- viduals are all Black (phenotypically identical), but that they are not genotypically alike is clearly shown by inbreeding them. In experi- ment (i) we get only individuals of the four color types, while in experiment (4) we get, in addition to the four color types, four albino types. castle's guinea pigs Professor W. E. Castle was one of the first zoologists to use Men- del's methods. He soon discovered that in the determination of the coat characteristics of guinea pigs at least three sets of factors were necessary, as follows: C = colored fur c = albinism (absence of C) 5 = short fur 5 = long fur (recessive to S) i? = rosetted fur r = smooth fur (absence of R) An example will show how these factors segregate: P Colored, Short, Smooth X Albino, Long, Rosetted {CSr) (csR) Fi 100 per cent Colored, Short, Rosetted (CSr-csR) Colored Short Rosetted 27 Albino Long Rosetted 3 Colored Long Rosetted 9 Albino Long Smooth I Colored Short Smooth 9 Albino Short Rosetted 9 Colored Long Smooth 3 Albino Short Smooth 3 432 READINGS IN EVOLUTION, GENETICS, AND EUGENICS The ratio of 27, 9, g, g, 3, 3, 3, i shows clearly that the three factors independently segregate and are all three concerned in the determi- nation of the characters of the fur. A fourth factor, a pattern factor, is often present that further complicates the factorial analysis. Usually the self-color dominates the pattern, but certain special patterns are dominant over self-color. These two examples for animals are sufficient to illustrate the nature of Mendelian factors and their workings. Numerous other factors have been discovered. Castle, for example, found a factor associated with the occurrence of brown pigment in guinea pigs. Some rabbits have the pigment distributed evenly over the body; others have it in the eye only. These conditions are allelomorphic to each other, E (extension) being dominant over e (restriction to eyes). Inhibiting factors are distinguished, the presence of which prevents the appearance of a character represented in the germ plasm. Lethal factors result in the loss of something necessary for the life of the individual. Modifying factors change the expression of a character that depends on another gene. These and various other types of factors have been discovered by the large school of neo-Mendelians now so actively at work. CHAPTER XXXI SEX-LINKED AND OTHER KINDS OF LINKED INHERIT- ANCE IN DROSOPHILA AND OTHER SPECIES^ WILLIAM E. CASTLE All the facts of sex-linked inheritance in Drosophila harmonize with Morgan's hypothesis that the genes of sex-linked characters lie in a common cell structure (X-chromosome) which is duplex in females, simplex in males. Accordingly, in a race which breeds true for a sex-linked character, that character may be transmitted by every egg, but by only half the sperms, namely by such as possess an X-chromosome and by virtue of that fact determine as female all zygotes into which they enter. To male zygotes the sperm will not transmit sex-linked characters. This hypothesis is supported by some curious facts already alluded to but deserving of fuller consideration in this connection, viz., facts observed in reciprocal crosses involving a sex-linked character, as for example white-eye in Drosophila. TABLE I Reciprocal Crosses of White-eyed with Red-eyed Drosophila Male Female Male Female p White X Red Red X White Fx Red Red White Red Fa I Red: i White Red I Red: i White i Red: i White It has already been stated that a white-eyed male Drosophila crossed with normal females has only normal children of both sexes, while the white-eyed grandchildren are all of the male sex. In the reciprocal cross, between a white-eyed female and a normal male all the daughters are normal, but the sons are white-eyed, and among the grandchildren white-eyed individuals occur in both sexes. Diagrams will best explain these facts on the basis of Morgan's hypothesis. (See Figs. 87 and 88 and Table I.) To state the foregoing facts in another way, it will be observed that the recessive sex-linked character in Drosophila, when introduced in a cross by the male parent, disappears entirely in Fi and reappears in Yi ^ From W. E. Castle, Genetics and Eugenics (copyright 1920). Used by special permission of the publishers, The Harvard University Press. 433 434 READINGS IN EVOLUTION, GENETICS, AND EUGENICS only in male individuals. But if the recessive sex-linked character is introduced by the female parent, it appears in Fi in male individuals but in F2 in both sexes. Suppose now a cross is made between two races, each of which possesses a different sex-linked recessive character, as for example white eye and yellow body. (See Table II, p. 436.) If the white-eyed Flies CliTOjnosomes 6 d 9 X 6 Parents Gametes (^ XI XX X ? ? C? Fi Gaifietes F2 Fig. 87. — Sex-linked inheritance of white and red eyes in Drosophila. Parents white-eyed male and red-eyed female; Fi, red-eyed males and females; Fj, red- eyed females and equal numbers of red-eyed and white-eyed males. A black X indicates an X chromosome bearing the gene for red eye, a white X bears white eye. (p) indicates that X is wanting; in recent publications Morgan replaces it by Y. {From Conklin, after Morgan.) parent is a female, there will be produced white-eyed males in Fj and white-eyed flies of both sexes in F2. But the male parent being yellow, there will be no yellow flies produced in Fj and only yellow males in F2. In the reciprocal cross (yellow female X white-eyed male) yellow males will be produced in Fi and yellow flies of both sexes in F2, while white-eyed flies will not appear until F2 and then only in the male sex. In either of the reciprocal crosses we expect the pro- duction in F2 both of yellow-bodied males and of white-eyed males. SEX-LINKED INHERITANCE 435 Usually no other sort of male is produced throughout the experiment except these two, but occasionally there is produced a male both yellow-bodied and white-eyed, or one which is gray-bodied and red- eyed, like wild flies. How do these arise? If in Fi females the paired X's were to exchange loads in part, so that G and R came to be attached to the same X and g and r to the other X, and if each of the Flies Chromosomes XX X X ? X iXi X 1 XX X^ X ? ? d' Parents Gametes Fi Gametes Fz S Fig. 88. — Reciprocal cross to that shown in Figure 87. Parents, red-eyed male and white-eyed female; Fj, white-eyed males and red-eyed females ("criss- cross inheritance" — Morgan); Fj equal numbers of red-eyed and white-eyed individuals of both sexes. The distribution of the sex chromosomes is shown at the right, as in Figure 87. {From Conklin, after Morgan.) eggs having such a constitution were to be fertilized with a sperm which lacked X (male determining sperm), this would make possible the production of F2 males possessing both dominant characters and others possessing both recessive characters or gray-red and yellow- white respectively, as actually observed in about one case in a hundred by Morgan. It may add interest to the case to state parenthetically that in man occur a number of sex-linked variations which are inherited in this same curious fashion. Among them may be mentioned color blindness and 436 READINGS IN EVOLUTION, GENETICS, AND EUGENICS bleeding {haemophilia), which occur chiefly in males, but are never transmitted by males to their sons but only through their daughters to their grandsons. Morgan and his pupils have described between forty and fifty characters in Drosophila which are sex-linked in heredity; they also have discovered a large number of other %X Mendelizing characters in Drosophila which ^•\\^ are not sex-linked but which nevertheless are inherited in groups, characters in the same Fig. 89.— Drawing group showing coupling when introduced in a showing the four pairs . 010 of chromosomes seen in cross from the same parent, and repulsion the dividing egg of Droso- when introduced from different parents. The phila. (After Dr. C.E.V. number of these groups exactly corresponds with the number of the chromosomes and Morgan believes that their genes are located in the chromosomes, an hypothesis which seems reasonable but which would be severely strained if an additional group of characters should be discovered. There are three groups of the non-sex-linked characters. (See Fig. 90.) In one of these referred to as Group II (the sex-linked group being called Group I), are found variations known as black body and vestigial wings respec- tively, together with some thirty-five other variations. In Group III are found the variations known as pink eye, spread wings, and ebony body, together with some twenty other variations. In Group IV are included as yet only two characters, bent wings and eyeless, which how- ever show linkage with each other. No inherited characters have been discovered in Drosophila which are not inherited in one or another of the four linkage groups. TABLE II Reciprocal Crosses of White-eyed and Yellow-bodied Flies Male Female Male Female P Yellow-red X Gray-white Gray-white X Yellow-red Fi Gray-white Gray-red Yellow-red Gray-red J I Gray- white: i Gray-red: i Gray- white: i Gray- red: ^ \ I Yellow- red i Gray- white i Yellow-red i Yello\v-red DROSOPHILA TYPE AND POULTRY TYPE OF SEX-LINKED INHERITANCE I. Drosophila type. — The same type of sex-linked inheritance which is found in Drosophila is found also in man, in cats (inheritance of yellow color), and in the plants, Lychnis and Bryonia. The essen- tial feature of the "Drosophila type" of inheritance is this. In a race .0.0 YELLOW. SPOT. -.Of LKTflAL I. «-0 A\ IIJTE. EOSIN CHERBV. a.o AUNORMAL. •ao STREAK. .0.0 SEPIA. ••O BIFID. •BEVT. 14.' CLfR ■i».0 SHIFTED. ■!•.• DACeS. ■ »•.» LETHAL irr. •a».a TAN. ' a».o riNK, PEACH. •EYELESS. 0) C o c o o c c 'z: -c C r -:2 tA ■33.0 VERMIUON. 3«.a Ml MA TUBE. • -".T LETH.4L V. •■•9.0 SABLE. •»4.7 BLACK. -♦0.0 PURPLE. •■•o. KIDNET. •■*•,» LETHAL IV. ■MJ RUDIMENTARY. »••• FORKED. ■»7.o BARRED. •»».5 FUSED. •••.• LETHAL S. ••a.0 VESTIGLAl. •e».EBONT. SCOTT. • •OA CUBVED. • ■'•. BEADED. '•44 ARC ••o.o SPECK. ••JO MORULA. -••ORorCH Q q ii C 3 X c £ O £ .£• i^ ir n I"' C -C C _, rt o •^ iH ^ o ~ o X •-< .« 5j c g ^ § -^ ^ o '^ £ ^ i^£ tc ii a C ^ CJ ^ »-: o £ o tn O £ o «*- CQ c a o -5: -c c: o 2 .c rC -G ^ i5 c o rt CJ »-i '• s ~ o t« < 6 § X O c -^ o p^ Cj O 52 S c '■B .£ ^ c o 438 READINGS IN EVOLUTION, GENETICS, AND EUGENICS breeding true for a sex-linked character, the female is homozygous for the character in question while the male is heterozygous and incap- able of becoming homozygous. Reciprocal crosses with such a race give unlike results, because the female transmits the character to all her offspring, but the male_ transmits it to only half his offspring, viz., the females. Fig. 91. — Sex-linked inheritance of barred and unbarred (black) plumage in poultry. P, parents, barred male, unbarred female; Fi, barred males and females; F2, males all barred, females in equal numbers barred and unbarred. {After Morgan.) 2. Poultry type. — Another type of sex-linked inheritance exists in which the sex relations are exactly reversed. This was first observed in the moth, Abraxas, but more familiar cases occur in poultry, for which reason it may be called the poultry type of sex-lmked inherit- ance. Here the male is the homozygous sex, the female being hetero- zygous. This condition is found in moths and in certain birds, viz., SEX-LINKED INHERITANCE 439 in domestic fowls, pigeons, ducks, and canaries. As an example we may take the inheritance of the color pattern, barring, in crosses of barred Plymouth Rock fowls. In reciprocal crosses between pure- bred barred Plymouth Rocks and black Langshans (or another unbarred breed), the results are not identical. If the barred parent is the male (Fig. 91 and Table III), all Fi ofTspring are barred and in Fj all males are barred, but half the females are black and half are barred. If, however, the barred parent is the female (Fig. 92 and Table III), TABLE III Reciprocal Crosses of Barred and Black Breeds of Fowls Male Female Male Female P Barred X Black Black X Barred Fi Barred Barred Barred Black F2 Barred i Barred: i Black I Barred: i Black i Barred: i Black (See Fig. 91) (See Fig. 92) Fig. 92. — Reciprocal cross to that shown in Fij^ure 91. P, parents, unbarred male, barred female; F,, barred males, unbarred females (crisscross inheritance); F2, barred and unbarred birds equally numerous in both sexes. {From Ciisllc.) 440 READINGS IN EVOLUTION, GENETICS, AND EUGENICS all Fi males are barred, but all Fi females are black. In F2 barred birds and black birds occur in both sexes. These curious facts, which have been repeatedly verified, suggest the occurrence of a vehicle of inherit- ance which is duplex in males but simplex in females. What this is we do not know. No chromosome has been found which has a distribu- tion of this sort in fowls, but it is possible that some chromosome component, or other cell constituent has such a distribution and may be the actual vehicle of inheritance in such cases. The most important character economically, which appears to be affected by some sex- linked factor in poultry, is fecundity. Pearl has shown that when reciprocal crosses are made between Cornish Indian Games, a poor breed for winter egg production, and barred Plymouth Rocks, a fairly cm M O ur B M a Fig. 93-— -B and C illustrate Morgan's idea of the linear arrangement of the genes in the chromosomes. A and D show how the composition of the chromo- somes is supposed to change as the result of the crossover. On the right, a pair of chromosomes, a, before; h, during; and c, after a double crossover. {After Morgan.) good breed for winter egg production, the Fi females in each case resemble the father's race more strongly than the mother's race as regards egg production. Pearl did not maintain, however, nor do his experiments suggest, that the inheritance of fecundity depends exclu- sively upon a sex-linked factor. Goodale, however, has not been able to confirm Pearl's observations, in the case of Rhode Island Red fowls. He finds no evidence of superior influence of the sire in the transmission of racial fecundity. CHAPTER XXXII LINKAGE AND CROSSING-OVER^ WILLIAM E. CASTLE In ordinary Mendelian inheritance, if two characters, A and B, enter a cross in the same gamete (either egg or sperm), it will be wholly a matter of chance whether they continue together or are found apart in the following generation. If in the formation of gametes by the cross-bred, A and B separate from each other and pass into differ- ent gametes, it is evident that one of them has crossed over from the gametic group in which both originally lay to enter the alternative group. This event may be called simply a crossover. Crossovers and non-crossovers will be equally numerous (50 per cent each) where no linkage occurs. Also, if A and B enter a cross in different gametes, one in the egg, the other in the sperm, it will in ordinary Mendelian inheritance be a matter of chance whether they emerge from the cross together or apart. If together, it is evident that a crossover has occurred; if apart, a non-crossover, that is a persistence of their previous relations. Again, crossovers and non-crossovers will be equally numerous (50 per cent each) if no linkage occurs. Linkage may be defined as the tendency sometimes shown by genes to maintain in hereditary transmission their previous relations to each other. Thus if two linked genes, A and B, enter a cross together in the same gamete, they will oftener than not be found together in the gametes formed by fhe cross-bred individual. Crossovers in that case will be less than 50 per cent, and non-crossovers more. And if the same two genes enter the cross separately, one in the egg, the other in the sperm, then oftener than not they will be found apart, in different gametes formed by the cross-bred individual. Again crossovers will be less than 50 per cent. The number of genes in a linkage group varies in known cases from 2 to 50 or more. However many genes there are in a linkage group, each gene shows linkage with every other gene belonging to the same group, but the apparent strength of the linkage varies greatly. Under uniform environmental conditions, A and B show a fairly constant ^ From W. E. Castle, Genetics and Eugenics (copyright 1920). Used by special permission of the publishers, The Harvard University Press. 441 442 READINGS IN EVOLUTION, GENETICS, AND EUGENICS linkage with each other, A and C show a different and likewise fairly- constant linkage strength, and so on through the entire group. This leads to the conclusion that the genes of a linkage system are bound together, gene with gene, with bonds of definite strength in each case. In order to visualize the matter and get a more objective view of link- age relations, Morgan and his associates have developed the chromo- some theory of linkage. Its essential parts are: 1. Genes which show linkage with each other are located in the same pair of chromosomes. It is the substance of the chromosome which binds the genes to each other and causes A to be inherited when B is. 2. Genes close together in the same chromosome show strong link- age, genes farther apart show less linkage. 3. Homologous chromosomes, those containing corresponding sets of genes, one set derived from the father, one from the mother, lie side by side (in synapsis) previous to the formation of gametes. At this time breaks are likely to occur in the chromosomes and parts of one are likely to replace corresponding parts of the other. 4. Such replacement is called crossing-over. 5. Breaks are commoner in long chromosomes than in short ones, and between distant points than between near points on the same chromosome. 6. The genes occur in a chromosome, like beads on a string, in a single row and in definite order. The supposed order of the genes in the four linkage groups of Dro- sophila and their relative distances apart are shown in Fig. 90 (p. 437) . In these diagrams or "maps," when the probable order of the genes in a system has once been determined, the supposed end gene of the system is placed at position o and the gene next to it is placed at a distance (in centimeters or other units) corresponding to the average cross-over percentage between the two, this process being repeated from gene to gene until the whole chain is plotted. The "map" is thus based on a summation of the distances (measured in cross-over per- centages) from gene to gene. But if we compare the "map distances" between genes not adjacent to each other in the chain with the observed cross-over percentages between the same genes, we find that the map distance is regularly greater than the cross-over percentage, except for very short distances (5 or less). Thus if three genes occur in the order ABC, it is usually found that AB+BC is greater than AC. In other words, the cross-over percentage between B and C is commonly LINKAGE AND CROSSL\G-OVER 443 greater than the cross-over percentage between A and C, and the dis- crepancy increases with the magnitude of the values involved. This fact has been accounted for in two different ways. First, it may be supposed that the arrangement of the genes is really not linear, that B lies out of line with A and C, so that AC will be less than the sum of AB and BC, and that the more distant genes are no farther apart than indicated by the cross-over percentages between them. This ex{)la- nation has met with more difficulties than it has cleared away. The second explanation is that the map-distances indicate proportionate numbers of breaks in the linkage chain between points, not propor- tionate numbers of changes of relation between genes at particular points. Thus, suppose genes ABCDE of a linkage system meet their allelomorphs, abcde, in a cross and gametes are later formed by the cross-bred as follows, (i) ABcde, (2) ABcdE, and (3) AbcDe. Assum- ing that the arrangement is linear, we must suppose that one break in the linkage chain has occurred in (i), two breaks in (2), and three breaks in (3). But if we did not have genes BCD under observation, and merely noted the relation of A to E, we should infer that in case (i) and in case (3) a single crossover had occurred. We should on that basis underestimate the amount of breaking in the linkage chain. Accordingly the construction of maps on the basis of short distances summated is justifiable, provided the arrangement is linear, as it seems to be. But it must be borne in mind that the map distances do not correspond with cross-over percentages (although they are based on them) except in the case of very short distances. Alap distances often exceed 50, but cross-over percentages cannot do so, as already pointed out. To get a distinctive name for the map units, Haldane has called them units of Morgan or simply ''morgans." Haldane has computed a formula for converting cross-over percentages into ''morgans" and vice versa. He finds that the two correspond only for very low values (5 or less) and diverge more and more as the observed cross-over per- centages approach 50. Haldane's formula may be stated thus. If three genes, A, B, and C, occur in a common hnkage group, and ihc cross-over percentages are known between A and B and between B and C, we may predict with a probable error of not over 2 per cent, what cross-over percentage will be found to occur between A and C. Call- ing the cross-over percentage between A and B, w, and that between B and C, n, the cross-over percentage between A and C will lie between (w+n) and (m+n — inm). It will approach the former for amounts of 5 or less, and the latter for amounts of 45 or over. In a useful table 444 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Haldane shows the calculated map distances (morgans) for all cross- over percentages between 5 and 50. This table is based on the rela- tions of the genes observed in the sex-linked group of Drosophila, but it applies equally well to the second linkage group of Drosophila, and to a group of three genes in the plant, Primula. Provisionally it may be considered to be applicable generally to linkage systems in animals and in plants. TABLE IV A Table for Converting Cross-Over Percentages into Map Distances ("Morgans") and Vice Versa (After Haldane) Cross-over percentage k . 0.0 5-0 8.0 10. II .0 12.0 13.0 ]Map distance . 0.0 5-1 8.2 10.3 II. 4 12.5 13-6 14.0 I5-0 16.0 17.0 18.0 19.0 20.0 21 .0 22.0 14.7 15-9 17.0 18. 1 19-3 20.5 21 .7 22.9 24.1 23.0 24.0 25.0 26.0 27.0 28.0 29.0 30.0 31.0 25-3 26.6 27.9 29.2 30-5 31-9 33-3 34-7 36.2 32.0 33-0 34 -o 35-0 36.0 37-0 38.0 39.0 40.0 37-7 39-3 40.9 42.6 44-3 46.1 48.0 50.0 52.2 41.0 42.0 43-0 44 -o 45 -o 46.0 47 -o 48.0 49 54-4 56.8 59-6 62.6 66.0 70.1 75-1 81.9 930 49-5 49-7 49.8 49.9 50.0 99.2 109.4 117. 7 128. 