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THIS BOOK IS DUE ON THE E 
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 JECT TO AN OVERDUE FIN 
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 APR^ 1 1998 
 
YALE UNIVERSITY 
 MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES 
 
 PROBLEMS OF GENETICS 
 
SILLIMAN MEMORIAL LECTURES 
 PUBLISHED BY YALE UNIVERSITY PRESS 
 
 ELECTRICITY AND MATTER. By Joseph John Thomson, 
 d.sc, ll.d., PH.D., f.r.s., Fellow of Trinity College, Cambridge, 
 Cavendish Professor of Experimental Physics, Cambridge. 
 
 Price $1.25 net; postage 10 cents extra. 
 
 THE INTEGRATIVE ACTION OF THE NERVOUS SYSTEM. 
 By Charles S. Sherrington, d.sc, m.d., hon. ll.d.. tor., f.r.s., 
 Holt Professor of Physiology in the University of Liverpool. 
 
 Price $3.50 net; postage 25 cents extra. 
 
 RADIOACTIVE TRANSFORMATIONS. By Ernest Ruther- 
 ford, d.sc, ll.d., f.r.s., Macdonald Professor of Physics, McGill 
 University. 
 
 Price $3.50 net; postage 22 cents extra. 
 
 EXPERIMENTAL AND THEORETICAL APPLICATIONS OF 
 THERMODYNAMICS TO CHEMISTRY. By Dr. Walther 
 Nernst, Professor and Director of the Institute of Physical Chemistry 
 in the University of Berlin. 
 
 Price $1.25 net; postage 10 cents extra. 
 
 THE PROBLEMS OF GENETICS. By William Bateson, m.a., 
 f.r.s., Director of the John Innes Horticultural Institution, Mer ton 
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 Price $4.00 net; postage 25 cents extra. 
 
 STELLAR MOTIONS. With Special Reference to Motions 
 Determined by Means of the Spectrograph. By William 
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 University of California. 
 
 Price $4.00 net; postage 30 cents extra. 
 
 THEORIES OF SOLUTIONS. By Svante August Arrhenius, 
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 Price $2.25 net; postage 15 cents extra. 
 
 IRRITABILITY. A Physiological Analysis of the General 
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 Price $3-50 net; postage 20 cents extra. 
 
 THE EVOLUTION OF MODERN MEDICINE. By Sir William 
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 Oxford University. 
 
 Price $3.00 net; postage 40 cents extra. 
 
PROBLEMS OF GENETICS 
 
 BY 
 
 WILLIAM BATESON, M.A., F.R.S. 
 
 DIRECTOR OF THE JOHN INNES HORTICULTURAL INSTITUTION, HON. FELLOW OF ST. JOHN* 
 COLLEGE, CAMBRIDGE, AND FORMERLY PROFESSOR OF BIOLOGY IN THE UNIVERSITY 
 
 WITH ILLUSTRATIONS 
 
 New Haven: Yale University Press 
 
 London: Humphrey Milford 
 
 Oxford University Press 
 
 MCMXVI 
 
Copyright, 1913 
 By Yale University 
 
 First printed August, 1913, 1000 copies 
 July, iqi6, soo copies 
 
THE SILLIMAN FOUNDATION 
 
 In the year 1883 a legacy of about eighty-five thousand 
 dollars was left to the President and Fellows of Yale College 
 in the city of New Haven, to be held in trust, as a gift from her 
 children, in memory of their beloved and honored mother, Mrs. 
 Hepsa Ely Silliman. 
 
 On this foundation Yale College was requested and directed 
 to establish an annual course of lectures designed to illustrate 
 the presence and providence, the wisdom and goodness of God, 
 as manifested in the natural and moral world. These were to be 
 designated as the Mrs. Hepsa Ely Silliman Memorial Lectures. 
 It was the belief of the testator that any orderly presentation 
 of the facts of nature or history contributed to the end of this 
 foundation more effectively than any attempt to emphasize the 
 elements of doctrine or of creed ; and he therefore provided that 
 lectures on dogmatic or polemical theology should be excluded 
 from the scope of this foundation, and that the subjects should be 
 selected rather from the domains of natural science and history, 
 giving special prominence to astronomy, chemistry, geology, and 
 anatomy. 
 
 It was further directed that each annual course should be made 
 the basis of a volume to form part of a series constituting a 
 memorial to Mrs. Silliman. The memorial fund came into the 
 possession of the Corporation of Yale University in the year 1901 ; 
 and the present volume constitutes the fifth of the series of 
 memorial lectures. 
 
 ***«te 
 
 74-at 
 
PREFACE 
 
 This book gives the substance of a series of lectures delivered 
 in Yale University, where I had the privilege of holding the office 
 of Silliman Lecturer in 1907. 
 
 The delay in publication was brought about by a variety of 
 causes. 
 
 Inasmuch as the purpose of the lectures is to discuss some of 
 the wider problems of biology in the light of knowledge acquired 
 by Mendelian methods of analysis, it was essential that a fairly 
 full account of the conclusions established by them should first 
 be undertaken and I therefore postponed the present work till 
 a book on Mendel's Principles had been completed. 
 
 On attempting a more general discussion of the bearing of 
 the phenomena on the theory of Evolution, I found myself 
 continually hindered by the consciousness that such treatment 
 is premature, and by doubt whether it were not better that the 
 debate should for the present stand indefinitely adjourned. 
 That species have come into existence by an evolutionary 
 process no one seriously doubts; but few who are familiar with 
 the facts that genetic research has revealed are now inclined to 
 speculate as to the manner by which the process has been ac- 
 complished. Our knowledge of the nature and properties of 
 living things is far too meagre to justify any such attempts. 
 Suggestions of course can be made: though, however, these 
 ideas may have a stimulating value in the lecture room, they 
 look weak and thin when set out in print. The work which may 
 one day give them a body has yet to be done. 
 
 The development of negations is always an ungrateful task 
 apt to be postponed for the positive business of experiment. 
 Such work is happily now going forward in most of the centers 
 of scientific life. Of many of the subjects here treated we already 
 know more than we did in 1907. The delay in production has 
 made it possible to incorporate these new contributions. 
 
 The book makes no pretence at being a treatise and the 
 
Vlll 
 
 PREFACE 
 
 number of illustrative cases has been kept within a moderate 
 compass. A good many of the examples have been chosen from 
 American natural history, as being appropriate to a book in- 
 tended primarily for American readers. The facts are largely 
 given on the authority of others, and I wish to express my 
 gratitude for the abundant assistance received from American 
 colleagues, especially from the staffs of the American Museum 
 in New York, and of the Boston Museum of Natural History. 
 In connexion with the particular subjects personal acknowledg- 
 ments are made. 
 
 Dr. F. M. Chapman was so good as to supervise the prepara- 
 tion of the coloured Plate of Colaptes, and to authorize the loan 
 of the Plate representing the various forms of Helminthophila, 
 which is taken from his North American Warblers. 
 
 I am under obligation to Messrs. Macmillan & Co., for per- 
 mission to reproduce several figures from Materials for the Study 
 of Variation, illustrating subjects which I wished to treat in 
 new associations, and to M. Leduc for leave to use Fig. 9. 
 
 In conclusion I thank my friends in Yale for the high honour 
 they did me by their invitation to contribute to the series of 
 Silliman Lectures, and for much kindness received during a 
 delightful sojourn in that genial home of learning. 
 
TABLE OF CONTENTS. 
 
 CHAPTER PAGE 
 
 I. Introductory. The Problem of Species and 
 
 Variety i 
 
 II. Meristic Phenomena 31 
 
 III. Segmentation, Organic and Mechanical 60 
 
 IV. The Classification of Variation and the Nature 
 
 of Substantive Variation 83 
 
 Note to Chapter IV 94 
 
 V. The Mutation Theory 97 
 
 Note to Chapter V 116 
 
 VI. Variation and Locality 118 
 
 VII. Local Differentiation — continued. Overlapping 
 
 Forms 146 
 
 VIII. Locally Differentiated Forms — continued. Cli- 
 matic Varieties 164 
 
 IX. The Effects of Changed Conditions 187 
 
 X. The Effects of Changed Conditions — continued. 
 
 The Causes of Genetic Variation 212 
 
 XI. The Sterility of Hybrids. Concluding Re- 
 marks 233 
 
 Index 251 
 
PROBLEMS OF GENETICS 
 
PROBLEMS OF GENETICS 
 
 CHAPTER I 
 
 INTRODUCTORY 
 
 The purpose of these lectures is to discuss some of the 
 familiar phenomena of biology in the light of modern discoveries. 
 In the last decade of the nineteenth century many of us per- 
 ceived that if any serious advance was to be made with the group 
 of problems generally spoken of as the Theory of Evolution, 
 methods of investigation must be devised and applied of a kind 
 more direct and more penetrating than those which after the 
 general acceptance of the Darwinian views had been deemed 
 adequate. Such methods obviously were to be found in a 
 critical and exhaustive study of the facts of variation and heredity, 
 upon which all conceptions of evolution are based. To construct 
 a true synthetic theory of Evolution it was necessary that vari- 
 ation and heredity instead of being merely postulated as axioms 
 should be minutely examined as phenomena. Such a study 
 Darwin himself had indeed tentatively begun, but work of a 
 more thorough and comprehensive quality was required. In 
 the conventional view which the orthodoxy of the day prescribed, 
 the terms variation and heredity stood for processes so vague 
 and indefinite that no analytical investigation of them could be 
 contemplated. So soon, however, as systematic inquiry into 
 the natural facts was begun it was at once found that the ac- 
 cepted ideas of variation were unfounded. Variation was seen 
 very frequently to be a definite and specific phenomenon, af- 
 fecting different forms of life in different ways, but in all its 
 diversity showing manifold and often obvious indications of 
 regularity. This observation was not in its essence novel. 
 Several examples of definite variation had been well known to 
 
 2 i 
 
2 PROBLEMS OF GENETICS 
 
 Darwin and others, but many, especially Darwin himself in his 
 later years, had nevertheless been disposed to depreciate the 
 significance of such facts. They consequently then lapsed into 
 general disparagement. Upon more careful inquiry the abun- 
 dance of such phenomena proved to be far greater than was 
 currently supposed, and a discussion of their nature brought 
 into prominence a consideration of greater weight, namely 
 that the differences by which these definite or discontinuous 
 variations are constituted again and again approximate to and 
 are comparable with the class of differences by which species 
 are distinguished from each other. 
 
 The interest of such observations could no longer be denied. 
 The more they were examined the more apparent it became that 
 by means of the facts of variation a new light was obtained on 
 the physiological composition and capabilities of living things. 
 Genetics thus cease to be merely a method of investigating 
 theories of evolution or of the origin of species but provide a 
 novel and hitherto untried instrument by which the nature of 
 the living organism may be explored. Just as in the study of 
 non-living matter science began by regarding the external 
 properties of weight, opacity, colour, hardness, mode of occur- 
 rence, etc., noting only such evidences of chemical attributes 
 and powers as chance spontaneously revealed; and much later 
 proceeded to the discovery that these casual manifestations of 
 chemical properties, rightly interpreted, afford a key to the 
 intrinsic nature of the diversity of matter, so in biology, having 
 examined those features of living things which ordinary obser- 
 vations can perceive, we come at last to realize that when studied 
 for their own sake the properties of living organisms in respect 
 of heredity and variation are indications of their inner nature 
 and provide evidences of that nature which can be obtained from 
 no other source. 
 
 While such ideas were gradually forming in our minds, came 
 the rediscovery of Mendel's work. Investigations which before 
 had only been imagined as desirable now became easy to pursue, 
 and questions as to the genetic inter-relations and compositions 
 of varieties can now be definitely answered. Without prejudice 
 
INTRODUCTORY 3 
 
 to what the future may disclose whether by way of limitation 
 or extension of Mendelian method, it can be declared with 
 confidence and certainty that we have now the means of be- 
 ginning an analysis of living organisms, and distinguishing many 
 of the units or factors which essentially determine and cause the 
 development of their several attributes. 
 
 Briefly put, the essence of Mendelism lies in the discovery 
 of the existence of unit characters or factors. For an account 
 of the Mendelian method, how it is applied and what it has 
 already accomplished, reference must be made to other works. 1 
 With this part of the subject I shall assume a sufficient ac- 
 quaintance. In these lectures I have rather set myself the task 
 of considering how certain problems appear when viewed from 
 the standpoint to which the application of these methods has 
 led us. It is indeed somewhat premature to discuss such ques- 
 tions. The work of Mendelian analysis is progressing with 
 great rapidity and anything I can say may very soon be super- 
 seded as out of date. Nevertheless a discussion of this kind 
 may be of at least temporary service in directing inquiry to the 
 points of special interest. 
 
 The Problem of Species and Variety 
 Nowhere does our new knowledge of heredity and variation 
 apply more directly than to the problem what is a species and 
 what is a variety? I cannot assert that we are already in a 
 position to answer this important question, but as will presently 
 appear, our mode of attack and the answers we expect to re- 
 ceive are not those that were contemplated by our predecessors. 
 If we glance at the history of the scientific conception of Species 
 we find many signs that it was not till comparatively recent 
 times that the definiteness of species became a strict canon of 
 the scientific faith and that attempts were made to give precise 
 limits to that conception. When the diversity of living things 
 began to be accurately studied in the sixteenth and seventeenth 
 
 *In Mendel's Principles of Heredity (Cambridge University Press, 1909) 
 I have dealt with this subject, giving an account of the principal facts discovered 
 up to the beginning of 1909. 
 
4 PROBLEMS OF GENETICS 
 
 centuries names were applied in the loosest fashion, and in giving 
 a name to an animal or a plant the naturalists of those times had 
 no ulterior intention. Names were bestowed on those creatures 
 about which the writer proposed to speak. When Gesner or Aldro- 
 vandi refer to all the kinds of horses, unicorns, dogs, mermaids, 
 etc., which they had seen or read of, giving to each a descriptive 
 name, they do not mean to "elevate" each named kind to "spe- 
 cific rank"; and if anyone had asked them what they meant by 
 a species, it is practically certain that they would have had not 
 the slightest idea what the question might imply, or any suspicion 
 that it raised a fundamental problem of nature. 
 
 Spontaneous generation being a matter of daily observation, 
 then unquestioned, and supernatural events of all kinds being 
 commonly reported by many witnesses, transmutation of species 
 had no inherent improbability. Matthioli, 2 for instance, did not 
 expect to be charged with heresy when he declared Stirpium 
 mutatio to be of ordinary occurrence. After giving instances 
 of induced modifications he wrote, "Tantum enim in plantis 
 naturae germanitas potest, ut non solum saepe praedictos 
 praestet effectus, sed etiam ut alteram in alteram stirpem facile 
 vertat, ut cassiam in cinnamomum, sisymbrium in mentham, 
 triticum in lolium, hordeum in avenam, et ocymum in serpyllum." 
 
 I do not know who first emphasized the need for a clear 
 understanding of the sense in which the term species is to be 
 applied. In the second half of the seventeenth century Ray 
 shows some degree of concern on this matter. In the intro- 
 duction to the Historia Plantarum, 1686, he discusses some 
 of the difficulties and lays down the principle that varieties 
 which can be produced from the seed of the same plant are to 
 be regarded as belonging to one species, being, I believe, the 
 first to suggest this definition. That new species can come into 
 existence he denies as inconsistent with Genesis 2, in which it is 
 declared that God finished the work of Creation in six days. 
 Nevertheless he does not wholly discredit the possibility of a 
 "transmutation" of species, such that one species may as an 
 exceptional occurrence give rise by seed to another and nearly 
 
 2 Matthioli Opera, Ed. 1598, p. 8, originally published 1565. 
 
INTRODUCTORY 5 
 
 allied species. Of such a phenomenon he gives illustrations the 
 authenticity of which he says he is, against his will, compelled to 
 admit. He adds that some might doubt whether in the cases 
 quoted the two forms concerned are really distinct species, but 
 the passage is none the less of value for it shews that the con- 
 ception of species as being distinct unchangeable entities was not 
 to Ray the dogma sacrosanct and unquestionable which it 
 afterwards became. 3 
 
 In the beginning of the eighteenth century Marchant, 4 having 
 observed the sudden appearance of a laciniated variety of Mer- 
 curialis, makes the suggestion that species in general may have 
 arisen by similar mutations. Indeed from various passages it is 
 manifest that to the authors of the seventeenth and early eigh- 
 teenth centuries species appeared simply as groups more or less 
 definite, the boundaries of which it was unnecessary to determine 
 with great exactitude. Such view? were in accord with the 
 general scientific conception of the time. The mutability of 
 
 3 Ray's instances relate to Kales, and in. most of these examples we can 
 see that there was no question of mutation or transmutation at all, but that the 
 occurrence was due either to mistake or to cross-fertilisation. Sharrock, to whom 
 Ray refers, was inclined to discredit stories of transmutation, but he has also this 
 passage (History of the Propagation and Improvement of Vegetables by the Concurrence 
 of Art and Nature, Oxford, 1660, p. 29): 
 
 " It is indeed growen to be a great question, whether the transmutation of a 
 species be possible either in the vegetable, Animal, or Minerall Kingdome. For the 
 possibility of it in the vegetable; I have heard Mr. Bobart and his Son often report it, 
 and proffer to make oath that the Crocus and Gladiolus, as likewise the Leucoium, 
 and Hyacinths by a long standing without replanting have in his garden changed 
 from one kind to the other: and for satisfaction about the curiosity in the presence 
 of Mr. Boyle I tooke up some bulbs of the very numericall roots whereof the re- 
 lation was made, though the alteration was perfected before, where we saw the 
 diverse bulbs growing as it were on the same stoole, close together, but no bulb 
 half of the one kind, and the other half of the other: But the changetime being past 
 it was reason we should believe the report of good artists in matters of their own 
 faculty." 
 
 Robert Sharrock was a fellow of New College, Oxford. Both the Bobarts were 
 professional botanists, the father was author of a Catalogue of the plants in the 
 Hortus Medicus at Oxford, and the son was afterwards Curator of the Oxford 
 Garden. 
 
 i Mem. Ac. roy. des Set. for 1719 (1721), p. 59' 
 
6 PROBLEMS OF GENETICS 
 
 species is for example sometimes likened (see for instance Shar- 
 rock, loc. cit.) to the metamorphoses of insects, and it is to be 
 remembered that the search for the Philosopher's Stone by which 
 the transmutation of metals was to be effected had only recently 
 fallen into discredit as a pursuit. 
 
 The notion indeed of a peculiar, fixed meaning to be attached 
 to species as distinct from variety is I think but rarely to be 
 found categorically expressed in prae-Linnaean writings. 
 
 But with the appearance of the Systema Naturae a great 
 change supervened. Linnaeus was before all a man of order. 
 Foreseeing the immense practical gain to science that must come 
 from a codification of nomenclature, he invented such a system. 
 
 It is not in question that Linnaeus did great things for us and 
 made Natural History a manageable and accessible collection of 
 facts instead of a disorderly heap ; but orderliness of mind has 
 another side, and inventors and interpreters of systems soon attri- 
 bute to them a force and a precision which in fact they have not. 
 
 The systematist is primarily a giver of names, as Ray with 
 his broader views perceived. Linnaeus too in the exordium to 
 the Systema Naturae naively remarks, that he is setting out to 
 continue the work which Adam began in the Golden Age, to give 
 names to the living creatures. Naming however involves very 
 delicate processes of mind and of logic. Carried out by the light 
 of meagre and imperfect knowledge it entails all the mischievous 
 consequences of premature definition, and promotes facile 
 illusions of finality. So was it with the Linnaean system. An 
 interesting piece of biological history might be written respecting 
 the growth and gradual hardening of the conception of Species. 
 To readers of Linnaeus's own writings it is well known that his 
 views cannot be summarized in a few words. Expressed as they 
 were at various times during a long life and in various connexions, 
 they present those divers inconsistencies which commonly 
 reflect a mind retaining the power of development. Nothing 
 certainly could be clearer than the often quoted declaration of the 
 Philosophia Botanica, ''Species tot numeramus quot diversae 
 formae in principio sunt creatae," with the associated passage 
 "Varietates sunt plantae ejusdem speciei mutatae a caussa 
 
INTRODUCTORY 7 
 
 quacunque occasionali." Those sayings however do not stand 
 alone. In several places, notably in the famous dissertation 
 on the peloric Linaria he explicitly contemplates the possibility 
 that new species may arise by crossing, declaring nevertheless 
 that he thinks such an event to be improbable. In that essay 
 he refers to Marchant's observation on a laciniate Mercurialis, 
 but though he states clearly that that plant should only be 
 regarded as a variety of the normal, he does not express any 
 opinion that the contemporary genesis of new species must be an 
 impossibility. In the later dissertation on Hybrid Plants he 
 returns to the same topic. Again though he states the belief 
 that species cannot be generated by cross-breedings, he treats 
 the subject not as heretical absurdity but as one deserving 
 respectful consideration. 
 
 The significance of the aphorisms that precede the lectures 
 on the Natural Orders is not easy to apprehend. These are 
 expressed with the utmost formality, and we cannot doubt that 
 in them we have Linnaeus's own words, though for the record 
 we are dependent on the transcripts of his pupils. 
 
 The text of the first five is as follows: 
 
 1. Creator T. O. in primordio vestiit Vegetabile Medullar e 
 principiis constitutivis diversi Corticalis unde tot difformia 
 individua, quot Ordines Naturales prognata. 
 
 2. Classicas has (1) plantas Omnipotens miscuit inter se, 
 unde tot Genera ordinum, quot inde plantae. 
 
 3. Genericas has (2) miscuit Natura, unde tot Species con- 
 generes quot hodie existunt. 
 
 4. Species has miscuit Casus, unde totidem quot passim 
 occurrunt, Varietates. 
 
 5. Suadent haec (1-4) Creatoris leges a simplicibus ad 
 Composita. 
 
 Naturae leges generationis in hybridis. 
 
 Hominis leges ex observatis a posteriori. 
 
 I am not clear as to the parts assigned in the first sentence 
 respectively to the "Medulla" and the "Cortex," beyond that 
 Linnaeus conceived that multiformity was first brought about 
 by diversity in the "Cortex" The passage is rendered still 
 
8 PROBLEMS OF GENETICS 
 
 more obscure if read in connection with the essay on "Generatio 
 Ambigena," where he expresses the conviction that the Medulla 
 is contributed by the mother, and the Cortex by the father, both 
 in plants and animals. 5 
 
 But however that may be, he regards this original diversity 
 as resulting in the constitution of the Natural Orders, each rep- 
 resented by one individual. 
 
 In the second aphorism the Omnipotent is represented as 
 creating the genera by intermixing the individual plantae classicae, 
 or prototypes of the Natural Orders. 
 
 The third statement is the most remarkable, for in it he 
 declares that Species were formed by the act of Nature, who by 
 inter-mixing the genera produced Species congeneres, namely 
 species inside each genus, to the number which now exist. 
 Lastly, Chance or Accident, intermixing the species, produced 
 as many varieties as there are about us. 
 
 Linnaeus thus evidently regarded the intermixing of an 
 originally limited number of types as the sufficient cause of 
 all subsequent diversity, and it is clear that he draws an an- 
 tithesis between Creator, Natura, and Casus, assigning to each 
 a special part in the operations. The acts resulting in the 
 formation of genera are obviously regarded as completed within 
 the days of the Creation, but the words do not definitely show 
 that the parts played by Nature and Chance were so limited. 
 
 Recently also E. L. Greene 6 has called attention to some 
 curious utterances buried in the Species Plantarum, in which 
 Linnaeus refers to intermediate and transitional species, using 
 language that even suggests evolutionary proclivities of a 
 modern kind, and it is not easy to interpret them otherwise. 
 
 Whatever Linnaeus himself believed to be the truth, the 
 effect of his writings was to induce a conviction that the species 
 
 5 Amoen. Acad., 1789, vol. 6. I do not know whether attention has been called 
 to the curious mistake which Linnaeus makes in the course of this argument. He 
 cites the differences between the Mule and the Hinny in illustration of his thesis, 
 pointing out that the Mule is externally more like a horse and the Hinny more like 
 an ass. This, he says, is because the Mule has the horse for a father, and the 
 Hinny the ass, thus inverting the actual facts! 
 
 6 Proc. Washington Ac. Set., 1909, XI, pp. 17-26. 
 
INTRODUCTORY 9 
 
 of animals and plants were immutably fixed. Linnaeus had 
 reduced the whole mass of names to order and the old fantastical 
 transformations with the growth of knowledge had lapsed into 
 discredit; the fixity of species was taken for granted, but not 
 till the overt proclamation of evolutionary doctrine by Lamarck 
 do we find the strenuous and passionate assertions of immutability 
 characteristic of the first half of the nineteenth century. 
 
 It is not to be supposed that the champions of fixity were 
 unacquainted with varietal differences and with the problem 
 thus created, but in their view these difficulties were apparent 
 merely, and by sufficiently careful observation they supposed 
 that the critical and permanent distinctions of the true species 
 could be discovered, and the impermanent variations detected 
 and set aside. 
 
 This at all events was the opinion formed by the great body 
 of naturalists at the end of the eighteenth and beginning of the 
 nineteenth centuries, and to all intents and purposes in spite 
 of the growth of evolutionary ideas, it remains the guiding 
 principle of systematists to the present day. There are 'good 
 species' and 'bad species' and the systematists of Europe and 
 America spend most of their time in making and debating them. 
 
 In some of its aspects the problem of course confronted earlier 
 naturalists. Parkinson for instance (1640) in introducing his 
 treatment of Hieracium wrote, "To set forth the whole family of 
 the Hawkeweedes in due forme and order is such a world of 
 worke that I am in much doubt of mine own abilitie, it having 
 lyen heavie on his shoudiers that hath already waded through 
 them ... for such a multitude of varieties in forme pertaining 
 to one herbe is not to be found againe in rerurn natura as I 
 thinke," and the same idea, that the difficulty lay rather in 
 man's imperfect powers of discrimination than in the nature of 
 the materials to be discriminated, is reflected in many treatises 
 early and late. 
 
 It was however with the great outburst of scientific activity 
 which followed Linnaeus that the difficulty became acute. 
 Simultaneously vast masses of new material were being collected 
 from all parts of the world into the museums, and the products 
 
io PROBLEMS OF GENETICS 
 
 of the older countries were re-examined with a fresh zeal and on 
 a scale of quantity previously unattempted. But the problem 
 how to name the forms and where to draw lines, how much 
 should be included under one name and where a new name was 
 required, all this was felt, rather as a cataloguer's difficulty 
 than as a physiological problem. And so we still hear on the 
 one hand of the confusion caused by excessive "splitting" and 
 subdivisions, and on the other of the uncritical "lumpers" who 
 associate together under one name forms which another collector 
 or observer would like to see distinguished. 
 
 In spite of Darwin's hopes, the acceptance of his views has 
 led to no real improvement — scarcely indeed to any change at 
 all in either the practice or aims of systematists. In a famous 
 passage in the Origin he confidently declares that when his 
 interpretation is generally adopted "Systematists will be able 
 to pursue their labours as at present; but they will not be in- 
 cessantly haunted by the shadowy doubt whether this or that 
 form be a true species. This, I feel sure, and I speak after 
 experience, will be no slight relief. The endless disputes whether 
 or not some fifty species of British brambles are good species 
 will cease." Those disputes nevertheless proceed almost ex- 
 actly as before. It is true that biologists in general do not, 
 as formerly, participate in these discussions because they have 
 abandoned systematics altogether; but those who are engaged 
 in the actual work of naming and cataloguing animals and 
 plants usually debate the old questions in the old way. There 
 is still the same divergence of opinion and of practice, some in- 
 clining to make much of small differences, others to neglect 
 them. 
 
 Not only does the work of the sytematists as a whole proceed 
 as if Darwin had never written but their attitude towards these 
 problems is but little changed. In support of this statement I may 
 refer to several British Museum Catalogues, much of the Biologia 
 Centr all- Americana, Ridgway's Birds of North America, the 
 Fauna Hawaiensis, indeed to almost any of the most important 
 systematic publications of England, America, or any other 
 country. These works are compiled by the most proficient 
 
INTRODUCTORY n 
 
 systematists of all countries in the several groups, but with 
 rare exceptions they show little misgiving as to the fundamental 
 reality of specific differences. That the systematists consider 
 the species-unit as of primary importance is shown by the 
 fact that the whole business of collection and distribution of 
 specimens is arranged with regard to it. 
 
 Almost always the collections are arranged in such a way that 
 the phenomena of variation are masked. Forms intermediate 
 between two species are, if possible, sorted into separate boxes 
 under a third specific name. If a species is liable to be constantly 
 associated with a mutational form, the mutants are picked out, 
 regardless of the circumstances of their origin, from the samples 
 among which they were captured, and put apart under a special 
 name. Only by a minute study of the original labels of the 
 specimens and by redistributing them according to locality and 
 dates, can their natural relations be traced. The published 
 accounts of these collections often take no notice of variations, 
 others make them the subject of casual reference. Very few 
 indeed treat them as of much importance. From such indi- 
 cations it is surely evident that the systematists attach to the 
 conception of species a significance altogether different from that 
 which Darwin contemplated. 
 
 I am well aware that some very eminent systematists regard 
 the whole problem as solved. They hold as Darwin did that 
 specific diversity has no physiological foundation or causation 
 apart from adaptation, and that species are impermanent groups, 
 the delimitations of which are ultimately determined by en- 
 vironmental exigency or "fitness." The specific diversity of 
 living things is thus regarded as being something quite different 
 in nature from the specific diversity of inorganic substances. 
 In practice those who share these opinions are, as might be an- 
 ticipated, to be found among the 'lumpers' rather than among 
 the 'splitters.' In their work, certainly, the Darwinian theory 
 is actually followed as a guiding principle; unanalysed inter- 
 gradations of all kinds are accepted as impugning the integrity 
 of species; the underlying physiological problem is forgotten, 
 and while the product is almost valueless as a contribution to 
 
12 PROBLEMS OF GENETICS 
 
 biological research, I can scarcely suppose that it aids greatly 
 in the advances of other branches of our science. 
 
 But why is it that, with these exceptions, the consequences 
 of the admittedly general acceptance of a theory of evolution 
 are so little reflected in the systematic treatment of living things? 
 Surely the reason is that though the systematist may be con- 
 vinced of the general truth of the evolution theory at large, he 
 is still of opinion that species are really distinct things. For 
 him there are still 'good' species and 'bad' species and his ex- 
 perience tells him that the distinction between the two is not 
 simply a question of degree or a matter of opinion. 
 
 To some it may seem that this is mere perversity, a refusal 
 to see obvious truth, a manifestation of the spirit of the collector 
 rather than of the naturalist. But while recognising that from 
 a magnification of the conception of species the systematists 
 are occasionally led into absurdity I do not think the grounds 
 for their belief have in recent times been examined with the 
 consideration they deserve. The phenomenon of specific 
 diversity is manifested to a similar degree by living things be- 
 longing to all the great groups, from the highest to the lowest, 
 Vertebrates, Invertebrates, Protozoa, Vascular Plants, Algae, 
 and Bacteria, all present diversities of such a kind that among 
 them the existence of specific differences can on the whole be 
 recognised with a similar degree of success and with very similar 
 limitations. In all these groups there are many species quite 
 definite and unmistakable, and others practically indefinite. 
 The universal presence of specificity, as we may call it, simi- 
 larly limited and characterised, is one of their most remarkable 
 features. Not only is this specificity thus universally present 
 among the different forms of life, but it manifests itself in respect 
 of the most diverse characteristics which living things display. 
 Species may thus be distinguished by peculiarities of form, of 
 number, of geometrical arrangement, of chemical constitution 
 and properties, of sexual differentiation, of development, and of 
 many other properties. In any one or in several of these features 
 together, species may be found distinguished from other species. 
 It is also to be observed that the definiteness of these distinctions 
 
INTRODUCTORY 13 
 
 has no essential dependence on the nature of the characteristic 
 which manifests them. It is for example sometimes said that 
 colour-distinctions are of small systematic importance, but every 
 systematist is familiar with examples (like that of the wild species 
 of Gallus) in which colours though complex, show very little 
 variation. On the other hand features of structure, sexual dif- 
 ferentiation, and other attributes which by our standards are 
 estimated as essential, may be declared to show much variation 
 or little, not according to any principle which can be detected, 
 but simply as the attention happens to be applied to one species 
 or group of species, or to another. In many groups of animals and 
 plants observers have hit upon characters which were for a time 
 thought to be finally diagnostic of species. The Lepidoptera and 
 Diptera for instance, have been re-classified according to their 
 neuration. Through a considerable range of forms determinations 
 may be easily made on these characters, but as is now well 
 known, neuration is no more immune from variation than any 
 other feature of organisation, and in some species great varia- 
 bility is the rule. Again it was once believed by some that the 
 genitalia of the Lepidoptera provided a basis of final determination 
 — with a similar sequel. In some groups, for example the Lycae- 
 nidae, or the Hesperidae, there are forms almost or quite in- 
 distinguishable on external examination, but a glance at the 
 genitalia suffices to distinguish numerous species, while on the 
 contrary among Pieridae a great range of species show scarcely 
 any difference in these respects: and again in occasional species 
 the genitalia show very considerable variations. 
 
 The proposition that animals and plants are on the whole 
 divisible into definite and recognisable species is an approxi- 
 mation to the truth. Such a statement is readily defensible, 
 whereas to assert the contrary would be palpably absurd. For 
 example, a very competent authority lately wrote: "In the 
 whole Lepidopterous fauna of England there is no species of 
 really uncertain limits." 7 Others may be disposed to make 
 certain reservations, but such exceptions would be so few as 
 scarcely to impair the validity of the general statement. The 
 
 7 J. W. Tutt, in Ent. Rec, 1909, XXI, p. 185. 
 
i 4 PROBLEMS OF GENETICS 
 
 declaration might be extended to other orders and other 
 lands. 
 
 We know, of course, that the phenomenon of specific diversity 
 is complicated by local differentiation: that, in general, forms 
 which cannot disperse themselves freely exhibit a multitude of 
 local races, and that of these some are obviously adaptative, 
 and that a few even owe their peculiarity to direct environmental 
 effects. Every systematist also is perfectly aware that in dealing 
 with collections from little explored countries the occurrence 
 of polymorphism or even of sporadic variation may make the 
 practical business of distinguishing the species difficult and 
 perhaps for the time impossible; still, conceding that a great 
 part of the diversity is due to geographical differentiation, and 
 that some is sporadic variation, our experience of our own floras 
 and faunas encourages the belief that if we were thoroughly 
 familiar with these exotic productions it would usually be 
 possible to assign their specific limitations with an approach 
 to certainty. 
 
 For apart from any question of the justice of these wider 
 inferences, if we examine the phenomenon of specificity as it 
 appears in those examples which are nearest to hand, surely we 
 find signs in plenty that specific distinction is no mere consequence 
 of Natural Selection. The strength of that proposition has 
 lain mainly in the appeal to ignorance. Steadily with the growth 
 of knowledge has its cogency diminished, and such a belief 
 could only have been formulated at a time when the facts of 
 variation were unknown. 
 
 In Darwin's time no serious attempt had been made to ex- 
 amine the manifestations of variability. A vast assemblage of 
 miscellaneous facts could formerly be adduced as seemingly 
 comparable illustrations of the phenomenon "Variation." 
 Time has shown this mass of evidence to be capable of analysis. 
 When first promulgated it produced the impression that varia- 
 bility was a phenomenon generally distributed amongst living 
 things in such a way that the specific divisions must be arbitrary. 
 When this variability is sorted out, and is seen to be in part a 
 result of hybridisation, in part a consequence of the persistence 
 
INTRODUCTORY 15 
 
 of hybrids by parthenogenetic reproduction, a polymorphism 
 due to the continued presence of individuals representing various 
 combinations of Mendelian allelomorphs, partly also the tran- 
 sient effect of alteration in external circumstances, we see how 
 cautious we must be in drawing inferences as to the indefiniteness 
 of specific limits from a bare knowledge that intermediates exist. 
 Conversely, from the accident of collocation or from a mislead- 
 ing resemblance in features we deem essential, forms genetically 
 distinct are often confounded together, and thus the divergence 
 of such forms in their other features, which we declare to be 
 non-essential, passes as an example of variation. Lastly, and 
 this is perhaps the most fertile of all the sources of confusion, 
 the impression of the indefiniteness of species is created by the 
 existence of numerous local forms, isolated geographically from 
 each other, forms whose differences may be referable to any one 
 of the categories I have enumerated. 
 
 The advance has been from many sides. Something has 
 come from the work of systematists, something from cultural 
 experiments, something from the direct study of variation as it 
 appears in nature, but progress is especially due to experimental 
 investigation of heredity. From all these lines of inquiry we 
 get the same answer; that what the naturalists of fifty years 
 ago regarded as variation is not one phenomenon but many, 
 and that what they would have adduced as evidence against 
 the definiteness of species may not in fact be capable of this 
 construction at all. 
 
 If we may once more introduce a physical analogy, the dis- 
 tinctions with which the systematic naturalist is concerned in 
 the study of living things are as multifarious as those by which 
 chemists were confronted in the early days of their science. 
 Diversities due to mechanical mixtures, to allotropy, to differences 
 of temperature and pressure, or to degree of hydration, had all 
 to be severally distinguished before the essential diversity due 
 to variety of chemical constitution stood out clearly, and I 
 surmise that not till a stricter analysis of the diversities of animals 
 and plants has been made on a comprehensive scale, shall we 
 be in a position to declare with any confidence whether there is 
 
16 PROBLEMS OF GENETICS 
 
 or is not a natural and physiological distinction between species 
 and variety. 
 
 As I have said above, it is in the cases nearest to hand that 
 the problem may be most effectively studied. Comparison 
 between forms from dissimilar situations contributes something; 
 but it is by a close examination of the behaviour, especially the 
 genetic behaviour, of familiar species when living in the presence 
 of their nearest allies that the most direct light on the problem 
 is to be obtained. I cannot understand the attitude of those who, 
 contemplating such facts as this examination elicits, can com- 
 placently declare that specific difference is a mere question of 
 degree. With the spread of evolutionary ideas to speak much 
 of the fixity of species has become unfashionable, and yet how 
 striking and inscrutable are the manifestations of that fixity! 
 
 Consider the group of species composing the agrestis section 
 of the genus Veronica, namely Tournefortii, agrestis, and 
 polita. 
 
 These three grow side by side in my garden, as they do in 
 suitable situations over a vast area of the temperate regions. 
 I have for years noticed them with some care and become familiar 
 with their distinctions and resemblances. Never is there any 
 real doubt as to the identity of any plant. The species show 
 some variability, but I have never seen one which assumed any 
 of the distinguishing features of the others. A glance at the 
 fruits decides at once to which species a plant belongs. I find 
 it impossible to believe that the fixity of these distinctions is 
 directly dependent on their value as aids in the struggle for 
 existence. The mode of existence of the three forms in so far 
 as we can tell is closely similar. By whatever standard we reckon 
 systematic affinity I suppose we shall agree that these species 
 come very near indeed to each other. Bentham even takes 
 the view that polita is a mere variety of agrestis. 
 
 Now in such cases as this it has been argued that the specific 
 features of the several types have been separately developed 
 in as many distinct localities, and that their present association 
 is due to subsequent redistribution. Of these Veronicas indeed 
 we know that one, Tournefortii (=Buxbaumii) is as a matter of fact 
 
INTRODUCTORY 17 
 
 a recent introduction from the east. 8 But this course of argument 
 leads to still further difficulties. For if it is true that the peculiari- 
 ties of the several species have been perfected and preserved on 
 account of their survival-value to their possessors, it follows 
 that there must be many ways of attaining the same result. 
 But since sufficient adaptation may be ensured in so many ways, 
 the disappearance of the common parent of these forms is dif- 
 ficult to understand. Obviously it must have been a plant 
 very similar in general construction to its modern representatives. 
 Like them it must have been an annual weed, with an organisation 
 conformable to that mode of life. Why then, after having been 
 duly perfected for that existence should it have been entirely 
 superseded in favour of a number of other distinct contrivances 
 for doing the same thing, and — if a gradual transition be predi- 
 cated — not only by them, but by each intermediate stage 
 between them and the original progenitor? Surely the obvious 
 inference from such facts is that the burden cast upon the theory 
 of gradual selection is far greater than it can bear; that adapta- 
 tion is not in practice a very close fit, and that the distinctions 
 between these several species of Veronica have not arisen on 
 account of their survival-value but rather because none of their 
 diversities was so damaging as to lead to the extermination of 
 its possessor. When we see these various Veronicas each rigidly 
 reproducing its parental type, all comfortably surviving in 
 competition with each other, are we not forced to the conclusion 
 that tolerance has as much to do with the diversity of species 
 as the stringency of Selection? Certainly these species owe their 
 continued existence to the fact that they are each good enough 
 to live, but how shall we refer the distinctions between them di- 
 rectly or indirectly to the determination of Natural Selection? 
 
 8 E. Lehmann {Bull. VHerb. Boissier, Ser. 2, VIII, 1908, p. 229) has published 
 an admirable paper on the interrelationships of these species and has instituted 
 cultural experiments which will probably much elucidate the nature of their specific 
 distinctness. As regards the existence of intermediate forms he comes to the conclu- 
 sion that two only can be so regarded. The first was described by Kuntze from 
 specimens found on a flower-pot on board a Caspian steamer, for which Leh- 
 mann proposes the new specific name Siaretensis. This comes between polita and 
 filiformis, a close ally of Tournefortii. The other, which combines some of the 
 features of both polita and Tournefortii, was found in the province of Asterabad. 
 3 
 
1 8 PROBLEMS OF GENETICS 
 
 The control of Selection is loose while the conformity to 
 specific distinction is often very strict and precise, and no less 
 so even when several closely related species co-exist in the same 
 area and in the same circumstances. 
 
 The theory of Selection fails at exactly the point where it 
 was devised to help: Specific distinction. 
 
 Let us examine a somewhat different set of facts in the case 
 of another pair of nearly allied species Lychnis diurna and ves- 
 pertina. The two plants have much in common. Both are 
 dioecious perennials, with somewhat similar flowers, the one 
 crimson, the other white. Each however has its peculiarities 
 which are discernible in almost any part of its structure, whether 
 flower, leaf, fruit or seed, distinctions which would enable a 
 person thoroughly familiar with the plants to determine at once 
 from which species even a small piece had been taken. There 
 is so much resemblance however as readily to support the surmise 
 that the two were mere varieties of one species. Bentham, 
 following Linnaeus, in fact actually makes this suggestion, 
 with what propriety we will afterwards consider. Now this case 
 is typical of many. The two forms have a wide distribution, 
 occurring sometimes separately, sometimes in juxtaposition. 
 L. diurna is a plant of hedgerows and sheltered situations. L. 
 ves pertina is common in fields and open spaces, where diurna 
 is hardly ever found; but not rarely vespertina occurs in associa- 
 tion with diurna in the places which that plant frequents. In 
 this case I do not doubt that we have to do with organisms of 
 somewhat different aptitudes. That L. vespertina has powers 
 which diurna has not is shown very clearly by the fact that 
 diurna is sometimes entirely absent from areas where vespertina 
 can abound. 9 But in order to understand the true genetic 
 relations of the two plants to each other it is necessary to observe 
 their behaviour when they meet as they not unfrequently do. 
 