1 As an example of how the table may be used in predicting undeter- mined linkage values, suppose that A is linked with B, and B with C and that between A and B there are 10 per cent of crossovers. What will be the cross-over percentage between A and C ? Converting the observed cross-over percentages into map distances with the aid of the table, we find the distance AB to be 10.3 and the distance BC to be 15.9. On the linear theory the distance AC will equal either the sum or the difference of AB and BC, that is will be either 26.2 or 5.4. Converting these distances into cross-over percentages by interpola- tion in the table, we find that the cross-over percentage between A and C should be either 23.7 or 5.1, according as the linear arrangement is ABC or ACB. Measurement of linkage. — It will be observed that as the strength of linkage increases, the cross-over percentage decreases. With a cross-over percentage of 50, there is no linkage. With a cross-over percentage of o, the linkage is complete, two characters so related behaving as allelomorphs. Accordingly we depend upon the observed cross-over percentage both for the detection of linkage and for the measurement of its strength. But unfortunately the linkage strength varies inversely as the cross-over percentage. This makes the cross- over percentage directly considered, a rather poor measure of linkage LINKAGE AND CROSSING-OVER 445 strength. It is really the amount by which the cross-over percentage falls below 50 that measures directly the strength of linkage. Thus with cross-over percentages of 50, 40, 30, 20, 10, and o, we should have linkage strengths of o, 10, 20, 30, 40, and 50. We should then have a standard for measuring linkage strength directly, on a scale of 50. But as we are more accustomed to grading on a scale of 100, it seems preferable to double the values indicated above. We then have grades of linkage strength on a scale of 100, as follows: Cross-over Linkage Percentage Strength 50 O 40 20 30 40 20 60 10 80 o 100 Accordingly, to estimate the strength of linkage in a particular case, we multiply by 2 the difference between the observed cross-over per- centage and 50. But suppose the observed cross-over percentage were greater than 50, what then? Such an occurrence would not indicate linkage, a tendency of characters to remain grouped as they were, but an oppo- site tendency, to assume new groupings. No such tendency has been observed. If it should be, it would need a different name and method of measurement. We may now consider some further examples of linkage. In the plant, Primula sinensis, Gregory observed the occurrence of linkage in a group of five characters, viz., Dominant Recessive 1. Short style vs. long style (1) 2. Magenta corolla vs. red corolla (r) 3. Tinged corolla vs. full-colored corolla 4. Green stigma vs. red stigma (s) 5. Pale stem vs. full red stem Altenburg later determined the strength of the linkage existing between three of these five pairs of characters, viz., 1,2, and 4 of the above list. His results may be expressed in a linkage map as follows : - r s 34 o 45-6 The cross-over percentage between 1 and r was found to be 34.02, between r and s, 11.62. The sum of these two, 45.64, is the total (uncorrected) map distance. The observed cross-over percentage 446 READINGS IN EVOLUTION, GENETICS, AND EUGENICS between 1 and s was 40.6, which falls short of the map distance by almost exactly the amount indicated by Haldane's table. In the sweet pea the earliest discovered examples of linkage are found. Here are known two linkage groups containing each three pairs of characters as follows : Dominant Recessive {I. Blue VS. red flower color 2. Long vs. round pollen 3. Erect vs. hooded standard ( I. Dark vs. light leaf-axil Group 2 I 2. Fertile vs. sterile anthers 3. Normal vs. cretin flowers Results described by Bateson and by Punnett indicate that in Group i the map relations of the three genes are : E— B L 0.78 12.5 The group is a compact one, with E and B very closely linked, cross- over percentage less than one, with B and L showing between 1 1 and 12 per cent crossovers, and with E and L showing about 12.5 per cent of crossovers. In Group 2, the cross-over percentage between D and F is about 6.2, between F and N about 25.0. Until the cross-over percentage between D and N has been experimentally determined, it cannot be stated whether the "map" order is FDN or END. In the former case, the total map distance will be 25, or about double the length of Group I ; in the latter case, it will be still longer, or about 31.2. In garden peas two independent pairs of linked characters are known and two more are suspected (White) . In one of the estabhshed cases close linkage is found between round starchy seeds and tendrils on the leaves, with about 1.5 per cent of crossing-over. In the other case a gene for late flowering is linked with red flower color with an estimated cross-over percentage of between 12 and 16. In the snapdragon, Antirrhinum, two factors for flower color were found by Baur to be linked, with about 20 per cent of cross-overs occur- ring. In maize three linkage groups are known, one of four factors and two of two factors each. Group i includes a factor for waxy endo- sperm and the factor C for aleurone color. These show a cross-over percentage of 26.7. Group 2 includes four linked factors, aleurone fa ctor R, chlorophyll factor G, chlorophyll factor L, and aleurone LINKAGE AND CROSSING-OVER 447 TABLE V Cases of Linkage in Plants or in Animals Other Than Drosophila Species a 3 O li O I I I 2 2 2 I 2 I I 2 2 2 3 I 2 I I I I I I I I I I I I I I I Linked Characters Cross-over Percentage Linkage Strength Authority Sweet pea Purple flowers, long pollen Purple flowers, erect standard Long pollen, erect standard Dark axil, fertile anthers Dark a.xil, normal (not cretin) flower Fertile anthers, normal (not cretin) flowers ri or 12 0.78 12.5 6.2 ? 25.0 76-78! 98.4 75 87.6 , 50 . 32 18.8 • 76.8, Bateson and Punnctt Primula sinensis Short style, magenta corolla Short style, green stigma Magenta corolla, green stigma Tinged corolla, green stigma Pale stem, green stigma 340 40.6 II. 6 ? ? Altenburg Gregory Garden pea Round seeds, tendrils on leaves Late flowering, colored flow- ers 1-5 12-16 97 68-76 Bateson and Vilmorin Hoshino Antirrhinum Red flower color, "pictur- atum" pattern 20.0 60 Baur Maize Waxy endosperm, Aleurone C Aleurone R, Chlorophyl G Aleurone R, Chlorophyl L Chlorophyl G, Chlorophyl L Starchy endosperm, tunicate seed 26.7 19.0 0.0 23.0 8.3 46.6 62? ' 100 54 . 83.4 Breggar Lindstrom Jones Tomato Vine habit, fruit shape Green foliage, 2-celled fruit 20.0 0? 60 1 100? J Jones Beans Seed pattern, vine habit 0? 100? Surface Silkworm Pattern Q of larva, yellow silk 26. 1 47.8 Tanaka Apotetlix Patterns G and M Patterns M and K Patterns K and Y Patterns Y and R Patterns Y and T Patterns R and T Patterns ]M and R Patterns Y and Z 4 (in?; I (in?) 6 (in?) 10 (in?) 12 (in?) (in?) 10 (in?) 10 (in?) 92 98 88 76 100 80 80 Nabours Pigeon Sex-linked factors I and A 40 (in<5) 20 Cole and KcUey Rat Albinism, red-eye Albinism, pink-eye Red-eye, pink-eye I.O? 21 .0 18.3 98?! 58 • 63-4] Castle and Dunn Mouse Albinism, pink-eye 14 3 71.4 Castle and Dunn 448 READINGS IN EVOLUTION, GENETICS, AND EUGENICS spotting factor, S. No crossovers have been observed between R and L which behave as if they were allelomorphs, or "completely linked." The cross-over percentage between L and G has been determined as 23, that between R and G has been determined less accurately as 19, and that between R and S as 12.5. The order of the genes is accordingly R— L S G. Group 3 includes the two characters, starchy endosperm and tunicate ("podded") seeds. The cross-over percentage in this case is S.;^ (Jones and Gallastegui). • In the cultivated tomato two cases of linkage have been reported. A gene for "standard" vine habit and a gene for '^constricted" fruit shape show about 20 per cent of crossing over. In another linkage group, no crossovers have been observed between green foliage color and two-celled fruit, as opposed to yellow foliage color and many- celled fruit, in a total of 24 F2 plants. It seems probable that the linkage in this latter case is close, though the number of observations is too small to do more than establish a probability. In rats a group of three linked characters has been found, albinism (c), red-eye (r) and pink-eye (p), which may be mapped, thus c — r p 01 20 In mice albinism (c) and pink-eye (p) are linked, as they are in rats, but the cross-over percentage is less, viz., 14.3. In the silkworm, linkage occurs between a factor, Q, which gives to the larva characteristic pattern markings, and a factor, Y, which gives to the blood of the larva and the silk of the cocoon a yellow color. Crossing-over occurs only in males, and in a percentage of 26.1 (in a large series of backcrosses of Fi hybrid male with double recessive female, producing 24,918 individuals). In Drosophila crossing-over occurs only in the female parent, that is in the maturation of the eggs. This is true of all linkage groups, whether they involve sex-linkage or not. In the grouse-locust, Apotettix, a linkage group of seven or more characters has been discovered by Nabours, which have this curious feature, that crossing-over seems to occur much more fre- quently in females than in males. In all other known cases of linkage, crossing-over occurs with about the same frequency in the gametes formed by both sexes. This accordingly is to be regarded as the normal condition. Failure of crossing-over to occur in the oogenesis of Drosophila and in the spermatogenesis of the silkworm would seem to imply unusual cytological conditions in those cases. CHAPTER XXXIII SEX DETERMINATION VARIOUS THEORIES OF SEX DETERMINATION H. H. Newman In earlier chapters it has been necessary to introduce a few neces- sary facts about sex determination and sex-linked heredity. The mechanism of sex determination has been clearly described and illus- trated for Drosophila (pp. 411 ff.), and the close connection that exists between sex-linked heredity and sex determination has been shown in chapters xxxi and xxxii. A more detailed consideration of sex deter- mination and sex differentiation is now to come. The question as to what determines whether an animal shall be a male or a female is a very ancient one, and it is only during the present century that we have solved the puzzle. A great many theories of sex determination have been proposed, some of which are as follows: a) Hippocrates and some subsequent theorists believed that the sex of the offspring depended on the relative vigor of the parents, the more vigorous parent giving his or her sex to the offspring. h) Thury thought that the sex of the offspring depended on the degree of ripeness of the ovum at the time of fertilization. c) Various writers claim that statistics show that germ cells from the right ovary produce males and those from the left ovary females. d) The nutrition theory. — The egg is a much more highly nourished cell than the spermatozoon, and the idea seems natural that high degrees of nourishment of the mother produce female offspring and lower degrees of nourishment male offspring. Professor Schenk of Vienna gained a huge reputation by controlling the diet of certain royal prospective mothers and predicting the sex of the offspring accordingly. He was correct in his predictions several times, but his success was short-lived. His early predictions were merely lucky, just as one might be who could guess heads or tails correctly several times in succession. Some color is lent to the nutrition hypothesis by the fact, if it is a fact, that after war or famine, when the nutrition of mothers has been 449 450 READINGS IN EVOLUTION, GENETICS, AND EUGENICS low, more males than females are born. This is probably a case of differential prenatal mortality. By that we mean that more females die unborn than males, because the latter are hardier and stand pre- natal malnutrition better. e) Sex is deter?nined at the time of fertilization. — Perhaps the best evidence that sex is determined at the very beginning of development is derived from one-egg twins and quadruplets. In the nine-banded armadillo practically every female gives birth to quadruplets, four essentially identical young being produced in each litter. All in a given set of quadruplets are invariably of the same sex, either four males or four females. Newman and Patterson have shown that each set of quadruplets comes from a single egg which at a very early stage divides into four parts to form four foetuses (Fig. 94). The conclusion is that sex was determined before the separation took place. Human identical twins, also always of the same sex, furnish further evidence in favor of very early sex determination. These and numerous other similar facts justify the conclusion that sex is determined at the time of fertilization. THE CHROMOSOMAL MECHANISM OF SEX DETERMINATION The well-established case of Drosophila (pp. 411 ff.) will serve as a basis for comparison. Many cases similar to that of Drosophila have been worked out. The chief differences have to do with the X and Y chromosomes. Sometimes the X chromosome is distinct and inde- pendent during synapsis and maturation, but cases are known in which the X chromosome is attached to the end of one of the ordinary chromosomes, and will, of course, always follow this chromosome in reduction division. The Y chromosome varies considerably in dif- ferent species. Sometimes the Y chromosome and the X chromosome are optically indistinguishable. Sometimes the Y element is repre- sented by a group of as many as five small chromosomes which keep together in a group and always go either one way or the other in the reduction division. Again, the Y element may be very small or vesti- gial, or, finally, it may be wanting altogether, so that X is an entirely unpaired chromosome that goes to one cell only at the reduction division. In spite of all of these various modifications of the X-Y type of chromosomal sex-determining mechanism, the method of producing male-forming and female-forming spermatozoa is the same in each case. The female gametes all have one X chromosome, while half of SEX DETKRMIXATIOM 451 the male gametes have one X chromosome and the other half have no X, but sometimes Y and sometimes simply one chromosome less than the first type of male gametes. We can then speak of the two Fig. 94. — An armadillo egg about six weeks after fertilization, showing the quadruplet foetuses are derived from the single egg and all destined to be of the same sex. (Frotn Newtnan.) sexes produced by union of male and female gametes simply in terms of the X chromosomes, females being characterized by XX (duplex) and males by X (simplex). SEX DETERMINATION IN PARTHEXOGENETIC SPECIES Although it was at first thought that the facts of parthenogenesis (development of eggs without fertilization) was opposed to the chromosomal mechanism of sex determination, further study of this 45^ IIEADINGS IX EVOLUTION, GENETICS, AND EUGENICS phenomenon only served to line up this category of sex determina- tion with the type already explained. In the hees and wasps it has long been known that eggs which are extruded without fertilization produce drones (males), while fertilized eggs produce workers or queens (both females). It has now been dis- covered that the bee egg undergoes the reduction division before fertilization so that all eggs will have only one X chromosome. The eggs that are fertilized always have the XX condition and will produce only females, while the eggs that are not fertilized keep the X con- dition and produce males. These males with only half the normal number of chromosomes cannot carry out the reduction division, but produce spermatozoa always with an X chromosome. In aphids parthenogenetic individuals are always females and in this case it has been discovered that the egg develops without under- going the reduction division, thus retaining the XX condition. In all of the cases hitherto mentioned the female is said to be homo- zygous for sex, because she produces gametes of only one kind, from the sex standpoint, each matured egg having the X chromosome pres- ent. The male, on the contrary, is said to be heterozygous for sex since two kinds of sperms are produced, one with X and the other with- out X. The great majority of animals appear to have a similar mechanism, but there are a few groups of animals which are just the reverse of what we have described, since the female is heterozygous for sex and the male homozygous. THE POULTRY TYPE OF SEX DETERMINATION There is now evidence both cytological and genetic that in poultry, and probably in all birds, there are two kinds of maturated eggs, one having the X chromosome and the other with the Y chromosome, or at least without an X chromosome. All of the spermatozoa are believed to have the X chromosome. As has already been seen (chapter xxxi), the sex linkage is just the reverse of that seen in the majority of animals when the male is the heterozygous sex. Moths and butterflies are also probably of the same type as poultry. In the classic case of Abraxas, the currant moth, a pale mutant occurred which was female and was sex-linked to females just as white eye color was linked to males in Drosophila. Apart from the fact that the XX con- dition seems to have shifted from one sex to the other in these two groups (birds and butterflies) the mechanism of sex determination seems to be exactly the same as in the majority of animals studied. SEX DETKRMIXATIOX 45^ SEX DIFFERENTIATION It now becomes necessary to distinguish clearly between sex determination and sex differentiation. When we say that by means of a chromosomal mechanism sex is determined, exactly what do we mean ? We answer that the sex of an individual arising from a fertil- ized egg (in the case of parthenogenesis, an unfertilized egg) has been settled. Now as a matter of fact only one thing has been settled irrevo- cably, and that is that one individual will have the chromosome composition characteristic of a male and another individual that of a female. A male is usually an individual that produces spermatozoa and a female one that produces ova. Is it irrevocably settled beyond possibility of reversal that a zygote with the XX chromosome com- position must produce eggs and one with the X composition, sper- matozoa ? This question has apparently been answered by Geoffrey Smith in his work on parasitically castrated crabs and by Richard Goldschmidt on Gypsy moths. In the first case, individual crabs whose testes had been infested by the parasitic cirripede, Sa^cidina, were gradually changed over in their whole metabolism to such an extent that cells destined to produce spermatozoa produced ova. In the second case, when certain varieties of moth were crossed, all of the germ cells produced females with ova, whereas half of the eggs had the XX and half the X chromosome content. This evidently means that some individuals with the male chromosome character produced eggs. From these results we may be justified in conclud- ing that not even this most fundamental difference of sexes, that of the female producing ova and the male spermatozoa, is irrevocabh' predetermined at fertilization. Lest the reader be confused, however, we hasten to add that under natural conditions of life an individual with the male chromosomal content produces spermatozoa and one with the female chromosomal content produces eggs, and that only rare accidental or unnatural conditions disturb the normal course of events. For purj:)oses of practical genetics we may then define a female as an individual that produces ova and a male as one that produces spermatozoa. Secondary sexual characters. — Usually males and females differ from each other in many other characters besides the production of eggs or sperm. Often one sex is larger, stronger, more elaborately ornamented and colored than the other and possesses characteristic accessory sex organs whose function it is to facilitate the bringing together of the eggs and the sperm. All of the differences between the 454 READINGS IN EVOLUTION, GENETICS, AND EUGENICS sexes other than the primary difference of egg or sperm production are called secondary sexual characters. Usually very young animals show only slight differences in secondary sexual characters and the differ- ences increase markedly at sexual maturity. We speak of the gradual divergent development of the two sex types as sex differentiation. The question arises as to whether or not the chromosomal differences are the causes of the differentiation of secondary sexual characters. These secondary sexual characters are all somatic, and, since the soma is the product of cell division of the zygote, the soma cells must have either the male or the female chromosomal character. That the chromosomal mechanism in the somatic cells is not sufficient of itself to bring about, unaided, the differentiation of secondary sexual charac- ters can be shown readily in at least many animals. In the mammals, for example, it is known that the early removal of the testes or ovaries results in a retention of the juvenile or undif- ferentiated condition of secondary sexual characters. Evidently some influence is exerted by the tissues of the gonad that is necessary for the full differentiation of sex characters. The current theory is that certain glandular cells that form part of the body of ovary or testes excrete materials into the blood that stimulate various tissues in different ways and produce dimorphic results. The specific sub- stances produced by these glands are called "hormones," for want of a better name. To test the efficiency of these hormones the crucial experiment of taking out the gonads of a young rat or guinea pig and implanting the gonad of an individual of the opposite sex has been many times performed. For example, Steinach castrated young male rats and then successfully grafted into them ovaries from young female rats. The result was that these young rats which started to be males became much altered in a female direction, the mammary glands becoming greatly enlarged, their instincts more feminine than masculine, and in a number of other particulars they showed more or less pronounced evidences of feminization. Conversely, spayed females with engrafted testes showed a tendency toward male differ- entiation, especially in instincts. These experiments have been largely confirmed by C. R. Moore. Similar experiments with similar results have been performed with fowls and ducks. All indicate that the glandular part of the gonads has a determinative effect on sex differentiation, and this in spite of the chromosome make-up of the somatic cells; for the male differentiation in cells with the male chromosome characters can be SEX DETKRMLvvTrON 455 456 READINGS IN EVOLUTION, GENETICS, AND EUGENICS inhibited and female differentiation superimposed if the female gonad is introduced in the absence of the male gonad. A beautiful experiment conducted by nature herself helps to drive home the hormone theory of sex differentiation. In cattle, as shown recently by F. R. Lillie, twins occur in a small percentage of cases and involve the simultaneous fertilization of two eggs. These eggs lie as a rule in opposite horns of the forked uterus, but owing to the growth of their embryonic membranes the two individuals come to fuse cir- culations so that there is an admixture of blood (Fig. 95). The result is that if the twins are zygotically of the same sex, no untoward effect of blood admixture is apparent, but when the twins are zygotically a male and a female, the female individual is always stopped in its female differentiation and becomes more or less completely trans- * formed in a male direction. It appears, however, that at the time when blood admixture occurs, the female individual has already differentiated so far with respect to the external genitalia and in other respects that, even though subsequent development be entirely male in character, the resultant individual is always a sterile creature, neither fully a female nor a complete male. Such individuals have long been known as " freemartins." As a rare exception to the general rule an occasional case has appeared in which a male and a female pair fail to undergo blood admixture. In such cases both develop into normal animals. It now appears that the reason why the female sex is the one to suffer is that the male gonads differentiate precociously, before the female, and inhibit the subsequent development of female gonads. Hence the only hormones in the blood of both twins are the male hormones. In conclusion we may say then that though chromosoines tend to determine the primary sex differences, they have no effect on the differentiation of secondary sexual characters. These are due to substances secreted by the gonads that has been called a hormones. PART V EUGENICS CHAPTER XXXIV THE INHERITANCE OF HUMAN CHARACTERS, PHYSICAL AND MENTAL^ ELLIOT R. DOWNING Anyone who undertakes to trace the ancestry of an individual is soon impressed with the fact that it is a difficult task even to find the names of the persons involved three or four generations back; it is much more difficult to determine with certainty their physical and mental characteristics. One can more surely find the pedigree of a horse or hog that he may own than he can of a child in whom he is interested, for we do have registry books for good stock, but none ordinarily for human family relations (in Illinois not even compulsory birth registrations until very recently), so that a child born in this state may not even legally prove his existence or parentage by official records. It is not an easy matter, therefore, to find human data that illustrate the various phases of heredity concerning which we are reasonably sure in dealing with animals and plants. Fortunately, there are some studies of the inheritance of physical characters that are quite satisfactory. There is an increasing number of studies of the inheritance of insanity, feeble-mindedness, epilepsy, and alcoholism by the scientific staff of institutions deahng with such cases, and we do have a fairly good mass of material in the lines of descent of the royal families of Europe, where the matings and the characters of the individuals are more or less matters of history. Thanks to the generosity of some men of wealth and foresight, appre- ciative of the importance of a better knowledge of the laws of human heredity, we have in several countries well-endowed laboratories with expert staffs founded on purpose to study this topic; such as the Galton Laboratory of Eugenics in England and the Eugenics Labora- tory of the Carnegie Institution, Cold Springs Harbor, New York. Occasionally a family is found in which one or more members have five fingers instead of four; such a condition is known as polydactyl- ism. Sometimes a case is recorded in which a person has fingers with ^ From E. R. Downing, The Third atid Fourth Generation (The University of Chicago Press, copyright 1920). 459 46o READINGS IN EVOLUTION, GENETICS, AND EUGENICS two joints instead of three and a thumb with one joint in place of two (brachydactyUsm). Such human abnormahties are inherited. There •.-'• o 3 ©- c i: o c ^0- a o •o • a- .'• -•• •ti •'^ -•*■■ l-o- ■*•- ^'•-(, l-o o ID © ©- r©- Ll -0 (J P a -4-' o (Tt ^ "0 S O oo >> l/l r o ri ^ o O u tA •X fTl >. o 3 ° I o CI ^ o -^ t/3 C en tC O ^ C/} > o C •r' o a; o C < a 4-) o O u a -d C O k3 o ^ QJ c3 5 W) rt C s O o o 3 X [/3 o to -*-! ci " a c o -a rt 'r o • O t/2 a l-H O a; o o o O I— I O -ti! If >% o o '"> « c3 O IH ;-! O o fO. is given on this page a chart (Fig. 96) of a family tree in which brachydactylism is very common; it is based on a study made by Drinkwater. Males in the chart are represented by 6, females by ?, INHERITANCE OF HUMAN CHARACTERS 461 matings by = . The circles are of solid color • in individuals afTccted with the deformity, open O in normal individuals. The character seems to behave like a Mendelian dominant, though one could make no very positive assertion on this point from so few individuals. Hut it is very evident that such a physical character once in the stock is transmitted generation after generation, reappearing continually in the offspring. Below there is presented a chart (Fig. 97) of the transmission of cataract. This disease is characterized by the aj^pearance of an opaque area in the usually transparent parts of the eye, f I 1 1 r 4 ^ € 4 f 99 fxd'x9 99(D(S)ff ®® ®® o© o 909 4 f • 9 ®4Q)f^(5 Fig. 97. — Inheritance of one form of cataract. Modified from Nettleship's chart. The diagram reads thus: A man with cataract married a normal woman; of their eight children six were affected with the disease. One of these married an unaffected man; three of the children of this union were normal, sex unrecorded, two defective. This same man married a second wife who was normal; their eight children were all unaffected. So continue reading through five generations. {From Downing.) ultimately rendering the person blind. In the particular form of the disease here considered it does not develop until middle life. Clarence Loeb in a study of hereditary blindness pul^lished in 1909 tabulated the results of a study of 304 families in which such blindness occurs. There were 1,012 children, of whom 58 per cent were affficted, which is about the percentage expected when hybrid defectives mate with normal individuals and the defect is a dominant character. Similar extensive studies of congenital deafness and deaf-mutism show that these are similarly heritable, though just how the character behaves is not yet known, for undoubtedly under "deafness" are included a variety of diseased conditions that must be 462 READINGS IN EVOLUTION, GENETICS, AND EUGENICS studied separately before we shall know how each is inherited. Care must be taken, too, to distinguish between congenital deafness and blindness — that which inheres in the germ plasm — and those forms, due to accident or contagious disease, which are acquired modifications and so not heritable. Thus measles often produces deafness as one of its after effects. Persons so rendered deaf would not transmit the affliction to their children any more than they would transmit blind- ness if the eyes of the parents were put out by accident. Feeble-mindedness apparently behaves as a Mendelian recessive. Goddard's studies of the family pedigree of the inmates of the Vine- land, New Jersey, institution for the care of the feeble-minded gives us an abundance of material to show the heritability of this defect and its relation to alcoholism, insanity, syphilis, etc. Briefly, syphilitic infection is a fairly common cause of feeble-mindedness in children. There is a higher percentage of feeble-mindedness in the offspring of alcoholic parents than among those of parents not addicted to it. There seems little or no causal relation between feeble-mindedness and insanity. But aside from feeble-mindedness that may be pro- duced by such causes or by occasional accidents such as falls, blows on the head, there is the great mass of feeble-mindedness that is wholly a matter of heredity. If a feeble-minded individual comes from parents both of whom are congenitally feeble-minded or who both have a great deal of feeble- mindedness in their ancestry, such a one is taken to be a pure recessive as far as this character is concerned, and his germ cells have a double dose of the factor for feeble-mindedness (FF). When two such per- sons mate, their offspring would be expected to be all feeble-minded, for all eggs and sperm contain the factor F, and when any egg is fertil- ized the person produced is an FF individual. Out of 144 such mat- ings resulting in 482 offspring whose records are known, Goddard found that 476 were feeble-minded. This type of mating as well as others cited below are illustrated in the family pedigrees shown on pages 463 and 464, selected from Goddard's book. If a person comes from parents one of whom is entirely normal and one is feeble-minded with many feeble-minded ancestors, it is probable that such an individual is a hybrid with germ cells that, as far as this one character is concerned, can be designated NF. Such a person will pass for normal, since feeble-mindedness is recessive. If such a one mates with the type described above (FF), it would be expected that half the offspring would be normal, half feeble-minded. Out of INHERITANCE OF HUMAN CHARACTERS 463 122 such matings producing 371 children, 193 were found to be feeble- minded, 178 normal, which is remarkably close to expectation con- sidering the diflficulty of determining with certainty the real character of the parents. When two individuals of the NF type mate, their offspring would be expected to give 3 normals to i feeble-minded. Out of 146 children produced by t^t, such matings Goddard found 39 were feeble-minded. The first of Goddard's charts (Fig. 98) illustrates the family tree of Gertie K., a girl of 12 years, with the mental development of a child of 7. Males in this and the following chart are represented by squares, females by circles. Note* that this girl has a feeble-minded brother and that both her parents are feeble-minded and see the appalling array of feeble-minded cousins, aunts, uncles, and other relatives. Her grandmother passed for a normal individual, although it would seem from her children she must have been an NF individual. The T i ■ o~-= ■ = ® ■ • • ■ aoo ■ ® a I r— * — F 1 r — I 1 : 1 1 1 1 1 1 ©• [n][n][n|E][^|n]Q ■•■ # T Fig. 98. — Family of Gertie K. {From Downing, after Goddard.) second chart (Fig. 99) is quite exactly Mendelian, if we suppose the grandparents were NF individuals. This case is particularly interest- ing, for the parents of these six feeble-minded children were high-grade morons, both immigrants. The public must support the children because we have as yet instituted no expert examinations to detect such defectives among our immigrants in order to refuse them admis- sion to this country. See what a single unfortunate alliance can produce. A young man to whom Goddard gives the fictitious name of Martin Kallikak had children by a feeble-minded girl in the days before the Civil War. There have been traced some 480 descendants from this mating, and all of them are below normal intelligence. Later this same man married a good Quaker girl, and 496 of the descendants of this marriage have been traced, all of normal mentality. The contrast is strikingly instructive, for the conditions are almost those demanded by a scien- tific demonstration. 464 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Such cases as those cited are interesting from the standpoint of the student of heredity. They are tremendously significant to the average citizen because there is in the United States a very large feeble-minded population, estimated at 200,000, nine-tenths of whom are at large, free to reproduce their kind, and very prone to interbreed, because the feeble-minded are seldom sought as legitimate mates by persons of normal mentality. The number of feeble-minded is apparently increasing much more rapidly than the general population. How rapidly, it is impossible to determine, for we have no exact data on the number of feeble-minded; we are not yet awake to the enormity of the problem involved. From these feeble-minded come some d. AT 50 O «LAT60 56S LIVES IN ENGLAND IN GERMANY OUT WEST dTTS^SS^ k CHARLIE M. Fig. 99. — Family of Charlie M. {From Downing, after Goddard.) 40 per cent of our prostitutes, a fourth of our criminals, and at least a half of the inmates of our almshouses. A generation ago the valley of Aosta, in Northern Italy, was over- run with feeble-minded and idiotic individuals of the type known as cretins. It was estimated that fully 60 per cent of the population were affected with this abnormality. A law was passed and enforced segregating the really irresponsible cases and prohibiting the marriage of cretin with cretin. Now the condition has almost disappeared, and it is estimated that only a very small percentage of the population are cretins, these nearly all old, so that this particular form of idiocy will there very soon be a thing of the past. It seems only a rational proce- dure to accomplish at least a segregation of feeble-minded in this INHERITANCE OE HUMAN CHARACTERS 465 country, even if no more drastic action is taken. Otherwise the group is bound to be an increasing burden on the community, adding con- stantly to the tax needed for their support. Investigations of competent officials in the employ of insane hos- pitals have accumulated a mass of evidence demonstrating the herit- ability of many forms of nervous diseases which most commonly behave as recessives. Rosanoff and Orr,^ in a study of 206 matings between individuals from more or less insane stock, found 1,097 children, 146 of whom died in childhood. There were 351 afflicted offspring to 586 normal. The theoretical expectations, knowing with more or less certainty the character of the parents, were 359 to 578. There are presented (Figs. 100, loi) two typical family pedi- In the first an insane man was twice married, each time to an riji^£:i^i~^ Fig. 100. — (i) Ignorant, ''queer"; (2) Insane, was in sanitarium, com- mitted suicide; (3) eccentric, violent temper, ideas of persecution against neighbors; (4) eccentric, not well bal- anced; (5) alcoholic, lazy, indolent; (6) dementia praecox, paranoid, in state hospital; (7) violent temper, queer, extreme dolichocephaly; (8) defective, cranial malformation; (9) inferior, "slow." {From Do'd^ning, after Rosanoff afid Orr.) grees. [ti d) (t) 1 4 6,4 .iti] Fig. ioi. — (i) epileptic; (2) insane for a time, recovered; (3) epileptic, imbecile; (4) imbecile; (5) melancholy in early married life, recovered; (6) insane five years, was in state hospital, recovered; (7) insomnia, neuralgia; (8) daughter had spells of excitement; (9) feeble-minded; (10) dementia praecox, katatonic, in state hospital; (11) died of marasmus, had one convulsion. {From Downing, after Rosanoff and Orr.) eccentric woman, undoubtedly mildly insane. All the offspring were unbalanced. In the second case, those distinctly neurotic are indi- cated in soHd color; those having a neurotic element in the germ material are shaded. It might seem as if insane individuals would scarcely add materially to the general population, since they are com- monly in asylums. Often, however, the inherited insanity does not ^Eugenics Record Office (Cold Springs Harbor, N.Y.) Bulletin No. 5, 191 1. 466 READINGS IN EVOLUTION, GENETICS, AND EUGENICS manifest itself until past middle life, when they have already married and started a family. Moreover, those hybrid individuals in whom the insane tendency is present alongside of the normal determiner appear as normal individuals. Frequently they can be detected only by an examination of the pedigree. If such individuals mate, one- fourth of the offspring would be expected to be insane. Early modern European history centers about the doings of a few great men and women. Peter the Great of Russia, Ferdinand and Isabella and Charles V of Spain, Frederick the Great of Prussia,Gusta- vus Adolphus and Charles XII of Sweden, are among the most brilhant of these potent individuals that shaped the destinies of Europe during this period. It is interesting to note how their characters are deter- mined (and through them national destinies are apparently decided in no small measure) by the hereditary concentration of ability due to lucky royal matings, and how their genius is dissipated by unwise matings. Peter the Great of Russia came as a brilliant type from a good stock, though with a very evident taint of epilepsy and feeble- mindedness. He himself was an epileptic. His father, grandfather, and great-grandfather had been men of large ability. They had married peasant girls, as was the custom of the czars. Peter's own brothers and sisters were in no way remarkable. His half-sister Sophia was a woman of marked ability, although two of her brothers were imbeciles, one also an epileptic. As will be seen from the pedigree, the epilepsy, inbecility, and mediocrity appear in both Peter's children and grand- children, as well as in those of his imbecile half-brother, Ivan. It is interesting to note from the pedigree that the feeble-mindedness and epilepsy seem to cling to the males quite persistently. The females of the family are much more apt to be brilliant and virtuous. Peter the Great's own son Alexis was a poor dissolute specimen, and although he married Charlotte, the angelic daughter of a great line, the house of Brunswick, the son of this mating was Peter II, of unstable mind, while the daughter Natalia was as sweet as she was energetic. Isabella and Ferdinand were both descendants from lines of very great individuals, although in each case there is insanity in the family. Isabella herself comes from an insane mother and an imbecile father, but her grandparents and great-grandparents were well-balanced and able. The data for the charts of these royal families were taken largely from F. A. Woods's Mental and Moral Heredity in Royalty ^ supplemented with information from other sources. He grades the INHERITANCE OF HUMAN CHARACTERS 467 W Pi o w W H Pi; w H CO en o O < o w W H fcX) -t-> c a m O c s O o -o o ^ o -i\ to c ci (53 (U \ \ \ .2 o < *j >. CO rt o ft" I— I t> o ^ 1: o - C I- o C 3 3 u > •(-J •r' 11 — •~i *-> t: CO ■*-» «j - 0.0 go ^41 n: <^ O "J p p 4) (U c •c - 4> . M -t-t a U - C C < o 2 S in k 468 READINGS IN EVOLUTION, GENETICS, AND EUGENICS < pq < in < Q < I— I Q P^ W O o I— I o i-i bo w I— ( H < u o \ H I-I < o o w o CI • i-H - C • i-H d o < o p H O Ph O 12; O i3 C/3 O 3H I-I w - W H W Ph Pi 5^ t/3 I— I HH o 1-1 w PQ j3 o O ffi c o U o SI o H c3 (— 1 to 3 -4-1 )-i C 5 &H S t*-i w II H >. ;-i 03 ^H o5 T3 -tJ o .s 'l-l - u M H o -4-> hj 1-1 C J hH t-H n> ^ ^ s w o -<-> u n rt (— ( tu n o w W « tlH - C- oj o fey o < w h-l Ho I— I 'O © 0-is'^>' C/3 PL, o C/2 pq II © tf) ^ -4-1 <-t-l o bt) l-l > :3 O o < -4-> CO C! >H w @ w H H O ►-3 < u o C/2 ® - to O o (2) « -^^ "^"^ o o © -J3 ^© (Dg SI > < H O ® ^^ TO so X ' W" H-l < u UP nl O % m a, > - < H en o e •S - OJ ■<-> u N u o H CO K en U d C/3 u (A U I tn tn ffi *-• o fa CL, o o O I c en O K ® @ © o c I— ( '^ o fa o © I— I X - m M < CJ < y c o 05 H o XJ 4> 472 READINGS IN EVOLUTION, GENETICS, AND EUGENICS three other children one was insane and two weak. The children of Charles IX were both remarkably able. The daughter Catherine becomes the mother of a later succession of kings. Her son Charles X and his son Charles XI were rather mediocre; but Charles XI, with this fine stock behind him, married Ulrica Eleanor (7), granddaughter of Christian IV of Denmark, the most brilliant of all Danish sovereigns, and Charles XII, their son, is pronounced by Voltaire the most remarkable man who ever existed. Charles XII had no children: the succession passed to his sister's son, Adolph Frederick of Holstein- Gottorp, who married Louisa Ulrica, sister of Frederick the Great of Prussia. The result of this union of two great lines of hereditary ability was Gustavus III, a fit successor of Gustavus Vasa, Gustavus Adolphus, and Charles XII; he was " a prodigy of talents," statesman, poet, dramatist. CHAPTER XXXV HUMAN CONSERVATION^ HERBERT E. WALTER I. HOW MANKIND MAY BE IMPROVED There are two fundamental ways to bring about human better- ment, namely, by improving the individual and by improving the race. The first method consists in making the best of whatever heritage has been received by placing the individual in the most favorable environment and developing his capacities to the utmost through education. The second method consists in seeking a better heritage with which to begin the life of the individual. The first method is immediate and urgent for the present generation. The second method is concerned with ideals for the future, and consequently does not usually present so strong an appeal to the individual. The first is the method of euthenics, or the science of learning to live well. The second is eugenics, which Galton defines as '' the science of being well born." These two aspects of human betterment, however, are inseparable. Any hereditary characteristic must be regarded, not as an independent entity, but as a reaction between the germplas7n and its environment. The biologist who disregards the fields of educational endeavor and environmental influence, is equally at fault with the sociologist who fails sufficiently to realize the fundamental importance of the germ- plasm. Without euthenic opportunity the best of heritages would never fully come to its own. Without the eugenic foundation the best opportunity fails of accomplishment. The euthenic point of view, however, must not distract the attention now, for the present chapter is particularly concerned with the program of eugenics. 2. MORE FACTS NEEDED Since the point of attack in human heredity must be largely statistical, it is of the first importance to collect more facts. Our actual knowledge is confused with a mass of tradition and opinion, I From H. E. Waher, Genetics (copyright 1913). Used by special permission of the publishers, The Macmillan Company. 