 9 In Cambridgeshire for example vespertina is common but diurna is absent. 
 Whether this absence is connected with the general presence of chalk I cannot say. 
 When introduced artificially diurna establishes itself, for a time at least, without any 
 apparent difficulty and occasionally escapes from the garden on to the neighbouring 
 roadside. 
 
INTRODUCTORY i 9 
 
 If the Lychnis population of such a locality be examined it will 
 be found to consist of many undoubted and unmodified diurna, 
 a number — sometimes few, sometimes many — of similarly 
 unmodified vespertina, and an uncertain but usually rather small 
 proportion of plants obviously hybrids between the two. How 
 is it possible to reconcile these facts with the view that specific 
 distinction has no natural basis apart from environmental 
 exigency? 
 
 Darwinian orthodoxy suggests that by a gradual process of 
 Natural Selection either one of these two types was evolved from 
 the other, or both from a third type. I cannot imagine that 
 anyone familiar with the facts would propose the first hypothesis 
 in the case of Lychnis, nor can I conceive of any process, whether 
 gradual or sudden, by which diurna could have come out of 
 vespertina, or vespertina out of diurna. Both however may no 
 doubt have been derived from some original third type. It is 
 conceivable that Lychnis macrocarpa of Boissier, a native of 
 Southern Spain and Morocco, may be this original form. This 
 species is said to combine a white flower (like that of L. ves- 
 pertina), with capsule-teeth rolled back (like those of diurna). 10 
 But whatever the common progenitor may have been, if we are 
 to believe that these two species have been evolved from it by 
 a gradual process of Natural Selection based on adaptation, 
 enormous assumptions must be made regarding the special fitness 
 of these two forms and the special unfitness of the common 
 parent, and these assumptions must be specially invoked and 
 repeated for each several feature of structure or habits distin- 
 guishing the three forms. 
 
 Why, if the common parent was strong enough to live to give 
 rise to these two species, is it either altogether lost now, or at 
 least absent from the whole of Northern Europe? Its two 
 putative descendants, though so distinct from each other, are, 
 as we have seen, able often to occupy the same ground. If 
 they were gradually derived from a common progenitor — 
 necessarily very like themselves — can we believe that this original 
 
 10 Conceivably however it may be a segregated combination. For an account 
 of this plant see Boissier, Voy. Bot. Midi de I'Espagne, 1839, II, 722. 
 
20 PROBLEMS OF GENETICS 
 
 form should always, in all the diversities of soil and situation 
 which they inhabit, be unable to exist? Some one may fancy 
 that the hybrids which are found in the situations occupied by 
 both forms are this original parental species. But nothing can 
 be more certain than that these plants are simply heterozygous 
 combinations made by the union of gametes bearing the characters 
 of diurna and vespertina. 11 For they may be reproduced exactly 
 in Fi or in later generations of that cross when it is artificially 
 made; when bred from, their families exhibit palpable phenomena 
 of segregation more or less complex; and usually, if perhaps not 
 always, they are partially sterile. 12 In a locality on the Norfolk 
 coast that I know well, there is a strip of rough ground chiefly 
 sand-bank, which runs along the shore. This ground is full of 
 vespertina. Not a hundred yards inland is a lane containing 
 diurna, and among the vespertina on the sand-bank are always 
 some of the hybrid form, doubtless the result of fertilisation 
 from the heighbouring diurna population. Seed saved from these 
 hybrids gave vespertina and hybrids again, having obviously been 
 fertilised by other vespertina or by other hybrids, and I have 
 no doubt that such hybrid plants if fertilised by diurna would 
 have shown some diurna offspring. The absence of diurna 
 in such localities may fairly be construed as an indication that 
 diurna is there at a real disadvantage in the competition for 
 life. 
 
 But if, admitting this, we proceed to consider how the special 
 aptitude of vespertina is constituted, or what it is that puts 
 diurna at a disadvantage, we find ourselves quite unable to 
 show the slightest connexion between the success of one or the 
 
 11 A discussion of this subject with references to literature is given by Rolfe, 
 in an excellent paper on "Hybridisation viewed from the standpoint of Systematic 
 Botany" {Jour. R. Hort. Soc, XXIV, 1900, p. 197). He concludes: "The simple 
 fact is that the two plants (L. diurna and vespertina) are thoroughly distinct in 
 numerous particulars, and affect such different habitats that in some localities 
 one or the other of them is completely wanting. But when their stations are 
 adjacent they hybridise together very readily, and it is here that these intermediate 
 forms occur which have puzzled botanists so much." The same paper contains 
 valuable information concerning several cognate illustrations. 
 
 12 In only two cases have I seen such plants (both females) completely sterile. 
 
INTRODUCTORY 21 
 
 failure of the other on the one hand, and the specific character* 
 istics which distinguish the two forms on the other. The or- 
 thodox Selectionist would, as usual, appeal to ignorance. We 
 ask what can vespertina gain by its white flowers, its more lan- 
 ceolate leaves, its grey seeds, its almost erect capsule-teeth, 
 its longer fruits, which diurna loses by reason of its red flowers, 
 more ovate leaves, dark seeds, capsule-teeth rolled back, and 
 shorter fruits? We are told that each of these things may 
 affect the viability of their possessors. We cannot assert that 
 this is untrue, but we should like to have evidence that it is true. 
 The same problem confronts us in thousands upon thousands 
 of examples, and as time goes on we begin to feel that speculative 
 appeals to ignorance, though dialectically admissible, provide 
 an insufficient basis for a proposition which, if granted, is to 
 become the foundation of a vast scheme of positive construction. 
 
 One thing must be abundantly clear to all, that to treat two 
 forms so profoundly different as one, because intermediates of 
 unknown nature can be shown to exist between them, is a mere 
 shirking of the difficulties, and this course indeed creates artificial 
 obstacles in the way of those who are seeking to discover the 
 origin of organic diversity. 
 
 In the enthusiasm with which evolutionary ideas were re- 
 ceived the specificity of living things was almost forgotten. 
 The exactitude with which the members of a species so often 
 conform in the diagnostic, specific features passed out of account ; 
 and the scientific world by dwelling with a constant emphasis 
 on the fact of variability, persuaded itself readily that spe- 
 cies had after all been a mere figment of the human mind. 
 Without presuming to declare what future research only can 
 reveal, I anticipate that, when variation has been properly 
 examined and the several kinds of variability have been suc- 
 cessfully distinguished according to their respective natures, 
 the result will render the natural definiteness of species increas- 
 ingly apparent. Formerly in such a case as that of the two 
 Lychnis species, the series of "intermediates" was taken to be a 
 palpable proof that vespertina "graded" to diurna. It is this 
 fact, doubtless, upon which Bentham would have relied in sug- 
 
22 PROBLEMS OF GENETICS 
 
 gesting that both may be one species. 13 Genetic tests, though as 
 yet imperfectly applied, make it almost certain that these inter- 
 grading forms are not in any true sense variations from either 
 species in the direction of the other, but combinations of elements 
 derived from both. 
 
 The points in which very closely allied species are distin- 
 guished from each other may be found in the most diverse 
 features of their organisation. Sometimes specific difference 
 is to be seen in a character which we can believe to be important 
 in the struggle, but at least as often it is some little detail that 
 we cannot but regard as trivial which suffices to differentiate 
 the two species. Even when the diagnostic point is of such a 
 nature that we can imagine it to make a serious difference in the 
 economy we are absolutely at a loss to suggest why this feature 
 should be a necessity to species A and unnecessary to species B 
 its nearest ally. The house sparrow (Passer domesticus) is in 
 general structure very like the tree sparrow (P. montanus). 
 They differ in small points of colour. For instance montanus 
 has a black patch on the cheek which is absent in domesticus. 
 The presence in the one species and the absence in the other 
 are equally definite, and in both cases we are equally unable to 
 suggest any consideration of utility in relation to these features. 
 The two species are distinguished also by a characteristic that 
 may well be supposed to be of great significance. In domesticus 
 .the two sexes are strongly differentiated, the cock being more 
 ornate than the hen. On the other hand the two sexes in mon- 
 tanus are alike, and, if we take a standard from domesticus, we 
 may fairly say that in montanus the hen has the colouration of 
 the male. It is not unreasonable to suppose that such a dis- 
 tinction may betoken some great difference in physiological 
 economy, but the economical significance of this perhaps im- 
 portant distinction is just as unaccountable as that of the seem- 
 ingly trivial but equally diagnostic colour-point. 
 
 I have spoken of the fixed characteristics of the two species. 
 
 13 As is well known, in an even more notorious example, he proposed to unite 
 Primula vulgaris, P. elatior, and P. acaulis, similarly relying on the existence of 
 "intermediates," which we now well know to be mongrels between the species. 
 
INTRODUCTORY 23 
 
 If we turn to a very different feature, their respective liability 
 to albinistic variation, we find ourselves in precisely similar 
 difficulty. Passer domesticus is a species in which individuals 
 more or less pied occur with especial frequency, but in P. mon- 
 tanus such variation is extremely rare if it occurs at all. The 
 writer of the section on Birds in the Royal Natural History 
 (III., 1894-5, P- 393) cans attention to this fact and remarks 
 that in that species he knows no such instance. 
 
 The two species therefore, apart from any differences that we 
 can suppose to be related to their respective habits, are charac- 
 terised by small fixed distinctions in colour-markings, by a 
 striking difference in secondary sexual characters, and by a 
 difference in variability. In all these respects we can form 
 no surmise as to any economic reason why the one species 
 should be differentiated in the one way and the other in the other 
 way, and I believe it is mere self-deception which suggests the 
 hope that with fuller knowledge reasons of this nature would be 
 discovered. 
 
 The two common British wasps, Vespa vulgaris and Vespa 
 germanica, are another pair of species closely allied although 
 sharply distinguished, which suggest similar reflexions. Both 
 usually make subterranean nests but of somewhat ^different 
 materials. V. vulgaris uses rotten wood from which the nest 
 derives a characteristic yellow colour, while V. germanica scrapes 
 off the weathered surfaces of palings and other exposed timber, 
 material which is converted into the grey walls of the nest. The 
 stalk by which the nest is suspended (usually to a root) in the 
 case of germanica passes freely through a hole in the external 
 envelope, but vulgaris unites this external wall solidly to the 
 stalk. In bodily appearance and structure the two species are 
 so much alike that they have often been confounded even by 
 naturalists, and to the untrained observer they are quite indis- 
 tinguishable. There are nevertheless small points of difference 
 which almost though not quite always suffice to distinguish the 
 two forms. For example the yellow part of the sinus of the eyes 
 is emarginate in vulgaris but not emarginate in germanica. V, 
 vulgaris often has black spots on the tibiae while in germanica the 
 
24 PROBLEMS OF GENETICS 
 
 tibiae are usually plain yellow. In both species there is a hori- 
 zontal yellow stripe on the thorax, but whereas in vulgaris this is 
 a plain narrow stripe, it is in germanica enlarged downwards in 
 the middle. These and other apparently trivial details of colour- 
 ation, though not absolutely constant, are yet so nearly constant 
 that irregularities in these respects are quite exceptional. Lastly 
 the genitalia of the males, though not very different, present 
 small structural points of distinction which are enough to distin- 
 guish the two species at a glance. 14 
 
 In considering the meaning of the distinctions between these 
 two wasps we meet the old problem illustrated by the Sparrows. 
 The two species have somewhat different habits of life and we 
 should readily expect to find differences of bodily organisation 
 corresponding with the differences of habits. But is that what 
 we do find? Surely not. To suppose that there is a corre- 
 spondence between the little points of colour and structure which 
 we see and the respective modes of life of the two species is 
 perfectly gratuitous. We have no inkling of the nature of such 
 a correspondence, how it can be constituted, or in what it may 
 consist. 
 
 Is it not time to abandon these fanciful expectations which 
 are never realised? Everywhere both among animals and plants 
 does the problem of specific difference reiterate itself in the same 
 form. In view of such facts as I have related and might indefi- 
 nitely multiply, the fixity of specific characters cannot readily 
 be held to be a measure of their economic importance to their 
 possessors. The incidence of specific fixity is arbitrary and 
 capricious, sometimes lighting on a feature or a property which 
 can be supposed to matter much, but as often is it attached to the 
 most trifling of superficial peculiarities. 
 
 The incidence of variability is no less paradoxical, and without 
 investigation of the particular case no one can say what will be 
 
 14 For an account of the distinctions between Vespa vulgaris and germanica 
 see Ch. Janet, Etudes sur les Fourmis, les Guipes et les Abeilles, n e , Note. Sur 
 Vespa germanica et V. vulgaris. Limoges (Ducourtieux), 1895; and R. du Buysson, 
 Monographic des Guepes, Ann. Soc. Ent. France, 1903, Vol. LXXII, p. 603, PI, 
 VIII. 
 
INTRODUCTORY 25 
 
 found to show much or little variability. The very charac- 
 teristic which in one species may exhibit extreme variability 
 may in an allied species show extreme constancy. Illustrations 
 will occur to any naturalist, but nowhere is this truth more 
 strikingly presented than in the British Noctuid Moths. Many 
 are so variable that, in the common phrase, "scarcely two can 
 be found alike," while others show comparatively slight variation. 
 It need scarcely be remarked that, in the instances I have in 
 mind, the evidence of great variability is in no way due to the 
 abundance with which the particular species occurs, for common 
 species may show constancy, and less abundant species may show 
 great variability. The polymorphism seems to be now at least 
 a general property of the variable species, as the fixity is a 
 property of the fixed species. In illustration I may refer to the 
 following examples. 
 
 Dianthoecia capsincola is a common and widely distributed 
 moth which feeds on Lychnis. It shows little variation. Dian- 
 thoecia carpophaga is another species which feeds chiefly on 
 Silene. Its habits are very similar to those of capsincola. Like 
 that species it has a wide geographical range and is abundant 
 in its localities, but in contrast to the fixity of capsincola, car- 
 pophaga exhibits a complex series of varieties. Agrotis stiff usa 
 (= ypsilon) is a moth widely spread through the southern half 
 of England. It is very constant in colour and markings. Ag- 
 rotis segetum and tritici are excessively variable both in ground 
 colour and markings, being found in an immense profusion of 
 dissimilar forms throughout their distribution. Of these and 
 several other species of Agrotis there are many named varieties, 
 some of which have by various writers been regarded as speci- 
 fically distinct. Of the genus Noctua many species (e. g. f estiva) 
 show a similar polymorphism, but N. triangulum, though showing 
 some variation in certain respects, is usually very constant to 
 its type, and the same is true of N. umbrosa. 
 
 In several species of Taeniocampa, especially instabilis, the 
 multiplicity of forms is extreme, while cruda (= pulverulenta) 
 is a comparatively constant species. The genus Plusia contains 
 a number of constant species, but in Plusia interrogationis we 
 
26 PROBLEMS OF GENETICS 
 
 meet the fact that the central silvery mark undergoes endless 
 variation. "Truly no two are alike," says Mr. Tutt, "and to 
 look down a long series of inter? ogationis is something like looking 
 at a series of Chinese characters." In contrast to this we have 
 the fact that in Plusia gamma the very similar silvery mark is 
 by no means variable. 
 
 I have taken this series of cases from the Noctuid moths, 
 but it would be as easy to illustrate the same proposition from 
 the Geometridae or the Micro-Lepidoptera. 15 I have a long 
 series of Peronea cristana, for example, which was given tome by 
 Mr. W. H . B . Fletcher, of Bognor . All were beaten out of the same 
 hedge, and their polymorphism is such that no one unaccustomed 
 to such examples could suppose that they belonged to a single 
 species. Another common form, P. schalleriana, which lives in 
 similar circumstances, exhibits comparatively slight variability. 
 
 It should be expressly noted that the variation of which I am 
 speaking is a genuine polymorphism. Several of the species 
 enumerated exhibit also geographical variation, possessing defi- 
 nite and often strikingly distinct races peculiar to certain 
 localities; but apart from the existence of such local differen- 
 tiation, stands out the fact upon which I would lay stress, that 
 some species are excessively variable while others are by com- 
 parison constant, in circumstances that we may fairly regard 
 as comparable. 
 
 This fact is difficult to reconcile with the conventional view 
 that specific type is directly determined by Natural Selection 
 
 15 The statements made above are for the most part taken from Barrett, C. G., 
 Lepidoptera of the British Islands, and from Tutt, J. W., The British Noctuae and 
 their Varieties. The reader who is unfamilar with the amazing polymorphism 
 exhibited by some of these moths should if possible take an opportunity of looking 
 over a long series in a collection, or, if that be impossible, refer to the admirable 
 coloured plates published by Barrett. It may not be superfluous to observe that 
 plenty of similar examples are known in other countries. For instance Plotheia 
 frontalis, a Noctuid which often abounds in Ceylon, shows an equally bewildering 
 wealth of forms. If a dozen specimens of such a species were to be brought home 
 from some little known country, each individual would almost certainly be described 
 as the type of a distinct species. (See the coloured plate published by Sir G. Hamp- 
 son, Cat. Brit. Mus., Heterocera, Vol. IX.) 
 
INTRODUCTORY 27 
 
 and that the precision with which a species conforms to its 
 pattern is an indication of the closeness of that control. Anyone 
 familiar with the characteristics of Moths will agree that the 
 Noctuids, Geometrids and Tortricids are creatures whose existence 
 depends in some degree on the success with which they can escape 
 detection by their enemies in the imaginal state. We are there- 
 fore not surprised to find that some species of these orders 
 exhibit definite geographical variation in conformity with the 
 character of the ground, which may reasonably be supposed to 
 aid in their protection. If this were all, there would be nothing to 
 cause surprise. We might even be disposed to allow that varia- 
 bility might contribute to the perpetuation of animals so situ- 
 ated, on the principle that among a variety of surroundings 
 some would probably be in harmony with the objects on which 
 they rest. But we cannot admit the plausibility of an argument 
 which demands on the one hand that the extreme precision with 
 which species A adheres in the minutest details of its colour and 
 pattern to a certain type shall be ascribed to the protective fitness 
 of those details, and on the other hand that the abundant varia- 
 bility of species B shall be ascribed to the same determination. 
 If it is absolutely necessary for A to conform to one type how 
 comes it that B may range through some twenty distinct forms, 
 any two of which differ more from each other than the regular 
 species of many other genera? The only reply I can conceive 
 is a suggestion that there may be some circumstance which 
 differentiates the various classes of cases, that the exigencies 
 of the fixed species may be different from those of the variable. 
 Those who make such appeals to ignorance do not always perhaps 
 realise whither this course of reasoning may lead. If admissible 
 here the same argument would lead us to suggest that because 
 albino moles have for an indefinite period occurred on a certain 
 land near Bath there may be something in the soil or in the 
 conditions of life near Bath which requires a proportion of albinos 
 in its mole population. Or again, because the butterfly Thais 
 rumina in one locality, Digne in the south of France, has a per- 
 centage of individuals of the variety Honor atii (with certain 
 normally yellow spots on the hind wing coloured bright red) 
 
28 PROBLEMS OF GENETICS 
 
 and nowhere else throughout its distribution, that therefore we 
 may suggest that there is some difference in the condition of 
 life at Digne which makes the continuance of Honoratii there 
 possible and beneficial. 
 
 A polymorphism offering a parallel to that of the variable 
 moths is afforded by the breeding plumage of the Ruff, the 
 male of Machetes pugnax. The variety of plumage which these 
 cocks exhibit is such that the statement that no two can be 
 found alike is only a venial exaggeration. Newton remarks 16 
 ''that all this wonderful 'show' is the consequence of the poly- 
 gamous habit of the Ruff can scarcely be doubtful"; but even if 
 it be conceded that the great external differentiation of the 
 cocks may be a result of sexual selection, the problem of their 
 polymorphism remains unsolved, for, as we are well aware, 
 polygamy is not usually associated with polymorphism of the 
 male. The Black Cock (Tetrao tetrix), for example, is as polyga- 
 mous as the Ruff, but in that and countless other cases, both 
 sexes are constant to one type of plumage. 
 
 When we thus compare the polymorphism of one species with 
 the fixity of another, and attempt to determine the causes which 
 have led to these extraordinary contrasts, two distinct lines of 
 argument are open to us. We may ascribe the difference either 
 to causes external to the organisms, primarily, that is to say, 
 to a difference in the exigencies of Adaptation under Natural 
 Selection; or on the other hand we may conceive the difference 
 as due to innate distinctions in the chemical and physiological 
 constitutions of the fixed and the variable respectively. There 
 is truth undoubtedly in both conceptions. If the mole were 
 physiologically incapable of producing an albino that variety 
 would not have come into being, and if the albino were totally 
 incapable of getting its living it would not be able to hold its 
 
 16 Diet, of Birds, p. 800. It would be interesting and profitable to attempt in 
 a long series of Ruffs to determine the Mendelian factors which by their combinations 
 give r?se to this complex assemblage of varietal forms. A few such factors both of 
 colour and pattern can be at once distinguished, and it is noticeable that some of 
 the resulting types of barring, spangling and penciling show a perceptible corre- 
 spondence with some of the types of colouration found in the breeds of domestic 
 fowls. 
 
INTRODUCTORY 29 
 
 own. Were Plotheia frontalis constructed on a chemical plan 
 which admitted of no variation, the countless varieties would 
 not have been produced ; and if one of its varieties had an over- 
 whelming success out of all proportion to that of the rest, then 
 the species would soon become monomorphic again. We 
 cannot declare that Natural Selection has no part in the deter- 
 mination of fixity or variability; nevertheless looking at the whole 
 mass of fact which a study of the incidence of variation provides, 
 I incline to the view that the variability of polymorphic forms 
 should be regarded rather as a thing tolerated than as an element 
 contributing directly to their chances of life; and on the other 
 hand that the fixity of the monomorphic forms should be looked 
 upon not so much as a proof that Natural Selection controls 
 them with a greater stringency, but rather as evidence of a natural 
 and intrinsic stability of chemical constitution. 
 
 Compare the condition of a variable form like the male 
 Ruff (or in a less degree the Red Grouse in both its sexes) with 
 that of the common Pheasant which is comparatively constant. 
 In the Pheasant no doubt variations do occur as in other wild 
 birds, but apart from the effects of mongrelisation the species 
 is unquestionably uniform. Could it seriously be proposed 
 that we should regard the constancy of the pheasant's plumage 
 in this country as depending on the special fitness of that type 
 of colouration? Even if the pheasant be not an alien in Western 
 Europe, it has certainly been protected for centuries, and for a 
 considerable period has existed in a state of semi-domestication. 
 Such conditions should give good opportunity for polymorphism 
 to be produced. In some coverts various aberrations do of 
 course occur and persist, yet there is nothing indicative of a 
 general relaxation of the fixity of the specific type, and the pheas- 
 ant remains substantially a fixed species. 1 The common pheasant 
 (Phasianus colchicus) even shows little of that disposition to 
 
 1 Howard Saunders (Illust. Manual of British Birds, 1899, p. 499) states that 
 there is evidence that the pheasant had become naturalized in the south of England 
 before the Norman invasion. He adds, "little, if any, deviation from the typical 
 P. colchicus took place up to the end of last century, when the introduction of 
 the Chinese Ring-necked P. torquatus commenced, which has left almost indelible 
 marks, especially with regard to the characteristic white collar." 
 
3 o PROBLEMS OF GENETICS 
 
 form local races which appears in the species of Further India. 
 Are we not then on safer ground in regarding the fixity of our 
 species as a property inherent in its own nature and consti- 
 tution? Just as in ages of domestication no rose has ever given 
 off a blue variety so has the pheasant never broken out into the 
 polymorphism of the Ruff. 
 
 As soon as it is realised how largely the phenomena of vari- 
 ation and stability must be an index of the internal constitution 
 of organisms, and not mere consequences of their relations to 
 the outer world, such phenomena acquire a new and more 
 profound significance. 
 
 
CHAPTER II 
 
 MERISTIC PHENOMENA 
 
 Twenty years ago in describing the facts of Variation, argu- 
 ment was necessary to show that these phenomena had a special 
 value in the sciences of Zoology and Botany. This value is 
 now universally understood and appreciated. In spite however 
 of the general attention devoted to the study of Variation, and 
 the accumulation of material bearing on the problem, no satis- 
 factory or searching classification of the phenomena is possible. 
 The reason for this failure is that a real classification must pre- 
 suppose knowledge of the chemistry and physics of living things 
 which at present is quite beyond our reach. 
 
 It is however becoming probable that if more knowledge of 
 the chemical and physical structure of organisms is to be at- 
 tained, the clue will be found through Genetics, and thus that 
 even in the uncoordinated accumulation of facts of Variation 
 we are providing the means of analysis applicable not only to 
 them, but to the problems of normality also. 
 
 The only classification that we can yet institute with any 
 confidence among the phenomena of Variation is that which 
 distinguishes on the one hand variations in the processes of 
 division from variations in the nature of the substances divided. 
 
 Variations in the processes of division are most often made 
 apparent by a change in the number of the parts, and are therefore 
 called Meristic Variations, while the changes in actual composition 
 of material are spoken of as Substantive Variations. The Me- 
 ristic Variations form on the whole a natural and fairly well 
 defined group, but the Substantive Variations are obviously 
 a heterogeneous assemblage. 
 
 Though this distinction does not go very far, it is useful, 
 and in all probability fundamental. It is of value inasmuch as 
 it brings into prominence the distinct and peculiar part which 
 
 31 
 
32 PROBLEMS OF GENETICS 
 
 the process of division, or, more generally, repetition of parts, 
 plays in the constitution of the forms of living things. 
 
 That there may be a real independence between the M eristic 
 and the Substantive phenomena is evident from the fact both 
 that Meristic changes may occur without Substantive Variation, 
 and that the substances composing an organism may change 
 without any perceptible alteration in its meristic structure. 
 When the distinction between these two classes of phenomena 
 is perceived it will be realised that the study of genetics has on 
 the one hand a physical, or perhaps more strictly a mechanical 
 aspect, which relates to the manner in which material is divided 
 and distributed; and also a chemical aspect, which relates to 
 the constitution of the materials themselves. Somewhat as 
 the philosophers of the seventeenth and eighteenth centuries 
 were awaiting both a chemical and a mechanical discovery which 
 should serve as a key to the problems of unorganised matter, 
 so have biologists been awaiting two several clues. In Mendelian 
 analysis we have now, it is true, something comparable with the 
 clue of chemistry, but there is still little prospect of penetrating 
 the obscurity which envelops the mechanical aspect of our phe- 
 nomena. To make clear the application of the terms chemical 
 and mechanical to the problem of Genetics the nature of that 
 problem must be more fully described. In its most concrete 
 form this problem is expressed in the question, how does a cell 
 divide? If the organism is unicellular, and the single cell is 
 the whole body, then the process of heredity is accomplished 
 in the single operation of cell-division. Similarly in animals and 
 plants whose bodies are made up of many cells, the whole process 
 of heredity is accomplished in the cell-divisions by which the 
 germ-cells are formed. When therefore we see a cell dividing, 
 we are witnessing the process by which the form and the proper- 
 ties of the daughter-cells are determined. 
 
 Now this process has the two aspects which I have called 
 mechanical and chemical. The term " Entwicklungsmechanik" 
 has familiarised us with the application of the word mechanics 
 to these processes, but on reflexion it will be seen that this com- 
 prehensive term includes two sorts of events which are sometimes 
 
MERISTIC PHENOMENA 33 
 
 readily distinguishable. There is the event by which the cell 
 divides, and the event by which the two halves or their descend- 
 ants are or may be differentiated. It is common knowledge that 
 in some cell-divisions two similar halves, indistinguishable in 
 appearance, properties, and subsequent fate, may be produced, 
 while in other divisions daughter-cells with distinct properties 
 and powers are formed. We cannot imagine but that in the 
 first case, when the resulting cells are identical, the division 
 is a mechanical process by which the mother-cell is simply 
 cut in two; while in order that two differentiated halves may be 
 produced, some event must have taken place by which a chemical 
 distinction between the two halves is effected. 1 In any ordinary 
 Mendelian case we have a clear proof that such a chemical dif- 
 ference may be established between germ-cells. The facts of 
 colour-inheritance for instance prove that germ-cells, otherwise 
 identical, may be formed possessing the chromogen-f actor which 
 is necessary to the formation of colour in the flowers, or destitute 
 of that factor. Similarly the germ-cells may possess the ferment 
 which, by its action on the chromogenic substance, produces 
 the colour, or they may be without that ferment. The same 
 line of argument applies to a great range of cases. Nevertheless, 
 though differences in chemical properties are often thus consti- 
 tuted by cell-divisions, and though we are thus able to make a 
 quasi-chemical analysis of the individual by determining and 
 enumerating these properties, yet it is evident that the dis- 
 tribution of these factors is not itself a chemical process. This 
 is proved by the fact that similar divisions may be effected be- 
 tween halves which are exactly alike, and also by the fact that 
 the numbers in which the various types of germ-cells are formed 
 negative any suggestion of valency between them. The recog- 
 nition of the unit-factors may lead — indeed must lead — to great 
 advances in chemical physiology which without that clue would 
 have been impossible, but in causation the chemical phenomena 
 of heredity must be regarded as secondary to the physical or 
 
 1 In saying this we make no assumption as to the particular cell-division at 
 which differentiation occurs. This may be one of the maturation-divisions, or it 
 may perhaps be much earlier. 
 
34 PROBLEMS OF GENETICS 
 
 mechanical phenomena by which the cells and their constituents 
 are divided and separated. When therefore we speak of the 
 essential phenomena of heredity we mean the mechanics of divi- 
 sion, especially, though not, as we shall see, exclusively, of cell- 
 division ; and in the relation between the two halves of the divid- 
 ing cell we have the problem presented in what seems to be its 
 simplest form. 
 
 In attempting to form some conception of the processes by 
 which bodily characteristics are transmitted, or — to avoid that 
 confusing metaphor of "transmission" — how it comes about 
 that the offspring can grow to resemble its parent, continuity of 
 the germ-substance which in some animals is a visible phenom- 
 enon, 2 gives at least apparent help. An egg for example on be- 
 coming adult develops in certain parts a particular pigment. 
 The eggs of that adult when they reach the appropriate age de- 
 velop the same pigment. We have no clear picture of the 
 mechanism by which this process is effected, but when we realise 
 that the pigment results from the interaction of certain sub- 
 stances, and that since all the eggs are in reality pieces of the same 
 material, it seems, unless we inquire closely, not unnatural that 
 the several pieces of the material should exhibit the same colours 
 at the same periods of their development. The continuity of 
 the material of the germs suggests that there is a continuity 
 of the materials from which the pigment is formed, and that 
 thus an actual bit of those substances passes into each egg 
 ready at the appropriate moment to generate the pigment. 
 The argument thus outlined applies to all substantive character- 
 istics. In each case we can imagine, if we will, the appearance 
 of that characteristic as due to the contribution of its rudiment 
 from the germ tissues. 
 
 When we consider more critically it becomes evident that 
 the aid given by this mental picture is of very doubtful reality, 
 for even if it were true that any predestined particle actually 
 corresponding with the pigment-forming materials is definitely 
 
 2 From the recent discoveries of Erwin Baur we are led to surmise that in the 
 flowering plants the sub-epidermal layer, or some of its elements, may legitimately 
 be regarded as a similar germ-substance, continuous in Weismann's sense. 
 
MERISTIC PHENOMENA 35 
 
 passed on from germ to germ, yet the power of increase which 
 must be attributed to it remains so incomprehensible that the 
 mystery is hardly at all illuminated. 
 
 When however we pass from the substantive to the meristic 
 characters, the conception that the character depends on the 
 possession by the germ of a particle of a specific material becomes 
 even less plausible. Hardly by any effort of imagination can we 
 see any way by which the division of the vertebral column into x 
 segments or into y segments, or of a Medusa into 4 segments or 
 into 6, can be determined by the possession or by the want of a 
 material particle. The distinction must surely be of a different 
 order. If we are to look for a physical analogy at all we should 
 rather be led to suppose that these differences in segmental 
 numbers corresponded with changes in the amplitude or number 
 of dividing waves than with any change in the substance or 
 material divided. 
 
 Phenomena of Division 
 I have said that in the division of a cell we seem to see the 
 problem in its simplest form, but it is important to observe that 
 the problem of division may be presented by the bodies of animals 
 and plants in forms which are independent of the divisions be- 
 tween cells. The existence of pattern implies a repetition of 
 parts, and repetition of parts when developed in a material 
 originally homogeneous can only be created by division. Cell- 
 division is probably only a special case of a process similar to 
 that by which the pattern of the skeleton is laid down in a uni- 
 cellular body such as that of a Radiolarian or Foraminiferan. 
 Attempts have lately been made to apply mathematical treat- 
 ment to problems of biology. It has sometimes seemed to 
 me that it is in the geometrical phenomena of life that the most 
 hopeful field for the introduction of mathematics will be found. 
 If anyone will compare one of our animal patterns, say that of a 
 zebra's hide, with patterns known to be of purely mechanical 
 production, he will need no argument to convince him that there 
 must be an essential similarity between the processes by which 
 the two kinds of patterns were made and that parts at least of 
 
36 PROBLEMS OF GENETICS 
 
 the analysis applicable to the mechanical patterns are applicable 
 to the zebra stripes also. Patterns mechanically produced are 
 of many and very diverse kinds. One of the most familiar 
 examples, and one presenting some especially striking analogies 
 to organic patterns, is that provided by the ripples of a mackerel 
 sky, or those made in a flat sandy beach by the wind or the ebbing 
 tide. With a little search we can find among the ripple-marks, and 
 in other patterns produced by simple physical means, the closest 
 parallels to all the phenomena of striping as we see them in our 
 animals. The forking of the stripes, the differentiation of two 
 "faces," the deflections round the limbs and so forth, which in the 
 body we know to be phenomena of division, are common both to 
 the mechanical and the animal patterns. We cannot tell what in 
 the zebra corresponds to the wind or the flow of the current, but 
 we can perceive that in the distribution of the pigments, that is 
 to say, of the chromogen-substances or of the ferments which 
 act upon them, a rhythmical disturbance has been set up which 
 has produced the pattern we see; and I think we are entitled to 
 the inference that in the formation of patterns in animals and 
 plants mechanical forces are operating which ought to be, and 
 will prove to be, capable of mathematical analysis. The com- 
 parison between the striping of a living organism and the sand- 
 ripples will serve us yet a little farther, for a pattern may either 
 be formed by actual cell-divisions, and the distribution of dif- 
 ferentiation coincidently determined, or — as visibly in the pig- 
 mentation of many animal and plant tissues — the pattern may 
 be laid down and the pigment (for example) distributed through 
 a tissue across or independently of the cell-divisions of the tissue. 
 Our tissues therefore are like a beach composed of sands of 
 different kinds, and different kinds of sands may show distinct 
 and interpenetrating ripples. When the essential analogy be- 
 tween these various classes of phenomena is perceived, no one 
 will be astonished at, or reluctant to admit, the reality of dis- 
 continuity in Variation, and if we are as far as ever from knowing 
 the actual causation of pattern we ought not to feel surprised that 
 it may arise suddenly or be suddenly modified in descent. Biol- 
 ogists have felt it easier to conceive the evolution of a striped 
 
3& PROBLEMS OF GENETICS 
 
 animal like a zebra from a self-coloured type like a horse (or 
 of the self-coloured from the striped) as a process involving many 
 intergradational steps; but so far as the pattern is concerned, the 
 change may have been decided by a single event, just as the 
 multitudinous and ordered rippling of a beach may be created 
 or obliterated at one tide. 
 
 This point is well illustrated by the tusk of an Indian elephant 
 which I lately found in a London sale-room. This tusk is by 
 some unknown cause, presumably a chronic inflammation, 
 thrown up into thirteen well-marked ridges which closely simulate 
 a series of segments (Fig. i). Whatever the cause the condition 
 shows how easily a normally unsegmented structure may be 
 converted into a series of repeated parts. 
 
 The spread of segmentation through tissues normally unseg- 
 mented is very clearly exemplified in the skates' jaws shown in 
 Fig. 2. The right side of the upper figure shows the normal 
 arrangement in the species Rhinoptera jussieni, but the structure 
 on the left side is very different. The probable relations of the 
 several rows of teeth to the normal rows is indicated by the let- 
 tering, but it is evident that by the appearance of new planes 
 of division constituting separate centers of growth, the series has 
 been recast. The pattern of the left side is so definite that had 
 the variation affected the right side also, no systematist would 
 have hesitated to give the specimen a new specific name. The 
 other two drawings show similar variations of a less extensive 
 kind, the nature of which is explained by the lettering of the 
 rows of teeth. 
 
 This power to divide is a fundamental attribute of life, and 
 of that power cell-division is a special example. In regard to 
 almost all the chief vital phenomena we can say with truth that 
 science has made some progress. If I mention respiration, meta- 
 bolism, digestion, each of these words calls to mind something 
 more than a bare statement that such acts are performed by an 
 animal or a plant. Each stands for volumes of successful ex- 
 periment and research. But the expression cell-division, the 
 fundamental act which typifies the rest, and on which they all 
 depend, remains a bare name. We can see with the microscope 
 
MERISTIC PHENOMENA 
 
 39 
 
 the outward symptoms of division, but we have no surmise as 
 to the nature o the proeess by which the division is beg n o 
 accomplished. I know nothing which to a man well trained Z 
 scaent.Sc knowledge and method brings so vivid a reaTsatio„ 
 JJ In Oe Ob 
 
 a J' G : *' JaW ?/ f Skates C"***.»0 showing meristic variation. (For a detailed 
 d.senss.on see Materials for the Study of Variation, p. 259.) ed 
 
 of our ignorance of the nature of life as the mystery of cell- 
 tw'rT WhatisaIivi "g tWng? The best answer in few words 
 that I know is one which my old teacher, Michael Foster, used 
 to g, ve in his lectures introductory to biology. "A living thing 
 
40 PROBLEMS OF GENETICS 
 
 is a vortex of chemical and molecular change." This description 
 gives much, if not all, that is of the essence of life. The living 
 thing is unlike ordinary matter in the fact that, through it, matter 
 is always passing. Matter is essential to it; but, provided 
 that the flow in and out is unimpeded, the life-process can go 
 on so far as we know indefinitely. Yet the living "vortex" 
 differs from all others in the fact that it can divide and throw 
 off other "vortices," through which again matter continually 
 swirls. 
 
 We may perhaps take the parallel a stage further. A simple 
 vortex, like asmoke-ring, if projected in a suitable way will twist 
 and form two rings. If each loop as it is formed could grow and 
 then twist again to form more loops, we should have a model 
 representing several of the essential features of living things. 
 
 It is this power of spontaneous division which most sharply 
 distinguishes the living from the non-living. In the excellent 
 book dealing with the problems of development, lately published 
 by Mr. Jenkinson a special emphasis is very properly laid on the 
 distinction between the processes of division, and those of dif- 
 ferentiation. Too often in discussions of the developmental 
 processes the distinction is obscured. He regards differentiation 
 as the "central difficulty." "Growth and division of the nucleus 
 and the cells," he tells us, are side-issues. This view is quite 
 defensible, but I suspect that the division is the central difficulty, 
 and that if we could get a rationale of what is happening in 
 cell-division we should not be long before we had a clue to the 
 nature of differentiation. It may be self-deception, but I do 
 not feel it impossible to form some hypothesis as to the mode of 
 differentiation, but in no mood of freest speculation are we ever 
 able to form a guess as to the nature of the division. We see 
 differentiations occurring in the course of chemical action, in 
 some phenomena of vibration and so forth: but where do we 
 see anything like the spontaneous division of the living cell? 
 Excite a gold-leaf electroscope, and the leaves separate, but we 
 know that is because they were double before. In electrolysis 
 various substances separate out at the positive and negative 
 poles respectively. Now if in cell-division the two daughter- 
 
MERISTIC PHENOMENA 4I 
 
 cells were always dissimilar— that is to say, if differentiation 
 always occurred— we could conceive some rough comparison 
 with such dissociations. But we know the dissimilarity between 
 daughter-cells is not essential. In the reproduction of unicel- 
 lular organisms and many other cases, the products formed at the 
 two poles are, so far as we can tell, identical. Any assumption 
 to the contrary, if we were disposed to make it, would involve 
 us in difficulties still more serious. At any rate, therefore, if 
 differentiation be really the central difficulty in development, 
 it is division which is the essential problem of heredity. 
 
 Sir George Darwin and Professor Jeans tell us that "gravita- 
 tional instability" consequent on the condensation of gases is 
 "the primary agent at work in the actual evolution of the uni- 
 verse," which has led to the division of the heavenly bodies. The 
 greatest advance I can conceive in biology would be the dis- 
 covery of the nature of the instability which leads to the continual 
 division of the cell. When I look at a dividing cell I feel as an 
 astronomer might do if he beheld the formation of a double 
 star: that an original act of creation is taking place before me. 
 Enigmatical as the phenomenon seems, I am not without hope 
 that, if it were studied for its own sake, dissociated from the 
 complications which obscure it when regarded as a mere incident 
 in development, some hint as to the nature of division could be 
 found. It is I fear a problem rather for the physicist than for 
 the biologist. The sentiment may not be a popular one to utter 
 before an assembly of biologists, but looking at the truth imper- 
 sonally I suspect that when at length minds of first rate analytical 
 power are attracted to biological problems, some advance will 
 be made of the kind which we are awaiting. 
 
 The study of the phenomena of bodily symmetry offers 
 perhaps the most hopeful point of attack. The essential fact 
 in reproduction is cell-division, and the essential basis of heredi- 
 tary resemblance is the symmetry of cell-division. The phenom- 
 ena of twinning provide a convincing demonstration that this 
 is so. By twinning we mean the production of equivalent struc- 
 tures by division. The process is one which may affect the whole 
 body of an animal or plant, or certain of its parts. The term 
 
42 PROBLEMS OF GENETICS 
 
 twin as ordinarily used refers to the simultaneous birth of two 
 individuals. Those who are naturalists know that such twins 
 are of two kinds, (i) twins that are not more alike than any other 
 two members of the same family, and (2) twins that are so much 
 alike that even intimate friends mistake them. These latter 
 twins, except in imaginative literature, are always of the same 
 sex. 
 