473 474 READINGS IN EVOLUTION, GENETICS, AND EUGENICS much of which rests upon questionable foundations. The great present need is to learn more facts; to sift the truth from error in what is already known; and to reduce all these data to workable scientific form. Much progress is being made in this direction, owing to the impetus given by the revival of Mendel's illuminating work, but as yet the science of eugenics is in its infancy. The most systematic and effective attempt in this country to collect reliable data concerning heredity in man has been initiated by the Eugenics Section of the American Breeders' Association under the secretaryship of Dr. C. B. Davenport. In 1910 the Eugenics Record Office, with a staff of expert field and office workers and an adequate equipment of fire-proof vaults, etc., for the preservation of records, was opened at Cold Spring Harbor, Long Island, New York, with Mr. H. H. Laughlin as superintendent. "The main work of this office is investigation into the laws of inheritance of traits in human beings and their application to eugenics. It proffers its services free of charge to persons seeking advice as to the consequences of pro- posed marriage matings. In a word, it is devoted to the advance- ment of the science and practice of eugenics." The publication of results from the Eugenics Record Office has already been begun. The Volta Bureau, founded about twenty-five years ago in Washington by Dr. Alexander Graham Bell, is collecting data with reference to deafness and has now systematically arranged particu- lars concerning the history of over 20,000 individuals. In England, also, the Galton Laboratory for Eugenics, founded in 1905, is system- atically collecting facts about human pedigrees and publishing the results in a compendious ''Treasury of Human Inheritance." Besides these special bureaus of investigation, innumerable facts about the inheritance of particular traits are being incidentally brought together and made available in various institutions and asylums throughout the world which are immediately concerned with the care of defectives of different types. It is in connection with such institu- tions for defectives that much of the most successful ''field work" of the Eugenics Section of the American Breeders' Association is being accomphshed in the United States. 3. FURTHER APPLICATION OF WHAT WE KNOW NECESSARY Human performance always lags behind human knowledge. Many persons who are fully aware of the right procedure do not put their knowledge into practice. It follows, therefore, that any pro- HUMAN COXSERVATION 475 gram of eugenics which does not grip the imagination of the common people in such a way as to become an effective part of their very lives is bound to remain largely an academic affair for Utopians to quarrel and theorize over. It is not enough to collect facts and work out an analysis and interpretation of them, for, important as this preliminary step is, it must be followed by a convincing campaign of education. The lives of the unborn do not force themselves upon the average man or woman with the same insistency as the lives already begun. In the midst of the overwhelming demands of the present, the appeal of posterity for better blood is vague and remote. If every individual regarded the germplasm he carries as a sacred trust, then it would be the part of an awakened eugenic conscience to restrain that germplasm when it is known to be defective or, when it is not defective, to hand it on to posterity with at least as much foresight as is exercised in breeding domestic animals and cultivated plants. The eugenic conscience is in need of development, and it is only when this becomes thoroughly aroused in the rank and file of society as well as among, the leaders, that a permanent and increasing better- ment of mankind can be expected. 4. THE RESTRICTION OF UNDESIRABLE GERMPLASM A negative way to bring about better blood in the world is to follow the clarion call of Davenport, and "dry up the streams that feed the torrent of defective and degenerate protoplasm." This may be partially accomplished, at least in America, by employing the following agencies: control of immigration; more discriminating marriage laws; a quickened eugenic sentiment; sexual segregation of defectives; and finally, drastic measures of asexualization or steriliza- tion when necessary. a) CONTROL OF IMMIGRATION The enforcement of immigration laws tends to debar from the United States not only many undesirable individuals, but also inci- dentally to keep out much potentially bad germplasm that . if admitted, might play havoc with future generations. For example, during the year of 1908, 65 idiots, 121 feeble-minded, 184 insane, 3,741 paupers, 2,900 individuals having contagious dis- eases, 53 tuberculous individuals, 136 criminals, and 124 prostitutes were caught in the sieve at Ellis Island alone and turned back from this country by the immigration officials. These 7,000 and more 476 READINGS IN EVOLUTION, GENETICS, AND EUGENICS individuals probably were the bearers of very little germplasm that we are nationally not better off without. Eugenically, the weak point in the present application of immi- gration laws is that criteria for exclusion are phenotypic in nature rather than genotypic, and consequently much bad germplasm comes through our gates hidden from the view of inspectors because the bearers are heterozygous, wearing a cloak of desirability over undesir- able traits. It is not enough to lift the eyelid of a prospective parent of Ameri- can citizens to discover whether he has some kind of an eye-disease or to count the contents of his purse to see if he can pay his own way. The official ought to know if eye-disease runs in the immigrant's family and whether he comes from a race of people which, through chronic shiftlessness or lack of initiative, have always carried light purses. In selecting horses for a stock-farm an expert horseman might rely to a considerable extent upon his judgment of horsbffesh based upon inspection alone, but the wise breeder does more than take the chances of an ordinary horse trader. He wants to be assur^ of the pedigree of his prospective stock. It is to be hoped that the time will come when we, as a nation, will rise above the hazardous methods of the horse trader in selecting from the foreign applicants who knock at our portals, and that we will exercise a more fundamental discrimination than such a haphazard method affords, by demanding a knowledge of the germplasm of these candidates for citizenship, as displayed in their pedigrees. This may possibly be accomplished by having trained inspectors located abroad in the communities from which our immigrants come, whose duty it shall be to look up the ancestry of prospective applicants and to stamp desirable ones with approval. The national expense of such a program of genealogical inspection would be far less than the maintenance of introduced defectives, in fact it would greatly decrease the number of defectives in the country. At the present time this country is spending over one hundred million dollars a year on defectives alone, and each year sees this amount increased. The United States Department of Agriculture already has field agents scouring every land for desirable animals and plants to intro- duce into this country, as well as stringent laws to prevent the importa- tion of dangerous weeds, parasites, and organisms of various kinds. Is the inspection and supervision of human blood less important ? HUMAN COXSi:R\ ATIUX 477 b) MORE DISCRIMINATING MARRIAGE LAWS Every people, including even the more primitive races, make customs or laws that tend to regulate marriage. Of these, the laws which relate to the eugenic aspect of marriage are the only ones that concern us in this connection. ''Marriage," says Davenport, "can be looked at from many points of view. In novels as the climax of human courtship; in law largely as two lines of property descent; in society, as fixing a certain status; but in eugenics, which considers its biological aspect, marriage is an experiment in breeding." Certain of the United States have laws forbidding the marriage of epileptics, the insane, habitual drunkards, paupers, idiots, feeble- minded, and those afflicted with venereal diseases. It would be well if such laws were not only more uniform and widespread, but also more rigidly enforced. It is quite true that marriage laws in themselves do not necessarilv control human reproduction, for illegitimacy is a factor that must always be reckoned with; nevertheless such laws do have an important influence in regulating marriage and consequent reproduction. Marriage laws may, however, sometimes bring about a deplor- able result eugenically, as in the case of forced marriage of sexual offenders in order to legalize the offense and ''save the woman's honor." To compel, under the guise of legality, two defective streams of germplasm to combine repeatedly and thereby result in defective offspring just because the unfortunate event happened once illegiti- mately, is fundamentally a mistake. Darwin says: "E.xcept in the case of man himself hardly any one is so ignorant as to allow his worst animals to breed." c) AN EDUCATED SENTIMENT A far more effective means of restricting bad germplasm than placing elaborate marriage laws upon our statute-books is to educate pubHc sentiment and to foster a popular eugenic conscience, in the absence of which the safeguards of the law must forever be largely without avail. Such a sentiment already generally exists to a large extent with respect to incest, and the marriage of persons as noticeably defective as idiots or those afflicted with insanity, and also in America with respect to miscegenation, but a cautious and intelligent e.xamination of the more obscure defective traits, exhibited in the somatoplasms of the various members of families in question, is largely an ideal of the 478 READINGS IN EVOLUTION, GENETICS, AND EUGENICS future. Under existing conditions non-eugenic considerations such as wealth, social position, etc., often enter into the preliminary negotia- tions of a marriage alliance, but an equally unromantic caution with reference to the physical, moral, and mental characters that make up the biological heritage of contracting parties is less usual. The scientific attitude is not necessarily opposed to the romantic way of looking at things. Science is simple ''organized common sense," and romance, that dispenses with this balance-wheel, although it may be entertaining and always exciting at first, is sure to be dis- appointing in the end. Marriages may be "made in heaven," but, as a matter of fact, children are born and have to be brought up on earth. It follows without saying that it will be much easier to stamp out bad germplasm when an educated sentiment becomes common among all people everywhere. d) SEGREGATION OF DEFECTIVES Persons with hereditary defects, such as epileptics, idiots, and certain criminals, who become wards of the state, should be segregated so that their germplasm may not escape to furnish additional burdens to society. "We have become so used to crime, disease and degener- acy that we take them for necessary evils. That they were in the world's ignorance, is granted. That they must remain so, is denied" (Davenport). "The great horde of defectives once in the world have the right to live and enjoy as best they may whatever freedom is compatible with the lives and freedom of other members of society," says Kellicott, but society had a right to protect itself against repetitions of hereditary blunders. There is one grave danger connected with the administration of our humane and commendable philanthropies toward the unfortunate, for it frequently happens that defectives are kept in institutions until they are sexually mature or are partly self-supporting, when they are liberated only to add to the burden of society by reproducing their like. Furthermore, if defectives of the same sort are collected together in the same institutions, unless sexual segregation is strictly main- tained, they may by the very circumstance of proximity tend to reproduce their kind just as defectives in any isolated community tend to multiply. David Starr Jordan cites the interesting case of cretinism which occurs in the valley of Aosta in northern Italy, to prove the wisdom HUMAN' COXSERVATIOX 479 of the sexual segregation of defectives. Cretinism is an hereditary defect connected with an abnormal development of the thyroid gland which results in a peculiar form of idiocy usually associated with goitre. *'In the city of Aosta the goitrous cretin has been for centuries an object of charity. The idiot has received generous support, while the poor farmer or laborer with brains and no goitre has had the severest of struggles. In the competition of life a premium has thus been placed on imbecility and disease. The cretin has mated with cretin, the goitre with goitre, and charity and religion have presided over the union. The result is that idiocy is multiplied and intensified. The cretin of Aosta has been developed as a new species of man. In fair weather the roads about the city are lined with these awful paupers — human beings with less intelligence than a goose, with less decency than the pig." Whymper, writing in 1880, further observes: "It is strange that self-interest does not lead the natives of Aosta to place their cretins under such restrictions as would prevent their illicit intercourse; and it is still more surprising to find the Catholic Church actually legalizing their marriage. There is something horribly grotesque in the idea of solemnizing the union of a brace of idiots, and, since it is well known that the disease is hereditary and develops in successive generations the fact that such marriages are sanctioned is scandalous and infamous.'' Since 1890 the cretins have been sexually segregated, and in 1910 Jordan reported that they were nearly all gone. e) DRASTIC MEASURES A fifth method of restricting undesirable germplasm in the case of confirmed criminals, idiots, imbeciles, and rapists may be mentioned, namely, the extreme treatment of either asexualization or vasectomy. The latter is a minor operation confined to the male which occupies only a few moments and requires at most only the application of a local anaesthetic, such as cocaine. There are no disturbing or even inconvenient after effects from this operation. It consists in removing a small section of each sperm duct, and is entirely effectual in prevent- ing subsequent parenthood. In the female the corresponding operation, which consists in removing a portion of each Fallopian tube, is much more severe, but not impracticable or dangerous. Eight states already have sterilization laws providing lor certain (^ses and ''could such a law be enforced in the whole United States, 480 READINGS IN EVOLUTION, GENETICS, AND EUGENICS less than four generations would eliminate nine tenths of the crime, insanity and sickness of the present generation in our land. Asylums, prisons and hospitals would decrease, and the problems of the unem- ployed, the indigent old and the hopelessly degenerate would cease to trouble civilization." 5. THE CONSERVATION OF DESIRABLE GERMPLASM Not only negatively by the restriction of undesirable germplasm, but also positively by the conservation of desirable germplasm, may the eugenic ideal be approached. It is possible that if some of the philanthropic endeavor now directed toward alleviating the condition of the unfit should be directed to enlarging the opportunity of the fit, greater good would result in the end. In breeding animals and plants the most notable advances have been made by isolating and developing the best, rather than by attempting to raise the standard of mediocrity through the elimination of the worst. One leader is worth a score of followers in any community, and the science of genetics surely gives to educators the hint that it is wiser to cultivate the exceptional pupil who is often left to take care of him- self than to expend all the energies of the instructor in forcing the indifferent or ordinary one up to a passing standard. The campaign for human betterment in the long run must do more than avoid mis- takes. It must become aggressive and take advantage of those human mutations or combinations of traits which appear in the exceptionally endowed. There are various ways in which this. improvement of society may be brought about. a) BY SUBSIDIZING THE FIT The following unconfirmed newspaper clipping illustrates the point of what is meant by subsidizing the fit so far as certain physical characteristics are concerned. "Berlin, Dec. 11, 1911. The Emperor is reported to be interested in a plan proposed by Professor Otto Hauser for the propagation of a fixed German type of humanity — a type which will be as fixed as the Jewish in its characteristics, if the suggestions of the professor can ever be carried out. The fixed type is to be produced as follows : — Only ' typical ' couples are to be allowed to mate. The man is to be not more than thirty years old, the woman not over twenty-eight, and each have a perfect health certificate. The man should be at least five feet seven inches tall; the woman not undear HUMAN COXSERVATIOX 481 five feet six inches. Neither the man nor the woman should have dark hair. Its tint may range from blonde to auburn. 'I'he eyes of the pair should be pure blue without any tint of brown. The complexion should be fair to ruddy without any suggestion of heaviness or ' beefi- ness.' The nose ought to be strong and narrow, the chin square and powerful, and the skull well developed at the back. The man and the woman must be of German descent and must bear a German name and speak the language of Germany. These 'mated couples' are to get a wedding gift of $125 and an additional grant for each child born. The couples may settle in the United States if they prefer." This reported attempt to establish a Prussian iyn[)e of ''Hauser blondes'' at least points the way to one sort of a positive eugenic method that might possibly be employed with respect to certain physical charac- teristics. It should be remembered, however, that the eugenic ideal is not by any means confined to physical traits alone. b) BY ENLARGING INDR^IDUAL OPPORTUNITY Much good human germplasm goes to waste through inetTective- ness on account of unfavorable environment or lack of a suitable opportunity to develop. Every agency which contributes toward increasing the opportunity of the individual to attain to a better development of his latent possibilities is in harmony wdth a thoroughly positive eugenic practice. Thus better schools, better homes, better living conditions, in short, all euthenic endeavor, directly serves the eugenic ideal by making the best out of whatever germinal equipment is present in man. C) BY PREVENTING GERMINAL WASTE Much good protoplasm fails to find expression in the form of off- spring because one or the other of possible parents is cut off either by preventable death or by social hindrances. To avoid such calamities is a part of the positive program of eugenics. I. Preventable death. — War, from the eugenic point of view, is the height of folly, since presumably the brave and the physically fit march away to fight, while in general the unqualified stay at home to reproduce the next generation. When a soldier dies on the battlefield or in the hospital, it is not alone a brave man who is cut off, but it is the termination of a probably desirable strain of germplasm. The Thirty Years' War in Germany cost 6,000,000 lives, while Naix)leon in his campaigns drained the best blood of France. 482 READINGS IN EVOLUTION, GENETICS, AND EUGENICS David Starr Jordan has presented the matter very clearly. He points out that the ''man with a hoe" among the European peasantry is not the result of centuries of oppression, as he has been pictured, but rather the dull progeny resulting from generations of the unfit who were left behind when the fit went off to war never to return. Benjamin Franklin, with characteristic wisdom, sums up the situation in the following epigram: "Wars are not paid for in war time; the bill comes later." 2. Social Hindrances. — There are many conditions of modern society which act non-eugenically. For instance, the increasing demands of professional life prolong the period necessary for preparation, which, with the ''cost of high living," tends toward late marriage. In this way much of the best germplasm is very often withheld from circulation until it is too late to be effective in providing for the succeeding generation. Certain occupations such as school-teaching and nursing by women are filled by the best blood obtainable, yet this blood is denied a direct part in molding posterity, since marriage is either forbidden or regarded as a serious handicap in such lines of work. Advertisements concerning "unincumbered help" and "childless apartments" tell their own deplorable tale. One of the darkest features of the dark ages from a eugenic stand- point was the enforced celibacy of the priesthood, since this resulted, as a rule, in withdrawing into monasteries and nunneries much of the best blood of the times, and this uneugenic custom still obtains in many quarters today. 6. \VH.O SHALL SIT IN JUDGMENT? In the practical application of a program of eugenics there are many difficulties, for who is qualified to sit in judgment and separate the fit from the unfit ? There are certain strongly marked characteristics in mankind which are plainly good or bad, but the principle of the independence of unit characters demonstrates that no person is wholly good or wholly bad. Shall we then throw away the whole bundle of sticks because it contains a few poor or crooked ones ? The list of weakhng babies, for instance, who were apparently physically unfit and hardly worth raising upon first judgment, but who afterwards became powerful factors in the world's progress, is a notable one and includes the names of Calvin, Newton, Heine, Voltaire, Herbert Spencer, and Robert Louis Stevenson. HUMAN CONSERVATION 483 Or, take another example. Elizal^cth Tuttle, the grandmother of Jonathan Edwards whose remarkable progeny was referred to in a preceding chapter, is described as a "woman of great beauty, of tall and commanding appearance, striking carriage, of strong will, extreme intellectual vigor and mental grasp akin to rapacity," but with an extraordinary deficiency in moral sense. She was divorced from her husband "on the ground of adultery and other immoralities The evil trait was in the blood, for one of her sisters murdered her own son and a brother murdered his own sister." That Jonathan Edwards owed his remarkable qualities largely to his grandmother rather than to his grandfather is shown by the fact that Richard Edwards, the grandfather, married again after his divorce and had five sons and one daughter, but none of their numerous progeny " rose above mediocrity, and their descendants gained no abiding reputa- tion." As shown by subsequent events, it would have been a great eugenic mistake to have deprived the world of Elizabeth Tuttle's germp^asm, although it would have been easy to find judges to con- demn her. Dr. C. V. Chapin recently said with reference to the eugenic regulation of marriage by physician's certificate: "The causes of heredity are many and very conflicting. The subject is a difficult one, and I for one would hesitate to say, in a great many cases where I have a pretty good knowledge of the family, where marriage would, or would not, be desirable." Desirability and undesirability must always be regarded as rela- tive terms more or less indefinable. In attempting to define them, it makes a great difference whether the interested party holds to a puritan or a cavalier standard. To show how far human judgment may err as well as how radically human opinion changes, there were in England, as recently as 1819, 233 crimes punishable by death accord- ing to law. One needs only to recall the days of the Spanish Inquisition or of the Salem witchcraft persecution to realize what fearful blunders human judgment is capable of, but it is unlikely that the world will ever see another great religious inquisition, or that in applying to man the newly found laws of heredity there will ever be undertaken an equally deplorable eugenic inquisition. It is quite apparent, finally, that although great caution and broadness of vision must be exercised in bringing about the fulfilment of the highest eugenic ideals, nevertheless in this direction lies the future path of human achievement. CHAPTER XXXVI EUGENICS AND EUTHENICS^ PAUL POPENOE AND ROSWELL H. JOHNSON I Emphasis has been given, in several of the foregoing chapters, to the desirabihty of inheriting a good constitution and a high degree of vigor and disease-resistance. It has been asserted that no measures of hygiene and sanitation can take the place of such inheritance. It is now desirable to ascertain the limits within which good inheritance is effective, and this may be conveniently done by a study of the lives of a group of people who inherited exceptionally strong physical con- stitutions. The people referred to are taken from a collection of histories of long life made by the Genealogical Record Office of Washington. One' hundred individuals were picked out at random, each of whom had died at the age of ninety or more, and with the record of each indi- vidual were placed those of all his brothers and sisters. Any family was rejected in which there was a record of wholly accidental death (e.g., families of which a member had been killed in the Civil War). The loo families, or more correctly fraternities or sibships, were classified by the number of children per fraternity, as follows : Number of raternities Number of Children per Fraternity Total Number of Children in Group I 2 2 II 3 33 8 4 32 17 5 85 13 6 78 14 7 98 9 8 72 II 9 99 10 10 100 3 II 33 2 12 24 V I 13 13 TOO 669 I From P. Popenoe and R. H. Johnson, Applied Eugenics (copyright 1918). Used by special permission of the publishers, The Macmillan Company. 484 EUGENICS AND EUTHENICS 485 The average at death of these 669 persons was 64.7 years. The child mortahty (first 4 years of Ufe) was 7.5 per cent of the total mortality, 69 families showing no deaths of that kind. The group is as a whole, therefore, long-lived. The problem was to measure the resemblance between brothers and sisters in respect of longevity — to find whether knowledge of the age at which one died would justify a prediction as to the age at death of the others — or technically, it was to measure the fraternal correlation of longevity. A zero coefficient here would show that there is no association; that from the age at which one dies, nothing whatever can be predicted as to the age at which the others will die. Since it is known that heredity is a large factor in longevity, such a finding would mean that all deaths were due to some accident which made the inheritance of no account. In an ordinary population it has been found that the age at death of brothers and sisters furnishes a coefticient of correlation of the order of .3, which shows that heredity does determine the age at which one shall die to considerable extent, but not absolutely.^ The index of correlation^ between the lengths of life within the fraternity in these 100 selected families, furnished a coefficient of — .0163 ± .0672, practically zero. In other words, if the age is known at which a member of one of these families died, whether it be one month or 100 years, nothing whatever can be predicted about the age at which his brothers and sisters died. ' Mary Beeton, and Karl Pearson, Biometrika, I, p. 60. The actual correlation varies with the age and sex: the following are the results: COLLATERAL INHERITANCE Elder adult brother and younger adult brother 2290^ 0194 Adult brother and adult brother 2853=*= .0196 Minor brother and minor brother 1026=*= .0294 Adult brother and minor brother — .0262^ .0246 Elder adult sister and younger adult sister 3464=*= .0183 Adult sister and adult sister 3322± .0185 Minor sister and minor sister 1748='= .0307 Adult sister and minor sister — .0260=^ .0291 Adult brother and adult sister 2319=*= .0145 Minor brother and minor sister i43S=*= 0251 Adult brother and minor sister — .oo02± .0349 Adult sister and minor brother — .0274=*= .0238 2 The method used is the ingenious one devised by J. Arthur Harris (Biometrika, IX, p. 461). The probable error is based on ?i= 100. 486 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Remembering that longevity is in general inherited, and that it is found in the families of all the people of this study (since one in each fraternity lived to be 90 or over) how is one to interpret this zero coefficient? Evidently it means that although these people had inherited a high degree of longevity, their deaths were brought about by causes which prevented the heredity from getting full expression. As far as hereditary potentialities are concerned, it can be said that all their deaths were due to accident, using that word in a broad sense to include all non-selective deaths by disease. If they had all been able to get the full benefit of their heredity, it would appear that each of these persons might have lived to 90 or more, as did the one in each family who was recorded by the Genealogical Record Office. Geneti- cally, these other deaths may be spoken of as premature. In an ordinary population, the age of death is determined to the extent of probably 50 per cent by heredity. In this selected long- lived population, heredity appears not to be responsible in any meas- urable degree whatsoever for the differences in age at death. The result may be expressed in another, and perhaps more striking, way. Of the 669 individuals studied, a hundred — namely, one child in each family — lived beyond 90; and there were a few others who did. But some 550 of the group, though they had inherited the potentiality of reaching the average age of 90, actually died somewhere around 60; they failed by at least one-third to live up to the promise of their inheritance. If we were to generalize from this single case, we would have to say that five-sixths of the population does not make the most of its physical inheritance. This is certainly a fact that discourages fatalistic optimism. The man who tells himself that, because of his magnificent inherited constitution, he can safely take any risk, is pretty sure to take too many risks and meet with a non-selective — i.e., genetically, a pre- mature — death, when he might in the nature of things have lived almost a generation longer. It should be remarked that most of the members of this group seem to have lived in a hard environment. They appear to belong predominantly to the lower strata of society; many of them are immi- grants and only a very few of them, to judge by a cursory inspection of the records, possessed more than moderate means. This necessi- tated a frugal and industrious life which in many ways was favorable to longevity but which may often have led to overexposure, overwork, lack of proper medical treatment, or other causes of a non-selective EUGENICS AND EUTHEMCS 487 death. We would not push the conclusion too far, but we can not doubt that this investigation shows the folly of ignoring the environ- ment — shows that the best inherited constitution must have a fair chance. And what has here been found for a physical character, would probably hold good in even greater degree for a mental charac- ter. All that man inherits is the capacity to develop along a certain line under the influence of proper stimuli, food and exercise. The object of eugenics is to see that the inherent capacity is there. Given that, the educational system is next needed to furnish the stimuli. The consistent eugenist is therefore an ardent euthenist. He not only works for a better human stock but, because he does not want to see his efforts wasted, he always works to provide the best possible envi- ronment for this better stock. In so far, then, as euthenics is actually providing man with more favorable surroundings — not with ostensibly more favorable sur- roundings which, in reality, are unfavorable — there can be no antago- nism between it and eugenics. Eugenics is, in fact, a prerequisite of euthenics, for it is only the capable and altruistic man who can con- tribute to social progress; and such a man can only be produced through eugenics. Eugenic fatalism, a blind faith in the omnipotence of heredity regardless of the surroundings in which it is placed, has been shown by the study of long-lived families to be unjustified. It was found that even those who inherited exceptional longevity usually did not live as long as their inheritance gave them the right to expect. If they had had more euthenics, they should have lived longer. But this illustration certainly gives no ground for a belief that euthenics is sufficient to prolong one's life beyond the inherited limit. A study of these long-lived families from another point of view will reveal that heredity is the primary factor and that good environment, euthenics, is the secondary one. For this purpose we augment the 100 families of the preceding section by the addition of 240 more families like them, and we examine each family history to find how many of the children died before com- pleting the fourth year of Hfe. The data are summarized in the tal)le on page 488. The addition of the new families (which were not subjected to any different selection than the first 100) has brought down the child mortality rate. For the first 100, it was found to be 7.5 per cent. If in the above table the number of child deaths, 119, be divided by the 488 READINGS IN EVOLUTION, GENETICS, AND EUGENICS total number of children represented, 2,259, the child mortality rate for this population is found to be 5.27 per cent or 53 per 1,000. The smallness of this figure may be seen by comparison with the statistics of the registration area, U.S. Census of 1880, when the child mortality (0-4 years) was 400 per thousand, as calculated by Alexan- der Graham Bell. A mortality of 53 for the first four years of life is smaller than any district known in the United States, even to-day, can show for the first year of life alone. If any city could bring the deaths of babies during their first twelve months down to 53 per 1,000, it would think it had achieved the impossible; but here is a population CHILD MORTALITY IN FAMILIES OF LONG-LIVED STOCK, GENEALOGICAL RECORD OFFICE DATA Size of "amily Number of Families Investigated Number of Families Showing Deaths under Five Years Total Nun of Deaths I child 6 2 children 6 3 38 4 s 4 40 6 7 5 38 4 4 6 44 12 13 7 34 8 II 8 46 13 18 9 31 14 20 10 27 14 14 II 13 6 9 12 13 9 16 13 I 14 2 17 I I 2 340 91 119 in which 53 per 1,000 covers the deaths, not only of the fatal first 12 months, but of the following three years in addition. Now this population with an unprecedentedly low rate of child mortality is not one which had had the benefit of any Baby Saving Campaign, nor even the knowledge of modern science. Its mothers were mostly poor, many of them ignorant; they lived frequently under conditions of hardship ; they were peasants and pioneers. Their babies grew up without doctors, without pasteurized milk, without ice, without many sanitary precautions, usually on rough food. But they had one advantage which no amount of applied science can give after birth — namely, good heredity. They had inherited exceptionally good constitutions. EUGENICS AND EUTHENICS 489 It is not by accident that inherited lonp;evily in a family is associ- ated with low mortality of its children. The connection between the two facts was first discovered by Alary Beeton and Karl Pearson in their pioneer work on the inheritance of duration of life. They found that high infant mortality was associated with early death of parents, while the offspring of long-lived parents showed few deaths in child- hood. The correlation of the two facts was quite regular, as will be evident from a glance at the following tables prepared by A. Ploetz: LENGTH OF LIFE OF MOTHERS AND CHILI) MORTALITY OF THEIR DAUGHTERS (ENGLISH QUAKER F.VMILIES, DATA OF BEETON AND PEARSON, ARRANGED HV PLOETZ) All /\ges 1,846 511 27.7 Number of daughters Number of them who died in first five years Per cent of daughters who died Year of Life in Which Mothers Died to 38 39-53 54-68 69-83 84 up 234 304 395 666 247 122 114 118 131 26 52.1 37-5 29.9 19.7 10.5 LENGTH OF LIFE OF FATHERS AND CHILD IMORTALITV OF THEIR DAUGHTERS Number of daughters Number of them who died in first five years Per cent of daughters who died. . . Year of Life in Which Fathers Died to 38 39-53 54-68 69-83 84 up 105 51 48.6 284 98 34-5 585 156 26.7 797 177 22.2 236 40 17.0 At All Ages 2,009 522 26.0 To save space, we do not show the relation between parent and son; it is similar to that of parent and daughter which is shown in the preceding tables. In making comparison with the 340 families from the Genealogical Record Office, above studied, it must be noted that Dr. Ploetz's tables include one. year longer in the period of child mor- tality, being computed for the first five years of life instead of the first four. His percentages would therefore be somewhat lower if com- puted on the basis used in the American work. These various data demonstrate the existence of a considerable correlation between short life {bracJiybioty, Karl Pearson calls it) in parent and short life in offspring. Not only is the tendency to live long inherited, but the tendency not to live long is likewise inherited. 490 READINGS IN EVOLUTION, GENETICS, AND EUGENICS But perhaps the reader may think they show nothing of the sort. He may fancy that the early death of a parent left the child without sufficient care, and that neglect, poverty, or some other factor of euthenics brought about the child's death. Perhaps it lacked a mother's loving attention, or perhaps the father's death removed the wage-earner of the family and the child thenceforth lacked the necessities of life. Dr. Ploetz has pointed out that this objection is not valid, because the influence of the parent's death is seen to hold good even to the point where the child was too old to require any assistance. If the facts applied only to cases of early death, the supposed objection might be weighty, but the correlation exists from one end of the age- scale to the other. It is not credible that a child is going to be deprived of any necessary maternal care when its mother dies at the age of 69 ; the child herself was probably married long before the death of the mother. Nor is it credible that the death of the father takes bread from the child's mouth, leaving it to starve to death in the absence of a pension for widowed mothers, if the father died at 83, when the ^' child " herself was getting to be an old woman. The early death of a parent may occasionally bring about the child's death for a reason wholly unconnected with heredity, but the facts just pointed out show that such cases are exceptional. The steady association of the child death- rate and parent death-rate at all ages demonstrates that heredity is a common cause. But the reader may suspect another fallacy. The cause of this association is really environmental, he may think, and the same poverty or squalor which causes the child to die early may cause the parent to die early. They may both be of healthy, long-lived stock, but forced to live in a pestiferous slum which cuts both of them off prematurely and thereby creates a spurious correlation in the statistics. We can dispose of this objection most effectively by bringing in new evidence. It will probably be admitted that in the royal families of Europe, the environment is as good as knowledge and wealth can make it. No child dies for lack of plenty of food and the best medical care, even if his father or mother died young. And the members of this caste are not exposed to any such unsanitary conditions, or such economic pressure as could possibly cause both parent and child to die prematurely. If the association between longevity of parent and child mortality holds for the royal families of Europe and their princely EUGENICS AXD EUTHEXICS 491 relatives, it can hardly be regarded as anything but the effect of heredity — of the inheritance of a certain type of constitution. Dr. Ploetz studied the deaths of 3, 2 10 children in European royalty, from this viewpoint. The following table shows the relation between father and child : LENGTH OF LIFE OF FATPIERS AND CHILD MORTALITY' OF THEIR CHILDREN IN ROYAL AND PRLXCELY FAMILIES (PLOETZ DATA) Year of Life in Which Fathers Died At All Ages 16-25 23 12 52.2 26-35 36-45 46-55 56-65 66-75 1 76-85 86 up Number of children 90 29 32.2 367 115 31-3 545 171 31-4 725 200 27.6 983 254 25.8 444 105 23.6 33 I 3c 3210 887 27.6 Number who died in first five years. Per cent who died Allowing for the smallness of some of the groups, it is evident that the amount of correlation is about the same here as among the English Quakers of the Beeton-Pearson investigation, whose mortality was shown in the two preceding tables. In the healthiest group from the royal families — the cases in which the father lived to old age — the amount of child mortality is about the same as that of the Hyde family in America, which Alexander Graham Bell has studied — namely, somewhere around 250 per 1,000. One may infer that the royal families are rather below par in soundness of constitution. All these studies agree perfectly in showing that the amount of child mortahty is determined primarily by the physical constitution of the parents, as measured by their longevity. In the light of these facts, the nature of the extraordinarily low child mortality shown in the 340 families from the Genealogical Record Office, with which we began the study of this point, can hardly be misunderstood. These families have the best inherited constitution possible and the other studies cited would make us certain of finding a low child mortality among them, even if we had not directly investigated the facts. If the interpretation which we have given is correct, the conclusion is inevitable that child mortality is primarily a problem of eugenics, and that all other factors are secondary. There is found to be no warrant for the statement so often repeated in one form or another, that " the fundamental cause of the excessive rate of infant mortality in industrial communities is poverty, inadequate incomes, and low standards of living." Royalty and its princely relatives arc not 492 READINGS IN EVOLUTION, GENETICS, AND EUGENICS characterized by a low standard of living, and yet the child mortality among them is very high — somewhere around 400 per 1,000 in cases where a parent died young. If poverty is responsible in the one case, it must be in the other — which is absurd. Or else the logical absurdity is involved of inventing one cause to explain an effect today and a wholly different cause to explain the same effect tomorrow. This is unjustifiable in any case, and it is particularly so when the single cause that explains both cases is so evident. If weak heredity causes high mortality in the royal families, why, similarly, cannot weak heredity cause high infant mortality in the industrial communities ? We believe it does account for much of it, and that the inadequate income and low standard of living are largely the consequence of inferior heredity, mental as well as physical. The parents in the Genealogical Record Ofiice files had, many of them, inadequate incomes and low standards of living under frontier conditions, but their children grew up while those of the royal families were dying in spite of every attention that wealth could command and science could furnish. If the infant mortality problem is to be solved on the basis of knowledge and reason, it must be recognized that sanitation and hygiene cannot take the place of eugenics any more than eugenics can dispense with sanitation and hygiene. It must be recognized that the death-rate in childhood is largely selected, and that the most effective way to cut it down is to endow the children with better constitutions. This cannot be done solely by any euthenic cam- paign; it cannot be done by swatting the fly, abolishing the mid-wife, sterilizing the milk, nor by any of the other panaceas sometimes proposed. But, it may be objected, this discussion ignores the actual facts. Statistics show that infant mortality campaigns have consistently produced reductions in the death-rate. The figures for New York, which could be matched in dozens of other cities, show that the num- ber of deaths per 1,000 births, in the first year of life, has steadily declined since a determined campaign to ''Save the Babies" was started: 1902 181 1909 1 29 1903 152 1910 125 1904 162 1911 112 1905 159 1912 105 1906 153 1913 102 1907 144 1914 95 1908 128 EUGENICS AND EUTHENICS 493 To one who cannot see beyond the immediate consequences of an action, such figures as the above indeed give quite a different idea of the effects of an infant mortaUty campaign, than that which we have just tried to create. And it is a great misfortune that euthenics so often fails to look beyond the immecHate effect, fails to see what may happen next year, or 10 years from now, or in the next generation. We admit that it is possible to keep a lot of children alive who would otherwise have died in the ffrst few months of life. It is being done, as the New York figures, and pages of others that could be cited, prove. The ultimate result is twofold: 1. Some of those who are doomed by heredity to a selective death, but are kept alive through the first year, die in the second or third or fourth year. They must die sooner or later; they have not inherited sufficient resistance to survive more than a limited time. If they are by a great effort carried through the first year, it is only to die in the next. This is a statement which we have nowhere observed in the propaganda of the infant mortality movement; and it is perhaps a disconcerting one. It can only be proved by refined statistical methods, but several independent determinations by the English biometricians leave no doubt as to the fact. This work of Karl Pearson, E. C. Snow, and Ethel M. Elderton, was cited in our chapter on natural selection; the reader will recall how they showed that nature is weeding out the weaklings, and in proportion to the strin- gency with which she weeds them out at the start, there are fewer weaklings left to die in succeeding years. To put the facts in the form of a truism, part of the children born in any district in a given year are doomed by heredity to an early death; and if they die in one year they will not be alive to die in the succeeding year, and vice versa. Of course there are in addition infant deaths which are not selective and which if prevented would leave the infant with as good chance as any to live. In the light of these researches, we are forced to conclude that baby-saving campaigns accomplish less than is thought; that the supposed gain is to some extent temporary and illusory. 