 It is scarcely necessary for me to repeat the evidence from 
 which it has been concluded that without doubt such twins arise 
 by division of the same fertilised ovum. There is a perfect 
 series of gradations connecting them with the various forms of 
 double monsters united by homologous parts. They have been 
 shown several times to be enclosed in the same chorion, and the 
 proofs of experimental embryology show that in several animals 
 by the separation of the two first hemispheres of a dividing egg 
 twins can be produced. Lastly we have recently had the ex- 
 traordinarily interesting demonstration of Loeb, to which I may 
 specially refer. Herbst some years ago found that in sea water, 
 from which all lime salts had been removed, the segments of the 
 living egg fall apart as they are formed. Using this method 
 Loeb has shown that a temporary immersion in lime-free sea 
 water may result in the production of 90 per cent, of twins. 
 We are therefore safe in regarding the homologous or "identical" 
 twins as resulting from the divisions of one fertilised egg f while 
 the non-identical or "fraternal" twins, as they are called, arise 
 by the fertilisation of two separate ova. 3 
 
 In the resemblance of identical twins we have an extreme case 
 
 8 These fraternal twins, which show no special resemblance to each other, 
 are like the multiple births of other animals, and there is no disposition for them to 
 be of the same sex. In the sheep, for example, statistics show that the frequency 
 of pairs of twins, male and female, is approximately double that of the frequency 
 of pairs, both male or both female, as it should be if the sex-distribution were for- 
 tuitous. For instance Bernadin (La Bergerie de Rambouillet, 1890, p. 100) gives 
 the following figures for twin- lambs in Merinos: both male, 87; both female, 83; 
 sexes mixed, 187. The o-banded Armadillo (Dasypus novemcinctus) , in which 
 the young born in one litter are said to be always of one sex, is the only known 
 exception in Vertebrates, and is presumably a genuine case of normal polyembryony 
 (see especially, Rosner, Bull. Ac. Soc. Cracovie, 1901, p. 443, and Newman and 
 Patterson, Biol. Bull, XVII, 1909, p. 181, and an important paper lately published 
 by H. H. Newman and J. T. Patterson, Jour. Morph., 19 11, XXII, p. 855. 
 
MERISTIC PHENOMENA 43 
 
 of hereditary likeness 4 and a proof, if any were needed, that the 
 cause of individual variation is to be sought in the differentiation 
 of germ-cells. The resemblance of identical twins depends on 
 two circumstances, First, since only two germ-cells take part 
 in their production, difference between the germ cells of the 
 same individual cannot affect them. Secondly the division of 
 the fertilised ovum, the process by which they became two in- 
 stead of one, must have been a symmetrical division. The 
 structure of twins raises however one extremely significant 
 difficulty, which as yet we cannot in any way explain. The 
 resemblance between twins is a phenomenon of symmetry, 
 like the resemblance between the two sides of a bilaterally sym- 
 metrical body. Not only is the general resemblance readily 
 so interpreted, but we know also that in double monsters, namely 
 unseparated twins, various anatomical abnormalities shown by 
 the one half-body are frequently shown by the other half- 
 also. 6 The tw r o belong to one system of symmetry How then 
 does it happen that the body of one of a pair of twins does not 
 show a transposition of viscera? We know that the relation of 
 right and left implies that the one should be the mirror-image of 
 the other. Such a relation of images may be maintained even 
 in minute details. For example if the same pattern of finger- 
 print is given by the fingers of the two hands, one is the reverse 
 of the other. In double monsters, namely unseparated twins, 
 there is evidence that an inversion of viscera does occur with 
 some frequency. Evidence from such cases is not so clear and 
 simple as might be expected, because as a matter of fact, the 
 heart and stomach, upon which the asymmetry of the viscera 
 chiefly depend, are usually common to the two bodies. Du- 
 plicity generally affects either the anterior end alone, or the pos- 
 terior end alone. The division is generally from the heart forwards, 
 giving two heads and two pairs of anterior limbs on a common 
 trunk, or from the heart backwards, giving two pairs of posterior 
 limbs with the anterior body common. In either case, though 
 
 4 A good collection of evidence as to disease in homologous twins was lately 
 published by E. A. Cockayne, Brit. Jour. Child. Diseases, Nov., 191 1. 
 B Cp. Windle, B. C. A., Jour. Anat. Phys., XXVI, p. 295. 
 
44 PROBLEMS OF GENETICS 
 
 the bodies may be grouped in a common system of symmetry, 
 neither can be proved to show definite reversal of the parts. To 
 see that reversal recourse must be had to more extreme duplica- 
 tions, such as the famous Siamese Twins. They, as a matter of 
 fact, were an excellent instance of the proposition that twins 
 are related as mirror-images, for both of them had eleven pairs 
 of ribs instead of the normal twelve, and one of them had a 
 partial reversal of viscera. 6 (Kiichenmeister, Verlagerung, etc., 
 p. 204.) 
 
 If anyone could show how it is that neither of a pair of 
 twins has transposition of viscera the whole mystery of division 
 would, I expect, be greatly illuminated. 7 At present we have 
 simply to accept the fact that twins, by virtue of their detach- 
 ment from each other, have the power of resuming the polarity 
 which is proper to any normal individual. It was nevertheless 
 with great interest that I read Wilder's recent observation 8 that 
 occasionally in identical twins the finger-print of one or both 
 the index-fingers may be reversed, showing that there is after 
 all some truth in the notion that reversal should occur in them. 
 
 There is another phenomenon of twinning which, if we could 
 understand it, might help. I refer to the free-martin, the subject 
 of one of John Hunter's masterpieces of anatomical description. 
 In horned cattle twin births are rare, and when twins of opposite 
 sexes are born, the male is perfect and normal, but the repro- 
 
 6 Mr. E. Nettleship tells me that in the course of collecting pedigrees of families 
 containing colour-blind members he has discovered two cases (shortly to be pub- 
 lished) of pairs of twins, which on account of their very close resemblances must 
 be deemed homologous, one of each pair being colour-blind and the other normal. 
 Such a distinction between closely similar twins is most cuiious and unexpected. 
 
 7 Another paradoxical phenomenon of the same nature occurs in the Narwhal 
 The males normally have the left tusk alone developed, the corresponding right 
 tusk remaining as an undeveloped rudiment in its socket. The left tusk is a 
 left-handed screw. Occasionally the right tusk is also developed and grows to 
 the same length as that of the left side, but in such specimens the right tusk is 
 also a left-hand screw like the tusk of the other side, instead of being reversed 
 as we should certainly have expected. It need scarcely be remarked that in the 
 case of the horns of antelopes, and in other examples of spiral organs arranged in 
 pairs, that of one side of the body is the mirror image of that on the other side. 
 The Narwhal's tusks in being both twisted in the same direction are thus highly 
 anomalous, and are comparable with pairs of twins. 
 
 8 Wilder, H. H., Amer. Jour. Anat., 1904, III, p. 452. 
 
MERISTIC PHENOMENA 45 
 
 ductive organs of the female are deformed and sterile, being 
 known as a free-martin. The same thing occasionally happens 
 in sheep, suggesting that in sheep also twins may be formed by 
 the division of one ovum; for it is impossible to suppose that 
 mere development in juxtaposition can produce a change of this 
 character. I mention the free-martin because it raises a question 
 of absorbing interest. It is conceivable that we should interpret 
 it by reference to the phenomenon of gynandromorphism, seen 
 occasionally in insects, and also in birds as a great rarity. In 
 the gynandromorph one side of the body is male, the other female. 
 A bullfinch for instance has been described with a sharp line of 
 division down the breast between the red feathers of the cock 
 on one side and the brown feathers of the hen on the other. 
 (Poll,H., SB. Ges. Nat. Fr., Berlin, 1909, p. 338.) In such cases 
 neither side is sexually perfect. If the halves of such a gynan- 
 dromorph came apart, perhaps one would be a free-martin. 
 
 The behaviour of homologous twinning in heredity has been 
 little studied. It does not exist as a normal feature in any animal 
 which is amenable to experiment, and we cannot positively 
 assert that a comparable phenomenon exists in plants; for in 
 them — the Orange, for example — polyembryony may evidently 
 be produced by a parthenogenetic development of nucellar tissue. 
 It is possible that in Man twinning is due to a peculiarity of the 
 mother, not of the father. It may and not rarely does descend 
 from mother to daughter, but whether it can be passed on 
 through a male generation to a daughter again, there is not 
 sufficient evidence to show. The facts as far as they go are 
 consistent with the inference which may be drawn from Loeb's 
 experiment, that the twinning of a fertilized ovum may be de- 
 termined not by the germ-cells which united to form it, but by 
 the environment in which it begins to develop. The opinion that 
 twinning may descend through the male directly has been lately 
 expressed by Dr. J. Oliver in the Eugenics Review (191 2), on the 
 evidence of cases in which twins had occurred among the rela- 
 tions of fathers of twins, but I do not know of any comprehen- 
 sive collection of evidence bearing on the subject. 
 
 Besides twinning of the whole body a comparable duplicity 
 
46 PROBLEMS OF GENETICS 
 
 of various parts of the same body may occur. Such divisions 
 affect especially those organs which have an axis of bilateral 
 symmetry, such as the thumb, a cotyledon, a median petal, 
 the frond of a fern or the anal fin of a fish. From the little 
 yet known it is clear that the genetic analysis of these conditions 
 must be very difficult, but evidence of any kind regarding them 
 will be valuable. We want especially to know whether these 
 divisions are due to the addition of some factor or power which 
 enables the part to divide, or whether the division results from 
 the absence of something which in the normal body prevents 
 the part from dividing. Breeding experiments, so far as they 
 go, suggest that the less divided state is usually dominant to 
 the more divided. 9 The two-celled Tomato fruit is dominant to 
 the many-celled type. The Manx Cat's tail, with its suppression 
 of caudal segmentation is a partial dominant over the normal 
 tail. The tail of the Fowl in what is called the "Rumpless" 
 condition is at least superficially comparable with that of the 
 Manx Cat, and though the evidence is not wholly consistent, 
 Davenport obtained facts indicating that this suppressed con- 
 dition of the caudal vertebrae is an imperfect dominant. 10 
 
 Some evidence may also be derived from other examples of 
 differences which at first sight appear to be substantive though 
 they are more probably meristic in ultimate nature. The 
 distinction between the normal and the "Angora" hair of the 
 Rabbit is a case in point. We can scarcely doubt that one of 
 the essential differences between these two types is that in the 
 Angora coat the hair-follicles are more finely divided than they 
 are in the normal coat, and we know that the normal, or less- 
 divided condition, is dominant to the Angora, or more finely 
 divided. 
 
 In the case of the solid-hoofed or "mule-footed" swine, the 
 
 9 Polydactylism which is often a dominant, and the web-foot of Pigeons which 
 is recessive, should be remembered as possible exceptions (see p. 49). 
 
 10 Davenport inclined at first to regard rumplessness as a recessive, but in his 
 latest oublication on the subject he definitely concludes that it is an imperfect 
 dominant. This conclusion accords well with evidence quoted by Darwin (An. 
 and Pits., II, ed. 2, p. 4) that rumpless fowls may throw tailed offspring. (Amer. 
 Nat., 1910, XLIV, p. 134.) 
 
MERISTIC PHENOMENA 47 
 
 evidence shows, as Spillman has lately pointed out," that the 
 condition behaves as a dominant. The essential feature of 
 this abnormality is that the digits III and IV are partially 
 united. The union is greatest peripherally. Sometimes the 
 
 I 
 
 G 
 
 Fig. 3. I, II, III, various degrees of syndactyly affecting the medius and 
 annularis in the hand; IV, syndactyly affecting the index and medius in the foot 
 (After Annandale.) 
 
 third phalanges only are joined to form one bone, but the second 
 and even the first phalanges may also be compounded together. 
 Here the variation is obviously meristic and consists in a failure 
 "Spillman, W. J. t Amer, Breeders Mag., ioio, I, p. 178. 
 
4 8 
 
 PROBLEMS OF GENETICS 
 
 to divide, the normal separation of the median digits of the foot 
 being suppressed. 
 
 Webbing between the digits, in at least some of its mani- 
 festations, is a variation of similar nature. The family recorded 
 by Newsholme 12 very clearly shows the dominance of this con- 
 dition. The case is morphologically of great interest and must 
 undoubtedly have a bearing on the problems of the mechanics 
 of Division. In discussing the phenomena of syndactylism 
 I pointed out some years ago that the digits most frequently 
 united in the human hand are III and IV, while in the foot, 
 
 rn+m 
 
 Pig. 4. Case of complete syndactyly in the foot. II and III, digit apparently 
 representing the index and medius. c- + c 3 , bone apparently representing the 
 middle and external cuneiform; cb, cuboid; c 1 , internal cuneiform. (After Gruber.) 
 
 union most frequently takes place between II and III. 13 In 
 Newsholme's family the union was always between II and III 
 of the foot, except in the case of one male who had the digits 
 III and IV of the right hand alone webbed together. There 
 can be little doubt that the geometrical system on which the 
 foot is planned has an axis of symmetry passing between the 
 digits II and III, while the corresponding axis in the hand passes 
 between III and IV. Union between such digits may therefore 
 be regarded as comparable with any non -division or " coalescence " 
 of lateral structures in a middle line, and when as in these ex- 
 
 12 Newsholme, Lancet, December 10, 1910, p. 1690. 
 
 13 Materials for the Study of Variation, 1894, p. 358. 
 
MERISTIC PHENOMENA 49 
 
 amples such a condition is shown to be a dominant we cannot 
 avoid the inference that some concrete factor has the power of 
 suppressing or inhibiting this division. Figs. 3 and 4 illus- 
 trate degrees of union between digits in the human hand and 
 foot. 
 
 It is not in question that various other forms of irregular 
 webbing and coalescence of digits exist, and respecting the genetic 
 behaviour of these practically nothing is as yet known, Such 
 a case is described by Walker, 14 in which the first and second 
 metacarpals of both feet were fused in mother and daughter, 
 and several more are found in literature. Contrasted with these 
 phenomena we have the curious fact that in the Pigeon, Staples- 
 Browne found webbing of the toes a recessive character. The 
 question thus arises whether this webbing is of the same nature 
 as that shown to be a dominant in Man, and indeed whether the 
 phenomenon in pigeons is really meristic at all. There is some 
 difference perceptible between the two conditions; for in Man 
 there is not so much a development of a special web-like skin 
 uniting the digits as a want of proper division between the digits 
 themselves, and in extreme cases two digits may be represented 
 by a single one. In the Pigeon I am not aware that a real 
 union of this kind has ever been observed, and though the web-like 
 skin may extend the whole length of the digits and be so narrow 
 as to prevent the spread of the toes, it may, I think, be main- 
 tained that the unity of the digits is unimpaired. For the 
 present the nature of this variation in the pigeon's feet must be 
 regarded as doubtful, and we should note that if it is actually 
 an example of a more perfect division being dominant to a less 
 perfect division, the case is a marked exception to the general 
 rule that non-division is dominant to division. 
 
 Reference must also be made to the phenomenon of fasciation 
 in the stems of plants. As Mendel showed in the case of Pis urn 
 this condition is often a recessive. The appearances suggest 
 that the difference between a normal and a fasciated plant 
 consists in the inability of the fasciated plant to separate its 
 lateral branches. The nature of the condition is however very 
 
 "Walker, G., Johns Hnpliins Hospital Bulletin, XII, 1901. P- 129. 
 5 
 
5 o PROBLEMS OF GENETICS 
 
 obscure and it is equally likely that some multiplication of the 
 growing point is the essential phenomenon. 15 
 
 Stockard's interesting experiments 16 illustrate this question. 
 He showed that by treating the embryos of a fish (Fundulus 
 heteroclitus) with a dilute solution of magnesium salts, various 
 cyclopian monstrosities were frequently produced. These have 
 been called cases of fusion of the optic vesicles. I would prefer 
 to regard them as cases of a division suppressed or restricted by 
 the control of the environment. Conversely, the splendid dis- 
 covery of Loeb, that an unfertilised egg will divide and develop 
 parthenogenetically without fertilisation, as a consequence of 
 exposure to various media, may be interpreted as suggesting that 
 the action of those media releases the strains already present 
 in the ovum, though I admit that an interpretation based on the 
 converse hypothesis, that the medium acts as a stimulus, is as 
 yet by no means excluded. 
 
 In these cases we come nearest to the direct causation or 
 the direct inhibition of a division, but the meaning of the 
 evidence is still ambiguous. I incline to compare Loeb's par- 
 thenogenesis with the development (and of course accompany- 
 ing cell-division) of dormant buds on stems which have been 
 cut back. 
 
 It is interesting to note that sometimes as an abnormality, 
 the faculty of division gets out of hand and runs a course ap- 
 parently uncontrolled. A remarkable instance of this condition 
 is seen in Begonia " phyllomaniaca" which breaks out into buds 
 at any point on the stem, petioles, or leaves, each bud having, 
 like other buds, the power of becoming a new plant if removed. 
 We would give much to know the genetic properties of B. phyl- 
 lonianiaca, and in conjunction with Mr. W. O. Backhouse I have 
 for some time been experimenting with this plant. It proved 
 totally sterile. Its own anthers produce no pollen, and all at- 
 tempts to fertilise it with other species failed though the pollen 
 of a great number of forms was tried. 
 
 Recently however we have succeeded in making plants which 
 
 15 Cp. R. H. Compton, New Phytologist 191 1, p. 249. 
 
 16 Arch. f. Entwickelungsmech., 1907. XXIII, p. 249. 
 
MERISTIC PHENOMENA 
 
 5i 
 
 are in every respect Begonia phyllomaniaca, so far as the char- 
 acters of stems and leaves are concerned. These plants, of 
 which we have sixteen, were made by fertilising B. heracleifolia 
 with B. polyantha. They are all beginning to break out in 
 " phyllomania." They, also, are quite sterile and as they agree 
 in all details with phyllomaniaca there can be little doubt 
 that the original plant bearing that name was a hybrid similarly 
 produced. The production of " phyllomania" on a hybrid 
 Begonia has also been previously recorded by Duchartre. 17 
 In this case the cross was made between B. incarnata and lucida. 
 The synonymy of the last species is unfortunately obscure, and 
 I have not succeeded in repeating the experiment. 
 
 Fig. 5. Piece of petiole of Begonia phyllomaniaca. The proximal end is to the 
 
 right of the figure. 
 
 From these facts it seems practically certain that the condition 
 is one which is due to the meeting of complementary factors. 
 At first sight we may incline to think that the phyllomania is in 
 some way due to the sterility. This however cannot be seriously 
 maintained ; for not only is sterility in plants not usually associ- 
 ated with such manifestations, but we know a Begonia called 
 "Wilhelma" which is exactly phyllomaniaca and equally sterile, 
 though it has no trace of phyllomania. This plant arose in the 
 nurseries of MM. P. Bruant of Poitiers, and has generally been 
 described as a seedling of phyllomaniaca, but from the total 
 sterility of that form this account of its origin must be set aside. 
 
 The phenomenon in this case can hardly be regarded as 
 17 Bull. Soc. Bot. de France, xxxiv, 1887, p. 182. 
 
52 
 
 PROBLEMS OF GENETICS 
 
 due to the excitation of dormant buds, for it is apparent on 
 examination that the new growths are not placed in any fixed 
 geometrical relation to the original plant. They arise on the 
 
 Fig. 6. Two right hind feet of polydactyle cats. II shows the lowest de- 
 velopment of the condition yet recorded. The digit, d\, which stands as hallux is 
 fully formed and has three phalanges. Both it and the digit marked di are formed 
 as left digits. In the normal hind foot of the cat the hallux is represented by a 
 rudiment only. 
 
 J shows a further development of the condition. In this foot there are six 
 digits. d\ has two phalanges, but both it and d 2 and d 3 are shaped as left digits. 
 Thus d z , which in the normal foot would be shaped as a right digit, is transformed 
 so as to look like a left digit. 
 
 petiole, for example, as small green outgrowths each of which 
 gradually becomes a tiny leaf. The attitude of these leaves is 
 quite indeterminate, and they may point in any direction, 
 
MERISTIC PHENOMENA 53 
 
 some having their apices turned peripherally, some centrally, and 
 others in various oblique or transverse positions (Fig. 5). These 
 little leaves are thus comparable with seedlings, in that their 
 polarity is apparently not related to, or consequent upon that 
 of the parent plant. They have in fact that " individuality " 
 which we associate with germinal reproduction. 
 
 There are many curious phenomena seen in the behaviour of 
 parts normally repeated in bilateral symmetry which may some 
 day guide us towards an understanding of the mechanics of 
 division. A part like a hand, which needs the other hand to 
 complete its symmetry, cannot twin by mere division, yet by 
 proliferation and special modifications on the radial side of the 
 same limb, even a hand may be twinned. In the well known poly- 
 dactyle cats a change of this kind is very common and indeed 
 almost the rule. When extra digits appear at the inner (tibial) 
 side of the limb, they are shaped as digits of the other side, and 
 even the normal digit II (index) is usually converted into the 
 mirror-image of its normal self. The limb then develops a 
 new symmetry in itself. Nevertheless it is not easy to interpret 
 these facts as meaning that there has been some interruption 
 in the control which one side of the body exercises over the 
 other. The heredity of polydactylism is complex but there is 
 little doubt that the condition familiar in the Cat is a dominant. 
 In some human cases also the descent is that of a dominant, but 
 irregularities are so frequent that no general rule can yet be 
 perceived. The dominance of such a condition is an exception 
 to the principle that the less-divided is usually dominant to the 
 more-divided, a fact which probably should be interpreted as 
 meaning that divisions are of more than one kind. 
 
 Among ordinary somatic divisions, whether of organs, cells, 
 or patterns of differentiation, the control of symmetry is usually 
 manifested. There is however one class of somatic differenti- 
 ations which are exceptionally interesting from the fact that they 
 may show a complete independence of such geometrical control. 
 The most familiar examples of these geometrically uncontrolled 
 Variations are to be seen in bud-sports. The normal differ- 
 entiation of the organs of a plant is arranged on a definite geo- 
 
54 PROBLEMS OF GENETICS 
 
 metrical system, which to those who have never given special 
 attention to such things before, will often seem surprisingly 
 precise. The arrangement of the leaves on uninjured, free- 
 growing shoots can generally be seen to follow a very definite 
 order, just as do the flowers or the parts of the flowers. If 
 however bud sports occur, then though the parts included in 
 the sports show all the geometrical peculiarities proper to the 
 sport-variety, yet the sporting-buds themselves are not related 
 to each other according to any geometrical plan. 
 
 A very familiar illustration is provided by the distribution 
 of colour in those Carnations that are not self-coloured. The 
 pigment may, as in Picotees, be distributed peripherally with 
 great regularity to the edges of the petals; or, as in Bizarres and 
 Flakes, it may be scattered in radial sectors which show no 
 geometrical regularity. Now in this case the pigments are the 
 same in both types of flower, and the chemical factors concerned 
 in their production must surely be the same. The difference 
 must lie in the mechanical processes of distribution of the pig- 
 ment. In the Picotee we see the orderly differentiation which we 
 associate with normality; in the Bizarre we see the disorderly 
 differentiation characteristic of bud-sports. The distribution of 
 colour in this case lies outside the scheme of symmetry of the 
 plant. 
 
 Such a distribution is characteristic of bud-sports, and of 
 certain other differentiations in both plants and animals, which 
 I cannot on this occasion discuss. Now reflexion will show that 
 these facts have an intimate bearing on the mechanical problems 
 of heredity. For first in the bud-sports we are witnessing the 
 distribution of factors which distinguish genetic varieties. We 
 do not know the physical nature of those factors, but if we must 
 give them a name, I suppose we should call them "ferments" 
 exactly as Boyle did in 1666. He is discussing how it comes about 
 that a bud, budded on a stock, becomes a branch bearing the 
 fruit of its special kind. He notes that though the bud inserted 
 be "not so big oftentimes as a Pea," yet "whether by the help 
 of some peculiar kind of Strainer or by the Operation of some 
 powerful Ferment lodged in it, or by both these, or some other 
 
MERISTIC PHENOMENA 55 
 
 cause," the sap is "so far changed as to constitute a Fruit quite 
 otherwise qualify 'd." 18 We can add nothing to his speculation, 
 and we believe still that by a differential distribution of "fer- 
 ments" the sports are produced. All the factors are together 
 present in the normal parts; some are left out in the sport. 
 In an analogous case however, that of a variegated Pelargonium 
 which has green and also albino shoots, Baur proved that the 
 shoots pure in colour are also pure in their posterity. There 
 can be no doubt that the sports of Carnations, Azaleas, Chrysan- 
 themums, etc., would behave in the same way. 
 
 The well-known Azaleas Perle de Ledeburg, President 
 Kerchove, and Vervaeana are familiar illustrations. Perle de 
 Ledeburg is predominantly white, but it has red streaks in some 
 of its flowers. It not very rarely gives off a self-red sport. This 
 is evidently due to the development of a bud in a red-bearing 
 area of the stem. The red in this plant is not under "geometrical 
 control." Many plants have white flowers with no markings, 
 but if the red markings are geometrically ordered differentiations, 
 no self-coloured sports are formed. The case of Vervaeana is a 
 good illustration of this proposition. It has white flowers with 
 red markings arranged in an orderly manner on the lower parts 
 of the petals, especially on the dorsal petals. This is one of the 
 Azaleas most liable to have red sports, and at first sight it might 
 seem that the sport represented the red of the central marks. 
 Examination however of a good many flowers shows that irregular 
 red streaks like those of Perle de Ledeburg occur, about as com- 
 monly as in that variety. Vervaeana in fact is Perle de Ledeburg 
 with definite red markings added, and its red sports obviously are 
 those branches the germs of which came in a patch of the stem 
 bearing these red elements. That this is the true account is 
 rendered quite obvious by the fact that the red of the sport is a 
 colour somewhat different from that of the definite marks, and 
 that these marks are still present on the red ground of the sporting 
 flowers. 
 
 It will be understood that these remarks apply to those cases 
 in which the production of sports is habitual or frequent, and 
 
 18 R. Boyle, The Origine of Formes and Qualities, Oxford, 1666. 
 
56 PROBLEMS OF GENETICS 
 
 I imagine in all such examples it will be found that there are 
 indications of irregularity in the distribution of the differ- 
 entiations such as to justify the view that they are not under that 
 geometrical control which governs the normal differentiation 
 of the parts. The question next arises whether these consider- 
 ations apply also to the production of a bud-sport as a rare 
 exception, but by the nature of the case it is not possible to 
 say positively whether the appearance of an exceptional sport 
 is due to the unsuspected presence of a pre-existing fragment of 
 material having a special constitution, or to the origin, de novo, 
 of such a material. For instance one of the garden forms of 
 Pelargonium known as altum is liable perhaps once in some 
 hundreds of flowers to have one or two magenta petals. The 
 normal colour is a brilliant red ; and as we may be fairly sure that 
 this red is recessive to magenta the interpretation would be 
 quite different according as the appearance of the magenta is 
 regarded as due to the presence of small areas endowed with 
 magentaness, or to the spontaneous generation of the factor 
 for that pigment. Either interpretation is possible on the facts, 
 but the view that the whole plant has in it scarce mosaic particles 
 of magenta seems on the whole more consistent with present 
 knowledge. 
 
 In Pelargonium altum the enzyme causing the magenta colours 
 must be distributed in very small areas, but a case in which the 
 magenta is similarly arranged in a much coarser patchwork 
 may be seen in the Pelargonium " Don Juan," which often bears 
 whole trusses or branches of red flowers upon plants having the 
 normal dominant magenta trusses. In most cases there is little 
 doubt that though the magenta flowered parts can " sport" to 
 red, the red parts could not produce the magenta flowers. 
 
 The asymmetrical, or to speak more precisely, the disorderly, 
 mingling of the colours in the somatic parts is thus an indication 
 of a similarly disorderly mixing of the factors for those colours 
 in the germ-tissues, so that some of the gametes bear enough of 
 the colour-factors to make a self-coloured plant, while others 
 bear so little that the plant to which they give rise is a patch- 
 work. If this view is correct we may extend it so far as to con- 
 
MERISTIC PHENOMENA 57 
 
 sider whether the fineness or coarseness of the mixture visible 
 in the flowers or leaves may not give an indication of the degree 
 to which the factors are subdivided among the germ-cells. We 
 know very little about the genetic properties of striped varieties. 
 In both Antirrhinum and Mirabilis it has been found that the 
 striped may occasionally and irregularly throw self-coloured 
 plants, and therefore the striping cannot be regarded simply as 
 a recessive character. On the other hand in Primula Sinensis 
 there are well-known flaked varieties which ordinarily at least 
 breed true. Whether these ever throw selfs I do not know, 
 but if they do it must be quite exceptionally. The power of 
 these flaked plants to breed true is, I suspect, connected with 
 the fact that in their flowers the coloured and white parts are 
 intimately mixed, this intimate mixture thus being an indication 
 of a similarly intimate mixture in the germ-cells. It would be 
 important to ascertain whether self-fertilised seed from the oc- 
 casional flowers in which the colour has run together to form a 
 large patch gives more self-coloured plants than the intimately 
 flaked flowers do. 
 
 The next fact may eventually prove of great importance. 
 We have seen that in bud-sports the differentiation is of the same 
 nature as that between pure types, and also that in the sporting 
 plant this differentiation is distributed without any reference 
 to the plant's axis, or any other consideration of symmetry. 
 Now among the germ-cells of a Mendelian hybrid exactly such 
 characters are being distributed allelomorphically, and there 
 again we have strong evidence for believing that the distribution 
 obeys no pattern. For example, we can in the case of seeds still 
 in situ perceive how the characters were distributed among the 
 germ-cells, and there is certainly no obvious pattern connecting 
 them, nor can we suppose that there is an actual pattern obscured. 
 
 Of this one illustration is especially curious. Individual 
 plants of the same species are, as regards the decussations of 
 their leaves and in other respects, either rights or lefts. The fact 
 is not emphasized in modern botany and is in some danger of 
 being forgotten. When, as in the flowers of the Spring Gladioli, 
 Exacum, St. Paulia, or the fruits of Loasa, rights and lefts occur 
 
5 8 PROBLEMS OF GENETICS 
 
 on the same stem, they come off alternately. But if, as in the 
 seedlings of Barley the twist of the first leaf be examined, it 
 will be seen to be either a right- or left-handed screw. An 
 ear of barley, say a two-row barley, is a definitely symmetrical 
 structure. The seeds stand in their envelopes back to back in 
 definite positions. Each has its organs placed in perfectly 
 definite places. If these seeds were buds their differentiations 
 would be grouped into a common plan. One might expect that 
 the differentiations of these embryos would still fall into the 
 pattern; but they do not, and so far as I have tested them, any 
 one may be a right or a left, just as each may carry any of the 
 Mendelian allelomorphs possessed by the parent plant, without 
 reference to the differentiation of any other seed. The fertil- 
 isation may be responsible, but our experience of the allelo- 
 morphic characters suggest that the irregularity is in the egg- 
 cells themselves. 19 
 
 Germ cells thus differ from somatic cells in the fact that their 
 differentiations are outside the geometrical order which governs 
 the differentiation of the somatic cells. I can think of possible 
 exceptions, but I have confidence that the rule is true and I 
 regard it as of great significance. 
 
 The old riddle, what is an individual, finds at least a partial 
 ,-solution in the reply that an individual is a group of parts dif- 
 ferentiated in a geometrically interdependent order. With the 
 germ-cell a new geometrical order, with independent polarity is 
 almost if not quite always, begun, and with this geometrical inde- 
 pendence the power of rejuvenescence may possibly be associated. 
 
 The problems thus raised are unsolved, but they do not look 
 insoluble. The solution may be nearer than we have thought. 
 In a study of the geometry of differentiation, germinal and 
 somatic, there is a way of watching and perhaps analyzing what 
 may be distinguished as the mechanical phenomena of heredity. 
 If any one could in the cases of the Picotee and the Bizarre Carna- 
 tion, respectively, detect the real distinction between the two 
 
 19 Remarkable experiments on this question have lately been carried out by 
 R. H. Compton (Camb. Phil. Soc, XV, 19 10, p. 495). showing that in a certain 
 Barley, "Plumage Corn," the average ratio of left to right is about 1.5. A fuller 
 paper has since been published by Compton, Jour. Genetics, 191 2, II, 1, p. 53* 
 Compare also the case of Arum spathes examined by Christy, Jour. Bot., 191 4« 
 jiii, p. 258. 
 
MERISTIC PHENOMENA 59 
 
 types of distribution, he would make a most notable advance. 
 Any one acquainted with mechanical devices can construct a 
 model which will reproduce some of these distinctions more or less 
 faithfully. The point I would not lose sight of is that the analogy 
 with such models must for a long way be a true and valuable 
 guide. I trust that some one with the right intellectual equip- 
 ment will endeavor to follow this guide ; and I am sanguine enough 
 to think that a comprehensive study of the geometrical phenomena 
 of differentiation will suggest to a penetrative mind that critical 
 experiment which may one day reveal the meaning of spontaneous 
 division, the mystery through which lies the road, perhaps the 
 most hopeful, to a knowledge of the nature of life. 
 
CHAPTER III 
 
 SEGMENTATION, ORGANIC AND MECHANICAL 
 
 Models may be and often have been devised imitating some 
 of the phenomena of division, but none of them have reproduced 
 the peculiarity which characterises divisions of living tissues, 
 that the position of chemical differentiation is determined by those 
 divisions. For example, models of segmentation, whether 
 radial or linear, may be made by the vibration of plates as in 
 the familiar Chladni figures of the physical laboratory, or by 
 the bowing of a tube dusted on the inside with lycopodium 
 powder, and in various other ways. The sand or the powder 
 will be heaped up in the nodes or regions of least movement, and 
 the patterns thus formed reproduce many of the geometrical 
 features of segmentation. But in the segmentations of living 
 things the nodes and internodes, once determined by the dividing 
 forces, would each become the seat of appropriate and distinct 
 chemical processes leading to the differentiation of the parts, 
 and the deposition of the bones, petals, spines, hairs, and other 
 organs in relation to the meristic ground-plan. The "ripples" of 
 meristic division not merely divide but differentiate, and when 
 a "ripple" forks the result is not merely a division but a re- 
 duplication of the organ through which the fork runs. An 
 example illustrating such a consequence is that of the half-ver- 
 tebrae of the Python. On the left side the vertebra is single 
 (Fig. 7) and bears a single rib, but on the right side a division 
 has occurred with the result that two half -vertebrae, each 
 bearing a rib, are formed, one standing in succession to the 
 other. We cannot, indeed, imagine any operation of physio- 
 logical division carried out in such an organ as a vertebra, passing 
 through a plane at right angles to the long axis of the body, which 
 does not necessarily involve the further process of reduplication. 
 
 As the meristic system of distribution spreads through the 
 body, chemical differentiations follow in its track, with seg- 
 
 60 
 
SEGMENTATION 
 
 61 
 
 mentation and pattern as the visible result. Could we analyse 
 these simultaneous phenomena and show how it is that the places 
 of chemical differentiation are determined by the system of 
 division, progress would then be rapid. It is here that all 
 speculation fails. 
 
 E 
 
 Figs. 7 and 8. Two examples of imperfect division in the vertebrae of a python 
 
 I, the vertebrse 147-150 from the right side, showing imperfect division between 
 the 148th and 149th. The condition on the left side of this vertebra was the same 
 
 II, the dorsal surface of vertebrse 165-167. On the right side the 166th is double 
 and bears two ribs, but on the left side it is normal and has one rib only. 
 
 ^ Many attempts have been made to interpret the processes of 
 division and repetition, in terms of mechanics, or at least to 
 refer them to their nearest mechanical analogies, so far with 
 
62 PROBLEMS OF GENETICS 
 
 little success. The problem is beset with difficulties as yet 
 insurmountable and of these one must be especially noticed. In 
 the living thing the process by which repetition and patterns 
 come into being consists partly in division but partly also in 
 growth. We have no means of studying the phenomena of 
 pattern-formation except in association with that of growth. 
 Growth soon ceases unless division takes place, and if growth is 
 impossible division soon ceases also. In consequence of this 
 fact that the final pattern is partly a product of growth, it can 
 never be used as unimpeachable evidence of the primary geo- 
 metrical relations of the members as laid down in the divisions. 
 
 In the last chapter in referring to the problem of repetition 
 I introduced an analogy, comparing the patterns of the organic 
 world with those produced in unorganised materials by wave- 
 motion. In the preliminary stage of ignorance, having no more 
 trustworthy clue, I do not think it wholly unprofitable to consider 
 the applicability of this analogy somewhat more fully. It 
 possesses, as I hope to show, at least so much validity as to 
 encourage the belief that morphology may safely discard one 
 source of long-standing error and confusion. 
 
 Those who have studied the structure of parts repeated in 
 series will have encountered the old morphological problem of 
 "Serial Homology," which has absorbed so much of the attention 
 of naturalists and especially of zoologists at various periods. 
 This problem includes two separate questions. The first of 
 these is the origin in evolution of the resemblance between two 
 organs occurring in a repeated series, of which the fore and hind 
 limbs of Vertebrates are the prerogative instance. From the 
 fact that these resemblances can be traced very far, often into 
 minute details of structure, many anatomists have inclined to 
 the opinion that the resemblance must originally have been still 
 more complete, and that the two limbs, for instance, must have 
 acquired their present forms by the differentiation of two iden- 
 tical groups of parts. 
 
 Similar questions arise whenever parts are repeated in series, 
 whether the series be linear or radial, and, though less obviously, 
 even when the repetition is bilateral only. In each such example 
 
SEGMENTATION 63 
 
 the question arises, is the resemblance between the parts the 
 remains of a still closer resemblance, or is differentiation original? 
 Sometimes the view that these parts have arisen by the dif- 
 ferentiation of a series of identical parts is plausible enough, 
 as for example when the peculiarities of various appendages of a 
 Decapod Crustacean are referred to modifications of the Phyl- 
 lopod series. In application to other cases however we soon meet 
 with difficulty, and the suggestion that the segments of a verte- 
 brate were originally all alike is seen at once to be absurd, for 
 the reason that a creature so constituted could not exist, and that, 
 differentiation of at least one anterior and one posterior segment, 
 is an essential condition of a viable organism consisting of parts 
 repeated in a linear series. Between these two terminal segments 
 it is possible to imagine the addition of one segment, or of a 
 series of approximately similar segments; but when once it is 
 realised that the terminals must have been differentiated from 
 the beginning, it will be seen that the problem of the origin of 
 the resemblance between segments is not rendered more com- 
 prehensible by the suggestion that even the intervening members 
 were originally alike. Seeing indeed that some differentiation 
 must have existed primordially it is as easy to imagine that the 
 original body was composed of a series grading from the condition 
 of the anterior segment to that of the posterior, as any other 
 arrangement. The existence of a linear or successive series in 
 fact postulates a polarity of the whole, and in such a system the 
 conception of an ideal segment containing all the parts represented 
 in the others has manifestly no place. The introduction of that 
 conception though sanctioned by the great masters of com- 
 parative anatomy, has, as I think, really delayed the progress of 
 a rational study of the phenomena of division. The same notion 
 has been applied to every class of repetition both in animals and 
 plants, generally with the same unhappy results. In the cruder 
 forms in which this doctrine was taught thirty years ago it is 
 now seldom expressed, but modified presentations of it still 
 survive and confuse our judgments. 
 
 The process of repetition of parts in the bodies of organisms 
 is however a periodic phenomenon. This much, provided we 
 
6 4 
 
 PROBLEMS OF GENETICS 
 
 remain free from prejudice as to the nature and causation of the 
 period or rhythm, we may safely declare, and a comparison may 
 thus be instituted between the consequences of meristic repetition 
 
 Fig. 9. Osmotic growths simulating segmentation. (After Leduc.) 
 
 in the bodies of living things and those repetitions which in 
 the inorganic world are due to rhythmical processes. Of such 
 processes there is a practically unlimited diversity and we have 
 nothing to indicate with which of them our repetitions should 
 rather be compared. 
 
SEGMENTATION 65 
 
 In some respects perhaps the best models of living organisms 
 yet made are the "osmotic growths" produced by Leduc. 1 
 These curious structures were formed by placing a fragment of 
 a salt, for instance calcium chloride, in a solution of some col- 
 loidal substance. As the solid takes up water from the solution 
 a permeable pellicle or membrane is formed around it. The ves- 
 icle thus enclosed grows by further absorption of water, often 
 extending in a linear direction, and in many examples this growth 
 occurs by a series of rhythmically interrupted extensions. Some 
 of the growths thus formed are remarkably like organic structures, 
 and might pass for a series of antennary segments or many other 
 organs consisting of a linear series of repeated parts. In admitting 
 the essential resemblance between these "osmotic growths" 
 and living bodies or their organs I lay less stress on the general con- 
 formation of the growths, which often as Leduc points out, recall 
 the forms of fungi or hydroids, but rather on the fact that the 
 interruptions in the development of these systems are so closely 
 analogous to the segmentations or repetitions of parts character- 
 istic of living things (Fig. 9). In the same way I am less im- 
 pressed by Leduc's models of Karyokinesis, wonderful as they 
 nevertheless are, for the division is here imitated by putting 
 separate drops on the gelatine film. What we most want to know 
 is how in the living creature one drop becomes two. The models 
 of linear segmentation have the remarkable merit that they do in 
 some measure imitate the process of actual division or repetition. 
 So in a somewhat modified method Leduc, by causing the diffusion 
 of a solution in a gelatine film, produced rhythmical or periodic 
 precipitations strikingly reminiscent of various organic tissues, 
 for here also the process of periodic repetition is imitated with 
 success. 
 
 It is a feature common to these and to all other rhythmical 
 repetitions produced by purely mechanical forces that there 
 is resemblance between the members of the series, and that this 
 similarity of conformation may be maintained in most complex 
 detail. When however in the mechanical series some of the 
 members differ from the rest we have no difficulty in recognising 
 
 1 Stephane Leduc, Theorie Physico-Chymiquj. de la Vie, Paris, 1910. 
 6 
 
66 PROBLEMS OF GENETICS 
 
 that these differences — which correspond with the differenti- 
 ations of the organic series — are due to special heterogeneity in 
 the conditions or in the materials, and it never occurs to us to 
 suppose that all the members must have been primordially alike. 
 For example, in the case of ripple-marks on the sand, which I 
 choose as one of the most familiar and obvious illustrations 
 of a repeated series due to mechanical agencies, if we notice 
 one ripple different in form from those adjacent to it, we do 
 not suppose that this variation must have been brought about by 
 deformation of a ripple which was at first formed like the others, 
 but we ascribe it to a difference in the sand at that point, or to a 
 difference in the way in which the wind or the tide dealt with it. 
 We may press the analogy further by observing that in as much 
 as such a series of waves has a beginning and an end, it possesses 
 polarity like that of the various linear series of parts in organisms, 
 and even the formation of each member must influence the 
 shape of its successor. Since in an organism the beginning and 
 end of the series are always included, some differentiation among 
 the repetitions must be inevitable. If therefore it be conceded, 
 as I think it must, that segmentation and pattern are the con- 
 sequence of a periodic process we realize that it is at least as 
 easy to imagine the formation of such a series of parts having 
 family likeness combined with differentiation as it would be to 
 conceive of their arising primordially as a series of identical repe- 
 titions. The suggestion that the likenesses which we now per- 
 ceive are the remains of a still more complete resemblance 
 is a substitution of a more complex conception for a simpler one. 
 The other question raised by the problem of Serial Homology 
 is how far there is a correspondence between individual members 
 of series when the series differ from each other either in the 
 number of parts, or in the mode of distribution of differentiation 
 among them. Students, for example, of vertebrate morphology 
 debate whether the nth vertebra which carries the pelvic girdle 
 in Lizard A is individually homologous with the n+xth vertebra 
 which fulfils this function- in Lizard B,or whether it is not more 
 truly homologous with the vertebra standing in the nth ordinal 
 position, though that vertebra in Lizard B is free. 
 
SEGMENTATION 67 
 
 In various and more complex aspects the same question is 
 debated in regard to the cranial and spinal nerves, the branches 
 of the aorta, the appendages of Arthropoda, and indeed in re- 
 gard to all such series of differentiated parts in linear or suc- 
 cessive repetition. Persons exercised with these problems 
 should before making up their minds consider how similar 
 questions would be answered in the case of any series of rhyth- 
 mical repetitions formed by mechanical agencies. In the case 
 of our illustration of the ripples in the sand, given the same forces 
 acting on the same materials in the same area, the number of 
 ripples produced will be the same, and the nth. ripple counting 
 from the end of the series will stand in the same place whenever 
 the series is evoked. If any of the conditions be changed, the 
 number and shapes can be changed too, and a fresh "distribution 
 of differentiation " created. Stated in this form it is evident 
 that the considerations which would guide the judgment in the 
 case of the sand ripples are not essentially different from those 
 which govern the problem of individual homology in its applica- 
 tion to vertebrae, nerves, or digits. 
 
 The fact that the unit of repetition is also the unit of growth 
 is the source of the obscurity which veils the process. When we 
 compare the skeleton of a long-tailed monkey with that of a 
 short-tailed or tailless ape we see at once how readily the addi- 
 tional series of caudal segments may be described as a conse- 
 quence of the propagation of the "waves" of segmentation 
 beyond the point where they die out in the shorter column, and 
 we see that with an extension of the series of repetitions there is 
 growth and extension of material. 
 
 The considerations which apply to this example will be found 
 operating in many cases of the variation of terminal members of 
 linear series. Some of these series, like the teeth of the dog, 
 end in a terminal member of a size greatly reduced below that of 
 the next to it. Even when there is thus a definite specialisation 
 of the last member of the series it not infrequently happens that 
 the addition, by variation, of a member beyond the normal 
 terminal, is accompanied by a very palpable increase in size of 
 the member which stands numerically in the place of the normal 
 
68 PROBLEMS OF GENETICS 
 
 terminal. 2 So also with variation in the number of ribs, when a 
 lumbar vertebra varies homoeotically into the likeness of the 
 last dorsal and bears a rib, the rib placed next in front of this, 
 which in the normal trunk is the last, shows a definite increase 
 in development. 
 
 The consequences of such homoeoses are sometimes very- 
 extensive, involving readjustments of differentiation affecting 
 a long series of members, as may easily be seen by comparing 
 the vertebral columns of several individual Sloths 3 (whether 
 Bradypus or Choloepus) to take a specially striking example. 
 
 It may be urged that no feature as yet enables us to perceive 
 wherein lies the primary distinction which determines such 
 variation, whether it is due to a difference in the dividing forces 
 or in the material to be divided. If for instance we were to 
 imitate such a series of segments by pressing hanging drops of 
 a viscous fluid out of a paint- tube by successive squeezes, the 
 number of times the tube is contracted before it is empty will give 
 the number of the segments, but their size may depend either 
 on the force of the contractions or on the capacity of the tube, 
 or on various other factors. Nevertheless in the case of the 
 variation of terminal members, whatever be the nature of the 
 rhythmical impulse which produces the series of organs, the ele- 
 vation of the normally terminal member in correspondence with 
 the addition of another is what we should expect. 
 
 If the organism acquired its full size first and the delimitation 
 of the parts took place afterwards, there might be some hope that 
 the resemblance between living patterns and those mechanically 
 caused by wave-motion might be shown to be a consequence of 
 some real similarity of causation, but in view of the part played 
 by growth, appeal to these mechanical phenomena cannot be 
 declared to have more than illustrative value. Similarly in as 
 much as living patterns appear, and almost certainly do in reality 
 come into existence by a rhythmical process, comparisons of 
 these patterns with those developed in crystalline structures, and 
 in the various fields of force are, as it seems to me, inadmissible, 
 or at least inappropriate. 
 
 2 Materials for the Study of Variation, No. 249, p. 217; and p. 272. 
 
 3 Materials, p. 118. 
 
SEGMENTATION 
 
 69 
 
 However their intermittence be determined, the rhythms of 
 division must be looked upon as the immediate source of those 
 geometrically ordered repetitions universally characteristic of 
 organic life. In the same category we may thus group the seg- 
 mentation of the Vertebrates and of the Arthropods, the concen- 
 tric growth of the Lamellibranch shells or of Fishes' scales, the 
 ripples on the horns of a goat, or the skeletons of the Foraminifera 
 or of the Heliozoa. In the case of plant-structures Church 4 has 
 admirably shown, with an abundance of detail, how on analysis 
 the definiteness of phyllotaxis is an expression of such rhythm in 
 the division of the apical tissues, and how the spirals and "ortho- 
 stichies" displayed in the grown plant are its ultimate conse- 
 quences. The problem thus narrows itself down to the question 
 of the mode whereby these rhythms are determined. 
 
 It is natural that we should incline to refer them to a chemical 
 source. If we think of the illustration just given, of the seg- 
 mentation of a viscous fluid into drops by successive contractions 
 of a soft-walled tube we can, I think, conceive of such rhythmic 
 contractions as due to summations of chemical stimuli, somewhat 
 as are the beats of the heart. But when we recognize the vast 
 diversity of materials the distribution of which is determined by 
 an ostensibly similar rhythmic process it seems hopeless to look 
 forward to a directly chemical solution. That the chemical 
 degradation of protoplasm or of materials which it contains is 
 the source of the energy used in the divisions cannot be in dispute, 
 but that these divisions can be themselves the manifestations 
 of chemical action seems in the highest degree improbable. 
 
 We may therefore insist with some confidence on the dis- 
 tinction between the Meristic and the substantive constitution 
 of organisms, between, that is to say, the system according to 
 which the materials are divided and the essential composition 
 of the materials, conscious of the fact that the energy of division 
 is supplied from the materials, and that in the ontogeny the 
 manner in which the divisions are effected must depend secon- 
 darily on the nature of the substances to be divided. The me- 
 
 4 Church, A. H., On the Relation of Phyllotaxis to Mechanical Laws, London, 
 1904. 
 
70 PROBLEMS OF GENETICS 
 
 chanical processes of division remain a distinguishable group of 
 phenomena, and variations in the substances to be distributed in 
 division may be independent of variations in the system by which 
 the distribution is effected. 
 
 Modern genetic analysis supplies many remarkable examples 
 of this distinction. When formerly we compared the leaves of a 
 normal palmatifid Chinese Primula with the pinnatifid leaves 5 of 
 its fern-leaved variety we were quite unable to say whether the 
 difference between the two types of leaf was due to a difference 
 in the material cut up in the process of division or to a difference 
 in that process itself. Knowledge that the distinction is deter- 
 mined by a single segregable factor tends to prove that the 
 critical difference is one of substance. So also in the Silky fowl 
 we know that the condition of its feathers is due to the absence 
 of some one factor present in the normal form. We may con- 
 ceive such differences as due to change of form in the successive 
 "waves" of division, but we cannot yet imagine segregation 
 otherwise than as acting by the removal or retention of a material 
 element. Future observation by some novel method may suggest 
 some other possibility, but such cases bring before us very clearly 
 the difficulties by which the problem is beset. 
 
 In another region of observation phenomena occur which as 
 it seems to me put it beyond question that the meristic forces are 
 essentially independent of the materials upon which they act, 
 save, in the remoter sense, in so far as these materials are the 
 sources of energy. The physiology of those regenerations and 
 repetitions which follow upon mutilation supplies a group of 
 facts which both stimulate and limit speculation. No satis- 
 factory interpretations of these extraordinary occurrences has 
 ever been found, but we already know enough to feel sure that 
 in them we are witnessing indications which should lead to the 
 discovery of the true mechanics of repetition and pattern. 
 The consequences of mutilation in causing new growth or perhaps 
 more strictly in enabling new growth to take place, are such that 
 they cannot be interpreted as responses to chemical stimuli in 
 
 5 It is a question whether the dominance of the palmatifid leaf over the pin- 
 natifid is not really an example of the dominance of a lower number of segmentations 
 
SEGMENTATION 71 
 
 any sense which the word chemical at present connotes. Powers 
 are released by mutilation of which in the normal conditions of 
 life no sign can be detected. All who have tried to analyse the 
 phenomena of regeneration are compelled to have recourse to the 
 metaphor of equilibrium, speaking of the normal body as in a state 
 of strain or tension (Morgan) which when disturbed by muti- 
 lation results in new division and growth. The forces of division 
 are inacessible to ordinary means of stimulation. Applications, 
 for example, of heat or of electricty excite no responses of a 
 positive kind unless the stimuli are so violent as to bring about 
 actual destruction. 6 These agents do not, to use a loose expres- 
 sion, come into touch with the meristic forces. Changes in the 
 chemical environment of cells may, as in the experiments of 
 Loeb and of Stockard produce definite effects, but the facts 
 suggest that these effects are due rather to alterations in the 
 living material than to influence exerted directly on the forces 
 of division themselves. 
 
 By destruction of tissue however the forces both of growth 
 and of division also may often be called into action with a re- 
 sulting regeneration. Interruption of the solid connexion 
 between the parts may produce the same effects, as for example 
 when the new heads or tails grow on the divided edges of Pla- 
 narians (Morgan), or when from each half embryo partially sepa- 
 rated from its normally corresponding half, a new half is formed 
 with a twin monster as the result. 
 
 Often classed with regenerations but in essence quite distinct 
 from them are those special and most interesting examples 
 where the growth of a paired structure is excited by a simple 
 
 over a higher. From the uncertainty whether two given leaves of two separate 
 plants are actually comparable one cannot institute quite satisfactory numerical 
 comparisons, but I think the view that the "Fern" leaf has more lobes than an 
 otherwise similar "Palm" leaf may be fairly maintained. If this be admitted, 
 the "Palm" leaf represents the dominant low number and its round shape is a 
 consequence of the greater powers of growth which are so often possessed by the 
 members of a shorter series. 
 
 6 It is perhaps of importance to remember that in certain species of bacteria 
 (e. g. Bacillus Anthracis) division may cease where the organism is cultivated under 
 certain artificial conditions though growth continues. In this way very long un- 
 segmented threads are produced. 
 
72 PROBLEMS OF GENETICS 
 
 wound. Some of the best known of these instances are presented 
 by the paired extra appendages of Insects and Crustacea. Some 
 years ago I made an examination of all the examples of such 
 monstrosities to which access was to be obtained, and it was with 
 no ordinary feeling of excitement that I found that these super- 
 numerary structures were commonly disposed on a recognizable 
 geometrical plan, having definite spatial relations both to each 
 other and to the normal limb from which they grew. The more 
 recent researches of Tornier 7 and especially his experiments on 
 the Frog have shown that a cut into the posterior limb-bud 
 induces the outgrowth of such a pair of limbs at the wounded 
 place. Few observations can compare with this in novelty or 
 significance; and though we cannot yet interpret these phenomena 
 or place them in their proper relations with normal occurrences, 
 we feel convinced that here is an observation which is no mere 
 isolated curiosity but a discovery destined to throw a new light 
 on biological mechanics. The supernumerary legs of the Frog 
 are evidently grouped in a system of symmetry similar to that 
 which those of the Arthropods exhibit, and though in Arthropods 
 paired repetitions have not been actually produced by injury 
 under experimental conditions we need now have no hesitation 
 in referring them to these causes as Przibram has done. 
 
 At this point some of the special features of the super- 
 numerary appendages become important. First they may arise 
 at any point on the normal limb, being found in all situations 
 from the base to the apex. Nor are they limited as to the surface 
 from which they spring, arising sometimes from the dorsal, 
 anterior, ventral, or posterior surfaces, or at points intermediate 
 between these principal surfaces. 
 
 With rare and dubious exceptions, the parts which are con- 
 tained in these extra appendages are only those which lie periph- 
 eral to their point of origin. Thus when the point of origin is 
 in the apical joint of the tarsus, the extra growth if completely 
 developed consists of a double tarsal apex bearing two pairs 
 of claws. If they arise from the tibia, two complete tarsi are 
 
 » Arch. f. Entwm., XX, 1905, p. 76; Sitzungsb. d. Ges. Naturf., Berlin, 1907, 
 p. 41, etc. 
 
SEGMENTATION 73 
 
 added. If they spring from the actual base of the appendage 
 then two complete appendages may be developed in addition to 
 the normal one. We must therefore conclude that in any point 
 on a normal appendage the power exists which, if released may 
 produce a bud containing in it a paired set of the parts peripheral 
 to this point. 
 
 Next the geometrical relations of the halves of the super- 
 numerary pair are determined by the position in which they stand 
 
 D 
 
 DDF 
 
 DDA 
 
 Fig. ii. Diagrams of the geometrical relations which are generally exhibited 
 by extra pairs of appendages in Arthropoda. The sections are supposed to be 
 those of the apex of a tibia in a beetle. A, anterior, P, posterior. D, dorsal V 
 ventral. M\ M* are the imaginary planes of reflexion. The shaded figure is in 
 each case a limb formed like that of the other side of the body, and the outer 
 unshaded figures are shaped like the normal for the side on which the appendages 
 are. On the several radii are shown the extra pairs in their several possible re- 
 Iations to the normal from which they arise. The normal is drawn in thick lines 
 in the center. 
 
74 PROBLEMS OF GENETICS 
 
 in regard to the original appendage. These relations are best 
 explained by the diagram (Fig. n), from which it will be seen 
 that the two supernumerary appendages stand as images of each 
 other; and, of them, that which is adjacent to the normal ap- 
 pendage forms an image of it. Thus if the supernumerary pair 
 arise from a point on the dorsal surface of the normal appendage, 
 the two ventral surfaces of the extra pair will face each other. 
 If they arise on the anterior surface of the normal appendage, 
 their morphologically posterior surfaces will be adjacent, and so on. 
 
 These facts give us a view of the relations of the two halves 
 of a dividing bud very different from that which is to be derived 
 from the exclusive study of normal structures. Ordinary mor- 
 phological conceptions no longer apply. The distribution of the 
 parts shows that the bud or rudiment which becomes the super- 
 numerary pair may break or open out in various ways according 
 to its relations to the normal limb. Its planes of division are 
 decided by its geometrical relations to the normal body. 
 
 Especially curious are some of the cases in which the extra 
 pair are imperfectly formed. The appearance produced is then 
 that of two limbs in various stages of coalescence, though in 
 reality of course they are stages of imperfect separation. The 
 plane of "coalescence" may fall anywhere, and the two appen- 
 dages may thus be compounded with each other much as an 
 object partially immersed in mercury " compounds" with its 
 optical image reflected from the surface. 
 
 Supernumerary paired structures are not usually, if ever, 
 formed when an appendage is simply amputated. Cases oc- 
 casionally are seen which nevertheless seem to be of this nature. 
 Borradaile, 8 for example, described a crab {Cancer pagurus) 
 having in place of the right chela three small chelae arising from 
 a common base, where the appearances suggested that the three 
 reduced limbs replaced a single normal limb. From the details re- 
 ported however it seems still possible that one of the chelae 
 (that lettered F. I in Borradaile's figure) may be the normal 
 one, and the other two an extra pair. The chela which I suspect 
 to be the normal is in several respects deformed as well as being 
 
 8 Borradaile, L. A., Jour. Marine Zool, 1897, No. 8. 
 
SEGMENTATION 75 
 
 reduced in size, and this deformity may perhaps have ensued as 
 a consequence of the same wound which excited the growth of the 
 extra pair. Its reduced size may be due to the same injury, 
 which may quite well have checked its growth to full proportions. 
 
 Admitting doubt in these ambiguous cases it seems to be a 
 general rule that for the production of the extra pair the normal 
 limb should persist in connexion with the body. Moreover it is 
 practically certain that in no case can a single, viz. an unpaired, 
 duplicate of the normal appendage grow from it. Many examples 
 have been described as of this nature, but all of them may be with 
 confidence regarded as instances of a supernumerary pair in 
 which only the two morphologically anterior or the two mor- 
 phologically posterior surfaces are developed. We have thus 
 the paradox that a limb of one side of the body, say the right, 
 has in it the power to form a pair of limbs, right and left, as an 
 outgrowth of itself, but cannot form a second left limb alone. 
 
 A very interesting question arises whether it is strictly 
 correct to describe the extra pair as a right and a left, or whether 
 they are not rather two lefts or two rights of which one is reversed. 
 This question did not occur to me when in former years I studied 
 these subjects. It was suggested to me by Dr. Przibram. 
 The answer might have an important bearing on biological 
 mechanics, but I know no evidence from which the point can 
 be determined with certainty. In order to decide this question 
 it would be necessary to have cases in which the paired repetition 
 affected a limb markedly differentiated on the two sides of the 
 body, and of course the development of the extra parts in order 
 to be decisive must be fairly complete. One example only is 
 known to me which at all satisfies these requirements, that of the 
 lobster's chela figured (after Van Beneden) in Materials for the 
 Study of Variation, p. 531, Fig. 184, III. 
 
 Here the drawing distinctly suggests that one of the extra 
 dactylopodites, namely that lettered R, is differentiated as a 
 left and not merely a reversed right. For the teeth on this 
 dactylopodite are those of a cutting claw, not of a crushing claw, 
 whereas the dactylopodites R' and L' bear crushing teeth. The 
 figure makes it fairly certain also that the limb affected was a 
 
76 PROBLEMS OF GENETICS 
 
 crushing claw. Accepting this interpretation, we reach the 
 remarkable conclusion that the bud of new growth consisted of 
 halves differentiated into cutter and crusher as the normal claws 
 are, and that the extra crusher is geometrically a left but physi- 
 ologically a right. Though shaped as a left in respect of the 
 direction in which it points, the extra crusher is really an op- 
 tically reversed right, while the dactylopodite R, which is 
 placed pointing like a right, is really a reversed left (Fig. 12). 
 
 If these indications are reliable 9 and are established by further 
 observation we shall be led to the conclusion that the bud which 
 
 Ind£g 
 
 Fig. 12. Right claw of lobster bearing a pair of extra dactylopodites (after 
 van Beneden). The fine toothing on R suggests that this is part of a cutting 
 claw, though the limb bearing it is a crusher. 
 
 becomes an extra pair of limbs does not merely contain the parts 
 proper to the side on which it grows, but is comparable with 
 the original zygotic cell, and consists not simply of two halves, 
 but of two halves differentiated as a right and a left like the two 
 halves of the normal body. 
 
 Phenomena of this kind, evoked by mutilation or injury, 
 together with the cognate observations on regeneration throw 
 
 9 Dr. Przibram, I should mention, concludes that on the whole the facts are 
 against this interpretation, but as more evidence is certainly required, I call at- 
 tention to the possibility. 
 
SEGMENTATION 77 
 
 very curious lights on the nature of living things. To an under- 
 standing of the nature of the mechanics of living matter and its 
 relation to matter at large they offer the most hopeful line of 
 approach. I allude especially to the examples in which it has 
 been established that the part which is produced after mutilation 
 is a structure different from that which was removed The 
 term "regeneration" was introduced before such phenomena 
 were discovered, and though every one recognizes its inapplica- 
 bility to these remarkable cases, the word still misleads us by 
 presenting a wrong picture to the mind. The expression " hetero- 
 morphosis" (Loeb) has been appropriately applied to various 
 phenomena of this kind, and Morgan has given the name "mor- 
 phallaxis" to another group of cases in which the renewal occurs 
 by the transformation of a previously existing part. 10 But we 
 must continually remember that all these occurrences which we 
 know only as abnormalities and curiosities must in reality be 
 exemplifications of the normal mechanics of division and growth 
 The conditions needed to call them forth are abnormal, but the 
 responses which the system makes are evidences of its normal 
 constitution. When therefore, for example, the posterior end 
 of a worm produces a reversed tail from its cut end we have a 
 proof that there must be in the normal body forces ready to 
 cause this outgrowth. The new structure is not an ill-shaped 
 head-end, for, as Morgan shows, the nephridial ducts have their 
 funnels perforating the segments in a reversed direction. The 
 "tension" of growth is actually reversed." So also when in a 
 Planarian amputation of the body immediately behind the head 
 leads to the formation of a new reversed head at the back of 
 the normal head, while amputation further back leads to the 
 regeneration of a new tail, these responses give indications 
 of forces normally present in the body of the Planarian. Such 
 facts open up a great field of speculation and research. Espe- 
 cially important it would be to determine where the critical 
 region may be at which the one response is replaced by the 
 
 10 Morgan, T. H., Regeneration, 1901. 
 
 11 It would be interesting to know whether growth continues at the original 
 posterior end after the new "posterior" end has been formed in front. 
 
78 PROBLEMS OF GENETICS 
 
 other. I suppose it is even possible that there is some neutral 
 zone in which neither kind of response is made. 
 
 Physical parallels to the phenomena of regeneration are 
 not easy to find and we still cannot penetrate beyond the empirical 
 facts. Przibram has laid stress on the general resemblance 
 between the new growth of an amputated part in an animal and 
 the way in which a broken crystal repairs itself when placed in 
 the mother-solution. That the two processes have interesting 
 points of likeness cannot be denied. It must however never be 
 forgotten that there is one feature strongly distinguishing the 
 two; for I believe it is universally recognized by physicists that 
 all the phenomena of geometrical regularity which crystals 
 display are ultimately dependent on the forms of the particles 
 of the crystalline body. This cannot in any sense be supposed 
 to hold in regard to protoplasm or its constituents. The de- 
 finiteness of crystals is also an unlikely guide for the reason that 
 it is absolute and perfect, or in other words because this kind of 
 regularity cannot be disturbed at all without a change so great 
 that the substance itself is altered; whereas we know that the 
 forms of living things are capable of such changes, great and small, 
 that we must regard perfection of form, whether manifested in 
 symmetry or in number, as an ideal which will only be produced 
 in the absence of disturbance. The symmetry of the living 
 things is like the symmetry of the concentric waves in a pool 
 caused by a splash. Perfect circles are made only in the imagi- 
 nary case of mathematical uniformity, but the system maintains 
 an approximate symmetry though liable to manifold deformation. 
 
 Since the geometrical order of the living body cannot be a 
 direct function of the materials it must be referred to some more 
 proximate control. In renewing a part the body must possess 
 the power of seizing particles of many dissimilar kinds, and whirl 
 them into their several and proper places. The action in re- 
 newal, like that of original growth, may be compared — very 
 crudely — with the action of a separator which simultaneously 
 distributes a variety of heterogeneous materials in an orderly 
 fashion ; but in the living body the thing distributed must rather 
 be the appetency for special materials, not the materials them- 
 selves. 
 
SEGMENTATION 79 
 
 If the analogy of crystals be set aside and we seek for other 
 parallels to regeneration there are none very obvious. I have 
 sometimes wondered whether it might not be possible to institute 
 a fruitful comparison between the renewal of parts and the refor- 
 mation of waves of certain classes after obliteration. In several 
 respects, as I have already said, some curious resemblances with 
 the repetitions formed by wave-motion are to be traced in our 
 organic phenomena, and though admitting that I cannot develop 
 these comparisons, I think nevertheless they may be worth 
 bearing in mind. When, after obliteration, an eddy in a stream, 
 or a ripple-mark (a more complex case of eddy-formation) in 
 blown sand is re-formed, we have an example in which pattern is 
 reconstituted and growth takes place not by virtue of the com- 
 position of the materials— in this case the water or the sand- 
 but by the way in which they are acted upon by extraneous 
 forces. 
 
 A feature in the actual mode by which ripple-marks are 
 reconstituted may not be without interest in connexion with 
 our phenomena of regeneration. When, for example, the wind 
 is blowing steadily over a surface of fine, dry sand, the familiar 
 ripple-marks are formed by a heaping of the sand in lines trans- 
 verse to the direction of the wind. The heaping is due to the 
 formation of eddies corresponding with positions of instability. 
 W T hen the wind is steady and the sand homogeneous, the dis- 
 tances between the ripples, or wave-lengths, are sensibly equal. 
 If while the wind continues to blow, the ripples are obliterated 
 with a soft brush they will quickly be re-formed over the whole 
 area, but I have noticed that at first their wave-length is ap- 
 proximately half that of the ripples in the undisturbed parts of 
 the system. 12 The normal wave-length is restored by the gradual 
 accentuation of alternate ripples. Of course the sand-ripples are 
 in reality slowly travelling forward in the direction towards 
 which the wind is blowing, and for this our living segmentations 
 afford no obvious parallel, but the appearances in the area of 
 
 12 In the actual case observed, the ripples unsmoothed had a wave-length of 
 about 2Y 2 inches; and when the new ones were first formed, there were about 30 
 ridges in the length originally traversed by 15 or 16. 
 
80 PROBLEMS OF GENETICS 
 
 reformation, and especially the forking of the old ridges where 
 they join the new ones, are curiously reminiscent of the irregu- 
 larities of segmentation seen in regenerated structures. The 
 value of the considerations adduced in the chapter is, I admit, 
 very small. The utmost that can be claimed for them is that 
 mechanical segmentations, like those seen in ripple-mark, or 
 in Leduc's osmotic growths, show how by the action of a contin- 
 uous force in one direction, repeated and serially homologous 
 divisions can be produced having features of similarity common 
 to those repetitions by which organic forms and patterns are 
 characterised. The analogy supplies a vicarious picture of the 
 phenomena which in default of one more true may in a slight 
 degree assist our thoughts. It suggests that the rhythms of 
 segmentation may be the consequence of a single force definite 
 in direction and continuously acting during the time of growth. 
 The polarity of the organism would thus be the expression of 
 the fact that this meristic force is definitely directed after it has 
 once been excited, and the reversal seen in some products of re- 
 generation suggest further that it is capable of being reflected. 
 This polarity cannot be a property of the material, as such, 
 but is determined by a force acting on that material, just as the 
 polarity of a magnet is not determined by the arrangement of its 
 particles, but by the direction in which the current flows. 
 
 To some it may appear that even to embark on such discussions 
 as this is to enter into a perilous flirtation with vitalistic theories. 
 How, they may ask, can any force competent to produce chem- 
 ical and geometrical differentiation in the body be distinguished 
 from the "Entelechy" of Driesch? Let me admit that in this 
 reflexion there is one element of truth. If those who proclaim 
 a vitalistic faith intend thereby to affirm that in the processes 
 by which growth and division are effected in the body, a part is 
 played by an orderly force which we cannot now translate into 
 terms of any known mechanics, what observant man is not a 
 vitalist? Driesch's first volume, putting as it does into intel- 
 ligible language that positive deduction from the facts — es- 
 pecially of regeneration — should carry a vivid realisation of this 
 truth to any mind. If after their existence is realised, it is 
 
SEGMENTATION 81 
 
 desired that these unknown forces of order should have a name, 
 and the word entelechy is proposed, the only objection I have to 
 make is that the adoption of a term from Aristotelian philosophy 
 carries a plain hint that we propose to relegate the future study 
 of the problem to metaphysic. 
 
 From this implication the vitalist does not shrink. But 
 I cannot find in the facts yet known to us any justification of so 
 hopeless a course. It was but yesterday that the study of 
 Entwicklungsmechanik was begun, and if in our slight survey 
 we have not yet seen how the living machine is to be expressed 
 in terms of natural knowledge that is poor cause for despair. 
 Driesch sums up his argument thus: 13 
 
 "It seems to me that there is only one conclusion possible. 
 If we are going to explain what happens in our harmonious- 
 equipotential sytems by the aid of causality based upon the 
 constellation of single chemical factors and events, there must 
 be some such thing as a machine. Now the assumption of the 
 existence of a machine proves to be absolutely absurd in the 
 light of the experimental facts. Therefore there can be neither 
 any sort of a machine nor any sort of causality based upon con- 
 stellation underlying the differentiation of harmonious-equipotential 
 systems. 
 
 "For a machine, typical with regard to the three chief dimen- 
 sions of space, cannot remain itself if you remove parts of it 
 or if you rearrange its parts at will." 
 
 To the last clause a note is added as follows: 
 
 "The pressure experiments and the dislocation experiments 
 come into account here; for the sake of simplicity they have 
 not been alluded to in the main line of our argument." 
 
 I doubt whether any man has sufficient knowledge of all 
 possible machines to give reality to this statement. In spite also 
 of the astonishing results of experiments in dislocation, doubt 
 may further be expressed as to whether they have been tried in 
 such variety or on such a scale as to justify the suggestion that 
 the living organism remains itself if its parts are rearranged at 
 
 13 The Science and Philosophy of the Organism; GifTord Lectures, 1907. London, 
 1908, p. 141. 
 
82 PROBLEMS OF GENETICS 
 
 will. All we know is that it can "remain itself" when much is 
 removed, and when much rearrangement has been affected, 
 which is a different thing altogether. 
 
 I scarcely like to venture into a region of which my ignorance 
 is so profound, but remembering the powers of eddies to re-form 
 after partial obliteration or disturbance, I almost wonder whether 
 they are not essentially machines which remain themselves 
 when parts of them are removed. 
 
 Real progress in this most obscure province is not likely to be 
 made till it attracts the attention of physicists ; and though they 
 for long may have to forego the application of exact quantitative 
 methods, I confidently anticipate that careful comparison 
 between the phenomena of repetition formed in living organisms 
 and the various kinds of segmentation produced by mechanical 
 agencies would be productive of illuminating discoveries. 14 
 
 14 The conclusion expressed (p. 72) that the extra appendages of Arthropods 
 result from injury, may have to be modified in view of the remarkable evidence 
 lately obtained by Miss Hoge {Jour. Exp. Zool., 1915, xviii, p. 241) showing that 
 in Drosophila the production of extra paired legs is a Mendelian character behav- 
 ing as a recessive of a peculiar kind, easily modified by temperature. The case of 
 the Prionus {Materials, p. 544) found with all six legs bearing extra parts had 
 always seemed to me inconsistent with the suggestion that injury was the exciting 
 cause, especially since in a second specimen of Prionus two legs had been found 
 thus affected. Nevertheless here, and even in the Drosophila strain, it might 
 be argued that the essential distinction may consist in a natural liability to 
 autotomy at some developmental stage, the extra growths being produced in con- 
 sequence of this. 
 
CHAPTER IV 
 
 THE CLASSIFICATION OF VARIATION AND THE NATURE OF 
 SUBSTANTIVE FACTORS 
 
 We have now seen that among the normal physiological 
 processes the phenomena of division form a recognisable, and 
 in all likelihood a naturally distinct group. Variations in these 
 respects may thus be regarded as constituting a special class 
 among variations in general. 
 
 The substantive variations have only one property in com- 
 mon — the negative one that they are not Meristic. The work 
 of classifying them and distinguishing them according to their 
 several types demands a knowledge of the chemistry of life far 
 higher than that to which science has yet attained. In reference 
 to some of the simplest variations Garrod has introduced the 
 appropriate term "Chemical sports." The condition in man 
 known as Alkaptonuria in which the urine is red is due especially 
 to the absence of the enzyme which decomposes the excretory 
 substance, alkapton. The " chemical sport" here consists in the 
 inability to break up the benzene ring. The chemical feature 
 which distinguishes and is the proximate cause of several colour- 
 varieties can now in a few cases be declared. The work of Miss 
 Wheldale has shown that colour-varieties may be produced by 
 the absence of the chromogen compound the oxidation of which 
 gives rise to sap-colours, by differences in the completeness of 
 this process of oxidation, and by a process of reduction super- 
 vening on or perhaps suppressing the oxidation. Some of these 
 processes moreover may be brought about by the combined action 
 of two bodies, the one an enzyme, for example an oxygenase, and 
 the other a substance regarded as a peroxide, contributing the 
 oxygen necessary for the oxidation to take place. Variation in 
 colour may thus be brought about by the addition or omission 
 of any one of the bodies concerned in the action. 
 
 Similar variations, or rather similar series of variations will 
 
 83 
 
84 PROBLEMS OF GENETICS 
 
 undoubtedly hereafter be identified in reference to all the various 
 kinds of chemical processes upon which the structure and func- 
 tions of living things depend. The identification of these proc- 
 esses and of the bodies concerned in them will lead to a real 
 classification of Substantive Variations. 
 
 To forecast the lines on which such classification will proceed 
 is to look too far ahead. We may nevertheless anticipate with 
 some confidence that future analysis will recognise among the 
 contributing elements, some which are intrinsic and inalienable, 
 and others which are extrinsic and superadded. 
 
 We already know that there may be such interdependence 
 among the substantive characters that to disentangle them will 
 be a work of extreme difficulty. The mere fact that in our 
 estimation characters belong to distinct physiological systems is 
 no proof of their actual independence. In illustration may be 
 mentioned the sap-colour in Stocks and the development of 
 hoariness on the leaves and stems, which Miss Saunders's experi- 
 ments have shown to be intimately connected, so that in certain 
 varieties no hoariness is produced unless the elements for sap- 
 colour are already present in the individual plant. 
 
 The first step in the classification of substantive variations 
 is therefore to determine which are due to the addition of new 
 elements or factors, and which are produced by the omission of 
 old ones. A priori there is no valid criterion by which this can 
 be known, and actual experiments in analytical breeding can 
 alone provide the knowledge required. Some very curious results 
 have by this method been obtained, which throw an altogether 
 unexpected light on these problems. For example, in order that 
 the remarkable development of mesoblastic black pigment char- 
 acteristic of the Silky Fowl should be developed, it is practically 
 certain that two distinct variations from such a type as Gallus 
 bankiva must have occurred. I assume, as is reasonable, that 
 G. bankiva has genetic properties similar to those of the Brown 
 Leghorn breed which has been used in the experiments which Mr. 
 Punnett and I have conducted. Gallus bankiva was not available 
 but the Brown Leghorn agrees with it very closely in colouration, 
 and probably in the general physiology of its pigmentation. 
 
CLASSIFICATION OF VARIATIONS 85 
 
 Setting aside the various structural differences between the two 
 breeds, the Silky is immediately distinguished from the Leghorn 
 by the fact that the skin of the whole body including that of the 
 face and comb appears to be of a deep purplish colour. The 
 face and comb of the Leghorn are red and the skin of the body 
 is whitish yellow. On examination it is found that the purple 
 colour of the Silky is in reality due to the distribution of a deep 
 black pigment in the mesoblastic membranes throughout the 
 body. The somatopleura, the pleura, pia mater, the dermis, and 
 in most organs the connective tissue and the sheaths of the blood- 
 vessels, are thus impregnated with black. No such pigmentation 
 exists in the Leghorn. As the result of an elaborate series of 
 experimental matings we have proved that the distinction be- 
 tween the Leghorn and the Silky consists primarily in the fact 
 that the Silky possesses a pigment-producing factor, P, which is 
 not present in the Leghorn. 
 
 This variation must undoubtedly have been one of addition. 
 But besides this there is another difference of an altogether dis- 
 similar nature; for the Brown Leghorn possesses a factor which 
 has the power of partially or completely restricting the operation 
 of the pigment-producing factor, P. Moreover in respect of 
 this pigment-restricting factor which we may call D, the sexes 
 of the Brown Leghorn differ, for the male is homozygous or DD r 
 but the female is heterozygous, Dd. Thus in order that the 
 black-skinned breed could be evolved from such a type as a 
 Brown Leghorn it must be necessary both that P should be added 
 and that D should drop out. We have not the faintest concep- 
 tion of the process by which either of these events have come to 
 pass, but there is no reasonable doubt that in the evolution of 
 the Silky fowl they did actually happen. 
 
 We may anticipate that numerous interdependences of this 
 kind will be discovered. 
 
 Before any indisputable progress can be made with the prob- 
 lem of evolution it is necessary that we should acquire some real 
 knowledge of the genesis of that class of phenomena which 
 formed the subject of the last chapter. So long as the process 
 of division remains entirely mysterious we can form no conception 
 
86 PROBLEMS OF GENETICS 
 
 even of the haziest sort as to the nature of living organisms, or 
 of the proximate causes which determine their forms, still less 
 can we attempt any answer to those remoter questions of origin 
 and destiny which form the subject of the philosopher's con- 
 templation. It is in no spirit of dogmatism that I have ventured 
 to indicate the direction in which I look for a solution, though 
 I have none to offer. It may well be that before any solution 
 is attained, our knowledge of the nature of unorganised matter 
 must first be increased. For a long time yet we may have to 
 halt, but we none the less do well to prepare ourselves to utilise 
 any means of advance that may be offered, by carefully recon- 
 noitering the ground we have to traverse. The real difficulty 
 which blocks our progress is ignorance of the nature of division, 
 or to use the more general term, of repetition. 
 
 Let us turn to the more familiar problem of the causes of 
 variation. Now since variation consists as much in meristic 
 change as in alteration in substance or material, there is one 
 great range of problems of causation from which we are as yet 
 entirely cut off. We know nothing of the causation of division, 
 and we have scarcely an observation, experiment or surmise 
 touching the causes by which the meristic processes may be 
 .altered. 
 
 Of the way in which variations in the substantive composition 
 <-of organisms are caused we have almost as little real evidence, 
 but we are beginning to know in what such variations must con- 
 sist. These changes must occur either by the addition or loss 
 of factors. 
 
 We must not lose sight of the fact that though the factors 
 operate by the production of enzymes, of bodies on which these 
 enzymes can act, and of intermediary substances necessary to 
 complete the enzyme-action, yet these bodies themselves can 
 scarcely be themselves genetic factors, but consequences of their 
 existence. What then are the factors themselves? Whence do 
 they come? How do they become integral parts of the organism? 
 Whence, for example, came the power which is present in a White 
 Leghorn of destroying — probably reducing — the pigment in its 
 feathers? That power is now a definite possession of the breed, 
 
CLASSIFICATION OF VARIATIONS 87 
 
 present in all its germ-cells, male and female, taking part in their 
 symmetrical divisions, and passed on equally to all as much as 
 is the protoplasm or any other attribute of the breed. From the 
 body of the bird the critical and efficient substance could in all 
 likelihood be isolated by suitable means, just as the glycogen 
 of the liver can be. But even when this extraction has been 
 accomplished and the reducing body isolated, we shall know no 
 more than we did before respecting the mode by which the power 
 to produce it was conferred on the fowl, any more than we know 
 how the walls of its blood-vessels acquired the power to form a 
 fibrin-ferment. 
 
 It is when the scope of such considerations as this are fully 
 grasped that we realise the fatuousness of the conventional treat- 
 ment which the problem of the causes of variation commonly 
 receives. Environmental change, chemical injury, differences in 
 food supply, in temperature, in moisture, or the like have been 
 proposed as "causes." Admitting as we must do, that changes 
 may be produced — usually inhibitions of development — by sub- 
 jecting living things to changes in these respects, how can we 
 suppose it in the smallest degree likely that very precise, new, 
 and adaptative powers can be conferred on the germs by such 
 treatment? Reports of positive genetic consequences observed 
 comparable with those I have mentioned, become from time to 
 time current. We should I think regard them with the gravest 
 doubt. Few, so far as I am aware, have ever been confirmed, 
 though clear and repeated confirmation should be demanded 
 before we suffer ourselves at all to build upon such evidence. 
 In a subsequent chapter some of these cases will be considered in 
 detail. 
 
 In no class of cases would the transmission of an acquired 
 character superficially appear so probable as in those where power 
 of resisting the attack of a pathogenic organism is acquired in 
 the lifetime of the zygote. The possession of such a power is 
 moreover a distinction comparable with those which differentiate 
 varieties and species. It is due to the development in the blood 
 of specific substances which pervade the whole fluid. This 
 development is exactly one of those "appropriate responses to 
 
88 PROBLEMS OF GENETICS 
 
 stimuli" which naturalists who incline to regard adaptation as 
 a direct consequence of an environmental influence might most 
 readily invoke as an illustration of their views. And yet all 
 evidence is definitely unfavourable to the suggestion of an 
 inheritance of the acquired power of resistance. Such change as 
 can be perceived in the virulence of the attacks on successive 
 generations may be most easily regarded as due to the exter- 
 mination of the more susceptible strains, and perhaps in some 
 measure to variation in the invading organisms themselves, an 
 "acquired character" of quite different import. 
 
 The specific "anti-body" may have been produced in re- 
 sponse to the stimulus of disease, but the power to produce it 
 without this special stimulus is not included in the germ-cells 
 any more than a pigment. All that they bear is the power to 
 produce the anti-bodies when the stimulus is applied. 
 
 If we could conceive of an organism like one of those to 
 which disease may be due becoming actually incorporated with 
 the system of its host, so as to form a constituent of its germ-cells 
 and to take part in the symmetry of their divisions, we should 
 have something analogous to the case of a species which acquires 
 a new factor and emits a dominant variety. When we see the 
 phenomenon in this light we realise the obscurity of the problem. 
 The appearance of recessive varieties is comparatively easy to 
 understand. All that is implied is the omission of a constituent. 
 How precisely the omission is effected we cannot suggest, but 
 it is not very difficult to suppose that by some mechanical fault 
 of cell-division a power may be lost. Such variation by unpack- 
 ing, or analysis of a previously existing complex, though unac- 
 countable, is not inconceivable. But whence come the new 
 dominants? Whether we imagine that they are created by some 
 rearrangement or other change internal to the organism, or 
 whether we try to conceive them as due to the assumption of 
 something from without we are confronted by equally hopeless 
 difficulty. 
 
 The mystery of the origin of a dominant increases when it 
 is realised that there is scarcely any recent and authentic account 
 of such an event occurring under critical observation, which can 
 
CLASSIFICATION OF VARIATIONS 89 
 
 be taken as a basis for discussion. The literature of horticulture 
 for example abounds in cases alleged, but I do not think anyone 
 can produce an illustration quite free from doubt. Such evidence 
 is usually open to the suspicion that the plant was either intro- 
 duced by some accident, or that it arose from a cross with a pre- 
 existing dominant, or that it owed its origin to the meeting of 
 complementary factors. In medical literature almost alone how- 
 ever, there are numerous records of the spontaneous origin of 
 various abnormal conditions in man which habitually behave 
 as dominants, and of the authenticity of some of these there 
 can be no doubt. 
 
 When we know that such conditions as hereditary cataract 
 or various deformities of the fingers behave as dominants, we 
 recognize that those conditions must be due to the addition of 
 some element to the constitution of the normal man. In the 
 collections of pedigrees relating to such pathological dominants 
 there are usually to be found alleged instances of the origin of the 
 condition de novo. Not only do these records occur with such 
 frequency that they cannot be readily set aside as errors, but from 
 general considerations it must be obvious that as these mal- 
 formations are not common to normal humanity they must at 
 some moment of time have been introduced. The lay reader 
 may not be so much impressed with the difficulty as we are. He 
 is accustomed to regard the origin of any new character as equally 
 mysterious, but when once dominants are distinguished from 
 recessives the problem wears a new aspect. Thus the appearance 
 of high artistic gifts, whether as an attribute of a race or as a 
 sporadic event among the children of parents destitute of such 
 faculties, is not very surprising, for we feel fairly sure that the 
 faculty is a recessive, due to the loss of a controlling or inhibiting 
 factor; but the de novo origin of brachydactylous fingers in a 
 child of normal parents is of quite a different nature, and must 
 indicate the action of some new specific cause. 
 