2. There is still another consequence. If the gain is by great exertions made more than temporary; if the baby who would other- wise have died in the first months is brought to adult life and repro- duction, it means in many cases the dissemination of another strain of weak heredity, which natural selection would have cut off ruthlessly 494 READINGS IN EVOLUTION, GENETICS, AND EUGENICS in the interests of race betterment. In so far, then, as the infant mortahty movement is not futile it is, from a strict biological view- point, often detrimental to the future of the race. Do we then discourage all attempts to save the babies ? Do we leave them all to natural selection ? Do we adopt the ''better dead" gospel ? Unqualifiedly, no! The sacrifice of the finer human feelings, which would accompany any such course, would be a greater loss to the race than is the eugenic loss from the perpetuation of weak strains of heredity. The abolition of altruistic and humanitarian sentiment for the purpose of race betterment would ultimately defeat its own end by making race betterment impossible. But race betterment will also be impossible unless a clear distinc- tion is made between measures that really mean race betterment of a fundamental and permanent nature, and measures which do not. We have chosen the Infant Mortality Movement for analysis in this chapter because it is an excellent example of the kind of social better- ment which is taken for granted, by most of its proponents, to be a fundamental piece of race betterment; but which, as a fact, often means race impairment. No matter how abundant and urgent are the reasons for continuing to reduce infant mortality wherever pos- sible, it is dangerous to close the eyes to the fact that the gain from it is of a kind that must be paid for in other ways; that to carry on the movement without adding eugenics to it will be a short-sighted policy, which increases the present happiness of the world at the cost of diminishing the happiness of posterity through the perpetuation of inferior strains. While some euthenic measures are eugenically evils, even if necessary ones, it must not be inferred that all euthenic measures are dysgenic. Many of them, such as the economic and social changes we have suggested in earlier chapters, are an important part of eugenics. Every euthenic measure should be scrutinized from the evolutionary standpoint; if it is eugenic as well as euthenic, it should be whole- heartedly favored; if it is dysgenic but euthenic it should be con- demned or adopted, according to whether or not the gain in all ways from its operation will exceed the damage. In general, euthenics, when not accompanied by some form of selection (i.e., eugenics) ultimately defeats its own end. If it is accom- panied by rational selection, it can usually be indorsed. Eugenics, EUGENICS AND EUTHENlCS 495 on the other hand, is hkewise inadequate unless accompanied by constant improvement in the surroundings; and its advocates must demand euthenics as an accompaniment of selection, in order that the opportunity for getting a fair selection may be as free as possible. If the euthenist likewise takes pains not to ignore the existence of the racial factor, then the two schools are standing on the same ground, and it is merely a matter of taste or opportunity, whether one empha- sizes one side or the other. Each of the two factions, sometimes thought to be opposing, will be seen to be getting the same end result, namely, human progress. Not only are the two schools working for the same end, but each must depend in still another way upon the other, in order to make headway. The eugenist cannot see his measures put into effect except through changes in law and custom — i.e., euthenic changes. He must and does appeal to euthenics to secure action. The social reformer, on the other hand, cannot see any improvements made in civilization except through the discoveries and inventions of some citizens who are inherently superior in abihty. He in turn must depend on eugenics for every advance that is made. It may make the situation clearer to state it in the customary terms of biological philosophy. Selection does not necessarily result in progressive evolution. It merely brings about the adaptation of a species or a group to a given environment. The tapeworm is the stock example. In human evolution, the nature of this environment will determine whether adaptation to it means progress or retro- gression, whether it leaves a race happier and more productive, or the reverse. All racial progress, or eugenics, therefore, depends on the creation of a good environment, and the fitting of the race to that environment. Every improvement in the en\dronment should bring about a corresponding biological adaptation. The two factors in evolution must go side by side, if the race is to progress in what the human mind considers the direction of advancement. In this sense, euthenics and eugenics bear the same relation to human progress as a man's two legs do to his locomotion. Social workers in purely euthenic fields have frequently failed to remember this progress of adaptation, in their efforts to change the environment. Eugenists, in centering their attention on adaptation, have sometimes paid too little attention to the kind of environment to which the race was being adapted. The present book holds tliat the 496 READINGS IN EVOLUTION, GENETICS, AND EUGENICS second factor is just as important as the first, for racial progress; that one leg is just as important as the other, to a pedestrian. Its only con- flict with euthenics appertains to such euthenic measures as impair the adaptability of the race to the better environment they are trying to make. Some supposedly euthenic measures opposed by eugenics are not truly euthenic, as for instance the limitation of a superior family in order that all may get a college education. For these spurious euthenic measures, something truly euthenic should be substituted. Measures which show a real conflict may be typified by the infant mortality movement. There can be no doubt but that sanitation and hygiene, prenatal care and inteUigent treatment of mothers and babies, are truly euthenic and desirable. At the same time, as has been shown, these euthenic measures result in the survival of inferior children, who directly or through their posterity will be a drag on the race. Euthenic measures of this type should be accompanied by counterbalancing measures of a more eugenic character. Barring these two types, euthenics forms a necessary concomitant of the eugenic program; and, as we have tried to emphasize, eugenics is likewise necessary to the complete success of every euthenic program. How foohsh, then, is antagonism between the two forces! Both are working toward the same end of human betterment, and neither can succeed without the other. When either attempts to eliminate the other from its work, it ceases to advance toward its goal. In which camp one works is largely a matter of taste. If on a road there is a gradient to be leveled, it will be brought down most quickly by two parties of workmen, one cutting away at the top, and the other filling in the bottom. For the two parties to indulge in mutual scorn and recrimination would be no more absurd than for eugenics and euthenics to be put in opposition to each other. The only reason they have been in opposition is because some of the workers did not clearly understand the nature of their work. With the dissemination of a knowledge of biology, this ground of antagonism will disappear. chapi1':r xxxmi the promise of race culture' CALEB WILLIAMS S^VLEEBY Tlie best is yet to be. In its form of what we have called negative eugenics, the practice of our principle would assuredly reduce to an incalculable extent the amount of human defect, mental and physical, which each generation now exhibits. This alone, as has been said, would be far more than sufficient to justify us. A world without hereditary disease of mind and body would alone warrant the hint of Ruskin that posterity may some day look back upon us with " incredulous disdain." Yet, assum- ing that this could be accomplished, as it will be accomplished, what more is to be hoped for ? Must race-culture cease merely when it has raised the average of the community by reducing to a minimum the proportion of those who are thus grossly defective in mind or body ? Such disease apart, are we to be content, must we be content, with the present level of mediocrity in respect of intelligence and temper and moral sentiment ? Can we anticipate a London in which the present ratio of musical comedy to great opera will be reversed, in which the works of Mr. George Meredith will sell in hundreds of thousands, whilst some of our popular novelists will have to find other means of earning a living ? Can we make for a critical democracy which no political party can fool, and which will choose its best to govern it ? Yet more, can we undertake, now or hereafter, to provide every generation with its own Shakespeare and Beethoven and Tintoretto and Newton ? What, in a word, is the promise of positive eugenics? It is to this aspect of the question that Mr. Galton has mainly directed himself. Indeed he was led to formulate the princi- ples and ideals of the new science by his study of hereditary genius some four decades ago. Let us now attempt to answer some of these questions. The production of genius. — And first as to the production of genius. It is this, perhaps, that has been the main butt of the jesters who pass for philosophers with some of us today. It may be said ^ From C. W. Saleeby, Parenthood and Race Culture (copyright 1909). Used by special permission of the publishers, MolTat, Yard, and Company. 497 498 READINGS IN EVOLUTION, GENETICS, AND EUGENICS at once that neither Mr. Galton nor any other responsible person has ever asserted that we can produce genius at will. The difficulties in the way of such a project — at present — are almost innumerable. One or two may be cited. In the first place, there is the cardinal — but by no means univer- sal — difficulty that the genius is too commonly so occupied with the development and expansion of his own individuality that he has little time or energy for the purposes of the race. This, of course, is an example of Spencer's great generalization as to the antagonism or inverse ratio between individuation and genesis. Again, there is the generalization of heredity formulated by Mr. Galton, and named by him the law of regression towards mediocrity. It asserts that the children of those who are above or below the mean of a race, tend to return towards that mean. The children of the born criminal will be probably somewhat less criminal in tendency than he, though more criminal than the average citizen. The children of the man of genius, if he has any, will probably be nearer mediocrity than he, though on the average possessing greater talent than the average citizen. It is thus not in the nature of sheer genius to reproduce on its own level. It is only the critics who are totally ignorant of the elemen- tary facts of heredity that attribute to the eugenist an expectation of which no one knows the absurdity so well as he does. On the other hand, it is impossible to question that the hereditary transmission of genius or great talent does occur. One may cite at random such cases as that of the Bach family, Thomas and Matthew Arnold, James and John Stuart Mill; and the reader who is inclined to believe that there is no law or likelihood in this matter, must certainly make himself acquainted with Mr. Galton's Hereditary Genius, and with such a paper as that which he printed in Sociological Papers, 1904, furnishing an "index to achievements of near kinsfolk of some of the Fellows of the Royal Society." There is, of course, the obvious fallacy involved in the possibility that not heredity but environment was really responsible for many of these cases. It must have been a great thing to have such a father as James Mill. But it would be equally idle to imagine that the evidence can be dismissed with this criticism. A Matthew Arnold, a John Stuart Mill, could not be manufactured out of any chance material by an ideal education continued for a thousand years. The transmission of genius. — One single instance of the trans- mission of genius or great talent in a family may be cited. We shall THE PROMISE OF RACE CULTURE 499 take the family which produced Charles Darwin, the discoverer of the fundamental principle of eugenics, and his first cousin, Francis Galton. Darwin's grandfather was Erasmus Darwin, physician, poet and philosopher, and independent expounder of the doctrine of organic evolution. Darwin's father was a distinguished physician, described by his son as ''the wisest man I ever knew." Darwin's maternal grandfather was Josiah Wedgwood, the famous founder of the pottery works. Amongst his first cousins is Mr. Francis Galton. He has five living sons, each a man of great distinction, including Mr. Francis Darwin and Sir George Darwin, both of them original thinkers, honored by the presidency of the British Association. No one will put such a case as this down to pure chance or to the influence of environment alone. This is evidently, like many others, a greatly distinguished stock. The worth of such families to a nation is wholly beyond any one's powers of estimation. W'liat if Erasmus Darwin had never married! No student of human heredity can doubt that, however limited our immediate hopes, facts such as those alluded to furnish promise of great things for the future. But let us turn now from genius to what we usually call talent. The production of talent. — There can be no question that amongst the promises, of race-culture is the possibility of breeding such things as talent and the mental energy upon which talent so largely depends. In the Inquiries into Human Faculty, Mr. Galton shows the remark- able extent to which energy or the capacity for labor underlies intellec- tual achievement. He says, of energy: ''It is consistent with all the robust virtues, and makes a large practice of them possible. It is the measure of fullness of life ; the more energy the more abundance of it; no energy at all is death; idiots are feeble and listless. In the enquiries I made on the antecedents of men of science no points came out more strongly than that the leaders of scientific thought were generally gifted with remarkable energy, and that they had inherited the gift of it from their parents and grand- parents It m.aybe objected that if the race were too healthy and energetic there would be insufficient call for the exercise of the jiilying and self-denying virtues, and the character of men would grow harder in consequence. But it does not seem reasonable to preserve sickly breeds for the sole purpose of tending them, as the breed of foxes is preserved solely for sport and its attendant advantages. There is little fear that misery will ever cease from the land, or that the 500 READINGS IN EVOLUTION, GENETICS, AND EUGENICS compassionate will fail to find objects for their compassion; but at present the supply vastly exceeds the demand ; the land is over-stocked and over-burdened with the listless and the incapable. In any scheme of eugenics, energy is the most important quality to favor; it is, as we have seen, the basis of living action, and it is eminently transmissible by descent." Need it be pointed out that any political system which ceases to favor or actively disfavors energy, making it as profitable to be lazy as to be active, is antieugenic, and must inevitably lead to disaster ? That, however, by the way. Our present point is that eugenics can reasonably promise, when its principles are recognized, to multiply the human and diminish the vegetable type in the community. In so doing, it will greatly further the production of talent, and therefore of that traditional or acquired progress which men of talent and genius create. Such a result will also further, though indirectly, the production of genius itself. For, as Mr. Galton points out, "men of an order of ability which is now very rare, would become more frequent, because the level out of which they rose would itself have risen." This is by no means the only fashion in which an effective and practicable race-culture would serve genius, and I shall not be blamed for considering this matter further by any reader who realizes, however faintly, what the man of genius is worth to the world. If it were shown possible to establish such social conditions that genius could never flower in them, we should realize that their establishment would mean the putting of an end to progress and the blasting of all the highest hopes of the highest of all ages. The immediate need of this age, as of all ages, is perhaps not so much the birth of babies capable of developing into men and women of genius, as the full exploitation of the possibilities of genius with which, as I fancy, every generation on the average is about as well endowed as any other. There is, of course, the popular doctrine that there are no mute inglorious Miltons, that "genius will out," and that therefore if it does not appear, it is not there to appear. In expressing the com- pelling power of genius in many cases this doctrine is not without truth. Yet history abounds in instances where genius has been de- stroyed by environment — and we can only guess how many more instances there are of which history has no record. To take the single case of musical genius, it is a lamentable thought that there may be those now living whose natural endowments, in a favorable environ- ment, would have enabled them to write symphonies fit to place THE PROMISE OF RACE CULTURi: 501 beside Beethoven's, but whom some environmental factors — conven- tional, economic, educational, or what not — have silenced; or worse, have persuaded to write such sterile nullities as need not here be instanced. There is surely no waste in all this wasteful world so lamentable as this waste of genius. If, then, anyone could devise for us a means by which the genius, potentially existing at any time, were realized, he would have per- formed in effect a service equivalent to that of which eugenics rei)udi- ates the present possibility — the actual creation of genius. But if we consider what the conditions are which cause the waste of genius, we realize at once that they mainly inhere in the level of the human environment of the priceless potentiality in question. As we noted elsewhere, in an age like that of Pericles genius springs up on all hands. It is encouraged and welcomed because the average level of the human environment in which it finds itself is so high. But if eugenics can raise the average level of intelligence, in so doing not merely does it render more likely, as Mr. Galton points out, the production of men of the highest abiHty, but it provides those conditions in which men of genius, now swamped, can swim. We could not undertake to produce a Shakespeare, but we might reasonably hope to produce a generation which would not destroy its Shakespeares. And even if men of genius still found it necessary, as men of genius have found it necessary, to "play to the gallery," they would play, as IVIr. Galton says of the demagogue in a eugenic age, "to a more sensible gallery than at present." Darwin somewhere points out that it is not the scientific, but the unscientific man who denies future possibihties. Thus though an advocate of eugenics may be applauded for his judgment if he declares that the creation of genius will forever be impossible, yet I should not care to assert that the ultimate limitations of eugenics can thus be defined. We have yet to hear the last of IMendelism. Eugenics and unemployment. — Let us look now at another aspect of the promise of race-culture. When the time comes that quality rather than quantity is the ideal of those who concern themselves with the population question, it is quite evident that not a few of the social problems which we now find utterly insoluble will disapjx^ar. In this brief outline, we can only allude to one or two points. Take, for instance, the question of unemployment. We know that some by no means small proportion of the unem}:)loyed were really destined to be unemployable from the first, as for instance by reason of hereditary 502 READINGS IN EVOLUTION, GENETICS, AND EUGENICS disease. It were better for them and for us that they had never been born. Many more of the unemployed have been made unemployable by the influence of over-crowding, to which they were subjected in their years of development. Is there, can there be, any real and permanent remedy for overcrowding, but the_erection of parenthood into an act of personal and provident responsibility ? Eugenics and woman. — Take, again, the woman question. No one will deny that in many of its gravest forms, especially in its economic form, and the question of the employment of women, wisely or horribly, this depends (to a degree which few, I think, realize) upon the fact that there are now (1909), for instance, 1,300,000 women in excess in this country. Is it then proposed, the reader will say, by means of race-culture to exterminate the superfluous woman ? Indeed, no. But is the reader aware that Nature is not responsible for the existence of the superfluous woman ? There are more boys than girls born in the ratio of about 103 or 104 to 100; and Nature means them all to live, boys and girls alike. If they did so live, we should have merely the problem of the superfluous man, which would not be an economic problem at all. But we destroy hosts of all the children that are born, and since male organisms are in general less resistant than female organisms, we destroy a disproportionate number of boys, so that the natural balance of the sexes is inverted. Unlike ancient societies we largely practice male infanticide. Can the reader beheve that there is any permanent and final means of arresting this wastage of child- life, with its singular and far-reaching consequences, other than the elevation of parenthood, wholly apart from the question of the selec- tion of parents ? We shall not succeed in keeping all the children alive (with a trivial number of exceptions), thereby abolishing the super- fluous woman by keeping alive the boy who should have grown up to be her partner, until we greatly reduce the birth-rate; as it must and will be reduced when the ideal of race-culture is realized, and no child comes into the world that