 Whether such evidence is applicable to the general problem 
 of evolution may with some plausibility be questioned; but 
 there is an obvious significance in the fact that it is among these 
 pathological occurrences that we meet with phenomena most 
 
9 o PROBLEMS OF GENETICS 
 
 nearly resembling the spontaneous origin of dominant factors, 
 and I cannot see such pedigrees as these without recalling Vir- 
 chow's aphorism that every variation owes its origin to some 
 pathological accident. In the evolution of domestic poultry, 
 if Gallus bankiva be indeed the parent form of all our breeds, 
 at least some half dozen new factors must have been added 
 during the process. In bankiva there is, for example, no factor 
 for rose comb, pea comb, barring on the feathers, or for the 
 various dominant types of dark plumage. Whence came all 
 these? It is, I think, by no means impossible that some other 
 wild species now extinct did take part in the constitution of 
 domestic poultry. It seems indeed to me improbable that the 
 heavy breeds descend from bankiva. Both in regard to domestic 
 races of fowls, pigeons, and some other forms, the belief in origin 
 within the period of human civilization from one simple primi- 
 tive wild type seems on a balance of probabilities insecurely 
 founded, but allowing something for multiplicity of origin we 
 still fall far short of the requisite total of factors. Elements 
 exist in our domesticated breeds which we may feel with con- 
 fidence have come in since their captivity began. Such ele- 
 ments in fowls are dominant whiteness, extra toe, feathered 
 leg, frizzling, etc., so that even hypothetical extension of the 
 range of origin is only a slight alleviation of the difficulty. 
 
 Somehow or other, therefore, we must recognize that dominant 
 factors do arise. Whether they are created by internal change, 
 or whether, as seems to me not wholly beyond possibility, they 
 obtain entrance from without, there is no evidence to show. 
 If they were proved to enter from without, like pathogenic 
 organisms, we should have to account for the extraordinary 
 fact that they are distributed with fair constancy to half the 
 gametes of the heterozygote. 
 
 In proportion as the nature of dominants grows more clear 
 so does it become increasingly difficult to make any plausible 
 suggestion as to their possible derivation. On the other hand 
 the origin of a recessive variety by the loss of a factor is a process 
 so readily imagined that our wonder is rather that the phenom- 
 enon is not observed far more often. Some slip in the accurate 
 
CLASSIFICATION OF VARIATIONS 91 
 
 working of the mechanical process of division, and a factor 
 gets left out, the loss being attested by the appearance of a 
 recessive variety in some subsequent generation. 
 
 Consistently with this presentation of the facts we find that, 
 as in our domesticated animals and plants, a diversity of recessives 
 may appear within a moderately short period, and that when 
 variations come they often do not come alone. Witness the 
 cultural history of the Sweet Pea, Primula Sinensis, Primula 
 obconica, Nemesia strumosa and many such examples in which 
 variation when it did come was abundant. The fact cannot 
 be too often emphasized that in the vast proportion of these 
 examples of substantive variation under domestication, as well 
 as of substantive variation in the natural state, the change 
 has come about by omission, not by addition. To take, for 
 example, the case of the Potato, in which so many spontaneous 
 bud-variations have been recorded, East after a careful study 
 of the evidence has lately declared his belief that all are of this 
 nature, and the opinion might be extended to many other groups 
 of cases whether of bud or seminal variation. Morgan draws 
 the same conclusion in reference to the many varieties he has 
 studied in Drosophila. 
 
 In the Sweet Pea, a form which is beyond suspicion of having 
 been crossed with anything else, and has certainly produced 
 all the multitude of types which we now possess by variations 
 from one wild species, there is only one character of the modern 
 types which could, with any plausibility, be referred to a factor 
 not originally forming part of the constituents of the wild species. 
 This is the waved edge, so characteristic of the "Spencer" 
 varieties; for the cross between a smooth-edged and a waved 
 type gives an intermediate not unfrequently. Nevertheless 
 there is practically no doubt that this is merely an imperfection 
 in the dominance of the smooth edge, and we may feel sure 
 that any plant homozygous for smooth edge would show no wave 
 at all. Hence it is quite possible that even the appearance of 
 the original waved type, Countess Spencer, was due to the loss 
 of one of the factors for smooth edge at some time in the history 
 of the Sweet Pea. 
 
92 PROBLEMS OF GENETICS 
 
 In the case of the Chinese Primrose (Primula Sinensis) one 
 dominant factor has been introduced in modern times, probably 
 within the last six years at most. This is the factor which 
 causes suppression of the yellow eye, giving rise to the curious 
 type known as " Queen Alexandra." Mr. R. P. Gregory's 
 experiments proved that this was a very definite dominant, and 
 the element responsible for this development is undoubtedly an 
 addition to the original ingredient-properties, with which the 
 species was endowed. Unfortunately, as happens in almost every 
 case of the kind, the origin of this important novelty appears 
 to be lost. Its behaviour, however, when crossed with various 
 other types is that of a simple dominant giving an ordinary 3 : I 
 ratio. There is therefore no real doubt that it came into exist- 
 ence by the definite addition of a new factor, for if it was simply 
 a case of the appearance of a new character made by combination 
 of two previously existing complementary factors we should 
 expect that when Queen Alexandra was self-fertilised a 9 : 7 
 ratio would be a fairly common result, which is not in practice 
 found. 
 
 In Oenothera Gates * has observed the appearance, in a large 
 sowing of about 1,000 Oenothera rubrinervis, of a single individual 
 having considerably more red pigment in the calyx than is usual 
 in rubrinervis. The whole of the hypanthium in the flowers of 
 this plant was red instead of green as in rubrinervis, and the 
 whole of the sepals were red in the bud-stage, except for small 
 green areas at the base. This type behaved as a dominant over 
 rubrinervis, but so far a pure-breeding individual was not found. 
 Admittedly the variation of this plant from the type of rubrinervis 
 can be represented as one of degree, though there is a very 
 sensible gap in the series between the new form which Gates 
 names " rubricalyx" and the reddest rubrinervis seen in his cul- 
 tures. It must certainly be recognised as a new dominant. 
 Gates, rightly as I consider, regards the distinction between 
 rubrinervis and rubricalyx as a quantitative one, and the same 
 remark applies to certain other types differing in the amount of 
 anthocyanin which they produce. I do not understand the argu- 
 
 1 Gates, R. R., Zts.f. Abstammungslehre, 1911, IV, pp. 34* and 361. 
 
CLASSIFICATION OF VARIATIONS 93 
 
 ment which Gates introduces to the effect that the difference 
 between such quantitative types cannot be represented in terms 
 of presence and absence. We are quite accustomed to the fact 
 that in the rabbit self-colour segregates from the Dutch-marked 
 type. These two types differ in a manner which we may reason- 
 ably regard as quantitative. It is no doubt possible that the 
 self-coloured type contains an ingredient which enables the colour 
 to spread over the whole body, but it is, I think, perhaps more 
 easy to regard the Dutch type as a form from which a part of 
 the colour is absent. It may be spoken of in terms I have used, 
 as a subtraction-stage in colour. Following a similar method we 
 may regard rubricalyx as an addition-stage in colour-variation. 
 The fact that crosses between rubrinervis, or rubricalyx and 
 Lamarckiana give a mixture of types in Fi, does not I think show, 
 as Gates declares, that there is any system here at work to 
 which a factorial or Mendelian analysis does not apply; but that 
 question may be more fitly discussed in connexion with the other 
 problems raised by the behaviour of Oenothera species in their 
 crosses. 
 
 I do, however, feel that, interesting as this case must be 
 admitted to be, we cannot quite satisfactorily discuss it as an 
 illustration of the de novo origin of a dominant factor. The 
 difference between the novelty and the type is quantitative, and 
 it is not unreasonable to think of such a difference being brought 
 about by some " pathological accident" in a cell-division. 
 
 Recognition of the distinction between dominant and reces- 
 sive characters has, it must be conceded, created a very serious 
 obstacle in the way of any rational and concrete theory of evolu- 
 tion. While variations of all kinds could be regarded as mani- 
 festations of some mysterious instability of organisms this diffi- 
 culty did not occur to the mind of evolutionists. To most of 
 those who have taken part in genetic analysis it has become a 
 permanent and continual obsession. With regard to the origin 
 of recessive variations, there is, as we have seen, no special 
 difficulty. They are negative and are due to absences, but as 
 soon as it is understood that dominants are caused by an addition 
 we are completely at a loss to account for their origin, for we 
 
94 PROBLEMS OF GENETICS 
 
 cannot surmise any source from which they may have been 
 derived. Just as when typhoid fever breaks out in his district 
 the medical officer of health knows for certain that the bacillus of 
 typhoid fever has by some means been brought into that district 
 so do we know that when first dominant white fowls arose in the 
 evolution of the domestic breeds, by some means the factor for 
 dominant whiteness got into a bird, or into at least one of its 
 germ-cells. Whence it came we cannot surmise. 
 
 Whether we look to the outer world or to some rearrangement 
 within the organism itself, the prospect of finding a source of 
 such new elements is equally hopeless. 
 
 Leaving this fundamental question aside as one which it is 
 as yet quite unprofitable to discuss, we are on safe ground in 
 foreseeing that the future classification of substantive variations, 
 which genetic research must before long make possible, will be 
 based on a reference to the modes of action of the several factors. 
 Some will be seen to produce their effects by oxidation, some by 
 reduction, some by generating substances of various types, 
 sugars, enzymes, activators, and so forth. It may thus be 
 anticipated that the relation of varieties to each other and to 
 types from which they are derived will be expressible in terms 
 of definite synthetical formulae. Clearly it will not for an in- 
 definite time be possible to do this in practice for more than a 
 few species and for characters especially amenable to experi- 
 mental tests, but as soon as the applicability of such treatment 
 is generally understood the influence on systematics must be 
 immediate and profound, for the nature of the problem will at 
 length be clear and, though the ideal may be unattainable, its 
 significance cannot be gainsaid. 
 
 Note. — With hesitation I allow this chapter to appear in 
 the form in which it was printed a year ago, but in passing it for 
 the press after that interval I feel it necessary to call attention 
 to a possible line of argument not hitherto introduced. 
 
 In all our discussions we have felt justified in declaring that 
 the dominance of any character indicates that some factor is 
 
CLASSIFICATION OF VARIATIONS 95 
 
 present which is responsible for the production of that character. 
 Where there is no definite dominance and the heterozygote is 
 of an intermediate nature we should be unable to declare on 
 which side the factor concerned was present and from which side 
 it was absent. The degree of dominance becomes thus the 
 deciding criterion by which we distinguish the existence of factors. 
 But it should be clearly realized that in any given case the argu- 
 ment can with perfect logic be inverted. We already recognize 
 cases in which by the presence of an inhibiting factor a character 
 may be suppressed and purely as a matter of symbolical expres- 
 sion we might apply the same conception of inhibition to any 
 example of factorial influence whatever. For instance we say 
 that in as much as two normal persons do not have brachydacty- 
 lous children, there must be some factor in these abnormal persons 
 which causes the modification. Our conclusion is based on the 
 observed fact that the modification is a dominant. But it may 
 be that normal persons are homozygous in respect of some factor 
 N, which prevents the appearance of brachydactyly, and that in 
 any one heterozygous, Nn, for this inhibiting factor, brachy- 
 dactyly can appear. Similarly the round pea we say contains 
 R, a factor which confers this property of roundness, without 
 which its seeds would be wrinkled. But here we know that the 
 wrinkled seed is in reality one having compound starch-grains, 
 and that the heterozygote, though outwardly round enough, is 
 intermediate in that starch-character. If we chose to say that 
 the compoundness of the grains is due to a factor C and that two 
 doses of it are needed to make the seed wrinkled, I know no 
 evidence by which such a thesis could be actually refuted. That 
 such reasoning is seemingly perverse must be conceded; but 
 when we consider the extraordinary difficulties which beset 
 any attempt to conceive the mode of origin of a new dominant 
 factor, we are bound to remember that there is this other line of 
 argument which avoids that difficulty altogether. In the case of 
 the " Alexandra " eye in Primula, or the red calyx in Gates's 
 Oenothera, inverting the reasoning adopted in the text, we may 
 see that only the Primula homozygous for the yellow eye can 
 develop it and that two doses of the factor for the rubrinervis 
 
9 6 PROBLEMS OF GENETICS 
 
 calyx are required to prevent that part of the plant from being 
 red. 
 
 We may proceed further and extend this mode of reasoning to 
 all cases of genetic variation, and thus conceive of all alike as 
 due to loss of factors present in the original complex. Until we 
 can recognize factors by means more direct than are provided by 
 a perception of their effects, this doubt cannot be positively 
 removed. For all practical purposes of symbolic expression we 
 may still continue to use in our analyses the modes of representa- 
 tion hitherto adopted, but we must not, merely on the ground of 
 its apparent perversity, refuse to admit that the line of argument 
 here indicated may some day prove sound. 
 
CHAPTER V 
 
 THE MUTATION THEORY 
 
 When with the thoughts suggested in the last chapter we 
 contemplate the problem of Evolution at large the hope at the 
 present time of constructing even a mental picture of that process 
 grows weak almost to the point of vanishing. We are left 
 wondering that so lately men in general, whether scientific or 
 lay, were so easily satisfied. Our satisfaction, as we now see, 
 was chiefly founded on ignorance. 
 
 Every specific evolutionary change must represent a definite 
 event in the construction of the living complex. That event 
 may be a disturbance in the meristic system, showing itself in 
 a change in the frequency of the repetitions or in the distribution 
 of differentiation among them, or again it may be a chemical 
 change, adding or removing some factor from the sum total. 
 
 If an attempt be made to apply these conceptions to an actual 
 series of allied species the complexity of the problem is such that 
 the mind is appalled. Ideas which in the abstract are appre- 
 hended and accepted with facility fade away before the concrete 
 case. It is easy to imagine how Man was evolved from an 
 Amoeba, but we cannot form a plausible guess as to how Veronica 
 agrestis and Veronica polita were evolved, either one from the 
 other, or both from a common form. We have not even an 
 inkling of the steps by which a Silver Wyandotte fowl descended 
 from Gallus Bankiva, and we can scarcely even believe that it 
 did. The Wyandotte has its enormous size, its rose comb, its 
 silver lacing, its tame spirit, and its high egg production. The 
 tameness and the high egg production are probably enough both 
 recessives, and though we cannot guess how the corresponding 
 dominant factors have got lost, it is not very difficult to imagine 
 that they were lost somehow. But the rose comb and the silver 
 colour are dominants. The heavy weight also appears in the 
 crosses with Leghorns, but we need not at once conclude that it 
 
 97 
 
98 PROBLEMS OF GENETICS 
 
 depends on a simple dominant factor, because the big size of 
 the crosses may be a consequence of the cross and may depend on 
 other elements. 
 
 Now no wild fowl known to us has these qualities. May we 
 suppose that some extinct wild species had them? If so, may 
 we again make the same supposition in all similar cases? To do 
 so is little gain, for we are left with the further problem, whence 
 did those lost wild species acquire those dominants? Supposi- 
 tions of this kind help no more than did the once famous 
 conjecture as to the origin of living things — that perhaps they 
 came to earth on a meteorite. The unpacking of an original 
 complex, the loss of various elements, and the recombination of 
 pre-existing materials may all be invoked as sources of specific 
 diversity. Undoubtedly the range of possibilities thus opened 
 up is large. It will even cover an immense number of actual 
 examples which in practice pass as illustrations of specific dis- 
 tinction. The Indian Rock pigeon which has a blue rump 
 may quite reasonably be regarded as a geographically separated 
 recessive form of our own Columba livia, for as Staples-Browne 
 has shown the white rump of livia is due to a dominant factor. 
 The various degrees to which the leaves of Indian Cottons are 
 incised have, as Leake says, been freely used as a means of classi- 
 fication. The diversities thus caused are very remarkable, 
 and when taken together with diversities in habit, whether 
 sympodial or monopodial, the various combinations of points 
 of difference are sufficiently distinctive to justify any botanist 
 in making a considerable number of species by reference to them 
 alone. Nevertheless Leake's work goes far to prove that all of 
 these forms represent the re-combinations of a very small number 
 of factors. The classical example of Primula Sinensis and its 
 multiform races is in fact for a long way a true guide as to the 
 actual interrelations of the species which systematists have 
 made. That they did make them was due to no mistake in 
 judgment or in principle, but simply to the want of that ex- 
 tended knowledge of the physiological nature of the specific 
 cases which we now know to be a prime necessity. 
 
 But will such analysis cover all or even most of the ordinary 
 
THE MUTATION THEORY 99 
 
 cases of specific diversity between near allies? Postponing the 
 problem of the interrelations of the larger divisions as altogether 
 beyond present comprehension, can we suppose, that in general, 
 closely allied species and varieties represent the various con- 
 sequences of the presence or absence of allelomorphic factors 
 in their several combinations? The difficulty in making a 
 positive answer lies in the fact that in most of the examples in 
 which it has been possible to institute breeding experiments with 
 a view to testing the question, a greater or less sterility is en- 
 countered. Where, however, no such sterility is met with, as 
 for instance in the crosses made by E. Baur among the species 
 of Antirrhinum there is every reason to think that the whole 
 mass of differences can and will eventually be expressed in terms 
 of ordinary Mendelian factors. Baur has for example crossed 
 species so unlike as Antirrhinum majus and molle, forms differing 
 from each other in almost every feature of organisation. 1 The 
 F2 generation from this cross presents an amazingly motley 
 array of types which might easily if met with in nature be de- 
 scribed as many distinct species. Yet all are fertile and there 
 is not the slightest difficulty in believing that they can all be 
 reduced to terms of factorial analysis. 
 
 If allowance be made for the complicating effects of sterility, 
 is there anything which prevents us from supposing that such 
 good species as those of Veronica or of any other genus comprising 
 well-defined forms may not be similarly related? I do not know 
 any reason which can be pointed to as finally excluding such a 
 possibility. Nevertheless it has been urged with some plausi- 
 bility that good species are distinguished by groups of differ- 
 entiating characters, whereas if they were really related as the 
 terms of a Mendelian F 2 family are, we should expect to find 
 not groups of characters in association, but rather series of forms 
 corresponding to the presence and absence of the integral factors 
 composing the groups of characters. I am not well enough 
 versed in systematic work to be able to decide with confidence 
 how much weight should be attached to this consideration. Some 
 
 1 See Lotsy and Baur, Rep. Genetics Conf., Paris, 191 1, pp. 416-426. Com- 
 pare Lecoq on Mirabilisjalapa X longiflora, Fecondation des Vegetaux, 1862, p. 311,. 
 
ioo PROBLEMS OF GENETICS 
 
 weight it certainly has, but I cannot yet regard it as forming a 
 fatal objection to the application of factorial conceptions on 
 the grand scale. It may be recalled that we are no longer under 
 any difficulty in supposing that differences of all classes may be 
 caused by the presence or absence of factors. It seemed at first 
 for example that such characters as those of leaf shape might 
 be too subtle and complex to be reducible to a limited number of 
 factors. But first the 'work of Gregory on Primula Sinensis 
 showed that several very distinct types of leaves were related 
 to each other in the simplest way. In that particular example, 
 intermediates are so rare as to be negligible, but subsequently 
 Shull dealing with such a complicated example as Capsella, and 
 Leake in regard to Cottons, both forms in which intergrades occur 
 in abundance, have shown that a simple factorial scheme is 
 applicable. We need not therefore, to take an extreme case, 
 doubt that if it were possible to examine the various forms of 
 fruit seen in the Squashes by really comprehensive breeding 
 tests, even this excessive polymorphism in respect of structural 
 features would be similarly reducible to factorial order. 
 
 It must always be remembered also that in a vast number 
 of cases, nearly allied forms which are distinct, occupy distinct 
 -ground. Moreover, by whatever of the many available mechan- 
 isms that end be attained, it is clear that nature very often does 
 succeed in preventing intercrossing between distinct forms so 
 far that the occurrence of that phenomenon is a rarity under 
 natural conditions. The facts may, I think, fairly be summarized 
 in the statement that species are on the whole distinct and not 
 intergrading, and that the distinctions between them are usually 
 such as might be caused by the presence, absence, or inter-com- 
 bination of groups of Mendelian factors; but that they are so 
 caused the evidence is not yet sufficient to prove in more than a 
 very few instances. 
 
 The alternative, be it explicitly stated, is not to return to the 
 view formerly so widely held, that the distinctions between 
 species have arisen by the accumulation of minute or insensible 
 differences. The further we proceed with our analyses the more 
 inadequate and untenable does that conception of evolutionary 
 
THE MUTATION THEORY 101 
 
 change become. If the differences between species have not 
 come about by the addition or loss of factors one at a time, then 
 we must suppose that the changes have been effected by even 
 larger steps, and variations including groups of characters, must 
 be invoked. 
 
 That changes of this latter order are really those by which 
 species arise, is the view with which de Vries has now made us 
 familiar by his writings on the Mutation Theory. In so far as 
 mutations may consist in meristic changes of many kinds and 
 in the loss of factors it is unnecessary to repeat that we have 
 abundant evidence of their frequent occurrence. That they may 
 also more rarely occur by the addition of a factor we are, I think, 
 compelled to believe, though as yet the evidence is almost en- 
 tirely circumstantial rather than direct. The evidence for the 
 occurrence of those mutations of higher order, by which new 
 species characterized by several distinct features are created, 
 is far less strong, and after the best study of the records which 
 I have been able to make, I find myself unconvinced. The facts 
 alleged appear capable of other interpretations. 
 
 The most famous and best studied examples are of course the 
 forms of Oenothera raised by de Vries from Oenothera Lamarckiana 
 in circumstances well known to all readers of genetic literature. 
 Whatever be the true significance of these extraordinary "mu- 
 tations" there can be no question about the great interest which 
 attaches to them, and the historical importance which they will 
 long preserve. Apart also from these considerations it is be- 
 coming more and more evident that in their peculiarities they 
 provide illustrations of physiological phenomena of the highest 
 consequence in the study of genetics at large. 
 
 De Vries found, as is well known, that Oenothera Lamarckiana 
 gives off plants unlike itself. These mutational forms are of 
 several distinct and recognizable types which recur, and several 
 of them breed true from their first appearance. The obvious 
 difficulty, which in my judgment should make us unwilling at 
 present to accept these occurrences as proof of the genesis of new 
 species by mutation, is that we have as yet no certainty that the 
 appearance of the new forms is not an effect of the recombination 
 
102 PROBLEMS OF GENETICS 
 
 of factors, such as is to be seen in so many generations of plants 
 derived from a cross involving many genetic elements. The 
 'first question is what is Oenothera Lamarckiana? Is it itself a 
 plant of hybrid origin? To this fundamental question no satis- 
 factory answer has yet been given. All attempts to find it as a 
 wild plant in America have failed. It existed in Europe in the 
 latter half of the eighteenth century. Whence it came is still 
 uncertain, but the view that it came into existence in Europe and 
 perhaps in Paris, seems on the whole the most probable. The 
 question has been debated by Macdougal, Gates, and Davis. 
 From historical sources there is little expectation of further 
 light. Those who favour the notion of a hybrid origin look on 
 Oenothera biennis as one of the putative parents. It has been 
 conjectured that a species called grandiflora lately re-discovered 
 on the Alabama river was the other parent. Experiments have 
 been instituted by Davis to discover whether Lamarckiana can 
 be made artificially by crossing these two species. The results 
 so far have shown that while plants approximating in various 
 respects to Lamarckiana have thus been produced, none agree 
 exactly with that form. Davis, to whom reference should be 
 made for a full account of the present state of the enquiry, 
 points out that there are many strains of biennis in existence 
 and that it is by no means impossible that by using others of 
 these strains a still closer approximation can be made. None 
 of Davis's artificial productions as yet breed at all true, as 
 Lamarckiana on the whole does. In such a case, however, where 
 several characters are involved, this is perhaps hardly to be 
 expected. 
 
 One feature of the Oenotheras is very curious. Not only 
 Lamarckiana, but all the allied species so far as I am aware, 
 have a considerable proportion of bad and shrivelled pollen 
 grains. This is undoubtedly true of species living in the wild 
 state as well as of those in cultivation. I have had opportunities 
 of verifying this for myself in the United States. No one looking 
 at the pollen of an Oenothera would doubt that it was taken from 
 some hybrid plant exhibiting partial sterility. On the other 
 hand, it is difficult to suppose that numbers, perhaps all, of the 
 
THE MUTATION THEORY 103 
 
 "species" of the genus are really hybrids, and many of them breed 
 substantially true. I regard this constant presence of bad 
 pollen grains as an indication that the genetic physiology of 
 Oenothera is in some way abnormal, and as we shall presently 
 see, there are several other signs which point in the same direction. 
 
 Discussion of the whole series of phenomena is rendered 
 exceedingly difficult first, by reason of the actual nature of the 
 material . The characteristics of many of the types which de Vries 
 has named are evasive. A few of these types, for instance, gigas, 
 nanella, albida, brevistylis, and perhaps a few more are evidently 
 clear enough, but we have as yet no figures and descriptions 
 precise enough to enable a reader to appreciate exactly the pe- 
 culiarities of the vast number of forms which have now to be 
 considered in any attempt to gain a comprehensive view of the 
 whole mass of facts. It is also not in dispute that the forms are 
 susceptible of great variations due simply to soil and cultural 
 influences. 
 
 The fact that no Mendelian analysis has yet been found 
 applicable to this group of Oenotheras as a whole is perhaps largely 
 due to the fact that until recently such analysis has not been 
 seriously attempted. Following the system which he had 
 adopted before the rediscovery of Mendelism, or at all events, 
 before the development of that method of analysis, de Vries has 
 freely applied names to special combinations of characters and 
 has scarcely ever instituted a factorial analysis. Before we can 
 get much further this must be attempted. It may fail, but we 
 must know exactly where and how this failure comes about. 
 There are several indications that such a recognition of factorial 
 characters, could be carried some way. For example, the height, 
 the size of the flowers, the crinkling of the leaves, the brittleness 
 of the stems, perhaps even the red stripes on stems and fruits, 
 and many more, are all characters which may or may not depend 
 on distinct factors, but if such characters are really transmitted 
 in unresolved groups, the limitations of those groups should be 
 carefully determined. The free use of names for the several 
 forms, rather than for the characters, has greatly contributed 
 to deepen the obscurity which veils the whole subject. 
 
104 PROBLEMS OF GENETICS 
 
 I do not mean to suggest that these Oenotheras follow a simple 
 Mendelian system. All that we know of them goes to show that 
 there are curious complications involved. One of these, prob- 
 ably the most important of all, has lately been recognized by 
 de Vries himself, namely, that in certain types the characters 
 borne by the female and the male germ-cells of the same plant 
 are demonstrably different. There can be little doubt that 
 further research will reveal cognate phenomena in many unsus- 
 pected places. The first example in which such a state of things 
 was proved to exist is that of the Stocks investigated by Miss 
 Saunders. 2 By a long course of analysis she succeeded in estab- 
 lishing in 1908 the fact that if a plant of Matthiola is of that 
 eversporting kind which gives a large proportion of double- 
 flowered plants among its offspring (produced by self-fertili- 
 sation), then the egg-cells of such a plant are mixed in type, but 
 the pollen of the same plant is homogeneous. Some of the egg- 
 cells have in them the two factors for singleness, but some of 
 them are short of one or both of these factors. The pollen- 
 grains, however, are all recessives, containing neither of these 
 factors. The egg-cells, in other words, are mixed, ''singles" and 
 "doubles," while the pollen-grains are all "doubles." The same 
 is true of the factor differentiating "white," or colourless plastids 
 from cream-coloured plastids in Matthiola, the egg-cells being 
 mixed "whites" and "creams," while the pollen-grains are all 
 "creams," viz: recessives. Later in the same year (1908) 
 de Vries 3 announced a remarkable case which will be discussed 
 in detail subsequently. It relates to certain Oenotheras hetero- 
 zygous for dwarfness, in which (p. 113) the ovules were mixed, 
 tails and dwarfs, while the pollen is all dwarf. 
 
 Again in Petunia Miss Saunders's 4 work has shown that 
 a somewhat similar state of things exists, but with this remark- 
 able difference, that though the egg-cells are mixed, singles and 
 doubles, the pollen-grains are all singles, viz: dominants. All 
 the Petunias yet examined have been in this condition, including 
 
 2 Rep. Evol. Ctee. R. 5., IV, 1908, p. 38. 
 
 3 Ber. Deut. Bot. Ges., 1908, XXVI, a, p. 672. 
 
 4 Jour. Genetics, 1, 1910, p. 57. 
 
THE MUTATION THEORY 105 
 
 some which in botanic gardens pass for original species. Whether 
 actual wild plants from their native habitats are in the same 
 state, is not yet known, but it is by no means improbable. The 
 case may be compared with that of the moth Abraxas grossu- 
 lariata studied by Doncaster and Raynor, in which the females 
 are all heterozygous, or we may almost say " hybrids" of grossu- 
 lariata and the variety lacticolor. Similarly we may say that at 
 least garden Petunias are heterozygous in respect of singleness. 
 The proof of this is of course that when fertilised with the pollen 
 of doubles they throw a mixture of doubles and singles. The 
 statements which de Vries has published regarding the behaviour 
 of several of the Oenotheras go far to show that they must have 
 a somewhat similar organisation. On the present evidence it is 
 still quite impossible to construct a coherent scheme which will 
 represent all the phenomena in their interrelations, and among 
 the facts are several which, as will appear, seem mutually incom- 
 patible. The first indication that the Oenotheras may have 
 either mixed ovules or mixed pollen appears in the fact that 
 Lamarckiana and several of its "mutants" used as males, with 
 several other forms as females, give a mixed offspring. For 
 example, de Vries (1907) found that 
 
 biennis 9 X Lamarckiana o 71 
 biennis cruciata 9 X Lamarckiana o 71 
 muricata 9 X Lamarckiana o 71 
 biennis 9 X rubrinervis o 71 
 biennis cruciata 9 X rubrinervis o 71 
 all give a mixture of two distinct types which he names laeta 
 and velutina, consisting of about equal numbers of each. On 
 account of the fact that the two forms are produced in association 
 de Vries has called these forms "twin hybrids," a designation 
 which is not fortunate, seeing that it is impossible to imagine 
 that any kind of twinning is concerned in their production. The 
 distinction between these two seems to be considerable, laeta 
 having leaves broader, bright green in colour, and flat, with 
 pollen scanty, while velutina has leaves narrower, grayish green, 
 more hairy, and furrow-shaped, with pollen abundant. 
 
 We next meet the remarkable fact that these two forms, 
 
io6 PROBLEMS OF GENETICS 
 
 laeta and velutina breed true to their respective types, and do not 
 reproduce the parent-types among their offspring resulting from 
 self-fertilisation. This statement must be qualified in two 
 respects. When muricata d 1 is fertilised by brevistylis the forms 
 laeta and velutina are produced, but each of them subsequently 
 throws the short-styled form as a recessive (de Vries, 1907, 
 p. 406). It may be remembered that de Vries's previous publi- 
 cations had already shown that the short style of brevistylis, 
 one of the Lamarckiana " mutants," behaves as a recessive 
 habitually {Mutationstheorie, II, p. 178, etc.). 
 
 Also when nanella, the dwarf " mutant" of Lamarckiana is 
 used as male on muricata as female, laeta and velutina are pro- 
 duced, but one only of these, namely, velutina, subsequently 
 throws dwarfs on self -fertilisation. The dwarfs thus thrown are 
 said to form about 50 per cent, of the families in which they 
 occur (de Vries, 1908, p. 668). The fact that the two forms, 
 laeta and velutina, are produced by many matings in which 
 Lamarckiana and its mutant rubrinervis are used as males is 
 confirmed abundantly by Honing, who has carried out extensive 
 researches on the subject. After carefully reading his paper, 
 I have failed to understand the main purport of the argument 
 respecting the "double nature" of Lamarckiana which he founds 
 on these results, but I gather that in some way laeta is shown to 
 partake especially of the nature of Lamarckiana, while velutina 
 is a form of rubrinervis. The paper contains many records which 
 will be of value in subsequent analysis of these forms. 
 
 Before considering the possible meaning of these facts we 
 must have in our minds the next and most novel of the recent 
 extensions of knowledge as to the genetic properties of the 
 Oenotheras. In the previous statement we have been concerned 
 with the results of using either Lamarckiana itself or one of its 
 "mutants" rubrinervis, brevistylis, or nanella as male, on one of 
 the species biennis or muricata. The new experiments relate 
 to crosses between the two species biennis and muricata 
 themselves. 
 
 De Vries found : 
 
 1. That the reciprocal hybrids from these two species differed, 
 
THE MUTATION THEORY 107 
 
 biennis Xmuricata producing one type of Fi and muricataX 
 biennis producing another. Each Fi resembled the father more 
 than the mother. 
 
 2. That each of the hybrids so produced breeds true on self- 
 fertilisation. 
 
 3. That if we speak of the hybrid from biennis Xmuricata 
 as BM and of the reciprocal as MB, then 
 
 BMX MB 
 
 gives exclusively offspring of biennis type but that 
 
 MB XBM 
 
 gives exclusively offspring of muricata type. Evidently, apart 
 from all controversy as to the significance of the "mutants" 
 of Lamarckiana, we have here a series of observations of the first 
 importance. 
 
 The fact that reciprocal crossings give constantly distinct 
 results must be taken to indicate that the male and female sides 
 of one, if not of both, of the parents are different in respect of 
 characters which they bear. This is de Vries's view, and he 
 concludes rightly, I think, that the evidence from all the experi- 
 ments shows that both biennis and muricata are in this condition, 
 having one set of characters represented in their pollen-grains 
 and another in their ovules. The plants breed true, but their 
 somatic structures are compounded of the two sets of elements 
 which pass into them from their maternal and paternal sides 
 respectively. This possibility that species may exist of which 
 the males really belong to one form and the females to another, 
 is one which it was evident from the first announcement of the 
 discovery of Mendelian segregation might be found realised in 
 nature. 5 
 
 Oe. biennis and muricata were crossed reciprocally with each 
 other and with a number of other species, and the behaviour of 
 each, when used as mother, was consistently different from its 
 behaviour when used as father. De Vries is evidently justified 
 
 5 In Rep. 1 to Evol. Committee, 1902, p. 132, attention was called to this pos- 
 sibility, though of course at that date it was in sexual animals alone that it was 
 supposed to exist. It had not occurred to me that even a hermaphrodite plant 
 might be in this condition. 
 
108 PROBLEMS OF GENETICS 
 
 by the results of this series of experiments in stating that the 
 "Bild," as he terms it, or composition of the male and female 
 sides of these two species, biennis and muricata, are distinct. 
 On the evidence before us it is not, however, possible to form a 
 perfectly clear idea of each, and until details are published, a 
 reader without personal knowledge of the material cannot do 
 more than follow the general course of the argument. For fuller 
 comprehension a proper analysis of the characters with a clear 
 statement of how they are distributed among the several types 
 and crosses is absolutely necessary. According to de Vries the 
 female of biennis possesses a group of characters which he defines 
 as "conica" in allusion to the shape of the flower-buds. Besides 
 the conical buds, this group of features includes imperfect 
 development of wood, rendering the plant very liable to attacks 
 of Botrytis, and comparatively narrow leaves. 
 
 The female of muricata carries a group of features which he 
 calls "frigida," and, though this is not quite explicitly stated in a 
 definition of that type, it is to be inferred 6 that its characteristics 
 are regarded as greater height, strong development of wood with 
 comparative resistance to Botrytis, and broad leaves. 
 
 The characters borne by the male parts of the two species 
 are in general those by which they are outwardly distinguished. 
 For example, the leaves of Oe. biennis are comparatively broad 
 and are bright green, while those of muricata are much narrower 
 and of a glaucous green, and I understand that de Vries regards 
 these properties as contributed by the male side in each case and 
 to be carried by the male cells of each species. The suggestion 
 as regards biennis and muricata comes near the conception often 
 expressed by naturalists in former times (e. g., Linnaeus) and 
 not rarely entertained by breeders at the present day, that the 
 internal structure is contributed by the mother and the external 
 by the father. 
 
 On the other hand, the offspring of each species when used 
 as mother is regarded as possessing in the main the features of 
 the maternal "Bild," but the matter is naturally complicated 
 by the introduction of features from the father's side, and it is 
 
 « From the description of the offspring of muricata used as mother. 
 
THE MUTATION THEORY 109 
 
 here especially that the account provided is at present unsatis- 
 factory and inconclusive. There seems, however, to be no serious 
 doubt that biennis and muricata each in their outward appearance 
 exhibit on the whole the features which their pollens respectively 
 carry, and that the features borne by their ovules are in many 
 respects distinct. 
 
 The types are thus "hybrids" which breed true. The results 
 of intercrossing them each way are again "hybrids" which breed 
 true. It will be remembered that on former occasions de Vries 
 has formulated a general rule that secies-hybrids breed true, 
 but that the cross-breds raised by interbreeding varieties do not. 
 One of these very cases was quoted 7 as an illustration of this 
 principle, viz: muricata X biennis. The grounds for this general 
 statement have always appeared to me insufficient, and with the 
 further knowledge which the new evidence provides we are 
 encouraged to hope that when a proper factorial analysis of the 
 types is instituted we shall find that the phenomenon of a con- 
 stant hybrid will be readily brought into line with the systems 
 of descent already worked out for such cases as that of the Stocks, 
 and others already mentioned. 
 
 In further discussion of these facts de Vries makes a suggestion 
 which seems to me improbable. Since the egg-cells of muricata, 
 for instance, bear a certain group of features which are missing 
 on the male side, and conversely the pollen bears features absent 
 from the female side, he is inclined to regard the bad pollen grains 
 as the bearers of the missing elements of the male side and to 
 infer that there must similarly be defective ovules representing 
 the missing elements of the female side. No consideration is 
 adduced in support of this view beyond the simple fact that the 
 characters borne by male and female are dissimilar, whereas 
 it would be more in accord with preconception if the same sets 
 of combinations were represented in each — as in a normal 
 Mendelian case. There is as yet no instance in which the absence 
 of any particular class of gametes has been shown with any 
 plausibility to be due to defective viability, though there are, of 
 course, cases in which certain classes of zygotes do not survive 
 
 7 de Vries, Species and Varieties, 1905, p. 259. 
 
no 
 
 PROBLEMS OF GENETICS 
 
 owing to defective constitution (e. g., the albinos of Antirrhinum 
 studied by Baur, and the homozygous yellow mice). I am 
 rather inclined to suppose that in these examples of hybrids 
 breeding true we shall find a state of things comparable with 
 that to which we formerly applied the terms "coupling" and 
 "repulsion." In these cases certain of the possible combinations 
 of factors occur in the gametic series with special frequency, 
 being in excess, while the gametes representing other combina- 
 tions are comparatively few. In a recent paper on these cases 
 Professor Punnett and I have shown that these curious results 
 vary according to the manner in which the factors are grouped 
 in the parents. If A and B are two factors which exhibit these 
 phenomena we find that the gametic series of the double heterozy- 
 gote differs according as the combination is made by crossing 
 ABXab, or by crossing AbXaB. In a normal Mendelian case 
 the Fi form, AaBb, produces gametes AB, Ab, aB, ab, in equal 
 numbers; but in these peculiar cases those gametes which contain 
 
 Partial repulsion 
 
 from zygote 
 
 of form 
 
 AbXaB 
 
 Partial coupling 
 
 from zygote 
 
 of form 
 
 ABXab 
 
 Partial repulsion 
 
 from zygote 
 
 of form 
 
 AbXaB 
 
 Partial coupling 
 
 from zygote 
 
 of form 
 
 ABXab 
 
 Garnet 
 
 ic series 
 
 
 Number 
 of gametes 
 in series 
 
 Number of 
 zygotes 
 formed 
 
 AB Ab 
 
 aB 
 
 ab 
 
 I (n-l) 
 
 (»— I 
 
 ) I 
 
 2tt 
 
 4« 2 
 
 I 31 
 
 31 
 
 
 64 
 
 4096 
 
 1 is 
 
 15 
 
 
 32 
 
 1024 
 
 1 7 
 
 7 
 
 
 16 
 
 256 
 
 1 3 
 
 3 
 
 
 8 
 
 64 
 
 1 1 
 
 1 
 
 
 4 
 
 16 
 
 3 1 
 
 1 
 
 3 
 
 8 
 
 64 
 
 7 1 
 
 1 
 
 7 
 
 16 
 
 256 
 
 IS 1 
 
 1 
 
 15 
 
 32 
 
 1024 
 
 3i 1 
 
 1 
 
 3i 
 
 64 
 
 4096 
 
 63 1 
 
 1 
 
 63 
 
 128 
 
 16384 
 
 1 — 1) 1 
 
 1 
 
 (w— 1 
 
 ) 2» 
 
 4n 2 
 
 Nature of zygotic series 
 
 
 AB 
 
 Ab 
 
 aB 
 
 ab 
 
 
 2W 2 + I 
 
 w 2 -i 
 
 W 2 -I 
 
 
 
 2049 
 
 1023 
 
 IO23 
 
 
 
 513 
 
 255 
 
 255 
 
 
 
 129 
 
 63 
 
 63 
 
 
 
 33 
 
 15 
 
 15 
 
 
 
 9 
 
 3 
 
 3 
 
 
 
 41 
 
 7 
 
 7 
 
 9 
 
 
 177 
 
 15 
 
 15 
 
 49 
 
 
 737 
 
 3i 
 
 3i 
 
 225 
 
 
 3009 
 
 63 
 
 63 
 
 961 
 
 
 12161 
 
 127 127 
 
 3969 
 
 ^3W 2 — (2«— i) 2n— I 2«— I M 2 — (2»— i) 
 
THE MUTATION THEORY in 
 
 the parental combinations are in excess. This excess almost 
 certainly follows the system indicated by the accompanying 
 table. In the general expressions n is half the number of gametes 
 required to express the whole system. Now if we imagine that 
 sex-factors are involved with the others concerned in such a re- 
 lationship as this we have a system of distribution approximating 
 to that found in biennis and muricata. The difference in re- 
 ciprocals is represented in a not improbable way. It cannot yet 
 be said that the rarer terms in the series are formed at all, and 
 perhaps they are not. As we pointed out in our discussion of 
 these phenomena, the peculiar distribution of factors in these 
 cases must be taken to mean that the planes of division at some 
 critical stage in the segregation are determined with reference 
 to the parental groups of factors, or in other words, that the 
 whole system has a polarity, and that the distribution of factors 
 with reference to this polarity differs according to the grouping 
 of factors in the gametes which united in fertilization to produce 
 the plant. Subsequent proliferation of cells representing certain 
 combinations would then lead to excess of the gametes bearing 
 them. It is on similar lines that I anticipate we shall hereafter 
 find the interpretation of the curious facts discovered by de Vries, 
 though it is evident that a long course of experiment and analysis 
 must be carried through before any certainty is reached. The 
 work must be begun by a careful study of the descent of some 
 single factor, for example, that causing the broader leaf of 
 biennis, and we may hope that the study of Oenothera by proper 
 analytical methods will no longer be deferred. 
 