is not already loved and desired in antici- pation. Eugenics and cruelty to children. — ^This ideal, also, offers us in its realization the only complete remedy for the present ghastly cruelty under which so many children suffer even in Great Britain, even in the twentieth century. Is the reader aware that the National Society for the Prevention of Cruelty to Children inquired into the ill-treat- ment or cruel neglect of 115,000 children in the year beginning April I St, 1906 ? It has been reasonably and carefully estimated that " over THE PROMISE OF RACE CULTURE 503 half a million children are involved in the total of the wastage of child- life and the torture and neglect of child-Ufe in a single year." Surely Mr. G. R. Sims, to whom I would olTer a hearty tribute for his recent services to childhood, is justified in saying, "Against the guilt of race suicide our men of science are everywhere preaching their sermons to-day. It is against the guilt of race murder that the cry of the children should ring through the land." As regards race suicide and the men of science, I am not so sure as to the assertion. But the truth of the second sentence quoted is as indisputable as it is horriljle. Now no legislation conceivable will wholly cure this evil nor avert its consequences. At bottom it depends upon human nature, and you can cure it only by curing the defect of human nature. This, in general, is of course beyond the immediate powers of man, but evi- dently we should gain the same end if only we could confme the advent of children to those parents who desired them — that is to say, those in whom human nature displayed the first, if not indeed almost the only, requisite for the happiness of childhood. To this most beneficent and wholly moral end we shall come, notwithstandin'g the blind and pitiable guidance of most of our accredited moral teachers today. By no other means than the realization of the ideal defined, that every new baby shall be loved and desired in anticipation — an ideal which is perfectly practicable — can the black stain of child murder and child torture and child neglect be removed from our civilization. Ruskin and race-culture. — The name of Ruskin, perhaps, would not occur to the reader as likely to afford support to the fair hopes of the eugenist. Consider then, these words from Time and Tide: "You leave your marriages to be settled by supply and demand, instead of wholesome law. And thus, among your youths and maid- ens, the improvident, incontinent, selfish, and foolish ones marry, whether you will or not; and beget families of children necessarily inheritors in a great degree of these parental dispositions; and for whom, supposing they had the best dispositions in the world, you have thus provided, by way of educators, the foolishest fathers and mothers you could find; (the only rational sentence in their letters, usually, is the invariable one, in which they declare themselves 'incapable of providing for their children's education'). On the other hand, who- soever is wise, patient, unselfish, and pure among your youth, you keep maid or bachelor; wasting their best days of natural life in j^ain- ful sacrifice, forbidding them their best help and best reward, and care- fully excluding their prudence and tenderness from any ofiices of 504 READINGS IN E\^OLUTION, GENETICS, AND EUGENICS parental duty. Is not this a beatific and beautifully sagacious system for a Celestial Empire, such as that of these British Isles?" Apart from the point as to wholesome law rather than the educa- tion of opinion as the eugenic means, the foregoing passage must win the assent and respect of every eugenist. It indicates the promise of race-culture as it appeared to John Ruskin. The passage has been quoted in full, not for the benefit of the ordinary thoughtful reader but for that of the professional literary man who, in this remarkable age, so far as I can judge, reads nothing but what he writes, and thus quali- fies himself for dismissing Spencer or Darwin or Galton by any casual phrase. Race-culture and human variety. — Now let us turn to another question. Let it be asserted most emphatically that, if there is any- thing in the world which eugenics or race-culture does not promise or desire, it is the production of a uniform type of man. This delusion, for which there has never been any warrant at all, possesses many of the critics of eugenics, and they have made pretty play with it, just as they do with their other delusions. Let us note one or two facts which bear upon this most undesirable ideal. In the first place, it is unattainable because of the existence of what we call variation. No apparatus conceivable would suffice to eliminate from every generation those who varied from the accepted type. In the second place, this uniformity is supremely undesirable from the purely evolutionary point of view, because its attainment would mean the arrest of all progress. All organic evolution, as we know, depends upon the struggle between creatures possessing various varia- tions and the consequent selection of those variations which con- stitute their possessors best adapted or fitted to the particular environ- ment. If there is no variation there can be no evolution. To aim at the suppression of variation, therefore, on supposed eugenic grounds (which would be involved in aiming at any uniform type of mankind) would be to aim at destroying the necessary condition of all racial progress. The mere fact that all the critics of race-culture attribute to evolutionists, of all people, the desire to suppress variation, is a pathognomonic symptom of their critical quality. And, of course, quite independently of the evolutionary function of variation — though this is cardinal and must never be forgotten by the politician of any school, since what we call individuality is variation on the human plane — the value of variation in ordinary life is wholly THE PROMISE OF RACE CULTURE 505 incalculable. It is not merely that, as Mr. Galton says, "There are a vast number of conflicting ideals, of alternative characters, of incom- patible civilizations; but they are wanted to give fullness and interest to life. Society would be very dull if every man resembled the highly estimable Marcus Aurelius or Adam Bede." The question is not merely as to the interest of life. Much more important is the fact that it takes all sorts to make a world. What is the development of society but the result of the psychological division of labor in the social organism ? And how could such division of labor be carried out if we had not various types of laborers? What would be the good of science if there were no poetry or music to live for? How would poetry and music help us if we had not men of science to protect our shores from plague ? Obviously the existence of men of most various types is a necessity for any highly organized society. Even if eugenics were capable — as it is not — of producing a complete and balanced type, fit up to a point to turn out a satisfactory poem, a satisfactory symphony or a satisfactory sofa, the utmost could not be expected of such a man in any of these directions. In a word, as long as their activities are not antisocial, men cannot be of too various types. We require mystic and mathematician, poet and pathologist. Only, we want good specimens of each. ''The aim of eugenics," says Mr. Galton, "is to represent each class or sect by its best specimens; that done, to leave them to work out their common civilization in their own way Special aptitudes w^ould be assessed highly by those who possessed them, as the artistic faculties by artists, fearlessness of inquiry and veracity by scientists, rehgious absorption by mystics, and so on. There would be self-sacrificers, self-tormentors, and other exceptional idealists." But at least it is better to have good rather than bad specimens of any kind, whatever that kind may be. Mr. Galton thinks that all except cranks would agree as to including health, energy, ability, manliness, and courteous disposition amongst quaUties uniformly desirable — alike in poet and pathologist. We should desire also uniformity as to the absence of the antisocial proclivities of the born criminal. So much uniformity being granted, let us have with it the utmost conceivable variety — more, indeed, than most of us can conceive. This point, of course, is cardinal from the point of view of jiracticc. No progress could be made with eugenics, it would be impossible even to form a Eugenics Education Society, if each of us were to regard the particular type he belongs to as the ideal, and were to seek merely 5o6 READINGS IN EVOLUTION, GENETICS, AND EUGENICS to obtain the best specimens of that type. The doctrine that it takes all sorts to make a world — a doctrine very hard for youth to learn, yet unconsciously learnt by all who are capable of learning at all — must be regarded as cardinal truth for the eugenist. All he asks for, all he is wise in seeking, is good specimens rather than bad. Poets certainly but not poetasters; jesters certainly, but not clever fools. Time and its treasure.— Taking the modern estimates of the physicists, we are assured that the total period of past human existence is very brief compared with what may reasonably be predicted. Granted, then, practically unlimited time, what inherent limits are there to the upward development of man as a moral and intellectual being? Shall we answer this question by a study of the nature of matter ? Plainly not. Shall we answer it by a study of the nature of mind ? Surely not, for the study of the mind cannot inform us as to what mind might be. One source of guidance alone we have, and this is the amazing contrast which exists between the mind of man at its highest, and mind in its humblest animal forms; or shall we say even between the highest and lowest manifestations of mind within the human species ? The measureless height of the ascent thus indi- cated offers us no warrant for the conclusion that, as we stand on the heights of our life, our "glimpse of a height that is higher" is only a hallucination. On the contrary. There is no warrant whatever for supposing that the forces which have brought us thus far are yet exhausted ; they have their origin in the inexhaustible. Who, gazing on the earth of a hundred million years ago, could have predicted life — could have recognized, in the forces then at work and the matter in which they were displayed, the promise and potency of all terrestrial hfe ? Who, contemplating life at a much later stage, even later mammalian, could have seen in the simian the prophecy of man ? Who, examining the earliest nervous ganglia, could have foreseen the human cerebrum ? The fact that we can imagine nothing higher than ourselves, that we make even our gods in our own image, offers no warrant for supposing that nothing higher will ever be. What ape could have predicted man, what reptile the bird, what amoeba the bee ? ''There are many events in the womb of time which will be delivered" and the fairest of her sons and daughters are yet to be. But even grant, for the sake of the argument, that the intelligence of a Newton, the musical faculty of a Bach, the moral nature of any good mother anywhere, represent the utmost limits of which the THE PROMISE OF RACE CULTURE 507 evolution of the psychical is capable. There is every reason lo deny this, but let us for the moment assume it true. There still remains the thought of Wordsworth, "What one is, why may not millions be?" — a thought to which Spencer has also given utterance. What is shown possible for human nature here and there, he says, Ls con- ceivable for human nature at large. It is possible for a human being, whilst still remaining human, to be a Shakespeare or a St. Francis; these things are thus demonstrably within the possibilities of human nature. It is therefore at the least conceivable that, in the course of almost infinite time (even assuming, say, that intelligence must ever be limited, as even Newton's intelligence was limited) — some such capacities as his may be common property amongst men of the scientific type; and so with other types. We may answer Words- worth that there is no bar thrown by Nature in the way of such a hope. Whal is possible. — This of course is speculation and of no immediate value. I would merely remind the reader that the doctrine of optimism, as regards the future of mankind, which the principles of race-culture assume and which they desire to justify, was definitely shared by the great pioneers to whom we owe our understanding of those principles. Notwithstanding grave nervous disorder, such as makes pessimists of most men, both Darwin and Spencer were compelled by their study of Nature to this rational optimism as regards man's future. The doctrine of organic evolution, and of the age-long ascent of man through the selection of the fittest (who have, on the whole, been the best) for parenthood, is one not of despair but of hope. Exactly half a century ago it struck horror into the minds of our predecessors. Man, then, is only an erected ape, they thought — as if any historical doctrine, however true, could shorten the dizzy distance to which man has cHmbed since he was simian; and man being an ape, they thought his high dreams palpably vain. But the measure of the accomplished hints at the measure of the possible, and the value of the historical facts lies not in themselves, all facts as such being as dead as are the individual atoms of the living body, but in the principles which grow out of them. It is of no imj^ortance as such that man has simian ancestors; it is of immeasurable importance that he should learn by what processes he has become human, and by what, indeed, they became simian -which would have been a proud adjective for its own day. The principles of organic progress matter for us because they are the principles of race-culture, the only sure means of human progress. Our looking backwards does not turn us 5o8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS into pillars of salt, but teaches us that the best is yet to be, and how alone it is to be attained. Elsewhere the optimistic argument of Wordsworth is quoted. Here also John Ruskin: "There is as yet no ascertained limit to the nobleness of person and mind which the human creature may attain, by persevering observance of the laws of God respecting its birth and training." And Herbert Spencer: ''What now characterizes the exceptionally high may be expected eventually to characterize all. For that which the best human nature is capable of, is within the reach of human nature at large." And Francis Galton: "There is nothing either in the history of domestic animals or in that of evolution to make us doubt that a race of sane men may be formed, who shall be as much superior, mentally and morally, to the modern European, as the modern European is to the lowest of the Negro races. "It is earnestly to be hoped that inquiries will be increasingly directed into historical facts, with the view of estimating the possible effects of reasonable political action in the future, in gradually raising the present miserably low standard of the human race to one in which the Utopias in the dreamland of philanthropists may become practical possibilities." Conclusion — eugenics and religion. — In an early chapter it was attempted to show that eugenics is not merely moral, but is of the very heart of morality. We saw that it involves taking no life, that, rather it desires to make philanthrophy more philanthropic, that, at any rate so far as this eugenist is concerned, it recognizes and bows to the supreme law of love; and claims to serve that law, and the ideal of social morality, which is the making of human worth. Eugen- ics may or may not be practicable, it may or may not be based upon natural truth, but it is assuredly moral; though I, for one, would pro- claim eternal war between this real moraUty and the damnable sham, which approves the unbridled transmission of the most hideous diseases, rotting body and soul, in the interests of good. And if religion, whatever its origin and the more questionable chapters in its past, be now "morality touched with emotion," I claim that eugenics is religious, is and will ever be a religion. Else- where I have attempted to show that religion has survived and will survive because of its survival-value — its services to the Hfe of the THE PROMISE OF RACE CULTURE 509 societies wherein it flourishes. The rehgion of the future, it was sought to argue, will be that which "best serves Nature's unswerving desire — fullness of life." The Founder of liic Christian religion said, ''I am come that ye might have life, and that ye might have it more abundantly." It is higher and more abundant life that is the eugenic ideal. Progress I define as the emergence and increasing dominance of mind. Of progress, thus conceived, man is the highest fruit hitherto. He is also its appointed agent and eugenics is his instrument. To this end he must use all the j)owers which have blossomed in him from the dust. He must claim Art: and indeed in Wagner's great music-drama, at the moment when the prophetic Briinnhilde tells Sieglinde who has just lost her mate that she, the expectant mother, may look for the resurrection of the dead and the life of the world to come in the child Siegfried; and when the heroic theme is pronounced for the first time and followed by that which signifies redemption by love; then, I think, the eugenist may thrill not merely to the music, nor the humanity of the story, but to the spiritual and scientific truth which it symbolizes. If the struggle towards individual perfection be religious, so, assuredly, is the struggle, less egoistic indeed, towards racial perfection. If the historic meaning and purport of rehgion are as I conceive them, and if its future evolution may thence be inferred, there can be no doubt in the prophecy that in ages to come those high aspirations and spiritual visions which astronomy has dishoused from amongst the stars, and which, at their best, were ever selfish, will find a place on this human earth of ours. If we have transferred our hopes from heaven to earth and from ourselves to our children, they are not less religious. And they that shall be of us shall build up the old waste places; for we shall raise up the foundations of many generations. "We feel the high tradition of the world And leave our spirits on our children's breasts." rnOFERTY UBnART Af. C- Stiij^ C -'iff BIBLIOGRAPHY LIST OF FIFTY BOOKS FROM WHICH EXCERPTS HAVE BEEN QUOTED Babcock, Ernest Brown, and Clausen, Roy Elwood. Genetics in Relation to Agriculture. The McGraw-Hill Book Co., 1918. Bateson, William. Problems oj Genetics. Yale University Press, 1913. Castle, William E. Genetics and Eugenics. Harvard University Press, 2d ed., 1920. Child, Charles Manning. "Regulatory Processes in Organisms." Jour. Morph., XXII (191 1). Conklin, Edwin Grant. Heredity and Environment. Princeton Uni- versity Press, 3d ed., 1920. Coulter, John M., and Coulter, Merle C. Plant Genetics. The University of Chicago Press, 1918. Craivh'TON, Henry Edward. The Doctrine oj Evolution. The Columbia University Press, 191 1. Darwin, Charles. The Origin of Species. D. Appleton & Co., with additions from the sixth and last edition, 1893. Darwtn, Francis. Life and Letters of Charles Darwin. John Murray, London, 1888. Dendy, Arthur. Outlines of Evolutionary Biology. D. Appleton & Co., 1916. DeVries, Hugo. Species and Varieties. The Open Court Publishing Co., 1904. Downing, Elliot Rowland. The Third and Fourth Generation. The University of Chicago Press, 191 8. Eimer, Theodore. On Orthogenesis. The Open Court Publishing Co., 1898. Gadow, Hans. The Wanderings of Animals. Cambridge University Press, 1913. Guyer, Michael F. Being Well-Born. The Bobbs-Merrill Co., 1918. . "Immune Sera and Certain Biological Problems." Amer. Natur., LV (1921). Henderson, Lawrence J. The Fitness of the Environment. The Mac- millan Co., 1913. Herbert, S. The First Principles of Evolution. Adam and Charles Black, London, 1913. Jordan, David Starr, and Kellogg, Vernon L. Evolution and Animal Life. D. Appleton & Co., 1908. JuDD, John W. The Coining of Evolution. Cambridge University Press, 1911. 510 BIBLIOGRAPHY 511 Kellogg, Vernon L. Darwinism To-Day. Henry Holt & Co., 1907. Le Conte, Joseph. Evolution. D. Appleton & Co., 2d ed.. 1897. LocY, William A. The Main Currents oj Zoology. Henry Holt & Co., 1918. Lull, Richard Swann. Organic Evolution. The Macmillan Co., 191 7. McFarland, Joseph. Biology, General and Medical. W. B. Saunders Co., 3d ed., 1918. Metcalf, Maynard M. An Outline of the Theory of Organic Evolulion. The Macmillan Co., 191 1. Morgan, Thomas H. Evolution and Adaptation. The Macmillan Co., 1903. . A Critique of the Theory oj Evolution. Princeton University Press, 1916. Newman, Horatio H. "The Limits of Hereditary Control in Armadillo Quadruplets: A Study of Blastogenic Variation." Jour. Morph., XXII (1911). . The Biology of Twins. The University of Chicago Press, 191 5. . Vertebrate Zoology. The IMacmillan Co., 1920. NuTTALL, G. H. F. Blood Immunity and Blood Relationship. Cambridge, at the University Press, 1904. Nutting, C. C. "The Relation of Mendelism and the Mutation to Theory Natural Selection." Science, N.S., LIII (Feb. 11, 192 1). OsBORN, Henry Fairfield. From the Greeks to Darwin. The Macmillan Co., 1908. . The Origin and Evolution of Life. Charles Scribner's Sons, 19 18. Plate, Lltdwig. Uher die Bedeutung des Darwin' schen Selectionsprincips und Proleme der Artbildung. Engelmann, 2d ed., 1903. Popenoe, Paul, and Johnson, Roswell H. Applied Eugenics. The Macmillan Co., 1918. Romanes, George J. Darwin atul after Darwin. The Open Court Publishing Co., 1892. Saleeby, Caleb Williams. The Promise of Race Culture. MofTat, Yard & Co., 1909. ScHUCHERT, Charles. Text-Book of Geology: Part II, Historical Geology. John Wiley, 191 5. Scott, William Berryman. The Theory of Evolution. The ^Llcmillan Co., 1911. Shelford, Victor E. Animal Communities in Temperate America. The University of Chicago Press, 191 3. Shull, a. Franklin. Principles of Animal Biology. The McGraw-Hill Book Co., 1920. Steinmann, Gustav. Die Abstammungslchrc. Engelmann, Leipzig, 1908. Tayler, J. L. "The Scope of Natural Selection." Natural Science, Vol. XV., 1899. 