 We have now to return to the relations of laeta and velutina. 
 These two forms, it will be remembered are frequently produced 
 when Lamarckiana or one of its derivatives is used as male, 
 and the most unexpected feature in their behaviour is that both 
 breed true as regards their essential characteristics, on self -fertili- 
 sation. If one only bred true the case might, in view of the 
 approximate numerical equality of the two types, be difficult 
 to interpret on ordinary lines, but as both breed true it must be 
 clear that some quite special system of segregation is at work. 
 What this may be cannot be detected on the evidence, but with 
 
ii2 PROBLEMS OF GENETICS 
 
 the results from the biennis-muricata experiments before us, 
 it is natural to suspect that we may here again have to recognise 
 a process of allocation of different factors to the male and female 
 sides in laeta and velutina. That some such system is in operation 
 becomes the more probable from the new fact which de Vries 
 states in describing the group of characters which he calls conica, 
 namely that this type is the same as that of velutina. 
 
 There are many collateral observations recorded both by 
 de Vries and others which have a bearing on the problems, but 
 they do not yet fall into a coherent scheme. For example, we 
 cannot yet represent the formation of laeta and velutina from the 
 various species fertilised by Lamarckiana o 71 . That this is not 
 due to any special property associated with the pollen of La- 
 marckiana is shown by the fact that a species called Hookeri 
 gives laeta and velutina in both its reciprocal crosses with La- 
 marckiana (de Vries, 1909, p. 3), and also by the similar fact that 
 Lamarckiana 9 fertilised by the pollen of a peculiar race of 
 biennis named biennis Chicago throws the same types. Before 
 these very complicated phenomena can be usefully discussed 
 particulars must be provided as to the individuality of the various 
 plants used. This criticism applies to much of the work which 
 de Vries has lately published, for, as we now know familiarly, 
 plants to which the same name applies can be quite different in 
 genetic composition. 
 
 Attention should also be called to one curiously paradoxical 
 series of results. When the dwarf "mutant" of Lamarckiana 
 which de Vries names lt nanella" is used as father on muricata, 
 Fx consists of laeta and velutina in approximately equal numbers. 
 Both forms breed true to their special characteristics, but 
 velutina throws dwarfs of its own type, while laeta does not 
 throw dwarfs. Subsequent investigation of the properties of 
 these types has led to some remarkable conclusions, and it was 
 in a study of these plants that de Vries first came upon the phe- 
 nomen of dissimilarity between the factors borne by the male 
 and female cells of the same plant, a condition which had been 
 recently detected in the Stocks as a result of Miss Saunders's 
 investigations. The details are very remarkable. We have 
 
THE MUTATION THEORY 113 
 
 first the fact that muricata 9 X dwarf nanella cf gives about 
 50 per cent, laeta and about 50 per cent, of velutina. 
 As regards Velutina it was shown that: 
 
 1. Velutina selfed gave 
 r Velutina $ X dwarf nanella cf gave 
 
 2. -i do. X do. gave 
 
 I do. X dwarf cf derived from velutina gave 43 
 
 3. Dwarfs X velutina c? gave 
 
 Tails, 
 
 Dwarfs, 
 
 per cent. 
 
 per cent. 
 
 38 
 
 62 
 
 39 
 
 61 
 
 49 
 
 51 
 
 ■ve 43 
 
 57 
 
 — 
 
 all dwarfs 
 
 The three experiments taken together prove, as de Vries says, 
 that the ovules of velutina are mixed, tails and dwarfs, and that 
 the pollen is all dwarf. The condition is almost the same as 
 that of the Stocks. It may be noted also that in the Stocks the 
 egg-cells of the "double" type are in excess, being approximately 
 9 to 7 of the " single" type, but de Vries regards the two types 
 in velutina as probably equal in number. The figures (169 1231) 
 rather suggest some excess of the recessives, perhaps 9:7, and 
 the point would be worth a further investigation. 
 
 As regards laeta, by self-fertilisation no dwarfs were produced, 
 but in all other respects it behaved almost exactly like velutina. 
 The ovules are evidently mixed tails and dwarfs, and whether 
 fertilised by dwarfs or by the pollen of velutina, which is already 
 proved to be all dwarf, the result was a steady 50 per cent, of 
 tails and 50 per cent, of dwarfs. The pollen of laeta used on 
 dwarfs gives nothing but dwarfs, and in three series of such ex- 
 periments 226 dwarfs were produced. 
 
 We are thus faced with this difficulty. Since the egg-cells of 
 laeta are evidently mixed, tails and dwarfs, and the pollen used 
 on dwarfs gives all dwarfs, why does not self-fertilisation give 
 a mixed result, tails and dwarfs, instead of all tails? De Vries 
 regards the result of self-fertilisation as showing the real nature 
 of the pollen, and declares it to be all tails, while he represents 
 the behaviour of the same pollen used on dwarfs by stating that 
 in these combinations the dwarf character dominates. This 
 does not seem to me a natural interpretation. I should regard 
 the pollen of laeta as identical with that of velutina, namely dwarf, 
 and I suspect the difficulty is really created by the behaviour of 
 laeta on self -fertilisation. Until a proper analysis is made in 
 
ii4 PROBLEMS OF GENETICS 
 
 which the identity of the different individuals used is recorded, 
 no further discussion is possible. 8 
 
 Other results of a complicated kind involving production of 
 laeta and velutina together with a third form have been published 
 by de Vries in his paper on "Triple Hybrids." To these also 
 the same criticism applies. Some of the observations seem cap- 
 able of simple factorial representation and others are conflicting. 
 
 Taking the work on Oenothera as a whole we see in it con- 
 tinually glimpses of order which further on are still blocked by 
 difficulties and apparent inconsistencies. Through such a stage 
 all the successful researches in complicated factorial analysis 
 have passed and I see no reason for supposing that with the 
 application of more stringent methods this more difficult set of 
 problems will be found incapable of similar solutions. To 
 return to the original question whether in Oenothera we can claim 
 to see a special contemporaneous output of new species in actual 
 process of creation, it will be obvious that while the interrelation 
 of the several types is still so little understood, such a claim has 
 no adequate support. It is true that many of the "mutants" 
 of Lamarckiana can well pass for species, but this is equally true 
 of many new combinations of pre-existing factors as we have 
 seen in Primula Sinensis and other cases. Still less can it be 
 admitted that these facts of uncertain import supply a justi- 
 fication for the conception which has played a prominent part 
 in the scheme of the Mutationstheorie, namely that there are 
 special periods of Mutation, when the parent-species has peculiar 
 genetic properties. To conclude: The impression which the 
 evidence leaves most definitely on the mind is that further dis- 
 cussion of the bearing which the Oenotheras may have on the 
 problem of evolution should be postponed until we have before 
 us the results of a searching analysis applied to a limited part of 
 the field. In such an analysis it is to be especially remembered 
 that we have now a new clue in the well-ascertained fact that the 
 genetic composition of the male and female germ-cells of the 
 
 8 Zeijlstra in a recent paper announces that many nanella plants are the subject 
 of a bacterial disease to which he attributes their dwarfness, I gather that this 
 does not apply to all nanella plants and that some are dwarfs apart from disease. 
 The matter may no doubt be further complicated from this cause. 
 
THE MUTATION THEORY 115 
 
 same individual may be quite different. When with this pos- 
 sibility in view the behaviour of the types is re-examined I 
 anticipate that many of the difficulties will be removed. 
 
 Outside the evidence from Oenothera, which, as we have seen, 
 is still ambiguous, I know no considerable body of facts favour- 
 able to that special view of Mutation which de Vries has pro- 
 mulgated. Of variation, or if we will, Mutation, in respect of 
 some one character, or resulting from recombination, there is 
 proof in abundance ; but of that simultaneous variation in several 
 independent respects to which de Vries especially attributes the 
 origin of new specific types I know only casual records which 
 have yet to undergo the process of criticism. 
 
 Besides de Vries's " ' Mutationstheorie" and "Species and Vari- 
 eties" the chief publications relating to the subject of the be- 
 haviour of Oenothera are the following: (Many other papers 
 relating especially to the cytology of the forms have appeared.) 
 
 Davis, B. M. Genetical Studies on Oenothera, I. Amer. Nat., XLIV, 1910, p. 108. 
 
 Genetical Studies on Oenothera, II. Ibid., XLV, 1911, p. I03- 
 Gates, R. R. An Analytical Key to some of the Segregates of Oenothera. Twen- 
 tieth Annual Report of the Missouri Botanical Garden, 1909. 
 Studies on the Variability and Heritability of Pigmentation in Oenothera. 
 Ztsch.f. Abstammungslehre, 1911, IV, p. 337. 
 Honing, J. A. Die Doppelnatur der Oenothera Lamarckiana. Ztsch. f. Abstam- 
 mungslehre, 191 1, IV, p. 227. 
 Macdougal, D. T. (with A. M. Vail, G. H. Shull, and J. K. Small). Mutants and 
 Hybrids of the Oenotheras. Carnegie Institution's Publication, No. 24, 
 1905. 
 Macdougal, D. T., Vail, A. M., Shull, J. H. Mutations, Variations and Relation- 
 ships of the Oenotheras. Carnegie Institution's Publication, No. 81, 1907. 
 de Vries, H. On Atavistic Variation in Oenothera cruciata. Bull. Torrey Club, 
 1903. Vol. 30. P- 75- 
 On Twin Hybrids, Bot. Gaz., Vol. 44. 1907. P- 401. 
 Ueber die Zwillingsbastarde von Oenothera nanella. Ber. Deut. Bot. Ges.; 
 
 1908, XXVI, a, p. 667. 
 Bastarde von Oenothera gigas. Ibid., p. 754. 
 On Triple Hybrids. Bot. Gaz., 1909, Vol. 47. P- I« 
 
 Ueb. doppeltreziproke Bastarde von Oenothera biennis L. und Oenothera 
 muricata L. Biol. Cbltt., 191 1, XXXI, p. 97. 
 Zeijlstra, H. H. Oenothera nanella de Vries, eine krankhafte Pflanzenart. Biol. 
 Cbltt., 1911, XXXI, p. 129. 
 
n6 PROBLEMS OF GENETICS 
 
 Note. 
 
 Since this chapter was written two contributions of special 
 importance have been made to the study of the Oenothera prob- 
 lems. The first is that of Heribert-Nilsson. 9 The author begins 
 by giving a critical account of the evidence for de Vries's inter- 
 pretation of the nature of the mutants. In general this criticism 
 pursues lines similar to those sketched in the foregoing chapter, 
 concluding, as I have done, that the chief reason why factorial 
 analysis has been declared to be inapplicable to the Oenothera 
 mutants is because no one has hitherto set about this analysis 
 in the right way. He has also himself made a valuable beginning 
 of such an analysis and gives good evidential reasons for the belief 
 that at least the red veining depends on a definite factor which 
 also influences the size of certain parts of the plant. He argues 
 further that many of the distinctions between the mutants are 
 quantitative in nature. With great plausibility he suggests that 
 the system of cumulative factors which Nilsson-Ehle discovered 
 in the case of wheat (subsequently traced by East in regard to 
 maize) may be operating also in these Oenotheras. According 
 to this system several factors having similar powers may coexist 
 in the same individual, and together produce a cumulative effect. 
 Scope would thus be given for the production of the curious and 
 seemingly irregular numbers so often recorded in the "mutating" 
 families. 
 
 Another remarkable observation relating to the crosses of 
 muricata and biennis has been published by Goldschmidt. 10 
 He finds that in the formation of this cross the female pronucleus 
 takes no part in the development of the zygotic cell, but that 
 when the male pronucleus enters, the female pronucleus is 
 pushed aside and degenerates. As de Vries observed, the recip- 
 rocal hybrids are in each case very like the father ("stark 
 patroklin") , a consequence which finds a natural explanation in 
 the phenomenon witnessed by Goldschmidt. The results of 
 the subsequent matings can also be readily interpreted on the 
 same lines. Indications of maternal characters are nevertheless 
 
 9 Zts. f. Abstamm., 1912, VIII. 
 
 10 Arch. f. Zellforschung, 1912, IX, p. 331. 
 
THE MUTATION THEORY 117 
 
 mentioned by de Vries, and if Goldschmidt's account of the 
 cytology is confirmed, these must presumably be referred to the 
 influence of the maternal cytoplasm. Clearly this new work 
 opens up lines of exceptional interest. The interpretation I 
 have offered above must probably be reconsidered. 11 The dis- 
 tinction between the male and female cells of the types may no 
 doubt be ultimately factorial, but it is difficult to regard such a 
 distinction as created by a differential distribution of the ordinary 
 factors. 
 
 n Renner (Ber. Deut. Bot. Ges., 1913, xxxi, p. 334) has since declared that 
 Goldschmidt's observations were entirely mistaken. See also Renner, Flora, N. 
 F., 7, 1914, p. p. 115, Heribert-Nilsson, Die Spaltungserscheinungen der Oenothera 
 Lamarckiana (Lunds Univ. Arsskrift, N. F., Afd. 2, Bd. XII, Nr. 1). 
 
CHAPTER VI 
 
 VARIATION AND LOCALITY 
 
 In all discussions of the modes of Evolution the phenomena of 
 Geographical Distribution have been admitted to be of para- 
 mount importance. First came the broad question, were the 
 facts of distribution consistent with the Doctrine of Descent? 
 I suppose all naturalists are now agreed that they are thus 
 consistent, and that though some very curious and as yet in- 
 explicable cases remain to be accounted for, the distribution of 
 animal and plant life on the face of the earth is much what we 
 might expect as a result of a process of descent with modification. 
 Passing from this general admission to the more particular ques- 
 tion whether the facts of distribution favour one special con- 
 ception of the mode of progress of evolution rather than another, 
 no agreement has yet been reached. One outstanding feature 
 is hardly in dispute, namely that prolonged isolation is generally 
 followed by greater or less change in the population isolated. 
 Croups of individuals which from various causes are debarred 
 from free intermixture with other groups almost always exhibit 
 peculiarities, but on the other hand, cosmopolitan types which 
 range over wide areas are on the whole uniform, or nearly so 
 throughout their distribution. Examples of these two categories 
 will be familiar to all naturalists. The barriers to intercourse 
 may be seas, deserts, prairies, mountain-chains, or circumstances 
 of a much less obvious character which isolate quite as effectually. 
 The local unit is not necessarily an island, a district, or an area 
 of special geological formation, but may, as every collector knows, 
 be a valley, a pond, a creek, a "bank" in the sea, a clump of 
 trees, a group of rocks in a bay, or a particular patch of ground 
 on a mountain side. All the great groups provide examples of 
 such specially isolated forms. The botanist knows them well; 
 the conchologist, the entomologist, the ornithologist and the 
 student of marine life are all equally aware that special varieties 
 
 118 
 
VARIATION AND LOCALITY n 9 
 
 or special species come from special places and from nowhere 
 else. In one remarkable case the season of appearance plainly 
 acts as the isolating barrier. Tephrosia bistortata is a small 
 Geometric! moth which has two broods, appearing in March 
 and July respectively. It is closely allied to T. crepuscularia 
 which emerges in May and June. From the fact that occasional 
 specimens cannot be quite certainly referred to one or other of 
 the two, many have held that the two are one species. Never- 
 theless, in general they present distinctions which are plain 
 enough. Some localities have one form only, but in several 
 woods they co-exist. Experiment has shown that the two can 
 be crossed, and that the cross-breds can breed inter se and with 
 at least one of the parent stocks. 1 Some diminution in fertility 
 was observed, but perhaps not more than is commonly encountered 
 when wild forms are bred in captivity. In such a case it can 
 scarcely be doubted that the distinctness of the two forms in 
 the places where they co-exist is maintained by the seasonal 
 isolation. 
 
 Just as the consequences of isolation are to be seen in the 
 most different forms of life so may they also affect the most di- 
 verse features of organisation, such as size, colour, sculpture, 
 shape, or number of parts. In the Sloth (Choloepus) the geo- 
 graphical races differ in the number of cervical vertebrae — or 
 in other words, in the distribution of vertebral differentiation. 
 The geographical races of Cistudo differ in the number of claws 
 and phalanges. 2 
 
 In Shetland, the males of Hepialus humuli (the Ghost Moth) 
 are not sharply differentiated in colour from the females, as they 
 are elsewhere, but in varying degrees resemble them. 3 No such 
 males are found in other localities, and even in the other Scottish 
 islands they are normal. In the island of Waigiu the converse 
 phenomenon has been observed in Phalanger maculatus. Gen- 
 
 l Fov the evidence see Tutt, J. W., Trans. Ent. Soc, 1898, p. 17. Compare 
 the remarkable case given by Gulick {Evolution Racial and Habitudinal, p. 123) 
 of the two races of Cicada, which are separated by reason of their life-cycles, one 
 having a period of 13, the other 17 years. 
 
 2 For references see Materials, p. 396, and also G. Baur, Amer. Nat., 1893, 
 July, p. 677. 
 
 3 Jenner Weir, Entomologist, 1880, XIII, p. 251. 
 
120 PROBLEMS OF GENETICS 
 
 erally the male is spotted with white, and the female is without 
 spots, but in Waigiu the females are spotted like the males. 4 
 
 The following striking illustration was pointed out to me by 
 Dr. W. D. Miller. Euphonia elegantissima as it occurs in Mexico 
 and Central America has the two sexes very distinct from each 
 other. The male has the lower parts orange and the upper 
 parts a dark indigo blue, with a bright turquoise-blue head and 
 neck. The female, except for the head, is of a bright olive green. 
 A form in which the sexes are similarly differentiated exists in 
 Porto Rico and is known as E. Sclateri. But in many of the 
 other West Indian islands the representative "species" (E. 
 flavifrons) has the two sexes closely resembling the female of 
 E. elegantissima. This form is found in Antigua, Barbados, 
 St. Vincent, and Guadeloupe, from which localities the British 
 Museum has specimens. All three so-called species are very 
 much alike otherwise. 
 
 In the genus Pyrrhulagra (Loxigilla) to which Mr. Outram 
 Bangs called my attention, several distinct and alternative pos- 
 sibilities occur. The genus has many local species occurring 
 on the various West Indian islands. These species are char- 
 acterized by differences in size, colour, and the shape of the bill. 
 The colours have a narrow range, being black or greyish, with 
 or without chestnut marks about the head and throat. In 
 most of the islands the males are in general colour a full black, 
 and the females are distinctly grey. They are thus found in 
 San Domingo, Jamaica, Bahama, and most of the Lesser Antilles. 
 In Porto Rico we meet the peculiarity that the hens are almost 
 as black as the males (Ridgway describes the black of the hens 
 as slightly less intense). This form is called portoricensis. 
 A larger type, known as grandis, similarly coloured, inhabits 
 St. Kitt's. Then, on the contrary, in Barbados, both sexes are 
 a dull blackish grey, like the hens of the Lesser Antilles in general. 
 
 The local species of Agelaius show similarly capricious dis- 
 tinctions. A. phoeniceus is a widely spread species, found over 
 a great part of North America. The male is black with red-orange 
 
 * Jentink, Notes Leyden Mus., 1885, VII, p. in. Specimens illustrating this 
 peculiarity are in the British Museum. 
 
VARIATION AND LOCALITY 121 
 
 bars on the wings, but the female is somewhat thrush-like in 
 colour. In the island of Porto Rico there is a form called xan- 
 thomus, in which both sexes are like the males of the mainland. 
 A similar species called hunieralis, also with both sexes male-like, 
 lives in Cuba. The island of Cuba, curiously enough, has also a 
 distinct species named assimilis, in which the female is a dull 
 black all over, though the male is like the mainland type. 
 
 So also may local races differ in respect of variability. Ar- 
 gynnis paphia, the Silver Washed Fritillary, through a great 
 part of its distribution has only one female form. In the English 
 New Forest a second female form, valesina, co-exists with the 
 ordinary paphia female. But in the southern valleys of the Alps 
 the valesina female is much the commoner of the two, and indeed 
 in some localities where the species is abundant, I have seen no 
 paphia females in many days collecting. 
 
 The beetle Gonioctena variabilis furnishes an illustration of a 
 comparable phenomenon affecting the male sex. In 1894 and 
 1895 I studied the curious colour variations of this species es- 
 pecially in the neighbourhood of Granada, and Mr. Doncaster 
 ten years later repeated the observations on the same ground, 
 and also collected the insect in other places in the south of Spain. 
 The distinctions are not easy to give in words and the reader is 
 referred to the colour plate accompanying my paper. 5 The 
 essential fact is that the males commonly have the elytra red 
 with black spots and the females for the most part have greenish 
 grey elytra with black stripes. In some localities a large minority 
 of males closely resemble the female type, being identical in 
 colour and then only distinguishable by structural differences. 
 In two Granada localities I found the proportion of such males 
 quite different. In the Darro valley about 38 per cent, (in 
 
 5 Proc. Zool. Soc, 1895, p. 850. Plate. Many points beyond that mentioned 
 above are involved in this remarkable case. For example, not only are there males 
 like females, but a small proportion of females resemble the ordinary male type. 
 The stripes are not merely the spots produced, for they occupy different anatomical 
 positions. The spots almost always go with a black ventral surface, but the striped 
 forms nearly always have that region testaceous. Spartium retama, the food-plant, 
 will not grow in England, but if it could be naturalised in America the whole problem 
 might be investigated there and results of exceptional interest would almost cer- 
 tainly be attained. 
 
122 PROBLEMS OF GENETICS 
 
 718) were of this feminine type, but on the hills some 300 feet 
 above only 19 per cent, (in 3,230) were like the females. At 
 Castillejo, not far from Toledo I found no such male in 75 speci- 
 mens. 
 
 Mr. Doncaster collected from several localities, especially 
 from two areas near Malaga, about 5 miles apart. In one of 
 these the female-like males were, as usual, in a minority, but 
 in the other these were actually in great excess, amounting to 
 about 81 per cent, in the 173 taken. Doncaster found a doubtful 
 indication that the composition of the population varies with 
 the season, which is quite possible, but it is most interesting 
 to note that in my chief locality after the lapse of ten years he 
 found the proportions very much the same as I had done at the 
 same season, for where I had 19 per cent, of the female-like males 
 his collecting gave 16 per cent. In other respects also, his sta- 
 tistics corresponded very closely with mine. 6 
 
 The various forms of Heliconius erato are well known to en- 
 tomologists. They are strikingly distinguished by the colours 
 of the strong comb-like marking on the hind wing, which may be 
 red, yellow, green or blue. In various parts of the distribution 
 in South America sometimes two and sometimes three of these 
 distinct types co-exist. 7 
 
 The distribution of the varieties of Noctua castanea typifies 
 a large range of cases. The form which is reckoned the normal 
 of the species has red fore-wings. It is practically restricted to 
 Great Britain and Germany, according to Tutt. The other 
 common form, neglecta, has grey fore- wings, and in this pattern 
 it ranges through West Central Europe from North Italy to 
 Germany. In the British Isles it extends up to Orkney. In 
 Britain this grey form is by far the commoner, occurring where- 
 
 6 Doncaster, L., Proc. Zool. Soc, 1905, II, p. 528. 
 
 7 I am not aware that the details of this striking case have ever been worked 
 out. It should be noted that the green and blue forms are not due to simple modi- 
 fication of the red pigment; for these colours, due to interference, fork over the 
 area occupied by the red lines. The distinctions between these forms cannot 
 therefore be simply chemical, as we may suppose them to be, for instance, in the 
 case of many red and yellow forms, and the genetic relationships of the Hehconid 
 varieties would raise many novel problems and be well worth studying experi- 
 mentally. 
 
VARIATION AND LOCALITY 123 
 
 ever the species is found. The red form is much scarcer in 
 England, and does not occur at all in many localities where the 
 grey form is common. Mr. Woodforde, from whom this account 
 is taken, 8 states that in August, 1899, he saw considerably over a 
 hundred of the grey in the New Forest at sugar, but only two 
 red ones. In Staffordshire however the red is proportionately 
 more numerous and he estimates them as 40 per cent, of the 
 population. Lastly a form has been taken in Staffordshire as a 
 rarity in which the red is replaced by yellow, and this has hitherto 
 been seen nowhere else. It is beyond our immediate purposes 
 to discuss the genetic relationships of such forms, but the details 
 of this case are interesting as making fairly clear the fact that 
 the distinctions between castanea and neglecta are due to com- 
 binations of the presence of and absence of two pairs of factors, 
 of which one produces a red pigment in the ground colour of the 
 forew T ing and the other irrorates the same region with black 
 scales. Mr. Woodforde states that all intermediates exist, 
 and that in Staffordshire the greys always have a pinkish tinge. 
 The yellow is doubtless another recessive to the red. 
 
 Species which are uniform in some localities may be poly- 
 morphic in others. Such a phenomenon is well exemplified by 
 the orchid Aceras hircina. Of this species distinct varieties had 
 previously been known in Germany, but Galle 9 has lately given a 
 detailed account of a number of most diverse forms found growing 
 in a district of Eastern France. Without reference to his plates 
 it is impossible to give any adequate conception of the profusion 
 of types which the flowers of the species there assume. In some 
 the lip is elongated to many times its usual length, twisting 
 and dividing in a fashion suggesting some of the strangest of the 
 Tropical Orchids. In others the labellum and the lateral petals 
 are all comparatively short and wide (Fig. 13). Intermediates, 
 combining these qualities in various degrees, were abundant, and 
 the condition of the species, which was the only representative of 
 the genus in the locality, recalls the extreme polymorphism of 
 many of the Noctuid Moths. 
 
 8 Woodeforde, F. C, Trans. North Staffordshire Field Club, XXXV, 1901, Plate. 
 
 9 E. Galle, Compte Rendus du Congres Internet, de Bot. a VExpos. Univ., 1900, 
 p. 112. 
 
124 
 
 PROBLEMS OF GENETICS 
 
 Fig. 13. Various forms of Acer as hircina. (After Gall6.) This figure only shows 
 a few of the more striking forms illustrated in Galle's plates. 
 
VARIATION AND LOCALITY 125 
 
 Somewhat comparable variability has been seen in another 
 Orchid genus Ophrys. In Great Britain the species apifera, 
 aranifera and muscifera though variable are fairly distinct, but 
 Moggridge has published two series of plates 10 showing a very 
 different state of things as regards the Ophrys population of the 
 Riviera. Here the outward diversity is such that the ordinary 
 specific names cannot be applied with any confidence and the 
 limits of the species are quite uncertain. It may well be supposed 
 that these Riviera plants are interbreeding, and indeed we may 
 safely assume that they are. It is, however, to be remembered 
 that Darwin showed apifera in this country to be habitually self- 
 fertilised, so that the different behaviour on the Riviera may 
 itself constitute a local peculiarity. Moreover it is to be gathered 
 from Moggridge's account that in the districts which he examined 
 the condition was not to be described by the statement that our 
 three types were there co-existing and hybridising, but rather 
 we should say that the population was polymorphic, containing 
 these three types amongst others. Conchologists are aware 
 that on the Dogger Bank Modiola attains a size unparalleled 
 elsewhere. The same is true of the sponges Grantia compressa 
 and Grantia ciliata in the estuary of the Orwell. 11 Conversely, 
 as we know so well in the case of Man, dwarf races occur in 
 several special localities. Such examples may be multiplied 
 indefinitely. 
 
 The relation of local forms to species has often been dis- 
 cussed from many points of view, but I know no treatment of 
 the subject clearer or more comprehensive than an excellent 
 account of some of the various manifestations of local dif- 
 ferentiation as they appear in Helicidae published by Coutagne 12 
 and a reader interested in the problem which they raise would 
 
 10 Flora of Mentone, 1864-8, Nova Acta Acad. Caes., XXXV, 1869. 
 
 11 I owe these facts to Canon A. M. Norman, who showed me illustrative 
 specimens. They were originally described by Bowerbank (Monogr. Brit. Spongi- 
 adae, vol. II, pp. 18 and XX; vol. Ill, Pis. I and III). A specimen of G. compressa 
 measured 5 inches, with a greatest width of 3 x /i in. G. ciliata was found measuring 
 3 in. long and % in. wide. These dimensions are many times those of normal 
 specimens. 
 
 12 Coutagne, G., Recherches sur le Polymorphisme des Mollusques de France 
 Annates Soc. d'Agric. Sci. et Industr. Lyon, 1895. 
 
126 PROBLEMS OF GENETICS 
 
 do well to make himself acquainted with the original from 
 which the following notes are taken. He speaks for example 
 of Helix lapicida. This is on the whole a constant form ranging 
 up to the altitude of 1,300 m., common all over France except 
 at great heights and in the Olive regions where it is restricted 
 to moist places. Though subjected to such diverse conditions 
 it shows only trivial variations in colour and other respects 
 throughout its distribution, excepting that on both sides of the 
 Pyrenees it has a very distinct sporadic variety called Andorrica 
 or microporus. This variety occurs here and there, together 
 with the type-form, sometimes in colonies (pp. 26-30 and 86). 
 Bulimus detritus though more restricted in geographical range 
 is a much more variable form. It exhibits great variations in 
 colour, form, and size, and as Coutagne well insists, these are 
 independent of each other. Foreshadowing the methods of 
 factorial analysis he suggests that distinctions in each respect, 
 the "modes" as he calls them, should be denoted by a letter, 
 or if desired, by a name, and the several combinations of differ- 
 ences might thus be most logically and usefully expressed. Of 
 such combinations he says there are at least 18, all of which can 
 be found. The whole possible series does not necessarily occur 
 in the same place, and various localities are characterised by 
 the presence or absence of certain of the combinations as Cou- 
 tagne calls them, and by the relative frequency with which they 
 occur. The ideas thus enunciated are much in advance of the 
 ordinary practice of systematists, who give names to forms which 
 are nothing but accidental combinations of factors, just as the 
 horticulturists for practical reasons give names to similar com- 
 binations, which as we now know are merely specially noticeable 
 terms in a long series of possibilities. In each case it is rather 
 the factors which should be named than the forms which are 
 constituted by their casual collocation. In this special example 
 of Bulimus detritus the 18 forms are made by the combinations 
 of three pairs of independent factors. Besides these combina- 
 tions which may occur anywhere or almost anywhere in the dis- 
 tribution there are two more distinct local forms, each of which 
 is regarded by Coutagne as probably constituting a fresh " mode," 
 perhaps compatible with the others. 
 
VARIATION AND LOCALITY 127 
 
 Helix striata (Draparnauld) 13 is truly polymorphic; and its 
 various forms have been described under various specific names. 
 It abounds in the calcareous hills of Provence and Languedoc, 
 disappearing in the alluvial lowlands and equally in the upper 
 levels at about 800-1,000 m. From this district it extends 
 through regions of similar altitude over a great part of France 
 (details given). 
 
 Locard in his monograph of this group, which he calls col- 
 lectively the group of Helix Heripensis, tabulates 27 distinct 
 named forms. The characteristics in which these forms differ 
 have been reckoned as 17, and as several of these vary in degree 
 of development, the number of modes may be increased to 109. 
 For practical purposes however Coutagne considers that the 
 various developments of 7 characteristics in their several com- 
 binations are enough to express the various forms, and he gives 
 examples of this method of definition. As he observes, though 
 names may be required to define the modes, no one need be 
 alarmed at that, for the same names of modes will be applicable 
 to a great range of distinct species, and the formulae expressing 
 their combinations will replace the varietal names. 
 
 This particular example of polymorphism is but little limited 
 by locality. Occasional colonies present some special physiog- 
 nomy which may in a given place seem almost invariable, though 
 in this very respect the colonies found elsewhere may be highly 
 variable, but such limitations are exceptional for H. striata. 
 
 Some distinct and obvious susceptibilities to the influence 
 of soil and climate are however noticeable. For example on 
 siliceous ground the shells are thinner, while on calcareous soils 
 they are thicker; similarly those from the Northern districts 
 attain a larger size than those from further South. Moreover 
 those subjected to curtailed development, whether from drought, 
 heat or cold often show a shortening of the spire. In contrast 
 with this case Coutagne describes the varieties of Helix caespitum, 
 which he says are for the most part localised, quoting many il- 
 lustrative cases. 
 
 Another remarkable case in which locality plays a curious 
 part is provided by the two species Helix trochoides and pyra- 
 
 13 As to the synonymy and references see Coutagne, p. 45. 
 
128 PROBLEMS OF GENETICS 
 
 midata. In France generally they are distinct enough from 
 each other, trochoides being smaller and having a characteristic 
 keel. Coutagne says that after having collected these species 
 from more than a score of localities he came upon a colony of 
 trochoides on the island of Pomegues in which the shells were 
 relatively enormous, most of them having only a slight keel, 
 and a few none at all. On the other hand he received a con- 
 signment of pyramidata from four localities in Sicily, all small, 
 and one of them exactly like the trochoides from Pomegues. 
 Judging by the samples received from Sicily, trochoides is there 
 not more variable than it is in Provence, while the Sicilian 
 pyramidata is protean. 
 
 The relations of the two species Helix nemoralis and hortensis 
 provide an illustration of another kind of manifestation of local 
 peculiarity. H. hortensis and nemoralis as usually met with, 
 are two very distinct forms. H. hortensis is smaller and duller, 
 and its peristome is white. H. nemoralis is larger and more shiny, 
 and its peristome is brown. In several anatomical points, 
 moreover, especially in the shape of the dart, there are great 
 differences. For a full account of these peculiarities of the two 
 forms and a discussion of their inter-relations the reader is re- 
 ferred to the elaborate work of A. Lang 14 who has studied them 
 extensively and has also succeeded in experimentally raising 
 hybrids between them. These hybrids were in a slight degree 
 fertile with both the parent species, but up to the time of pub- 
 lication no young had been reared from hybrids inter se. 
 
 Coutagne describes the result of collections made in 62 
 French localities. Some had exclusively hortensis, some ex- 
 clusively nemoralis, and in some the two were found in associ- 
 ation. He gives details of five of these collections from which I 
 take the following summary of the more essential facts, omitting 
 much that is almost equally significant. 
 
 Locality A , near Honfleur. Both forms present, each sharply 
 and normally distinguished, without any intermediates. They 
 
 14 A. Lang, Die Bastarde von H. hortensis Midler H. nemoralis L. Jena, G. 
 Fischer, 1908; with a fine coloured plate showing the varieties of the species and 
 their hybrids. 
 
VARIATION AND LOCALITY 129 
 
 are thus found in many places. Coutagne instances M tiller's 
 observations in Denmark, his own series from the Jura, etc. 
 
 Locality B. Vonges (Cote d'Or), 242 hortensis taken at ran- 
 dom, showed 128 with light peristomes (either more or less pinkish 
 or quite white) and 114 with dark brown peristomes; together 
 with 26 nemoralis all with the usual brown peristomes. 
 
 Of the hortensis 50 were in ground-colour opalescens and 1 
 roseus; and in shape 5 were umbilicatus . 
 
 Locality C, about 3 kilometres from B. There were found 35 
 hortensis, of which 20 had light peristomes and 15 brown; to- 
 gether with 7 nemoralis. 
 
 Of the hortensis none were opalescens; 18 were roseus and none 
 has the shape of umbilicatus. 
 
 Locality D, about 1,200 metres from B. 147 hortensis, of 
 which 4 had light peristomes and 143 had brown. No nemoralis 
 were found. 
 
 None of the hortensis were opalescens or roseus, but 30 were 
 umbilicatus. 
 
 In these localities intermediates of every grade existed be- 
 tween the well-characterised opalescens, roseus, or umbilicatus, 
 and the other forms, but there were no intergrades between the 
 other nemoralis and the smaller hortensis, about which there 
 was no hesitation. In the next locality a very different state 
 of things was found. 
 
 Locality E. Banks of the Yvette at Orsay (Seine-et-Oise). 
 The actual numbers are not given, but we are told that 58 per 
 cent, were hortensis, 33 per cent, nemoralis, and 9 per cent, inter- 
 mediate. As at Honfleur, the hortensis had white peristomes, and 
 the nemoralis brown. Coutagne's visits to this locality were 
 in 1878 and 1880, and he calls attention to the fact that Pascal 
 found similar intermediates in the same neighbourhood in 1873. 
 
 The two species, in Coutagne's view, when they occur to- 
 gether, can generally be sorted from each other with perfect 
 confidence, and it is only in exceptional localities that these 
 intermediates occur. Whether they are hybrids, or whether 
 sometimes the species in their variations transgress their usual 
 limitations is regarded both by Coutagne and by Lang as a 
 
 10 
 
i 3 o PROBLEMS OF GENETICS 
 
 question not yet answerable with certainty. Coutagne moreover 
 lays stress on the fact that although each species may be easily 
 known from the other in its own district , yet when shells from 
 different districts are brought together it is sometimes impossible 
 to sort them. He mentions an example of such casual inter- 
 mixture occurring under natural conditions on an island in the 
 Rhone, to which it may well be supposed that floods had brought 
 immigrants from miscellaneous localities. This population con- 
 tained a very large number of uncertain specimens, and as he 
 says, it was much as if he were to mix the shells from his 62 local- 
 ities, after which it would certainly be impossible to separate 
 the two species again. 15 
 
 Further evidence is given in the same treatise as to other 
 examples of polymorphism, especially in the genus Anodonta, of 
 which Locard made 251 species for France alone. Here again 
 are cases like those already given, and many forms or "modes" 
 are found restricted to special localities, while occasionally 
 in the same locality dissimilar forms are found, collectively 
 forming a colony, without intermediates. 
 
 Taken as a whole the evidence shows the following conclusions 
 to be true. Local races, whether of animals or plants, may be 
 distinguished by characters which we are compelled to regard 
 as trivial, or again by features of such magnitude that if they 
 were known to us only as the characteristics of a uniform species 
 they would certainly be assumed without hesitation to be essential 
 for its maintenance. Local forms may be sharply differentiated 
 from the corresponding populations of other localities or they 
 may be connected with them by numbers of intermediates. 
 Not rarely also we find a fact which has always seemed to me of 
 special significance, that the peculiarity of the local population 
 or colony may show itself in a special liability to variation, and 
 this variability may show itself in one of many degrees, either 
 in the constant possession of a definite aberration, in a dimor- 
 phism, or in an extreme polymorphism. 
 
 At this stage attention should be called to two points. 
 
 15 With this evidence compare that given by A. Delcourt in his valuable 
 papers lately published relating to the variations of Notonecta. See especially 
 Bull. Sci. Fr. Belg., 1909, XLIII, p. 443; and C. R. Soc. Biol, 1909, LXVI, p. 589. 
 
VARIATION AND LOCALITY 131 
 
 First, that when the details of the geographical distribution of 
 any variable species are studied in that thorough and minute 
 fashion which is necessary for any true knowledge of the inter- 
 relations of the several forms, the conception of a species invented 
 by the popular expositions of Evolution under Selection is found 
 to be rarely if ever realised in nature. 
 
 A species in this generalised sense is an aggregate of indi- 
 viduals, none exactly alike, but varying round a normal type, 
 the characters of which are fixed in so far as they are adapted to 
 environmental exigency. In nature, however, the occurrence of 
 the varieties, and even the occurrence of the variability is 
 sporadic. In one place a population may be perfectly uniform. 
 In another it may be again uniform but distinct. In others 
 the two forms may occur together, sometimes with and sometimes 
 without intergrades. In some localities a sporadic variety 
 may be an element of the population, persisting through long 
 periods of time. In other localities there may be several such 
 aberrations occurring together which are absent elsewhere. 
 
 Secondly, I would remind the reader that in the light of genetic 
 analysis we know that intergrades, when they do occur, cannot 
 be assumed to represent conditions through which the species 
 must pass or has passed on its way to the extreme and definite 
 forms. 
 
 Often, perhaps generally, they are nothing but heterozygous 
 forms, and often also they are conditions corresponding with the 
 presence of factors in their reduction-stages. 
 
 A broad survey of the facts shows beyond question that it 
 is impossible to reconcile the mode of distribution of local forms 
 with any belief that they are on the whole adaptational. Their 
 peculiarities are occasionally the result of direct environmental 
 influence, as we shall hereafter notice in certain cases, but none 
 can attribute such sporadic and irregular phenomena to causes 
 uniformly acting. 
 
 Writers on systematics, especially those of former generations 
 often conjecture or assert that local distinctions are caused by 
 "differences of climate, soil, food, etc.," in vague general terms. 
 It is usually safe to assume that these remarks do not represent 
 
1 32 PROBLEMS OF GENETICS 
 
 conclusions drawn from actual evidence, for only rarely can they 
 be translated into more precise language. So thoroughly have 
 the biological sciences become permeated with the belief that all 
 distinctions are dependent upon adaptation, that the mere 
 existence of definite distinctions is felt by many to be sufficient 
 ground to warrant an assumption that these distinctions are 
 directly or indirectly due to special local conditions. For 
 example, Dr. J. A. Allen, who has done so much careful and valu- 
 able work in delimiting the local forms of the United States 
 fauna, writes of the Ground Squirrels (Tamias) 16 as follows: — 
 
 "From the extreme susceptibility of this plastic group to 
 the influences of environment, it is one of the most instructive 
 and fascinating groups among North American mammals. No 
 one can doubt its comparatively recent differentiation from a 
 common stock, and its dispersion from some common centre. 
 Whether the type originated at some point in North America, 
 or in the Northern part of Eurasia, it is perhaps idle to speculate, 
 but that it has increased, multiplied, spread, and become differ- 
 entiated to a wonderful degree in North America is beyond 
 question; as it is found from the Arctic regions to the high 
 mountain ranges of Central Mexico, and has developed some 
 twenty to thirty very palpable local phases." 
 
 "Some of them easily take rank as species, others as sub- 
 species. Probably a more striking illustration of evolution by 
 environment cannot be cited." 
 
 He proceeds to point out that the habits of these creatures 
 are such as lead to isolation. This may well be admitted, and 
 indeed no exception can possibly be taken to the passage as a 
 whole, save in the one respect that there is no real proof that 
 the local diversity is due to "evolution by environment" or an 
 indication of "susceptibility to the influences of environment." 
 
 Dr. Allen does indeed adduce the fact that California "ex- 
 tending through 800 miles of latitude, with numerous sharply 
 contrasted physiographic regions, has apparently no less than 
 six strongly differentiated forms, while the region east of the 
 Rocky Mountains from a little below the northern boundary of 
 
 i 6 Allen, J. A., Bull. Amer. Mus. N. H., Ill, 1891, pp. 5i~54. 
 
VARIATION AND LOCALITY 133 
 
 the United States northward to the limit of trees— a slightly 
 diversified region of at least ten times the area of California— has 
 only one"! But when one comes to ask how the various forms 
 are adaptational, and how the influences of environment have 
 led to their production, only conjectures of a preliminary and 
 tentative character could be expected in reply. Desert forms 
 are no doubt pallid as in so many instances, and forest forms are 
 more fully coloured, and we may readily enough accept such facts 
 as indications of a connection between bodily features and the 
 conditions of life, but further than that no one can go; so that 
 when we find size, length of ears or of tail, the number of dorsal 
 stripes, the pattern of the colours, not to speak of differences in 
 the pigments themselves, all exhibiting large modifications, we 
 cannot refer these peculiarities to the causation of environmental 
 difference, save as a simple expression of faith. I incline far 
 more to agree with Gulick who, after years of study of the local 
 variations of the Achatinellidae, came to the conclusion that it 
 was useless to expect that such local differentiation can be 
 referred to adaptation in any sense. 17 Even the most convinced 
 Selectionist must hesitate before such facts as those related by 
 A. G. Mayer regarding the distribution of Partula otaheitana, one 
 of these Achatinellidae. The island of Tahiti has been scored 
 by erosion so that a series of separated valleys radiate to the coast. 
 From four successive valleys Mayer collected the species, and 
 found that in the first (Tipaerui) valley all the shells were 
 dextral (115, containing 73 young) ; in the second valley 
 (Fautaua) 54 per cent, of adults and 55.5 per cent, of the young 
 contained were sinistral; in the third valley (Hamuta) 69 per 
 cent, of adults and 73 per cent, of young contained in them were 
 sinistral; and lastly, in the fourth valley (Pirae) all the shells 
 (131, containing 62 young) were sinistral. 18 In connection 
 with these observations I may mention the fact that in a certain 
 pond in the North of England 19 the sinistral form of Limnaca 
 
 17 J. T. Gulick, Evolution, Racial and Habitudinal, Carnegie Institution, Pub- 
 lication No. 25, 1905. 
 
 18 A. G. Mayer, Mem. Mus. Comp. Anat. Harvard, Vol. XXVI, 1902, p. 117. 
 From the tables given I cannot ascertain the actual numbers from the two inter- 
 mediate valleys, but they were considerable. 
 
 19 To which I was very kindly guided by Mr. C. T. Trechmann. 
 
134 PROBLEMS OF GENETICS 
 
 peregra has been known to occur for about fifty years. Visiting 
 it lately I found the left-handed shells to be about 3 per cent, of 
 the population. The species is the commonest British fresh- 
 water shell, but left-handed specimens are exceedingly rare. 
 Will anyone ask us to suppose that the persistence of a percentage 
 of this rarity in the same place is an indication of some specially 
 favouring circumstance in the waters of that pond? It is a 
 horse-pond to all appearances exactly like any other horse-pond ; 
 and I believe that in perfect confidence we may accept the 
 suggestion of common sense, which teaches us that there is 
 nothing particular in the circumstances which either calls such 
 varieties into existence or contributes in any direct way to their 
 survival. Had the phenomenon of local variation been studied 
 in detail before Darwin wrote, the attempt to make selection 
 responsible for fixity wherever found, could never have been 
 made. The proposition that not only the definiteness of local 
 forms but their variability also is sporadic, can be established 
 by countless illustrations taken from any group of either the 
 animal or the vegetable kingdoms. Only exceptionally can the 
 fixed differences be even suspected of contributing to adaptation, 
 and sporadic variability, which is a no less positive fact, must 
 manifestly lie outside the range of such suspicions. It is open 
 to any one to suggest speculatively that the persistence of 
 special varieties or of special variability in special places is an 
 indication that in those places the conditions of life are such 
 that the forms in question are tolerated though elsewhere 
 the same types are exterminated; but that consideration, even 
 if it could be proved to be well founded, is not one which lends 
 much force to the thesis that definiteness of type is a consequence 
 of Natural Selection. On the contrary, recourse to such reason- 
 ing implies the inevitable but very damaging admission that 
 the stringency of Selection is frequently so far relaxed that two 
 or more equally definite forms of the same species can persist 
 side by side. There is no doubt that this is the simple truth, 
 but when once that truth is perceived it is useless to invoke the 
 control of Selection as the factor to which definiteness of type 
 in general must be referred. 
 
VARIATION AND LOCALITY 135 
 
 The genetic relations of local forms to each other cannot in 
 the absence of actual breeding experiments be often ascertained. 
 Standfuss formerly enunciated as a general principle that when 
 two forms co-exist in the same locality and are able to interbreed, 
 they do not produce intermediates ; but that when the forms are 
 geographically separated as local races, crosses between them 
 result in a series of intermediates. 20 In this aphorism there is a 
 good deal of truth, but if in the light of Mendelian principles we 
 examine the two statements we see now that the first is in reality 
 only another way of saying that the distinctness of an aberrational 
 form co-existing with another is due to segregation, accompanied 
 by some degree of dominance of one type. Whether, however, 
 one geographically isolated race will give intermediates when 
 bred with another must depend entirely on the genetic physiology 
 of the special case, and no general rule can be laid down. It 
 may well be that, inasmuch as the distinctness of the variety is 
 maintained by isolation, the difference in factorial composition 
 between it and the representative form in another area is neither 
 simple nor sharp; but when two varieties co-exist, though inter- 
 breeding, it is now clear that their differences must depend on 
 the segregation of simple factors. Plainly such aberrations may 
 in one place co-exist with another type, and elsewhere be sep- 
 arated from it as local races. 
 
 Excellent illustrations of these two stages in evolution are 
 provided by the melanic varieties of British Lepidoptera. The 
 fact that black or blackish varieties of many species especially 
 of Geometridae have come into existence in recent years is well 
 known to British collectors, and it is not in dispute that they 
 have in several instances replaced the older type more or less 
 completely in certain districts. In the year 1900 the Evolution 
 Committee of the Royal Society instituted a collective inquiry 
 as to the contemporary distribution of these dark varieties. As 
 the change had happened within living memory and had greatly 
 progressed in recent years it was hoped that a record of the 
 existing distribution would serve as a point of departure for 
 future comparison. The records thus obtained were tabulated 
 
 20 Standfuss, Handbuch d. paldarkt Gross-schmet, 1896, p. 321. 
 
i 3 6 PROBLEMS OF GENETICS 
 
 by Mr. L. Doncaster. 21 From that account and from the state- 
 ments in Barrett's British Lepidoptera 22 this description of some 
 of the more notable cases is taken. 
 
 The most striking and familiar case is that of Amphidasys 
 betularia, of which only the ordinary type was known in any 
 locality until about 1 848-1 850, when the totally black var. 
 doubledayaria first appeared in the neighbourhood of Manchester. 
 This black form was subsequently recorded in Huddersfield 
 between i860 and 1870; Kendal about 1870; Cannock Chase, 
 1878; Berkshire, 1885; Norfolk, Essex and Cambridge about 
 1892; Suffolk, 1894; London, 1897. For the Southern Counties 
 of England, except in the London district, there are still very few 
 records. It cannot of course be asserted positively that the 
 variety spread from its place of first appearance into the other 
 localities, and that it did not arise de novo in them, but there 
 can be little doubt that the process was one of colonisation. 
 On the European Continent the first records are from Hanover 
 in 1884, Belgium 1886 and 1894, Crefeld 188-, Berlin 1903, 
 Dresden about the same date. 
 
 As regards the increase of the variety we have the fact that 
 in Lancashire, Cheshire and the West Riding of Yorkshire the 
 black is now the prevalent form ; and in some places, as for example, 
 Huddersfield, the black alone is now found, though it was un- 
 known there till between i860 and 1870. About 1870 at New- 
 port, Monmouth, the two forms were in about equal numbers, 
 but a few years later the type had almost vanished. Similarly 
 in Crefeld, where the black form was still very rare in the eighties, 
 it now forms about 50 per cent, of the population. In the 
 London district the black remains scarce and at the date of the 
 report it was still very scarce. From Ireland there is only one 
 record and there are hardly any from Scotland. 
 
 Boarmia repandata is another species which is behaving in a 
 somewhat similar way. Unlike betularia, however, the species 
 is a variable one, and has several colour-forms, amongst them 
 the banded var. conversaria, and many others. In addition 
 
 21 Ent. Rec, XVIII, No. 7- 1906. 
 
 22 This evidence was largely collected by Mr. G. T. Porritt, who has given 
 much attention to the subject. 
 
VARIATION AND LOCALITY 137 
 
 to these there is a black form in the North of England which 
 seems to be spreading. In Huddersfield the black was first 
 recorded in 1888, and in 1900 20-25 per cent, were black. At 
 Rotherham the black or very dark are now prevalent and have 
 increased in the last 15 years. From the Midlands, East Anglia 
 and Southern Counties the returns show only the light and 
 medium forms. 
 
 Of Odontoptera bidentata several intergrading dark forms exist, 
 and these are found exclusively in the North and the Midlands. 
 Unicolorous blacks have been found recently in the Lancashire 
 mosses and at Wakefield. At Huddersfield 50 years ago the 
 light forms were prevalent, but now a rather dark brown, not 
 infrequently suffused with black, is the commonest. In Southern 
 Counties only light forms are known. 
 
 Phigalia pilosaria in South England is always light, but in 
 the North the prevalent form is darker. About 35 years ago 
 a form with unicolorous sooty fore-wings and dull grey hind 
 wings was first seen in Yorkshire and a similar form is now taken 
 regularly in South Wales. 
 
 In the following cases the dark varieties were found originally 
 only in the South. 
 
 Boarmia rhomboidaria gave rise about 40 years ago to a uni- 
 colorous smoky variety called perfumaria. This was at first 
 peculiar to the London district, but it has since been taken in 
 Birmingham and other large cities. More lately coal-black 
 specimens have been found at Norwich, and others similar but 
 hardly so dark were taken in the South of Scotland and at 
 Cannock Chase. 
 
 Eupithecia rectangnlata is a similar case. Formerly the 
 light forms were prevalent but within sixty years they have 
 almost entirely been replaced in the South of London by a nearly 
 black form. 
 
 Tephrosia (Boarmia) consortaria and Tephrosia consonaria are 
 exceptionally interesting, for they have both given off dark 
 forms in the same wood near Maidstone, which is far from the 
 usual "centres of melanism." They were discovered in this 
 locality by Mr. E. Goodwin. That of consortaria is a dark 
 
i 3 8 PROBLEMS OF GENETICS 
 
 grey, but that of consonaria is a full black, and nothing like 
 either has been found anywhere else. 
 
 These examples are all taken from the Geometridae but 
 others, though of a less conspicuous kind, could be given from 
 the Noctuidae or the Micro-Lepidoptera. Acronycta psi, for 
 instance, has a suffused form which is believed to be becoming 
 more frequent in the London district. Polia chi has two dark 
 forms, olivacea, a. yellowish grey with dark markings, and suffusa 
 which is a darker, blackish-slate colour. Both occur in the North 
 of England, sometimes together, sometimes separately, or mixed 
 with the type and many intermediates. The distribution is 
 peculiarly irregular. At Huddersfield, where the very dark form 
 appeared suddenly about 1890, some 30 per cent, are said to be 
 now dark and about 6-7 per cent, very dark, but at Saddleworth, 
 12 miles away, only the pale forms occur. 
 
 Several questions of interest arise in regard to this evidence. 
 This progressive Melanism has arisen in certain families only, 
 and may be confined to certain species only, within those families. 
 As in almost all other examples in which variation has been much 
 observed, its incidence is capricious and specific. A collateral 
 line of inquiry relates to the degree of discontinuity which the 
 variation manifests. Here again there is no rule. Generally 
 speaking, in A. betularia, to take the case most fully studied, the 
 variation is discontinuous. Real intermediates between betu- 
 laria and doubledayaria are in most localities absent or rare. 
 The black spots of betularia may often be larger or more numerous 
 than in the normal, but this variation has nothing to do with 
 doubledayaria, and is not an intermediate stage towards it, 
 though sometimes wrongly so described. Doubledayaria owes its 
 characteristic appearance to a factor which blurs the surface 
 of the wings with a layer of black. Sometimes this blurring is 
 slighter than in the real doubledayaria, and these forms are real 
 intermediates. Occasionally the forewings alone are thus blurred. 
 These intermediates are clearly due to reduction-stages of the 
 doubledayaria factor, and are related to it as a blue mouse is to 
 a black, or a dutch rabbit to a self -colour. It cannot positively 
 be asserted that the full doubledayaria existed before the inter- 
 
VARIATION AND LOCALITY 139 
 
 mediate, but it almost certainly did. In certain places as for 
 instance in Belgium, there is evidence that intermediates have 
 at various times been fairly abundant, but they have never be- 
 come common, nor are they known to exist in the absence of 
 doiibledayaria. When the black variety and the light type breed 
 together they do not usually have intermediates among their 
 offspring, and the evidence is consistent with the view that the 
 black is a complete dominant. The same is probably true of 
 Tephrosia consonaria. 
 
 In some of the other species we know that the darkest forms 
 did not appear first. For example in Phigalia pilosaria and 
 Boarmia rhomboidaria dark forms existed and are believed to 
 have increased in number before the darkest made its appearance. 
 Hybernia progemmaria is said to have become darker gradually 
 both in Cheshire and in the West Riding, and a uniformly smoky 
 variety appeared in South Yorkshire less than 45 years ago which 
 has spread to neighbouring counties. The dark medium has 
 become the commonest form in Huddersfield district, where the 
 very dark variety is now about 20 per cent, of the population, 
 though the light form is still common. 
 
 Taking the evidence together we find it consistent with the 
 view that dark forms have appeared sporadically, in some species 
 the very dark appearing first and intermediates later, in others 
 the moderately dark came first and the darkest later in time. It 
 is practically certain that the change has in general come about 
 not by a gradual change supervening on the population at large, 
 but by the sporadic appearance of dark specimens as a new ele- 
 ment in the population, and strains derived from these dark 
 individuals have gradually superseded the normal type more or 
 less completely. 
 
 If it could be shown that these melanic novelties had a defi- 
 nite advantage in the struggle for existence they would provide 
 an instance of evolution proceeding much in the way which 
 Darwin contemplated. The whole process would differ from 
 that conceived by him as the normal method of evolution only 
 in so far as the change has come about with great rapidity and 
 in some instances largely by the appearance and success of dis- 
 
140 PROBLEMS OF GENETICS 
 
 continuous varieties. The question, however, must be asked 
 whether the dark form can reasonably be supposed to have 
 an advantage by reason of their darkness. Some naturalists 
 believe that the darkness of the colours does thus definitely con- 
 tribute to their protection by making the insects less conspicuous 
 and hence more likely to escape the search of birds. In support 
 of this view it may be pointed out that it is in the manufacturing 
 districts of Lancashire and Yorkshire, and again in the London 
 area that the melanics have attained their greatest development. 
 Consistently with this argument also, it is in the neighbourhood of 
 Crefeld and Essen, the black country of Germany, that they have 
 chiefly established themselves on the Continent, and Phigalia 
 pilosaria in the black form is now at home in South Wales. Thus 
 superficially regarded, the evidence looks rather strong, but it 
 is difficult to apply the reasoning in detail. We have first the 
 difficulty that the black form of betularia for instance has estab- 
 lished itself in thoroughly rural districts, notably near King's 
 Lynn in Norfolk, and in the neighbourhood of Kendal and 
 Windermere. The black form of consonaria and the dark 
 consortaria appeared in a wood near Maidstone, far from town 
 smoke, and the black rhomb oidaria was first found at Norwich, 
 which, as towns go, is clean. Then again the spread of the 
 melanics is very irregular and unaccountable. The black pilo- 
 saria is found both in the West Riding and in the Swansea 
 district, but not yet elsewhere. It rapidly increased at Hudders- 
 field, but made no noticeable progress at Sheffield though re- 
 corded there for ten years. It is also a remarkable fact that no 
 similar melanic development has been observed in America, 
 and, so far as I am aware, comparable melanic varieties have not 
 appeared on the European continent except in the case of the 
 few sorts which possibly may have come from England. 
 
 The whole subject is beset with complications. It must 
 not be forgotten that in a few species of moths there is an obvious 
 and recognised conformity between the colours of the perfect 
 insect and that of the soil on which they live, comparable with 
 that which is so striking in the case of some Oedipodidae and 
 other grasshoppers. Of this phenomenon the clearest example 
 
VARIATION AND LOCALITY 141 
 
 is Gnophos obscurata, which is a most variable species with many 
 local forms. Of these a well-known dark variety lives on the 
 peaty heaths of the New Forest and other districts, but on the 
 chalk hills of Kent, Sussex and Surrey various light varieties 
 are found, of which one is a bright silvery white, very near in 
 colour to the colour of a chalky bank. This case does not seem 
 to be one of direct environmental action, 23 for Poulton found no 
 change induced by rearing larvae among either white or black 
 surrounding objects. No one however can doubt that there is 
 some indirect connection between the colour of the ground and 
 that of the moths. 
 
 To my mind there is a serious objection to the theory of "pro- 
 tective resemblance in application to such a case as that of the 
 betularia forms, which arises from the fact that the black double- 
 dayaria is a fairly conspicuous insect anywhere except perhaps 
 on actually black materials, which are not common in any 
 locality. Tree trunks and walls are dirty in smoky districts but 
 they are not often black, and I doubt whether in the neighbour- 
 hood of Rotherham, for instance, which is one of the great 
 melanic centres, doubledayaria can be harder for a bird to find 
 than betularia would be. After all, too, many of the species 
 much affected are not urban insects. They live in country 
 places between the towns, and the general tone of these places 
 even in Lancashire and the West Riding is not very different 
 from that of similar places elsewhere. As against the objection 
 that the black varieties are much blacker than the case requires 
 it may be replied that we know nothing of the senses of birds, 
 and that perhaps to their eyes blackness does constitute a dis- 
 guise even though the surroundings are much less dark. • This is 
 undeniable, but recourse to such an argument is dangerous; for 
 if the sight of the insect-eating birds is so dull that it does not 
 distinguish dark things from dingy grey, we cannot subsequently 
 regard the keen sight of birds as the sufficient control which has 
 led to the minute and detailed resemblance of many insects to 
 their surroundings. Those who see in such cases examples of 
 
 23 Such direct action has of course been proved to occur in the case of several 
 dimorphic larvae (e. g„ A. betularia, itself) and pupae. 
 
142 PROBLEMS OF GENETICS 
 
 the omnipotence of Selection must frequently find themselves 
 in this dilemma. 
 
 Taking the evidence as a whole, we may say that it fairly 
 suggests the existence of some connection between modern urban 
 developments and the appearance and rise of the melanic vari- 
 eties. More than that we cannot yet affirm. It is a subject 
 in which problems open up on every side, and all of them are 
 profitable subjects for investigation. Unhappily such animals 
 are difficult to rear successfully in captivity for many generations, 
 owing to their extreme liability to disease. Not the least in- 
 teresting feature of the melanics is the fact that the black vari- 
 eties provide about the best and clearest example of a new dom- 
 inant factor attaching itself to a wild species in recent times. 
 None of the cases are satisfactorily recorded or analysed as yet, 
 but the evidence is clear that doubledayaria is a dominant to its 
 type, and in several other dark varieties, though the pigment 
 deposited is not black, the records show that the increased 
 amount of the pigment almost certainly is due to a positive factor. 
 Of this, Hemerophila abruptaria is a good example. 24 There are 
 some irregularities in the results, but taken together they leave 
 little doubt that the dark brown variety is a dominant and the 
 light, yellowish brown a recessive. 
 
 A curious parallel to the rise of the melanic moths in England 
 is provided by the case of the Honey-creepers or Sugar-birds, 
 in certain West Indian islands. 25 These birds of the genus 
 Coereba (Certhiola) range from Southern Mexico to the Northern 
 parts of South America and through the whole chain of the West 
 Indian islands and Bahamas except Cuba. There are numerous 
 local forms, and many of the islands have types peculiar to them- 
 selves, as is usual in such cases. Some of the types or species 
 range through several islands, but according to Austin Clark 26 
 no island has more than one of them. Cory 27 reckoned twelve 
 
 24 See Harris, Proc. Ent. Soc. London, 1904, p. lxxii, and 1905. P- lxiii ; also 
 Hamling, Trans. City of London Ent. Soc, 1905, p. 5. 
 
 25 1 am indebted to Mr. Outram Bangs of the Harvard Museum for calling my 
 attention to this remarkable case. 
 
 26 Auk, 1889, VI, p. 219. 
 
 27 Ann. N. Y. Acad. Set., 1878, I, p. 149. 
 
VARIATION AND LOCALITY 143 
 
 such species within the Antillean region. They are small birds 
 about the size of a nuthatch with a general colouring of black, 
 yellow, and white. From the island of St. Vincent the Smith- 
 sonian Institution received in the late seventies of last century 
 several completely black specimens in addition to two of the usual 
 type of colouring. The black were described by W. N. Lawrence 
 as atrata, and those marked with the usual yellow and white 
 were called saccharina. The collector (Mr. F. A. Ober) reported 
 that the black form was common, and that the saccharina form 
 was rarer. Lawrence remarks, "Had there been only a single 
 example (of the black form) I should have considered it as prob- 
 ably a case of abnormal colouring, but it seems to be a represent- 
 ative form of the genus in this island." 28 There is of course no 
 doubt of the correctness of the view taken by Austin Clark that 
 "atrata" is a black variety. The black bird is in every respect, 
 other than colour, identical with saccharina, and it is even possible 
 to detect a greenish colour in the areas which would normally be 
 yellow, showing plainly enough the yellow pigment obscured by 
 the black. 
 
 We have next the interesting fact that like our melanic moths 
 the dark form is replacing the "type." At the time of Ober's 
 visit the type was already in a minority, but now it is nearly 
 or perhaps actually extinct, though the black form is one of the 
 commonest birds on the island. Austin Clark found no specimen 
 when he collected there in 1903-4, though formerly it was not 
 uncommon in the vicinity of Kingston and in the immediate 
 windward district of St. Vincent. 
 
 The Grenadines are geographically just south of St. Vincent, 
 though separated by a deep channel. In these islands no black 
 forms have yet been taken, but Grenada, the next island to the 
 south, has both normals and blacks. There are trifling dif- 
 ferences of size between the Grenada birds and those from St. 
 Vincent, the Grenada specimens being slightly smaller and for 
 this reason they have received distinct names, the form marked 
 with yellow and white being called Godmani (Cory) and the black, 
 Wellsi (Cory), but this merely introduces a useless complication. 
 
 28 Ann. N. Y. Acad. Set., 1878, I, p. 149. 
 
144 PROBLEMS OF GENETICS 
 
 There is evidence that in Grenada, as in St. Vincent, the black is 
 gradually ousting the original type, but the process has not gone 
 so far as in St. Vincent. Austin Clark very properly compares 
 this case of the Sugar-birds with that of Papilio turnus, which as 
 is well-known, has a black female in the southern parts of its dis- 
 tribution, in addition to a female of the yellow type, but in the 
 Northern States the black female does not occur. 
 
 During the present year P. R. Lowe, who lately studied 
 Coerebas on a large scale in the West Indies, has published an 
 important paper on the subject. 29 He calls attention to the 
 fact that Cory recently found a black form of Coereba on Los 
 Roques Islands, and he himself discovered another on the 
 Testigos Islands. Both localities are on the coast of Venezuela, 
 far from St. Vincent and Grenada. The whole problem is thus 
 further complicated by the fact that the black varieties have, 
 as we are almost driven to admit, arisen independently in remote 
 places. Improbable as this conclusion may be, it is still more 
 difficult to regard all the black forms as derived from one source. 
 For first, they present definite small differences from each other; 
 and secondly we have to remember a consideration of greater 
 importance, that the very fact that each island has its own type 
 must be accepted as proving that the localities are effectively 
 isolated from each other, and that migration must be a very 
 rare event. 
 
 The rarity of such illustrative cases is, I believe, more ap- 
 parent than real. It is probably due to the extreme reluctance 
 of systematists to admit that such things can be, and of course 
 to the almost complete absence of knowledge as to the genetic 
 behaviour of wild animals and plants. Only in such examples 
 as this of the Coereba, where colour constitutes the sole differ- 
 ence, or that of the moths which have been minutely studied by 
 many collectors, does the significance of the facts appear. The 
 arrangement of catalogues and collections is such that much 
 practical difficulty of a quite unnecessary kind is introduced. For 
 example, in this very case of Coereba, I find the British Museum 
 has a fine series from Grenada including 3 normals and 1 1 black, 
 
 29 Ibis, 191 2, pp. 523-8. 
 
VARIATION AND LOCALITY i 45 
 
 and also 16 blacks from St. Vincent. If the black specimens from 
 Grenada were put with the normals which are almost certainly 
 nothing but a recessive form of the same bird, the variation would 
 strike the eye on even a superficial glance at the drawer. But 
 following the notions so naively expressed in the passage quoted 
 above from W. N. Lawrence, the blacks from Grenada are put 
 apart together with the other blacks from St. Vincent, though 
 two of them were shot on the same date as one of the normals. 
 
CHAPTER VII 
 
 LOCAL DIFFERENTIATION. Continued 
 
 Overlapping Forms 
 
 The facts of the distribution of local forms on the whole are con- 
 sistent with the view that these forms come into existence by the 
 sporadic appearance of varieties in a population, rather than by 
 transformation of the population as a whole. Of such sporadic- 
 ally occurring varieties there are examples in great abundance, 
 though by the nature of the case it can be but rarely that we are 
 able to produce evidence of a previous type being actually super- 
 seded by the variety. When the two forms are found co-existing 
 in the same area they are usually recorded as one species if inter- 
 grades are observed, and as two species if the intergrades are 
 absent. On the other hand when two forms are found occupying 
 separate areas, when, that is, the process of replacement is com- 
 pleted in one of the areas, then forthwith each is named separately 
 either as species or subspecies. Successive observations carried 
 out through considerable periods of time would be necessary to 
 establish beyond question that the history proceeds in one way 
 rather than another. Such continuity of observation has for 
 the most part never been attempted. The kind of information 
 wanted has indeed only been lately recognized, and really critical 
 collecting is a thing of only the last few decades. The methods of 
 the older collectors, who aimed at bringing together a few typical . 
 specimens of all distinct forms, are of little service in this class 
 of inquiry, which is better promoted by the indiscriminate col- 
 lection of large numbers of common forms from many localities. 
 When this has been done on a comprehensive scale we shall be 
 in a position to form much more confident judgments as to the 
 general theory of evolution. 
 
 Some little work of the kind has however been done and the 
 results are already of great value. Seeing that the differenti- 
 ation of local forms is only made possible by isolation, it neces- 
 
 146 
 
OVERLAPPING FORMS i 47 
 
 sarily happens that the collector finds one form in one locality 
 and another in a distinct locality, and there is no evidence as to 
 the behaviour which the two representative species might exhibit 
 if they came into touch with each other. In the most familiar 
 examples of such distinction each inhabits an island, completely 
 occupying it to the exclusion of any other similar form. It can 
 only be when the two representative species occupy parts of a 
 continental area connected with each other by regions habitable 
 for the organism in question, that there is a chance of seeing the 
 two forms in contact. Often also, even where this condition is 
 satisfied, the habits, social organisation, or some other special 
 cause may act as a barrier which prevents the distinguishable 
 forms from ever coming into such complete contact as to inter- 
 breed or to behave as a genetically continuous race. When 
 genetic continuity is ensured by a constant diffusion of the popu- 
 lation over the whole area which they inhabit there will mani- 
 festly be no formation of local races. The practical uniformity, 
 for example, of so many species of birds which inhabit widely 
 extended ranges of Western Europe is doubtless maintained by 
 such constant diffusion. When, as in the case of the Falcons, 
 many localities have peculiar forms, the fact may be taken as 
 conclusive evidence that there is little or no diffusion; and when 
 we find in such a species as the Goldfinch that in spite of mi- 
 gratory fluctuations there are nevertheless geographical races 
 fairly well differentiated, it may similarly be inferred that these 
 fluctuations habitually move up and down on paths which do 
 not intermingle. There are however a few examples of animals, 
 not given to much irregular wandering, which occupy a wide and 
 continuous range of diversified country and are differentiated as 
 local races in two or more districts, though the distinct races 
 meet in intervening areas. Of these the most notorious illus- 
 tration which has been investigated with any thoroughness is 
 that of the species of Colaptes (Woodpeckers) known in the United 
 States as Flickers. The study of the variations of these forms, 
 made by J. A. Allen 1 is an admirable piece of work, with which 
 
 1 J. A. Allen, The North American Species of the Genus Colaptes, Considered with 
 Special Reference to the Relationships of C. auratus and C. cafer. Bull. Am. Mus. 
 Nat. Hist., IV, 1892. 
 
148 PROBLEMS OF GENETICS 
 
 every student of variation and evolutionary problems should 
 make himself familiar. The two forms with which we are most 
 concerned are known as C. auratus and C. cafer, and are very 
 strikingly different in appearance. In size, proportions, general 
 pattern of colouration, habits, and notes, the two are alike, but 
 they differ in the following seven respects as stated by Allen. 
 
 Auratus Cafer 
 
 1. Quills yellow. I. Quills red. 
 
 2. Male with a black malar stripe. 2. Male with a red malar stripe. 
 
 3. Adult female with no malar stripe. 3. Adult female with usually a brown 
 
 malar stripe. 
 
 4. A scarlet nuchal crescent in both sexes. 4. No nuchal crescent in either sex. 
 
 5. Throat and fore neck brown. 5- Throat and fore neck grey. 
 
 6. Whole top of head and hind neck 6. Whole top of neck and hind neck 
 
 grey. brown. 
 
 7. General plumage with an olivaceous 7. General plumage with a rufescent 
 
 cast. cast. 
 
 These differences are illustrated in the coloured plate (frontis- 
 piece), which has been most kindly prepared for me under the in- 
 structions of Dr. F. M. Chapman of the American Museum of 
 Natural History. Before going further it is worth considering the 
 nature of these differences a little more closely. All but the last 
 are large differences which no one would overlook even in a hasty 
 glance at the birds. If the only distinction lay in the colour of the 
 quills we might feel fairly sure that auratus was a recessive form 
 of cafer, and so probably it is in this respect. Similarly the black 
 malar stripe of auratus is in all probability recessive to the red 
 malar stripe of cafer and I imagine the pigments concerned are 
 comparable with those in the Gouldian Finch (Poephila gouldiae) 
 of Australia. Both sexes in that species may have the head black, 
 red, or, less often, yellow, and though it is not any longer in 
 question that birds may breed in either plumage, I believe that 
 the young are always black-headed and I imagine that those 
 which become red-headed possess a dominant factor absent from 
 the permanently black-headed birds. 2 Yellow as a recessive 
 
 2 For a case in which a red-headed female X a black-headed male gave a 
 black-headed female and a red-headed male, see Avian Mag., N. S., IV, pp. 49 
 and 329. 
 
 **mcouaBflWy 
 
OVERLAPPING FORMS 149 
 
 form of a red is certainly very common, but red and black as 
 variants of the same pigment are less usual. In the Gouldian 
 Finch we seem to have a case where a pigment can assume all 
 three forms. It would be interesting to know whether the red 
 of the malar stripes in Colaptes is a pigment of the same nature 
 as the red of the quills. Both in Colaptes and in Poephila 
 gouldiae I have seen specimens intermediate between the black 
 and the red, and the appearance of the part affected was exactly 
 alike in the two cases, red feathers coming up among the black 
 ones, and many feathers containing both red and black pigments 
 mixed together. 
 
 The development of the scarlet nuchal crescent in auratus 
 and the absence of this conspicuous mark in cafer constitute from 
 the physiological point of view the most remarkable pair of dif- 
 ferences. When the red crescent is not formed, the feathers 
 which would bear it are exactly like the rest, and no special 
 pigment is visible in them which one can regard as ready to be 
 modified into red. If the crescent is due to a factor it must 
 therefore be supposed that this factor has the power of modifying 
 the pigment of the neck in one special place alone. Dr. W. D. 
 Miller called my attention to the fact that a similar variation 
 occurs in another American woodpecker, the Sapsucker, Sphy- 
 ropicus varius? 
 
 I do not suggest that such variations are without parallel: 
 indeed in P. gouldiae the factor which turns the black of the head 
 into scarlet affects one special region of the black only, being 
 sharply distinct from the unmodified black of the throat. These 
 regions of the head are however often the seat of special colours 
 in birds. 4 So also may be instanced the variety of the Common 
 
 3 The other variations of this bird are also interesting and important. The 
 normal male has a red head and a red throat. The female has a red head and a 
 white throat, but varieties of the female are known with a black head, thus again 
 illustrating the change from black to red. It should be noted that this is not a 
 mere retention of a juvenile character, but, as the birds mature, the red feathers 
 come up, or as an exception, the black. There is also a western species, ruber, 
 in which both sexes have a great extension of red, and are alike. The male of 
 nuchalis intergrades with this type, but the female does not. 
 
 4 Dr. W. Brewster, for example, has a remarkable specimen of the Teal (Ncttion 
 carolinense) with a white collar strongly developed at the front and sides of the 
 neck, in a place where the normal has no such mark. 
 
i 5 o PROBLEMS OF GENETICS 
 
 Guillemot ( Uria troile) which has a white line round the eyes and 
 at the sides of the head where the normal has no such mark; but 
 this line is formed in a very special place, the groove joining the 
 eye to the ear, whereas the feathers of the nuchal crescent are 
 not ostensibly distinguished from those adjacent. 5 
 
 The transposition of the brown and the grey on the back and 
 front of the neck also constitutes a very remarkable difference. 
 If either grey or brown depends on a factor then it must be sup- 
 posed that auratus has one of these factors and cafer the other. 
 
 From these several considerations it is quite clear that if 
 auratus and cafer are modifications of the same type produced 
 by presence or absence of factors, several independent elements 
 must be concerned, and to unravel their inter-relations would be 
 most difficult even if it were possible to breed the types under ob- 
 servation, which is of course quite beyond present possibilities. 
 
 The distribution of the two is as follows. On the east side of 
 the Continent C. auratus, relatively pure, occupies the whole of 
 Canada and the States from the North to Galveston. Westward 
 it extends across the whole continent in the more northern 
 region to Alaska, but in its pure form it only reaches down the 
 Pacific coast to about the northern border of British Columbia. 
 Its southern and western limit is thus roughly a line drawn from 
 north of Vancouver, southeast to North Dakota and then south 
 to Galveston. C. cafer in the comparatively pure form inhabits 
 iMexico, Arizona, California (except Lower California and the 
 -opposite coast), central and western Nevada, Utah, Oregon, and 
 is bounded on the east by a line drawn from the Pacific south of 
 Washington, south and eastward through Colorado to the mouth 
 
 B This variety is spoken of as the Ringed Guillemot and is sometimes regarded 
 as a distinct species to which the name ringvia was given by Briinnich. In sup- 
 port of this view Dr. William Brewster, to whom I am indebted for much assist- 
 ance in regard to the variation of birds, called my attention to observations of his 
 own and also of Maynard's, that the ringed birds were sometimes mated together, 
 though in a small minority (see Brewster, Proc. Boston Soc. N. H„ XXII, 1883, p. 
 410). It would however be possible to produce many instances of varieties mated 
 together though surrounded by a typical population (e. g., two varying Black- 
 birds, Zoologist, p. 2765; two varying Nightjars, ibid., p. 5278). I am inclined to 
 believe that in nature matings between brothers and sisters are frequent in many 
 species of animals, and that the production of sporadically varying colonies is 
 thus greatly assisted. 
 
OVERLAPPING FORMS 
 
 LSI 
 
 of the Rio Grande or the Gulf of Mexico. Between the two 
 lines thus roughly defined is a band of country about 1,200-1,300 
 miles long and 300-400 miles wide, which contains some normal 
 birds of each type, but chiefly birds exhibiting the characters of 
 both, mixed together in various and irregular ways. Even in 
 the areas occupied by the pure forms occasional birds are re- 
 corded with more or less indication of characteristics of the other 
 form, but within the area in which the two forms are conterminous, 
 the mixed birds are in the majority. The condition of these birds 
 of mixed character is described by Allen as follows: 
 
 "As has been long known — indeed, as shown by Baird in 
 1858 — the 'intermediates' or 'hybrids' present ever- varying 
 combinations of the characters of the two birds, from individuals 
 of C. auratus presenting only the slightest traces of the char- 
 acters of C. cafer, or, conversely — individuals of C. cafer present- 
 ing only the slighest traces of the characters of C. auratus — 
 to birds in which the characters of the two are about equally 
 blended. Thus we may have C. auratus with merely a few red 
 feathers in the black malar stripe, or with the quills merely 
 slightly flushed with orange, or C. cafer with either merely a few 
 black feathers in the red malar stripe, or a few red feathers at 
 the sides of the nape, or an incipient, barely traceable scarlet 
 nuchal crescent. Where the blending of the characters is more 
 strongly marked, the quills may be orange-yellow or orange-red, 
 or of any shade between yellow and red, with the other features 
 of the two birds about equally blended. But such examples are 
 exceptional, an unsymmetrical blending being the rule, the two 
 sides of the same bird being often unlike. The quills of the tail, 
 for example, may be part red and part yellow, the number of 
 yellow or red feathers varying in different individuals, and very 
 often in the opposite sides of the tail in the same bird. The 
 same irregularity occurs also, but apparently less frequently, 
 in the quills of the wings. In such cases the quills may be mostly 
 yellow with a few red or orange quills intermixed, or red with a 
 similar mixture of yellow. A bird may have the general coloura- 
 tion of true cafer combined with a well-developed nuchal crescent, 
 or nearly pure auratus with the red malar stripes of a cafer. 
 
i 5 2 PROBLEMS OF GENETICS 
 
 Sometimes the body plumage is that of C. auratus with the head 
 nearly as in pure cafer, or exactly the reverse may occur. Or we 
 may have the general plumage as in cafer with the throat and 
 crown as in auratus, and the malar stripe either red or black, 
 or mixed red and black, and so on in almost endless variations, 
 it being rare to find, even in birds of the same nest, two indi- 
 viduals alike in all their features of colouration. Usually the 
 first trace of cafer seen in auratus manifests itself as a mixture of 
 red in the black malar stripe, either as a few red feathers, or as a 
 tipping of the black feathers with red, or with merely the basal 
 portion of the feathers red. Sometimes, however, there is a 
 mixture of orange or reddish quills, while the malar stripe remains 
 normal. In C. cafer the traces of auratus are usually shown by a 
 tendency to an incipient nuchal crescent, represented often by 
 merely a few red-tipped feathers on the sides of the nape; at 
 other times by a slight mixture of black in the red malar stripe." 
 Such a state of things accords very imperfectly with expecta- 
 tions under any received theory of Evolution. As in some of 
 the instances discussed in the first chapter we have here two 
 fairly definite forms, nearly allied, which on any evolutionary 
 hypothesis must have been evolved either the one from the other, 
 or both from a third form at a time not very remote from the 
 present, as time must be measured in evolution. Yet though 
 intermediates exist in some quantity, no one can for a moment 
 suggest that they are that definite intermediate from which 
 auratus and cafer descend in common. One cannot imagine that 
 the immediate ancestor of these birds was a mosaic, made up of 
 asymmetrical patches of each sort: but that is what many of the 
 intermediates are. It is not much easier to suppose the ancestor 
 to have been a nondescript, with a compromise between the 
 developed characters of each, with quills buff, malar stripes 
 neither black nor red, with a trace of nuchal crescent, and so on. 
 Such Frankenstein-monsters have played, a considerable part in 
 the imaginations of evolutionary philosophers, but if it were 
 true that there was once a population of these monsters capable 
 of successful existence, surely they should now be found as a 
 population occupying the neutral zone between the two modern 
 
OVERLAPPING FORMS 153 
 
 forms. Yet, though much remains to be done in clearing up the 
 facts, one thing is certain, namely that the neutral zone has not a 
 definite and normally intermediate population, but on the con- 
 trary it is peopled by fragments of the two definite types and 
 miscellaneous mongrels between them. 
 
 On the other hand, one cannot readily suppose that either 
 form was the parent of the other. The process must have 
 involved both addition and loss of factors, for whatever hypoth- 
 esis be adopted, such changes must be supposed to have occurred. 
 A careful statistical tabulation of the way in which the characters 
 are distributed in the population of the mixed zone would be 
 of great value, and till that has been done there is little that can 
 be said with certainty as to the genetics of these characters. 
 In the collection of Dr. Bishop of New Haven I was very kindly 
 allowed to examine a sample, all taken at random, near together, 
 in Saskatchewan. There were females 4 adult, 2 young; males 
 4 adult and 5 young. This number, though of course insufficient, 
 is enough to give some guide as to the degree of definiteness which 
 the characters generally show in their variations. Of the 15 
 birds, 8 had simply yellow quills; 2 had red; I was almost red 
 but had one yellow tail-quill; 3 were intermediate and 1 was 
 buff. As regards the malar patch, which can only be determined 
 properly in the adult males, 1 was red, 1 was approximately red, 
 2 intermediate. As to nuchal crescent 4 females had none, 2 
 females very slight; 7 males had it, 1 had only a slight crescent, 
 and 1 had none. In point of quills therefore 10 were definite 
 out of 15; in point of crescent, 11 were definite out of 15; and in 
 point of malar patch 1 only was definite out of 4. The last is a 
 feature directly dependent on age and so counts for less, but as 
 regards the other two features there is some indication that the 
 factors show definiteness in their behaviour. It must be re- 
 membered that we have no knowledge what the heterozygous 
 form may be, and in the case of red and yellow it is probably a 
 reddish buff. The patch-works are no doubt to be compared with 
 other well-known pied forms, and in these we must suppose the 
 active factor broken up, which it probably can be very easily. 
 The asymmetry, which Allen notices as so marked a feature, in the 
 
154 PROBLEMS OF GENETICS 
 
 distribution of the red and yellow quills of the tail especially, 
 recalls that of the black markings in the pied Canaries. As is well 
 known to students of variations some pigment-factors in some 
 animals are apparently uncontrolled by symmetry, while in other 
 specific cases symmetry is the rule. On the other hand the 
 blackness or redness of the malar patches is, I think, as a rule 
 nearly symmetrical. It should be mentioned that two of Dr. 
 Bishop's young birds belonged to the same nest, one a female 
 with red quills, the other a male with yellow. Both are without 
 crescent. 
 
 As to the question whether certain combinations of characters 
 occur with special frequency, the evidence is insufficient to give a 
 definite answer. Among all the birds I have seen in America 
 or in England I have not yet found one having the malar patches 
 black without any nuchal crescent. Of Dr. Bishop's 8 adults 
 not one, however, showed the combination of the three chief 
 features normal for auratus or for cafer. 
 
 Besides the two forms that we have hitherto considered, 
 several other local types exist, and these throw some further 
 light on the problem. Of these the most important in this 
 connexion is chrysoides, which inhabits the whole of southern 
 California and the mainland opposite. This remarkable form 
 is as Allen says, very different from auratus except that it has 
 the quills yellow like auratus, not red like cafer. So that we 
 find here in the extreme west of the whole distribution a type 
 agreeing in one of its chief features with the eastern type. Be- 
 tween this and cafer intergrades have, according to Allen, not 
 been found. The relations of this chrysoides are, Allen thinks, 
 rather with mexicanoides , a southern, smaller race with colours 
 more intense, which inhabits Guatemala, but however that may 
 be, it must be regarded as a cafer which has lost its red quills. 
 The island of Guadeloupe off Lower California has an island 
 form. Beyond the other side of the continent there is also an 
 island form of auratus, inhabiting Cuba, so that clearly the yellow 
 quills can extend into the tropics. 
 
 The above account is in many respects incomplete, but it 
 suffices to give an outline of the chief facts. The whole problem 
 
OVERLAPPING FORMS 155 
 
 is complicated by the undoubted effects of an uncertain amount 
 of migration, and in many, perhaps all, districts, the winter 
 population differs from the summer population of the same 
 localities. The existence of these seasonal ebbs and flows is 
 now well known to ornithologists, and most of the bird species 
 of temperate regions are subject to them. 
 