512 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Thomson, J. Arthur. Darwinism and Human Life. Henry Holt & Co., 1909. . Heredity. John Murray, London, 1907. Walter, Herbert Eugene. Genetics. The Macmillan Co., 191 1. Whitman, Charles Otis. "The Problem of the Origin of Species." Proceedings of Congress of Arts and Science, Universal Expositiojt, St. Louis, 1906. Wright, Sewall. Principles of Livestock Breeding. U.S. Dept. Agri- culture, Bull. 905, 1920. INDEX INDEX Abiogenesis, 12 Abraxas, 438, 452 Acquired characters, inheritance of, 19, 20, 273; discussion by E. G. Conklin, 33°~3^'>^ lack of evidence for, 332, 33S; misunderstandings concerning, 323-30; other side of the question, 336-38; statement of the problem, 323, 331-32 Adaptation, 11, 30, 188-205; classi- fication of adaptations, 195, 196; Osborn's laws of, 192-95 Adaptive radiation, law of, 194, 195 Agassiz, L., 144, 145 Aggressive resemblance, 201 Albinos, different kinds, of, 431, 432 Allelomorphic, characters in heredity, 433 Allen, E. J., 209 Alluring coloration, 201 Ameghino, F., 86 Amphioxus, 178 Amphisbaenidae, 119 Analogous, versus homologous struc- tures, 136 Anaxagoras, 12 Anaximander, 11 Anaximenes, 11, 12 Ancestral inheritance, Galton's law of, 371, 372 Anti-lens serum, Guyer's experiments with, 338-45 Antirrhinum, 446 Apartness of the germ plasm, 31, 295, 296, 337 Apotetix, 448 Appendix vermiformis, in man and apes, 154, 155 Apteryx australis, 142, 143 Aquinas, Thomas, 8, 15 Archaesthetism, 35 Aristotle, 13, 14, 18 Arithmetical mean, 367 Armadillo quadruplets: and classifica- tion, 122, 123; and sex-determina- tion, 45 1 Armadillos, 97 Artificial selection, 25, 26 Aspergillus niger, 327 Atavism, 13 Atropa, 393 Augustine, 8, 15 Azores, fauna of, 103 Babcock, E. B., 287, 307-22, 363, 364, 401-12 Bacon, F., 15, 276 Balanoglossus, 177, 178 Bascanion anthonyi, 117 Bat, wing of, 134 Bateson, W., 7, 43, 346, 368, 369, 380, 393, 396, 414, 446 Bathmism, 35 Bauer, E., 335 "Beagle," Darwin's voyage on the, 23, 25, 26 Beebe, W., 316, 317 Beeton, M., 485, 489, 491 Begonia, 337, 338 Bell, A. G., 474, 491 Bembidium, 109 Bequerel, A. H., 275 Bergson, H., 34 Bermudas, fauna of, 103-5 Bibliography, 510-13 Billen, R. H., 394 Bimodal and multimodal curves, 368, 369 Biometr}': discussion of, 365-75; rise and vogue of, 38, 39 Birds: rudimentary teeth of, 181; wing of, 134 Birgus lalro, 138, 139, 140 Bison antiquus, 85 Blakeslee, A. F., 417 Blastoderm, 166 Blastula, 166 Blends, in heredity, 416-18 Blood-transfusion tests, evidences from Oo, 124-2S Bonnet, R., 16 515 5i6 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Boyd-Dawkins, 162 Brachydactylism, 399, 400; inheritance of, 460 Bridges, C. B., 402, 437 Briinn, 40, 41 Bryonia, 436 Buffon, G. L. L., 7, 15, 16, 17 Bullen, G. E., 209 Cameline: forefoot, 75; skull, evolu- tion of, 74 Camels, fossil pedigree of, 73-76 Cape de Verde Islands, fauna of, 106 Carnivora, 126 Carnot, S., 277 Carsinas maenas, 256 Castle, W. E., 39, 40, 41, 43, 261, 287, 333, 334, 378, 379. 399, 43^, 432, 433-48 Cataract, inheritance of, 461 Catastrophism, 22, 23 Cave animals, eyes of, 144, 145 Cebidae, 126 Cell: diagram of typical cell, 291; division, direct, 290, 291; division, indirect or mitotic, 291, 292, 403-5; division, somatic, 403-5; theory, 289 Cenogenetic, 174 Centrosome, 290, 291, 292 Cesnola, 257 Cetacea, 179 Chamberlin, T. C, 57 Chambers, R., 23, 26 Chapin, C. V., 483 Child, C. M., 191, 192, 337 Child mortality in long-lived families, 488 Chimpanzee, 88 Chromatin, 290; interchange between homologous chromosomes, 408; nu- cleolus, 290 Chromosomes, 291, 292; conjugation of pairs of, 407; of Drosophila, 297; in heredity, 304, 305; independent distribution of, 408, 409; individual- ity of, 296; maps to show loci of genes, 437, 442; of mosquito, 297; number and appearance, 293; pairs of, 297, 407; reduction of, 297-98, 406; and sex in Drosophila, 410-12; significance of, 292, 293 Circopithecidae, 126 Clark, J. M., 85 Classification: basis of, 11 7-19; evi- dences from, 60, 117-23; inter- national code of, 117; method of, 120-21 Clausen, R. E., 287, 307-22, 363, 364, 401-12 Cleavage of egg, 294 Cocoanut crab, 138, 139, 140 Coefficient of correlation, 369, 370 Coincident selection, 268, 269 Color in animals, 200-204 Coluber anthonyi, 117 Colubridae, 117 Colubrinae, 117 Commensalism, as adaptation, 198, 199 Communal life, as adaptation, 199, 200 Comparative Anatomy, evidences from, 60, 129-63 Confusing coloration, 204 Conklin, E. G., 330-37, 370-75. 434, 435 Conjugation, of homologous chromo- somes, 407 Convergence, Osborn's law of, 192-94 Cope, E. D., 35 Correlation: coefficient of, 369, 370; tables, 370 Correns, C, 40, 43, 380, 393, 394, 415, 416, 424 Cossonidae, 108 Coulter, J. M., 386-92, 413-28 Coulter, M. C, 386-92, 413-28 Coutagne, G., 396 Crampton, H. E., 5, 32, 271 Cretins of Aosta, 464, 465, 478, 479 Crossing-over, in Mendelian heredity, 441-48 Cuenot, L., 398 Curie, P., 275 Cuvier, G., 19, 21, 22 Cytoplasm, 290; in inheritance, 304 Dakin, W. J., 209 Daphnia, 335 Darbishire, A. D., 393, 398 Darwin, C, 4, 5, 6, 8, 10, 11, 17, 19, 23, 24, 25, 26, 27, 28, 29, 30, 31, 45, 61, 67, 97, 118, 120, 121, 160, 205, 210, 211, 213, 219-44, 259, 260, 307, 330, 427,477, 501 Darwin, E., 3, 16, 17, 18, 21 IXDEX :)i/ Darwmism,_ 7, 8; background of, 188- 216; critique of, 245-62; defense of, general, 252-56; objections to, 247- 52 Dasypus novemcinctus, 366, 375-78 Datura, 395 Davenport, C. B., 257, 333, 372, 393, 396, 474, 475 Davenport, G. C, 400 Davis, B. M., 360 De CandoUe, A., 23, 122 Defectives, segregation of, 479-80 Democritus, 12 Dendy, A., 71, 73 Descartes, R., 15 Determinants (Weismann's), 30, 31, 32, 265 Determination of sex, 449-56 Development: facts of, 164, 165; out- line of animal development, 165-72 De Vries, H., 7, 36, 37, 38, 39, 43, 258, 260, 261, 273, 335, 346-60, 380, 393, 429 Difficulties and objections to Natural Selection as seen by Darwin, 236-42. Difflugia, 378 Digby, L., 363, 364 Dihybrid ratio, 391 Dinornis gravis, 136, 137, 142 Dominance, Mendel's Law of, 40-42, 381, 382, 386 Doncaster, L., 399 Downing, E. R., 459-72 Driesch, H., 34 Drinkwater, H., 400, 460 Drosophila ampelophila, 318, 321, 380, 444; chromosomes of, 401, 402, 403; sex-linked heredity in, 433~38 Drummond, H., 214, 215 Durham, F. M., 429, 430 Ears of man and apes, 155, 156, 157 Earthworms and vegetable mold, 214 East, E. M., 43, 419 Eaton, Rev. A. E., 143 Ectoblast, 166 Edentates, distribution of, 98 Edwards, J., 483 Eimer, T., 34, 35, 264 Elderton, E. M., 493 Electric organ, of fishes, 196 IMcphants, evolution of, 76-80 l-^lephas, 76, 77, 78, 79, 80; K. autiquus, .Sg; E. columhi, 85; E. leidyi, H5 Embryology, evidences from ^o, 164-72 Ivmpedocles, 12, 13 Endoblast, 166 Kngrammcs of Rignano, 335 Enteleche, 34 Environment: cfTects of, on develop- ment, 317, 3i8;etTc(ts of, on hi-redity, 312-16, 318-20; and heredity, 312, in EoanthropHS dawsoni, 93 Epicurus, 14 Epigenesis, 13 Epilepsy, inheritance of, 465 Equidae, 70, 71, 72, 73 Equus, 71, 72, 73; E. leidyi, 85 P^scherich, 215 Eugenics: Carnegie Laboraton.' of, 459; and cruelty to children. 502, 503; defined, 473; Education Society, 505; and Euthenics, 484-96; Galton Laboratory of, 459; positive, 480-83; and religion, 508, 509; restrictive, 475-480; and unemployment, 501, 502; and woman, 502 Eupagurus, 246 Euthenics, 473, 484-96 Evolution, organic: causal factors of, 185; definitions of, 3, 4, 5 1 evidences of, 59, 60; experimental. 60; nature of proof of, 59; proof of, 57, 58, 59; what it is not, 8, 9 Fabre, M., 231 Factor hypothesis, 417*28 Factorial analysis of color in mice, 429- 30 Factors, in Xeo-Mendelian heredity: complementary, 417-20; cumulalive. 424; inhibitory. 421-23; lethal, 432; in quantitative inheritance, 424-28 Farmer. J. H.. 363, 364 Farrabee, \V. C\, 400 Feeble-mindedness, inheritance oi, 403, 464 Fertilization, 301, 302 Fierasfcr acus, 198, lOO Filiiria sanguinis hominis, 212 Filial Regression, Gallon's Uiw of, 372-74 Flower, Professor, 1O2 5l8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Forfictilata auricularia, 368, 369 Franklin, B., 482 Freemartin, 455, 456 Fossils: actual remains, 63 ; Cambrian, 62; casts and impressions, 64; classi- fication of, 63; conditions necessary for, 64, 65, 66; Dar.win's opinion as to the adequacy of the record, of the, 61, 62; definition of (by T. H. Huxley), 63; first recognized, 12; general facts revealed by, 69, 70; pedigrees of well-known vertebrates, 70-80; petrifications, 63 Gadow, H., 114, 115 Gager, C. S., 361 Galapagos Islands, fauna of, 105-7 Gallastegui, 448 Galton Laboratory of Eugenics, 459, 474 Galton, Sir F., 38, 39, 365, 370-75, 376, 473, 497-501, 505, 508 Gastrula, 166 Gates, R. R., 363 Gazelle-camels, 76 Gegenbaur, 175 Gemmules, 28, 30 Genealogical Records Ofiice, 484, 491, 492 Genetics: definitions of, 287; evi- dences from, 60; importance of cell theory in, 289; methods of study, 288; scope and methods of, 287-89; subject-matter of, 288-89 Genius: hereditary, 498; production of 497, 498; transmission of, 498, 499 Genotype, 377-79 Genotypic, 377-79 Geographic distribution, evidences from, 60, 97-116 Geologic time: lapse of, 67, 68; scale in millions of years, 68 Germ-cells: early setting apart of, 295, 296; origin of new, 294, 295; pro- duction of, 405-8 Germinal continuity, 31, 296 Germinal selection, 30, 31, 265-68 Germ-plasm theory, 31, 32 - Giekie, Sir A., 69 Gill arches in vertebrates, 176, 177 Giraffe-camels, 76 Glaser, O. C, 365 Glochidia, 211 Goddard, H. H., 462-64 Goethe, J. W., 21 Goldschmidt, R., 314, 315, 453 Goodale, H. D., 440 Gorilla, 149 Goss, J., 40 Graham-Smith, G. L., 125 Gray, A., 223 Greek evolutionists, 11-14 Gregory of Nyssa, 14 Gregory, R. P., 445 Gregory, W. K., 82 Guacanos, 73 Gulick, J. T., 32, 271 Guthrie, C. C, S33 Guyer, M. F., 289, 290-306, 336, 338-45 Habitat: preference, 190, 191; selec- tion, 190, 191 Haeckel, E., 19, 30, 172, 173 Hair of man and apes, 156-61 Haldane, 443, 444 Hamilton, D. J., 326 Hapalidae, 126 Harris, J. A., 335, 485 Harrison, R. G., 333 Harte, Bret, 85 Hartmann, C. G., 162 Harvey, W., 13 Hauser blonds, 481 Hegner, R. W., 295 Helix hortensls, 396; H. nenioralis, 396 Henderson, L. J., 189 Heraclitus, 12, 208 Herbert, S., 263, 264, 269 Heredity: Galton's Laws of, 371-75; in man, 398-400; in pure lines, 376, 379; statistical study of, 370-75 Hermit-crabs, 138 Heron, Sir R., 231 Herschel, Sir J., 3 Heterogenesis theory, 36 Heteromera, 106 Heterozygote, 390 Hialodaphnia, 315, 316 Hippocrates, 449 Homo: H. heidelhergensis, 88; H. sa- piens, 88, 90, 92, 93; H. neander- thalensis, 89, 90, 91, 92, 93; H. primagenius, 89 INDEX 519 Homologies, evidences from, 60 Homozygote, 390 Hooker, Sir J., 109, 223, 247, 330 Hormone theory of sex differentiation, 454-56 Hormones, 454 Horse: ancestr>' of, 8; feet and teeth in fossil pedigree of, 72; fossil pedigree of, 70, 71, 72, 73 Horseshoe-crab, 127 Hrdlicka, A., 86 Hudson, W. H., 206 Human antiquity, evidences of, 93, 94 Human conservation, 473-83 Humanity, future of, 95, 96 Humerus, perforations of, in Quadru- mana, 162 Hurst, C. C, 43> 393, 396, 399, 400 Hutton, J., 22, 57 Huxley, T. H., 28, 29, 62, 63, 91, 208 Hyatt, A., 35 Hybridization and the origin of species, 43 Hyracotherium, 71 Immigration and eugenics, 475-76 Immunity coloration, 203 Induction, a temporary change in germ-cells, 335, 336 Infant mortality movement, 494 Inheritance: of acquired characters {see Acquired characters); of brachy- dactylism, 460; of cataract, 461; of feeble-mindedness, 459, 462-64; of human characters, 459-72; of insanity, epilepsy, etc., 459, 465, 466; in royalty, 466-72; sex-linked, 433~4o Insanity, inheritance of, 465 Intraselection, 268 Isolation: biologic, 272; geographic, 269-72; theories of, 20, 32, 2;^, 269- 73; reproductive, 272, 273 Jennings, H. S., 261, 377, 378 Johannsen, W., 369, 376, 377 Johnson, R. H., 484-96 Jones, D. F., 448 Jordan, D. S.,4, 32, 33, 34, 63, 64, 65, 66, 121, 165-74, 188, 195, 202, 270, 271, 273, 478, 479, 482 Joule, J. P., 277 Judd, J. W., 10, 23, 24 Kallima, 201, 204, 250 Kammerer, P., 336 Kant, E., 15, 16 Kellogg, V. S., 4, 32, 33, 34, 63, 64, 65, 66, 121, 165-74, 188, 195, 202, 245- 47, 253, 266, 273 Kelvin, Lord, 67, 68, 277 Kinetogenesis, 35 King, C, 67 Klebs, E., 312, 313 Knight, T., 40 Kohlreuter, J. C, 40, 41 Korchinsky, H., 36 Lamarck, J. P., 7, 10, 11, 18, 19, 20, 21, 118, 247, 307, 330, 345 Lamarckism, 7, 21, 247 Lang, A., 395 Laplace, P. S., 57 Laplacian hypothesis, 67 Laughlin, H. H., 474 Le Conte, J., 3, 46-53 Leibnitz, 15 Leighty, C. E., 370 Lemuroidea, 126 Lepas, metamorphosis of, 171 Lepidosiren, 236 Lcptinotarsa deccmliniala, 321, 361, 377 Lillie, F. R., 455, 456 Liua lapponica, 396 Lincoln, A., 24 Linkage, in Mendelian heredity, 441- 48; chromosome thcor>' of, 442; measurements of, 444, 445 Linnaeus, 16, 43, 117 Locy, W. A., 41, 42, 43 Loeb, C, 461 Loess Man, 85 Lotsy, J. P., 43 Love, H. H., 370 Lowell, J. R., 330 Lucas, A. H. S., 218 Lucretius, 14 Lull, R. S., 3, 5, 24, 25, 73. 76, 79, Si-96 Lychnis, 393, 436 Lydekkcr, R., 178 Lyell, Sir C, 3, 8, '-3, 26, 57, 5^. 67 Maas, O., 180 Macdougal, D. T., 319, 320, 361, 362 McCracken, I., 396 520 READINGS IN EVOLUTION, GENETICS, AND EUGENICS McFarland, J,, 28 McGregor, J. H., 90 Madagascar, fauna of, no, in Maeterlinck, 200 Mallophaga, 271, 272 Malthus, 17, 23, 24, 26, 223 Mammalian dispersal, 1 14-15 Mammary glands, as adaptations, 196 Man: of Chappelle-aux-Saints, 91; Cro-Magnon Man, 93, 94; chrono- logical table of fossil man, 87 ; descent from trees, significance of, 84; evolu- tion of, 81-96; evolutionary changes of, 84; fossil man, 84-94; Heidel- berg Man, 88, 89, 91; impelling cause of origin, 83; Neanderthal Man, 89-92; origin of, 82-84; Pilt- down Man, 92, 93; place of origin, 82, 83; of Spy, 91; stock of, 82; time of origin, 83, 84 Mantis religiosa, 257 Marriage laws and eugenics, 477 Marsh, O. C, 72 Marshall, A. M., 214 Marsupial pouch, as adaptation, 196, 197 Mastodon, 76, 77, 78, 79, 85 Materialism, the relation of evolution to, 46-53 Matthew, W. T., 81 Matthiola, 394 Maturation: of egg-cell, 299, 300, 301; of sperm-cell, 298, 300 Maupertius, 15 Median, in variation, 367 Meek, 364 Megalonyx jefersoni, 85 Mendel, G., 40, 260, 346; his concep- tion of purity of gametes, 386, 387; his conception of unit characters, 386; his experiments, 381; his explana- tions, 386-92; his Law of Domi- nance, 40-42, 381, 382, 386; his Law of Segregation, 4C^42, 382-385; his life and character, 380; his results, 381,382 Mendelian heredity: in cats, 399; crossing-over in, 441-48; in guinea- pigs, 399> 432, 433; in Helix, 396; in Lina lapponica, 396; linkage in, 441-48; in maize, 394, 394; in man, 399, 400; in mice, 384, 385, 398; in nettles, 395; in numerous species, 393> 394; in peas, 384; in pigeons. 397,. 398; in poultry, 396, 397; in rabbits, 399; in silkworms, 395, 396 Mendelism: physical basis of, 401-12 Mesohippus, 72 Metcalf, M. M., 6, 32, 200-204 Metz, C. E. v., 297, 363, 436 Meyer, L., 155 Miastor americana, 294, 295 Millson, A., 214 Milton, J., 14 Mimicry, 203 Miohippus, 72 Mirabilis jalapa, 394, 415, 416, 424; M. rosea, 394 Mivart, 135, 136 Mneme theory of Semon, 335 Mode, in variation, 367 Modifications, 308, 309 Moeritherium, 76, 77, 78 Monohybrid ratio, 389 Moore, C. R., 454 Morgan, L., 264 Morgan, T. H., 42, 43, 44, 260, 261, 267, 318, 321, 355-60, 379, 433-38, 442, 443 Morphology, evidences from, 129-63 Moulton, F. R., 57 Miiller, P., 170, 172, 239 Miiller, H. J., 408, 437 Multiple factor hypothesis, 424-28 Mutation theory, 346-64; advantages over Natural Selection, 359; alterna- tive to Natural Selection, 272; criti- cism of, 359-60; De Vries' own account of, 348-55; historical account of, 36-38; Morgan's sum- mary of, 355-59 Mysis larva of Peneus, 170 Nabours, R. K., 448 Nageli, C. von, 34, 35, 7,2>2>, 380 Natural Selection, 4, 12, 24, 25, 26, 35, 37, 38, 223-43; experimental sup- port of, 256-58; present status of, 258; relation of Mendelism and mutation to, 258-62 Naudin, C, 41 Nauplius larva of Peneus, 170 Neanderthal Man, 88-92 Nebular hypothesis, 57 Neo-Lamarckism, 29, 335, 336 INDEX 521 Neo-Mendelism, 43, 44, 45, 413-32 Nest-making instincts, as adaptations, 197 Nettleship, E., 400, 461 Newman, Colonel, 211 Newton, Sir F., 7, 276, 277, 280, 283 New Zealand, fauna of, no, in Nictitating membranes of vertebrates, 146, 147 Nilsson-Ehle, H., 424-28 Nitsche, Dr., 155 Nucleolus: chromatin, 290; true, 290 Nutritive chains, 208 Nuttall, G. H. F., 124, 125 Nutting, C. C, 258-62 Oceanic islands, fauna of, loi-io Octopus, eye of, 135 Oenothera, 35; O. albida, _iS5> ^^ biennis, 349; 0. brevistylis, 349 ff- O. elliptica, 355; 0. gigas, 353 ff. O. lata, 355; O. laevifolia, 349 ff. O. lamarckiatta, 37, 346-60; leptocarpa, 357; 0. nannella, 349 ff. O. rubrinervis, 353 flf.; 0. sciniillans, 355; 0. spattdata, 357 Oglivie, Dr., 326 Oken, 12, 16 Onagra biennis, 362 Ontogenetic selection, 268 Oocyte, 300 Oogenesis, 299, 300 Oogonium, 300 Organic selection, 268, 269 Origin of Species, The, 4, 5, 7, 24, 27, 61, 62, 67 Ornithorhynchus, 236 Orohippus, 72, 73 Orr, H. B., 465 Orthogenesis, 33, 34, 35, 3^, 273 Orthoplasy, 268 Osborn, H. F., 8, 10, 11, 13, 20, 21, 45, 86, 87, 89, 94, 95, 192-95, 273, 274-83 Overspecializations, 31 Ovum, 164 Owen, R., 121, 150, 240 Pagurus bernhardus, 139 Palaeomastodon, 76, 77, 78 Palaeontology, evidences from, 60; opinions as to the adequacy of, 62, 63; strength and weakness of, 6r, 62 ralingenctic, 174 Pan vctus, 88 Pangenesis, 28, 30 Panmixia, 31, 263, 264 Panniculus carnosis, 148, 149 Papilio machaon, 315 Paramecium, 377 Parasitism, as adaptation, 197, 198 Parthenogenesis: as a method of asexual de\cl()|)ment, 164; sex deter- mination in connection with, 451, 452 Pearl, R., 440 Pearson, K., 39, 365, 371, 373, 484, 489, 491, 493 Pedigree: of brachydactylism, 460; of cataract, 461; of Charles the Great of Sweden, 471; of feeble- mindedness, 463, 464; of Ferdinand and Isabella, 468; of HohenzoUcms of Prussia, 470; of insanity, epilepsy, etc., 465; of Romanoffs of Russia, 467 Peneus potitniriiim, 170, 171 Phaseolus, 376 Phenotype, 377-79, 39°, 43 1 Phenotypic, 377-79, 39°, 43 1 Phillips, J. C, m Phocochaerus, 324, 325 Physiological units, 28 Pisum qiiadraium, 381 ; P. saccharalum, 381; P. sativum, iqy, P. umbdlatum, 381 Pithecanthropus ercctus, 86, 87, 88, 90, 93 Planetesimal hypothesis, 57 Plate, L., 264, 265, 371 PUny, 14 Pliohippus, 72 Ploetz, A., 489, 490, 491 Podocor>'ne, 246 Poebrotherium, 74, 75 Polar bodies, 299, 300, 301 Popenoe, P., 484--96 Post-Aristotelians, 14 Poulton, E. B., 257 Preformation doctrine, 176 Prenatal influences, 13, 14 Presence and absence hypothesis. 413- 15 Primates: geologic record of, 82; origin of, 81, 82; place of origin; slock of, 81; time of origin, 81 52 2 READINGS IN EVOLUTION, GENETICS, AND EUGENICS Primula kiwensis, 364; P, sinensis, 317, 394, 445 Priority, law of, 117 Probable error, 368 Procamelus, 74, 75 Promise of Race Culture, 497-509 Pronucleus: male, 301; female, 302 Protective resemblance, 200, 201 Protohippus, 72 Protylopus, 74, 75 Pterodactyl, wing of, 134 Punnett, R. C, 385, 395, 396, 446 Pure lines, heredity in, 376-79 Purity of gametes, 386, 387 Python, hind limbs of, 141, 142 Quadrumana, 148 Rabl, C, 166 Race culture and human variety, 504, 505 Rana sylvatica, 333; R. palustris, 333 Recapitulation, doctrine of, 60, 171, 172; critique of, 173-82 Reduction divisions, in maturation, 405 Reighard, J., 203 Remora, 199 Reversion, 13 Revival of Science, the, 15, 16 Rhodeus amarus, 212 Rignano, E., 335 Robinson, L., 151, 152 Rodentia, loss of teeth in, 180 Romanes, G. J., 35, loi-io, 129-63, 263, 264, 329 Roosevelt, T., 217 Rosanoff, A. J., 465 Roux, W., 268 Rumford, B. T., 277 Ruminants, collar-bone of, 180 Ruskin, J., 503, 504, 508 Ruskin and race culture, 503, 504 Rutherford, E., 275 Sacculina, 171, 181, 198, 453 Saleeby, C. W., 497-509 Saltatory, variations, 38 Sandwich Islands, fauna of, 109, no Saunders, 393 Scardafella inca, 316, 317; S. dialeucos, 316, 317; 5. braziliensis, 316, 317; S. ridgwayi, 316, 317 Schoetensack, Dr., 89 Schuchert, C, 67, 68, 69 Scott, W. B., 5, 6, 62, 63, 73-6, 82, 123- 28, 173-82 Scrophularia, 319, 320 Seals, comparative anatomy of, 129, 130 Secondary sexual characters, 453, 454 Sediim spectahile, 312-14 Segregation, Mendel's Law of, 40-42, 382-85 Semon, R., 335 Serology, evidences from, 60 Sex determination, 449-56; chromo- some mechanism of, in Drosophila, 410 ff.; in parthenogenetic species, 451, 452; nutrition theory of, 449; at time of fertilization, 449, 450; various theories of, 449-51 Sex differentiation, 453-56 Sex, Heredity and, 44, 305, 306 Sex-linked inheritance, 433, 440 Sexual coloration, 204 Sexual Selection, 26, 230-32 Shelford, V. E., 190 ShuU, A. F., 73, 76-80, 100, loi, 117-20 Shull, G. H., 43 Signals and recognition marks, 203, 204 Sims, G. R., 503 Smith, E. A., 339 Smith, G., 453 Snow, E. C, 493 Solidago virguarea, 324 Species, definitions of, 121, 122 Spencer, H., 4, 19, 28, 29, 264, 325, 508 Spermatid, 298, 299 Spermatocyte, 298, 299 Spermatogenesis, 298, 299 Spermatogonium, 298, 299 Spermatozoon, 165, 298, 299 Spontaneous generation, 12 Sports, 38 Sprengel, C. K., 210 Standard deviation, 367, 368 Staples-Brown, R., 397 Statistical study: of variation, 365-70; of heredity, 370-75 Stegodon, 76, 77, 78, 79 INDEX 523 Steinach, E., 454 Steinmann, G., 6 Stejneger, 117 Sterilization laws, 479, 480 Stock on graft, no influence of, m, 334 Stockard, C. R., 322, 335 St. Helena, fauna of, 107-9 St. Hilaire, E. G., 21, 22, 220 Strangevvays, T. S. P., 125 Sturtevant, A. H., 437 Subsidizing the fit, 480-82 Survival of the Fittest, 223-30 Swainson, 122 Synapsis, 298, 407 Systema Naturae, of Linnaeus, 117 Tail, vestigial in man, 152, 153 Talent, the production of, 499, 500 Tasmanian Wolf, 127 Tayler, J. L., 253-56 Taxonomy, the method of, 119-20 Teleology, 13 Termites, 214, 215 Tetrakinetic theory of Osborn, 274, 275- 83 Thales, 11 Theologians, the early Christian, 14, 15 Thompson, A., 153 Thomson, J. A., 97, 191, 205-18, 323- 30, 380-85, 393-400 Tibia, flattening of, in man, 162 Tomes, C. S., 161 Tower, W. L., 321, 322, 360, 361, 377 Toyama, K., 393, 395 Trihybrid ratio, 391, 392 Trilophodon, 76, 77, 78, 79 Tschermak, 40, 43, 380, 393 Turner, Sir W., 161 Tuttle, E., 483 Twins, evidences from in support of classification, 122 Typhlopidae, 119 Uhrschleim, 12, 16 Uniformitarianism, 22, 23 Unit characters, Mendelian, 386 Urtica dodarti, 394, 395; U. pilulifcra, 394, 395 Vam-ssa io, 314, 3^5 Variation, 307-22; classification of, 308-11; concept of, 308; continuous and discontinuous, 311, 346; and development, 311; and environment, 312; germinal, 308; nature of, 309, 310; polygons of, 366, 367; somatic, 308; statistical study of, 365-70; universality of, 307, 308 Vasectomy, 479 Vestigial structures, evidences from 60, 140-63 Virchow, R., 91, 240 Volta Bureau, 474 Wagner, M., 269, 270 Walcott, C. W., 62, 64, 67 Wallace, A. R., 17, 26, 27, 36, 97-100, 110-13, 122, 239 Walter, H. E., 373, 473-83 Warning coloration, 202, 203 Weismann, A., 3, 7, 30, 31, 32, 195, 247, 258, 260, 263-68, 281, 330-31, 428 Weismannism, 7 Weldon, W. F. R., 256, 257 Whales, comparative anatomy of, 131- Ss; embryology of, 179, iSo White, G., 206, 212, 213 White, T. H., 320 Whitman, C. O., 35 Whitney, D. D., 335 Whymper, 479 Wilberforce, Bishop, 28, 29 Williston, S. W., 35, 83, 85 Wilson, E. B., 6, 44, 401 Wings, comparative anatomy of, 134 Woltereck, R., 315, 316, 335 Woods, F. A., 466-72 Woodward, 88, 92 Woolner, 155 Wordsworth, W., 507 Wright, Sewall, 164, 165 Wyman, Professor, 150 Xenophanes, 12 Zca mays, 395 Zicgler, E., 326 Zoea larva of Pcneus, 1 70 Zygote, 165, 297