 Difficult as it may be to conceive the actual process of origin 
 of the two types auratus and cafer, it is I think still harder to 
 suggest any possible circumstance which can have determined 
 their development as distinct races, or which can maintain 
 that distinctness when created. Some will no doubt be disposed 
 to appeal once more to our ignorance and suggest that if we only 
 knew more we should see that the yellow quills, the black 
 "moustache" and the red crescent, specially qualify auratus 
 for the north and eastern region, and the red quills, red "mou- 
 stache" and absence of crescent fit cafer to the conditions of 
 its homes. Each can judge for himself, but my own view is 
 that this is a vain delusion, and that to cherish it merely blunts 
 the receptivity of the mind, which if unoccupied with such fancies 
 would be more ready to perceive the truth when at last it shall 
 appear. Think of the range of conditions prevailing in the 
 country occupied by auratus — a triangle with its apex in Florida 
 and its base the whole Arctic region of North America. Is it 
 seriously suggested that there is some element common to the 
 "conditions" of such an area which demands a nuchal crescent 
 in the Flickers, though the birds of the cafer area, almost equally 
 varied, can dispense with the same character? Curiously enough, 
 the geographical variation of Sphyropicus varius, another though 
 a very different Woodpecker 6 shows that conversely the nuchal 
 crescent can be dispensed with in the Eastern form though it 
 is assumed by the Western. 7 
 
 6 The Sap-suckers feed on trees and somewhat resemble our Spotted Wood- 
 peckers in general appearance. Colaptes feeds on the ground and corresponds 
 perhaps rather with the European Green Woodpecker. 
 
 7 For an introduction to this example I am indebted to Mr. W. D. Miller of 
 the American Museum of Natural History. Some account of the facts is given by 
 Baird, Brewer, and Ridgway (A Hist, of N. Amer. Birds, 1874, H. PP- 540, 544, 
 etc.). 5. varius occupies the whole country in suitable places from the Atlantic 
 to the eastern slopes of the Rockies, and all Mexico to Guatemala. 5. nuchalis was 
 
156 PROBLEMS OF GENETICS 
 
 Allen points out the interesting additional fact that super- 
 posed upon each of the two distinct forms, auratus and cafer, 
 are many geographical variations which can very naturally be 
 regarded as climatic. Each decreases in size from the North 
 southward, as so many species do. 8 They become paler in the 
 arid plains, and show the ordinary phases which are seen in 
 other birds having the same distribution. Such differences 
 we may well suppose to be determined directly or indirectly, by 
 environment, and we may anticipate with fuller knowledge it 
 will be possible to distinguish variations of this nature as in the 
 broad sense environmental, from the larger differences separating 
 the two main types of Colaptes, which I surmise are altogether 
 independent of such influences. 
 
 It is generally supposed that phenomena like those now so 
 well established in the case of Colaptes are very exceptional, and 
 as has already been stated a number of circumstances must 
 combine in order that they may be produced. I suspect however 
 that the examples are more numerous than is commonly thought. 
 In all likelihood the three forms Sphyropicus varius, nuchalis 
 and ruber are in a very similar condition though the details 
 have not, so far as I know, been worked out. A complex example 
 which is closely parallel to the case of Colaptes was described by 
 F M. Chapman 9 at the same date as Allen's work. This is the 
 case of Quiscalus, the Grackles, which in the North American 
 Continent have three fairly distinct forms which Chapman speaks 
 of as Q. aeneus, Q. quiscula, and Q. quiscula aglaeus. The birds 
 are all, so far as pigment is concerned, dark blackish brown, but 
 the head and mantle have superposed a metallic sheen of inter- 
 ference-colours which in the various forms take different tints, 
 
 first known from the Southern Rockies only, but many were afterwards taken in 
 Utah. 5. ruber is restricted to the Pacific coast. In Ridgway's opinion all three 
 are geographical forms of one species. In ruber the sexes are alike having both a 
 great extension of the red in the throat, and a red crescent. The male of nuchalis 
 grades to the ruber form, but the female does not. This female has some red in 
 the throat like the male of varius, whereas the female of varius has a whitish throat. 
 
 8 Not only vertebrates but the marine Crustacea and Mollusca illustrate this 
 curious "principle" of variation, as Canon Norman formerly pointed out to me 
 with abundant illustrations. There are of course cases to the contrary also. 
 
 9 Chapman, F. M., Bull. Amer. Mus., IV, 1892, p. 1; see also Ridgway, Birds 
 of North and Middle America, 1902, Part II, p. 214. 
 
OVERLAPPING FORMS 157 
 
 bluish green, bronze green, or bronze purple. The details are 
 complicated and difficult to appreciate without actual specimens, 
 but the two common types are sufficiently distinct. The birds 
 inhabit the whole area east of the Rockies, quiscula aglaeus oc- 
 cupying Florida and the Southern States southwest of a band 
 of country about a hundred miles broad extending roughly from 
 Connecticut to the mouth of the Mississippi; and aeneits taking 
 the area north and west of this band. In discussing this case 
 Chapman expresses the same view as Allen does in the Colaptes 
 case, that there are two distinct populations, substantially fixed, 
 and that the band of country in which they meet each other 
 has a mongrel population, with no consistent type, but showing 
 miscellaneous combinations of the character of the two chief 
 types. 
 
 The warblers of the genus Helminthophila provide another 
 illustration which has points of special interest. The two chief 
 species are H. pinus, which has a yellow mantle and lower parts, 
 white bars on the wings, a black patch behind the eyes and a 
 broad black mark on the throat ; and H. chrysoptera with dark grey 
 mantle and pale whitish grey lower parts, yellow bars on the 
 wings, and grey marks on cheeks and throat where pinus has 
 black. These two birds are exceeding distinct, and in addition 
 their songs are quite unlike. H. pinus ranges through the eastern 
 United States up to Connecticut and Iowa. H. chrysoptera is a 
 northern form extending down to Connecticut and New Jersey. 
 Both are migrants. 
 
 In these two States, where the two types overlap, certain 
 forms have been repeatedly found which have been described as 
 two distinct species, Lawrencei and leucobronchialis. Dr. L. B. 
 Bishop and Mr. Brewster showed me two long series of Hel- 
 minthophila containing various intergrades between the four 
 named kinds, and details regarding these may be found in 
 Chapman's North American Warblers and in Dr. Bishop's paper 
 in Auk, 1905, XXII. Though the characters evidently break 
 up to some extent, the series can be represented as due to re- 
 combinations of definite factors more easily than the others 
 which I have described. The differentiating characters are: 
 
158 PROBLEMS OF GENETICS 
 
 Pinus Chrysoptera 
 
 1. Mantle and lower parts yellow (Y 1 ). I. Mantle and lower parts grey (y 1 ). 
 
 2. Wing-bars white (y 2 ). 2. Wing-bars yellow (Y 2 ). 
 
 3. Cheek and throat not black (b). 3- Cheek and throat black (B). 
 
 The grey pigment of the mantle is common to both, but is 
 masked by the yellow in pinus, the net result being an olive- 
 green. 10 
 
 I am much indebted to Dr. F. M. Chapman for the loan of 
 the coloured plate in which these distinctions are shown. It first 
 appeared in his book, North American Warblers. 
 
 We cannot tell whether yellow or not-yellow is due to the 
 presence of a factor, but we may suppose that one or other 
 gives the special colour to the parts. The black of character 3 
 is no doubt a dominant. Thus pinus becomes Yyb and chry- 
 soptera in y x Y 2 B. The Lawrencei which has the underparts 
 yellow, wing-bars white, and black patches is Y x y 2 B and leuco- 
 bronchialis which has mantle and underparts not-yellow, wing- 
 bars yellow and no black patches is y x Y 2 b. This representation, 
 it should be clearly understood, is tentative and approximate 
 only. The characters are not really sharp, for there is much 
 grading; but allowing for the effects of heterozygosis and for some 
 actual breaking-up of factors I believe it gives a fairly correct 
 view of the case. In particular we can see how it meets the dif- 
 ficulty which Chapman felt in accepting leucobronchialis as in 
 any sense derived from pinus which has a yellow breast, and 
 chrysoptera which has a black throat, seeing that leucobronchialis 
 has neither. We now recognize at once that this form could be 
 produced by ordinary re-combination of the absence of Y 1 with 
 the absence of B. 
 
 I note also with great interest that the modern observers 
 agree that the so-called hybrids may have the song either of 
 the one species, or of the other, or a song intermediate between 
 the two. It may also be added that these two types have several 
 
 10 It would aid greatly in factorial analysis if the descriptive term "green" 
 could be avoided in application to cases where the green effect is due only to a mix- 
 ture of black and yellow pigments. The absence of yellow is the sole difference 
 between the mantle and underparts of pinus and chrysoptera. 
 
Fig. i. Helminthopkila pinus, male. 
 
 Fig. 2. Helminthopkila pinus, female. 
 
 F.G. 3. "l^wrenie-s Warbler." male; one of the intending forms 
 
 F.o. 4 . •• Broker's Warbler." male; another of the interring form. 
 
 Fig. 5. Helminthopkila chrysoptera, male. 
 
 Fig. 6. Helminthopkila chrysoptera, female. 
 
 X 
 
 
 

 
 .efjsrn , 
 
 m<>\ ,z^\u<\ aVu^V 
 
 ' 
 
 ■ 
 
 I 
 
^ 
 
 f.G ftccrt,. 
 
OVERLAPPING FORMS 159 
 
 times been seen, in the breeding season, paired with each other 
 or with one of the other combinations. 
 
 Allen 11 has described another excellent American example, 
 the Tits of the group Baeolophus bicolor-atricristatus. The form 
 bicolor belongs to the eastern States and ranges from the Atlantic 
 coast to the Great Plains, and atricristatus, of east Mexico, ex- 
 tends from Vera Cruz to central Texas. In southern and 
 central Texas the breeding ranges adjoin, and in this country 
 various intermediates occur. The chief types differ in two main 
 points. 
 
 B. bicolor B. atricristatus 
 
 Forehead varies from deep black to dull Forehead white to buffish white. 
 
 black, suffused with rusty brown. 
 
 Crown and crest grey, slightly darker Crown and crest black, abruptly con- 
 
 than the black. trasting with the back. 
 
 The intergrades between the two have, as usual, received specific 
 names. A detailed description is given by Allen, from which 
 it appears that the gradation is very complete. In one case a 
 series of 16 adults were all intermediates. It is not stated whether 
 the collector took these at random, but from the local lists it is 
 clear that the types are found not far away from the place where 
 the intergrades were shot. 
 
 Another very striking case is that of the Tanagers, of the 
 genus Rhamphocoelus. In this group there are several local forms 
 which are related to each other in remarkable ways. The forms 
 known as passerinii and icteronotus exhibit the clearest phenomena 
 of intergradation. The species passerinii has a brilliant scarlet 
 and black male, and it inhabits Honduras and Nicaragua. 
 Proceeding southwards along the isthmus we find next costari- 
 censis which has a male like that of passerinii (but a female 
 with more orange than the olive-grey female of passerinii). 
 Next we come to Panama which is occupied by icteronotus, 
 sharply distinguished from passerinii by the fact that the scarlet 
 is replaced by lemon-yellow. This same icteronotus occurs again 
 as a pure type in Ecuador and many other parts of South America; 
 but Colombia, between Panama and Ecuador, contains scarlets 
 like passerinii, yellows like icteronotus, and various intergrades 
 
 11 Bull. Amer. Mus. Nat. Hist., XXIII, 1907, p. 467. 
 
160 PROBLEMS OF GENETICS 
 
 of several shades of orange. The passerinii males from Nica- 
 ragua are indistinguishable from those of Colombia, and the 
 icteronotus of Ecuador are the same as those in Panama. The 
 orange intergrades, doubtless heterozygous forms, though col- 
 lected at the same locality (Medellin in Colombia) as several 
 pure yellows and pure scarlets, are in the British Museum series 
 sorted out as a separate species under the name chrysonotus! 
 Complications are introduced by the relations of these forms to 
 another named type, fiammigerus, but we may for our purpose 
 leave that out of consideration, and say that the order of geo- 
 graphical sequence from Honduras to Ecuador is (i) scarlet, 
 (2) yellow, (3) mixture of types, scarlet, yellow, orange, (4) 
 yellow. 
 
 Similar examples exist in the birds of the old world, but I do 
 not know of any that have been studied so fully as those of 
 America. The best known is that of the two Rollers, Coracias 
 indicus which spreads from Asia Minor through Persia, Balu- 
 chistan, the Indian Peninsula and Ceylon, and affinis which 
 ranges from Nepal, through Assam, Tenasserim and the Indo- 
 Chinese countries. The two types are very different and may be 
 distinguished as follows: 
 
 C. indicus C. affinis 
 
 Mantle drab brown-chestnut. Dark olive-green. 
 
 Breast chestnut. Dull purple brown. 
 
 Throat purplish, streaked with white. Purple, streaked with blue. 
 
 Upper tail-coverts indigo. Turquoise. 
 
 The wings are the same in both. In the provinces of Nepal, 
 Sikhim, and Darjiling the two species coexist, with the result 
 that intergrades have been frequently recorded. The line of 
 intergradation extends to the coast, and birds showing various 
 combinations of the two types from the Calcutta district exist 
 in collections. 12 The case is interesting inasmuch as like that of 
 Quiscalus it shows a series of combinations of various metallic 
 colours. Some of these are probably evoked by the development 
 of pigment behind striations or other interferences already exist- 
 ing, but in the present state of knowledge it would be quite im- 
 
 w References on this subject will be found in Brit. Mus. Cat. Birds, XVII, p. 13. 
 
OVERLAPPING FORMS i6r 
 
 possible to suggest what the actual factors producing these ap- 
 pearances may be. 
 
 There are, naturally, many other cases among birds which 
 are suspected of being in reality comparable, but in most of them 
 the evidence is still inadequate. Among Lepidoptera also there 
 are a few of these; perhaps the most striking is that of Basilar chia 
 "proserpina." 13 The genus is well known to European col- 
 lectors under the name Limenitis, of which we in England have 
 one species, L. sibylla, the " White Admiral." A species, arthe- 
 mis, very like sibylla in appearance is common in the northern 
 parts of the United States, ranging through Canada and Northern 
 New England, but rarely south of Boston. This species has 
 the conspicuous white bands across both wings like our sibylla. 
 
 There is also a more Southern type known as astyanax, 
 which is very different in its appearance, being without the white 
 bands and having a broad irroration of blue scales on the posterior 
 border of the hind wings. The two are so distinct that one would 
 not be tempted to suspect any very close relation between them. 
 In its distribution astyanax is described by Field as replacing 
 arthemis south of latitude 42 . About Boston it is much more 
 common than arthemis. 
 
 The two forms encroach but little on each other's territory, 
 but where they do coexist, a third form, known as proserpina, is 
 found which is almost intermediate, with the white bands much re- 
 duced. There is now no doubt that this proserpina is a hetero- 
 zygous form, resulting from a combination of the characters of 
 arthemis and astyanax. Field succeeded in rearing a brood of 16 
 from a proserpina mother caught wild which laid 31 eggs, and of 
 these, nine (five males, four females) resembled the mother, 
 being proserpina, and seven (four males, three females) were 
 arthemis. There can be no question therefore that the mother 
 had been fertilised by a male arthemis and that no-white-band 
 is a factor partially dominant over the white band. Another 
 point of interest which Field observed was that the proserpina 
 female refused to lay on birch, poplar or willow, but accepted 
 
 " For these facts I am indebted to Mr. W. L. W. Field, who has lately published 
 an account of his observations and experiments. See especially, Psyche, 1910. 
 XVII, No. 3, where full references to previous publications are given. 
 
162 PROBLEMS OF GENETICS 
 
 wild cherry {Prunus serotina) a species on which astyanax can 
 live, though that tree is not known to be eaten by arthemis. 
 Incidentally also the observations show that sterility cannot be 
 supposed to be the bar which maintains the distinctness of 
 arthemis and astyanax. 
 
 In this connection Papilio oregonia and bairdii should be 
 mentioned. 14 P. oregonia is one of the numerous forms like 
 machaon, but rather paler. It is a northern insect, inhabiting 
 British Colombia east of the Cascade Range, and reaching to 
 Colorado. P. bairdii is a much darker butterfly, representing 
 the asterias group of the genus Papilio. Like asterias it has the 
 abdomen spotted at the sides, not banded as in the machaon 
 group. It belongs to Arizona and Utah extending into Colorado. 
 From Colorado the form brucei is described, more or less inter- 
 mediate, like bairdii but with the abdomen banded as in oregonia. 
 W. H. Edwards records the results of rearing the offspring of the 
 bairdii-like and of the oregonia-like mothers. Each was found 
 able to have offspring of both kinds, that is to say, bairdii 
 females gave both forms, and oregonia females gave both forms. 
 It is not possible to say which is dominant, since the fathers were 
 unknown. On general grounds one may expect that the bairdii 
 form will be found to dominate, but this is quite doubtful. 
 
 From this particular discussion I omit reference to those 
 examples in which the permanently established types are ob- 
 viously associated with special conditions of life. Where con- 
 siderable climatic differences exist between localities, or when we 
 pass from South to North, or from the plains into Alpine levels 
 we often find that in correspondence with the change of climate 
 there is a change in the characteristics of a species common to 
 both. When I say "species" in such a connection I am obviously 
 using the term in the inclusive sense. Some would prefer to 
 say that in the two sets of conditions two representative species 
 exist. Whichever expression be preferred it is plain that such 
 examples present another phase of the problem we have been 
 just considering, and in them also we have an opportunity of 
 
 14 For the facts and further references see W. H. Edwards, Butterflies of N. 
 America, 2d series, Papilio VII and X; 3d series, 1897. Papilio IV, Can. Entom., 
 1895, XXVII, p. 239. 
 
OVERLAPPING FORMS 163 
 
 observing the consequences of the overlap of two closely related 
 types, but there are advantages in considering them separately. 
 In the examples hitherto given, with the possible exception of 
 the Papilios, 15 the two fixed types severally range over so ex- 
 tensive a region that it may fairly be supposed that in the dif- 
 ferent parts they are subject to considerable diversities of climate. 
 There is no outstanding difference that we know distinguishing 
 the habitats of the two forms; but in comparing Alpine with 
 Lowland forms, or essentially northern with essentially southern 
 forms we do know an external circumstance, temperature, that 
 may reasonably be supposed to have an influence, direct or indi- 
 rect, on the population. 
 
 15 I think this case is fairly included because the machaon type is so widespread 
 that it cannot be regarded as a product of a Northern climate, nor can asterias be 
 claimed as especially a warm country form, seeing that brevicauda, which is scarcely 
 distinguishable from asterias, inhabits Newfoundland (having a curious phase 
 there in which the yellow is largely replaced by red) . 
 
CHAPTER VIII 
 
 LOCALLY DIFFERENTIATED FORMS. Continued 
 
 Climatic Varieties 
 
 In this chapter we will examine certain cases which illustrate 
 phenomena comparable with those just considered, though as 
 I have already indicated, they form to some extent a special 
 group. The outstanding fact that emerges prominently from 
 the study of the local forms is that when two definite types, 
 nearly allied, and capable of interbreeding with production of 
 fertile offspring, meet together in the region where their dis- 
 tributions overlap, though intergrades are habitually found, 
 there is no normally or uniformly intermediate population oc- 
 cupying the area of intergradation. Such phenomena as these 
 must, I think, be admitted to have great weight in any attempt 
 to construct a theory of evolution. True we must hesitate in 
 asserting their positive significance, but I see no escape from the 
 conclusion that they throw grave doubt on conventional views. 
 Again and again the same question presents itself. If A and B 
 lately emerged from a common form why is that common form 
 so utterly lost that it does not even maintain itself in the region 
 of overlapping? Almost equally difficult is it, in the cases which 
 I have numerated, to apply concrete suggestions based on any 
 factorial scheme. We may see that in Heliconius erato the type 
 with the red mark on the hind wing probably contains a dominant 
 factor, and that where the red mark is absent the metallic colours 
 are exposed ; and that similarly the green metallic colour may have 
 another factor which distinguishes it from the blue. In this way 
 we can fairly easily represent the various types of erato on a 
 factorial system as the result of the various possible combinations 
 of two pairs of factors. But there we stop, and we are quite 
 unable to suggest any reason why one area should have the red 
 and the green type while another should have the blue also. So 
 again with Colaptes or the Warblers. By application of a fac- 
 
 164 
 
CLIMATIC VARIETIES 165 
 
 torial system, admittedly in a somewhat lax fashion, the genetic 
 interrelations of the types can be represented ; but how it comes 
 about that each type maintains a high degree of integrity in 
 its own region we can only imagine. Each has in actual fact 
 a stability which the intermediate forms have not, but we cannot 
 yet analyse the nature of that stability. Mendelian conceptions 
 show us how by segregation the integrity of the factors can be in 
 some degree maintained, but not why certain combinations of 
 factors should be exceptionally stable. All that is left us to 
 fall back on is the old unsatisfying suggestions that some com- 
 binations may have greater viability than others, that there 
 may be a tendency for like to mate with like, and so forth. 
 
 These difficulties acquire more than ordinary force in those 
 cases in which the two fixed types inhabit regions differing in 
 some respect so obvious and definite that we are compelled to 
 regard each type as climatic and as specially adapted to the 
 conditions. When for example an animal has a distinct type 
 never met with except in Arctic or Alpine conditions, and another 
 type proper to the plains and temperate regions, what are the 
 characteristics of the population of intermediate latitudes or 
 at intermediate levels? Some of the examples discussed in the 
 last chapter may be instances of this very nature, but even if 
 they are not, others are forthcoming which certainly are. The 
 evidence of these cases leads to the suspicion that with further 
 knowledge they will be found to consist of two classes, some in 
 which the observer as he passes from the one climate to the other 
 will find the intermediate area actually occupied by a population 
 of intermediate character, and others in which, though we may 
 presume the maintenance of intermediate conditions in the tran- 
 sitional area, there is no definite transitional population. This 
 interrupted or discontinuous distribution seems, so far as I 
 have means of judging, to be by far the more common of the two. 
 I do not doubt that by sufficient search individuals representing 
 every or almost every transitional form can be found, but it is 
 apparently rare that populations corresponding to these several 
 grades can be seen. The question has in few if any cases been 
 studied with precision sufficient to provide a positive answer; 
 
166 PROBLEMS OF GENETICS 
 
 but I suspect that real and complete continuity, in the sense 
 thus defined, will only be found where the character of the local 
 populations depends directly on the conditions of life, and shows 
 an immediate response to changes in them apart from that post- 
 poned response which we suppose to be achieved by selection. 
 Obviously the character must be one, like size for instance, 
 capable of sensibly complete gradation. 
 
 The only example I have met with of the phenomenon of 
 anything like a complete intergradation between local types 
 really distinct in kind is that provided by the butterfly Pararge 
 egeria. It is well known to entomologists that this insect exists 
 in two very different types, a northern one, the "Speckled Wood " 
 of England, in which the spots are a pale whitish yellow, and a 
 southern type having the full fulvous colour that we know as 
 characteristic of megaera, the "Gatekeeper." It appears that 
 Linnaeus gave the name egeria to the southern type, 1 and our 
 own is now called egerides. Broadly speaking, so far as Great 
 Britain, France, and the Spanish Peninsula are concerned, the 
 tawny-coloured egeria occupies Spain and western France up 
 to the latitude of Poitiers and the pale yellow egerides extends from 
 Scotland, where it has a scanty distribution, through southern 
 England, where in suitable localities it is common, and the north 
 of France to Paris. 2 The two types when placed side by side 
 are strikingly different from each other, and are an excellent 
 illustration of what is meant by climatic variation. The insect 
 is not a great traveller and probably scarcely ever wanders far 
 from its home. It should therefore be possible by collecting 
 from north to south to find out how the transition is effected, 
 whether suddenly or gradually. This at various times I have 
 endeavoured to do, but I am still without exact information as 
 to the population in certain critical areas. In addition to the 
 information derived from specimens which I have collected or 
 seen in the collections of others there is a good account of the 
 general distribution in Europe given by the Speyers, 3 who evi- 
 
 1 Often referred to by older writers as Meone, Esper's name. 
 
 2 There are also two distinct island forms, unlike the European, Xiphia of 
 Madeira, and a smaller variety, Xiphioides of Canary. See especially, Baker, 
 G. T., Trans. Ent. Soc. London, 1891, p. 292. 
 
 8 Speyer, Adolf, and August. Verbreitung der Schmetterlinge, 1858, I, p. 217. 
 
CLIMATIC VARIETIES 167 
 
 dently paid more attention to the subject than most lepidopterists 
 have done, and many more recent records. In particular 
 Oberthur 4 has published many details as to the distribution in 
 western France and I am especially indebted to Mr. H. Rowland- 
 Brown for a long series of notes as to the distribution in France 
 generally, and to Mr. H. E. Page and Dr. T. A. Chapman, Mr. 
 Oberthur, Prof. Arrigoni degli Oddi, Mr. H. Williams and other 
 correspondents, for showing me forms from many localities. The 
 butterfly is attached for the most part to woods of deciduous trees 
 and to country abounding in tall hedges or rough scrub. It is not 
 usually to be found in highly cultivated districts or in very dry 
 regions. Hence there is necessarily some want of continuity in 
 the distribution at the present time and I should think a mile or 
 two of arable land without big hedges would constitute a barrier 
 hardly ever passed. The larva feeds on several coarse grasses, 
 especially Dactylis glomerata. Barrett mentions also Triticum 
 repens. In this country the winter is usually passed in the larval 
 stage, but I have found that in captivity, at least, there is much 
 irregularity. The larvae feed whenever the weather is not very 
 cold and may pupate, but if sharp cold comes on when they are pu- 
 pating or nearly full-grown they often get killed unless protected. 
 
 Some writers speak of a difference between the early and 
 later broods, but I have never noticed this, and I do not think 
 that the general tone of the yellow is affected by the seasons 
 (see Tutt, Ent. Rec, IX, 1897, p. 37). 5 
 
 Beginning at the south of Spain the thoroughly fulvous type 
 egeria is common at Gibraltar in the Cork woods, at Granada, 
 and doubtless generally. Lederer is said to have found only 
 this type in Spain (Speyer), and though I have no precise in- 
 formation as to other places in the Peninsula north of Jaen I feel 
 tolerably sure that there is no change from south to north.* 
 
 4 Lepid. Comparee, fsc. Ill, p. 372. 
 
 6 Mr. Rowland-Brown has called my attention to a statement by Dr. Vaillantin 
 (Petites Nouv. Ent., II, 235) that in Indre-et-Cher the first brood is of the northern 
 type and the second of the southern. My experience is that in captivity these 
 distinctions do not occur, and I have true egeria as first brood from Vienne and as 
 the late brood from the Landes. I never collected in Indre-et-Cher. 
 
 6 1 have since seen true egeria from Ferrol in the extreme northwest, which was 
 in Mr. Tutt's collection. 
 
168 PROBLEMS OF GENETICS 
 
 Immediately north of the Pyrenees we still meet egeria exclusively, 
 and up to Poitiers at least there is no noticeable change. But 
 somewhere between Poitiers and the bottom of the Loire valley 
 at Tours, the genuine southern type comes to an end, and the 
 whole population begins at the Loire to be of an intermediate 
 type, easy to distinguish both from egeria and from egerides. As 
 to the exact condition of the species in the fifty miles separating 
 St. Savin on the Vienne from places on the Loire I have no ade- 
 quate information. I have only one small sample from there, 
 but it does contain insects both of the southern and intermediate 
 types taken on the same day, in a wood near Preuilly. Oberthlir 
 also states that at Nantes the true southern form exists in com- 
 pany with the northern. From this I infer that the southern 
 form extends up the coast further than it does inland, but I 
 imagine the representative spoken of as northern would be of 
 usual Brittany or intermediate type. 
 
 The Vienne river joins the Loire, so the true southern type 
 reaches over into the basin of the Loire. From the Loire (Tours, 
 Cormery) north to Calvados (Balleroy) only the intermediate 
 is found, so far as I know, and the same type extends over 
 Brittany. 7 In general, however, the woods near Paris have the 
 thoroughly northern type egerides, but at St. Germain-en-Laye 
 and at Etampes (Oberthlir) the population approaches the 
 intermediate type. 
 
 On the whole the intermediate type is certainly less homo- 
 geneous than either of the extremes, and females with the two 
 central spots either paler or more fulvous than the rest are not 
 uncommon, but I have never taken one on the Loire or in Brit- 
 tany which I should class with either of the extreme types. 
 
 Before speaking of the distribution in other parts of France 
 and in Europe generally I will briefly state the results of my breed- 
 ing experiments. The work was done many years ago before 
 we had the Mendelian clue, and it is greatly to be hoped that 
 some one will find opportunities of repeating it. Crossing the 
 English and the thoroughly southern type the families produced 
 
 7 Mr. G. Wheeler kindly showed me a series identical with this type, from 
 Guernsey, and others from near Laon. 
 
CLIMATIC VARIETIES 169 
 
 agree entirely with the intermediates of Brittany and the Loire. 
 Reciprocals are alike. Of F 2 I only succeeded in raising very 
 few and of those that I had (about 30) nearly all were intermediate 
 in character, though perhaps rather less uniform than Fi. One 
 family alone, containing only 4 specimens, had one egerides, 
 and three fulvous intermediates. As the case stands alone I 
 hesitate whether or not to suppose it due to some mistake. More- 
 over from Fi crossed back with the respective parental types I 
 had fairly long series, especially from Fi X the southern type, and 
 looking at these families I cannot see any clear evidence of segre- 
 gation. On the contrary, I think that though there are slight 
 irregularities, they would, taken as a whole, be classed as coming 
 between the intermediate type and the extreme form used as the 
 second parent. This at least is true when the second parent was 
 of the southern type. 
 
 On this evidence I have regarded the case as one in which 
 there is no good evidence of segregation and as conforming most 
 nearly with the conventional view of gradual transition in re- 
 sponse to climatic influences. Such influence must however be 
 indirect; for I reared five generations of the northern type in 
 England, and these, though they included several abnormal- 
 looking specimens in the last generation and then died out, did 
 not show any noticeable change from the fulvous colour of the 
 wild type. Merrifield 8 also found that heat applied to pupae 
 of the northern type produced no approach to the southern 
 type. 
 
 Looking at the facts now in the light of more experience it 
 seems to me just possible that the case may be one in which, as 
 in Nilson-Ehle's Wheats, the dominant differs from the re- 
 cessive in having two pairs of factors with similar effects. The 
 fulvous type for example may have two or more elements in 
 separate pairs which together produce the full effect, and the 
 intermediate may have one of these. If this were so, some 
 segregation should of course eventually be observable, but the 
 proportion of the various fulvous and fulvous-intermediate 
 individuals would be large, and the reappearance of actual repre- 
 
 8 £w/. Rec, V, 1894, p. 134- 
 
1 7 o PROBLEMS OF GENETICS 
 
 sentatives of the northern type might be rare. I admit that this 
 is a somewhat strained interpretation of the facts, and as yet it 
 is not entitled to serious consideration. Nevertheless I am led 
 to form some such expectation partly from the great difficulty 
 in the way of any other, partly from the evidence of the small 
 mixed sample found at Preuilly and partly from the statements 
 given by Oberthur. There are moreover other features in the 
 general distribution of the species which make it improbable 
 that the dependence on climate can after all be so close. Pub- 
 lished lists are unfortunately of little use in deciding which form 
 occurs at a particular place, because, since the name Meone has 
 ceased to be used for the southern form, there is no complete 
 unanimity among authors as to the application of the names 
 egeria and egerides, and unless more particulars are given, either 
 name may be used for either form. Besides this, difficulty arises 
 from the fact that the intermediate type is not generally dis- 
 tinguished at all, and English collectors finding it, may easily 
 record it as the southern type. From Staudinger's note on the 
 distribution, I gather that he, on the contrary, reckoned the 
 intermediate with the northern type, as do the Speyers also. 
 The late Mr. J. W. Tutt was careful to distinguish the three 
 forms and has left several useful records. Easy therefore as it 
 might seem to be to make out the distribution of such a familiar 
 insect in its various modifications, there are serious practical 
 difficulties, and until long series are brought together with this 
 special object in view many obscurities will remain. 
 
 With only the series from England, the west of France, and 
 Spain before one it would be easy to regard the successive series 
 of tones as a fair measure of climate ; the brighter the colour, the 
 hotter might one expect the locality to be. Such rough corre- 
 spondence is often to be observed in butterflies and birds. It 
 becomes impossible to take these simple views in the light of 
 more complete knowledge. Beginning with France the fulvous 
 egeria occupies the lower valley of the Rhone, probably from well 
 above Lyon, though I have no exact information respecting the 
 country above Avignon. According to Speyer it also takes the 
 department of Lozere. The same authority says that Puy-de- 
 
CLIMATIC VARIETIES 171 
 
 Dome has "egeria," meaning perhaps the intermediate form, 
 with the fulvous form much less commonly. Next comes the 
 curious fact that though the Lower Rhone (Avignon, Tarascon, 
 Nimes) has the true fulvous form, Hyeres, Cannes, Grasse, Nice, 
 Digne, and Alassio have the intermediate. Savoy has the inter- 
 mediate (Chambery) and even egerides perhaps, though in the 
 same latitude on the west of France there is nothing but the 
 fulvous type. At Chalseul and Besancon (Doubs) the ordinary 
 northern type is found. Switzerland generally, I believe, has the 
 northern type, but Staudinger gives egeria for Valais and the 
 intermediate occurs in Vaud. 9 The south side of the Alps has 
 probably colonies of the pale egerides, and of intermediates. 
 Orta, with a very hot summer, has the English type (Tutt, Ent. 
 Rec, XII, 1900, p. 328). Locarno has the intermediate (ibid., 
 XV, 1903, p. 321). North Italy in general and western Pied- 
 mont have the intermediate; but further south egeria begins, 
 at what region I do not know. Speyer gives on his own authority 
 the remarkable statement that at Florence both extremes occur, 
 but chiefly intermediates between the two. Mr. R. Verity 
 however kindly informs me that in his experience this is not so, 
 and that neither the real southern type nor the northern occur 
 there. Sardinia, Sicily, Crete all have the southern type. 
 Greece probably has various types. Staudinger (Hor. Ross., VII, 
 1870, p. 78) says intermediates resembling Nice types common 
 everywhere, but from "Greece" the British Museum has a series 
 that would pass for English specimens; and the same type occurs 
 near Constantinople. The island of Corfu has a pale inter- 
 mediate, distinct from egerides but approaching it. In Roumania 
 all three forms are recorded from various places : egeria in the 
 Dobrutscha; not quite typical (presumably an intermediate) 
 at Bukharest; intermediate in various mountainous localities 
 as well as in Macedonia and Dalmatia; but egerides in Azuga 
 at about 3,000 feet. 10 Hungary has the true egerides also. 
 (Cf . Caradja, Deut. Ent. Zt., IX, p. 58.) Mathew records the same 
 
 9 Mr. Wheeler has some pale but rather worn specimens from the Rhone Valley 
 at Vernayaz. 
 
 10 See Fleck, E., Die Macrolep. Rumaniens, Bui. Soc. Sciinte, VIII, 1899; 
 p. 720. 
 
1 7 2 PROBLEMS OF GENETICS 
 
 from Gallipoli (E. M. M., 1881, p. 95)- Staudinger does not 
 distinguish the intermediates from the northern, but he gives 
 "egerides" for Armenia and Fergana (Central Asia). As against 
 the mere proximity of a great mountain chain being the influence 
 which keeps the Riviera population intermediate may be 
 mentioned the fact that the northern foothills of the Pyrenees 
 have the pure southern type, and the climate of Cambo must 
 surely be far cooler than that of Nice. The exact locality of 
 the Greek specimens is not given, but there can be no part of 
 Greece which is not much hotter in summer than Brittany, or 
 Calvados, which have the intermediate, not the English type. 
 In face of these facts it can scarcely be maintained that 
 average temperature is the efficient cause of the particular tone 
 of colour which the butterfly shows in a given region. Never- 
 theless it is clear that climate counts for much in determining 
 the distribution. It is noticeable that though the pale egerides 
 can be established in a warm climate we never find egeria in cold 
 climates, and even the intermediate is not found in places that 
 have a hard winter. I suspect that the distribution of the 
 broods through the year and the condition of the animal at the 
 onset of hard frost are features which really determine whether 
 a strain can live in a particular place or not. Though the truth 
 of the suggestion cannot be tested by experiments in captivity, 
 which at once introduce disturbances, I incline to the idea that 
 egeria has not got the right periodicity for northern climates. 
 If it could arrange its life so that the population consisted either 
 of young larvae, or perhaps of thoroughly formed pupae 11 at 
 the onset of winter, it might, for any obvious reason to the 
 contrary, be able to live in England. It is irregularly "poly- 
 voltine," as the silk- worm breeders say, and as soon as a little 
 warmth encourages it, a new generation starts into being, which 
 if the frost comes at an untimely moment, is immediately 
 
 11 My experience agrees with that of Mr. H. Williams (Ent. Rec, VIII, 1896, 
 p. 181) that pupae, well-formed, can stand considerable frost; but I used to find 
 that half-grown larvae usually died if unprotected, and I believe that larvae which 
 attempted to pupate in warm autumn weather and then got caught by frosts, 
 always died. Small larvae which can creep into shelter at the bottom of the plants 
 survived, and I expect that in the north the winter is usually passed in that state 
 (see also Merrifield, F., Ent. Rec, VIII, 1896, p. 168, and Carpenter, J. H., ibid.). 
 
CLIMATIC VARIETIES 173 
 
 destroyed. Many species are continually throwing off indi- 
 viduals which feed up fast 12 and emerge at once if the temperature 
 permits, and I imagine a species of Satyrid wholly or largely 
 represented by such individuals could scarcely survive in a 
 country which had a hard winter. For such a climate some 
 definite periodicity in the appearance of the broods may well be 
 indispensable. But assuming that egeria is cut off from cold 
 climates for such a reason, there is nothing yet to connect these 
 habits with the fulvous colour, and until breeding can be carried 
 out on a satisfactory scale there is no more to be said. 
 
 From time to time records appear of individual specimens 
 more or less fulvous being caught in southern England, especially 
 in the New Forest. 13 It would be interesting to know what 
 offspring such individuals might produce. From the evidence 
 now given some notion both of the strength and the weakness 
 of the case considered as one of continuous climatic variation 
 can be formed. I know no other equally satisfactory. Whether 
 or not definite mixture of the intermediates with either of the 
 extremes will be proved to occur, the case differs materially from 
 those considered in the last chapter in the fact that at all events 
 there is no general overlapping of forms. In a species so little 
 given to wandering, overlapping could indeed scarcely be expected 
 to occur. It is this circumstance which makes the species 
 preeminently suitable as a subject for the study of climatic 
 influences, and I trust that entomologists with the right oppor- 
 tunities may be disposed to explore the facts further. 
 
 Just as many species, like egeria, have varieties which can 
 be regarded as adapted to northern and southern regions, so 
 there are also several which have lowland and Alpine forms quite 
 distinct from each other. Every such case presents an example 
 of the problem we have been considering. As the collector 
 passes from the plains to the Alpine region, how will he find the 
 
 12 Some most unlikely species do this. I once had a larva of Paruassius dclius, 
 found at about 5,500 feet, which emerged late in the autumn (in October I believe), 
 a season at which it must have perished in its own country. 
 
 13 See, for examples, Barrett, G. C, Lepidoptera of the Brit. Islands, I, 1893, 
 p 229; also Grover, W., Ent. Rec, IX, 1897, p. 314; Williams, H„ Proc. Ent.Soc, 
 1898, who reared several specimens from the New Forest which would pass for 
 Bretons, though the rest of the family were true egcrides. 
 
i 74 PROBLEMS OF GENETICS 
 
 transition from one form to the other effected? Does the low- 
 land form give place to the Alpine form suddenly, with a region 
 in which the two are mixed, or will he find a zone inhabited by 
 an intermediate population? I have spent a good deal of time 
 examining the facts in the case of Pieris napi and its Alpine 
 female variety bryoniae, and though there are many compli- 
 cations which still have to be cleared up, no doubt is possible 
 as to the main lines of the answer. If in any valley in the Alps 
 inhabited by both napi and bryoniae the collector catches every 
 specimen he can, beginning at the bottom and working up to 
 7,000 feet, he will at first get nothing but napi. At about 2,500 
 feet, he may catch an occasional bryoniae flying with the napi. 
 After 3,000 feet napi usually ceases, and only bryoniae are found. 
 As an exception a colony of napi may be met with at much 
 greater heights. I once found them in numbers at about 6,000 
 feet. 14 Not only were they free from any trace of modification 
 in the direction of bryoniae, but they were of the thoroughly 
 southern type of napi, being a late brood of that large and very 
 pale kind {meridionalis) almost destitute both of dark veining 
 above and of green veining below, which are common on the 
 shores of Lago Maggiore and in other hot southern localities. 
 Not far off at the same level were typical bryoniae in fair abund- 
 ance. Occasionally an intermediate may be met with. I have 
 taken a few, for example, at Macugnaga and at Fobello. These, 
 however, in my experience are rarities in the Alps. Fleck 15 
 gives notes on the distribution in Roumania which shows the 
 same state of things. The lowland form is not transformed 
 though found at great heights, and at Azuga (nearly 3,000 feet) 
 bryoniae occurs with only occasional "flavescens" viz., inter- 
 mediates of the second brood. 
 
 If this were all the evidence we should be satisfied that the 
 lowland and Alpine types keep practically distinct, overlapping 
 occasionally, but rarely interbreeding. The problem would 
 remain, how is the distinctness of the two types maintained in 
 the region of overlapping? Nowadays, I suppose, we should 
 
 14 Above the Tosa falls. 
 
 15 Bui. Soc. Sciinte, VIII, 1899, p. 691. 
 
CLIMATIC VARIETIES i 75 
 
 incline to answer this question by reference to segregation, and 
 perhaps by an appeal to selective mating. The suggestion that 
 segregation does take place is certainly true to some extent. 
 There are, however, difficulties in the way, and the whole subject 
 is one of great complexity. My own experiments were made in 
 pre-Mendelian times and were not arranged with the simplicity 
 which we now know to be essential. The results are neither 
 extensive enough nor clear enough to settle the many collateral 
 questions which have to be considered, and the work ought to 
 be done again. Nevertheless, some notes of the observations 
 may have a suggestive value. 
 
 When I began, I did not sufficiently appreciate that the 
 "napi" group, omitting the North American forms, and the 
 Asiatic representatives, has at least three chief types in western 
 Europe. The differences we have to deal with are manifested 
 by the females only, so in this account particulars as to the males 
 are omitted for the most part. These are (i) our own British 
 napi; (2) the form found in the south, from the Loire downwards, 
 and in the Italian Alps, which I think may be spoken of as 
 meridionalis; (3) bryoniae, which is a form clearly recognizable 
 in the female only, and is found only in the arctic regions and 
 in the Alps above 2,500 feet. The first two have several broods, 
 two, three, or more, according to opportunity, and the first 
 brood is different from the later ones. In napi the markings on 
 the upper surface are a dark grey but in meridionalis they are a 
 pale silvery grey and much less extensive. In the later broods 
 of napi there is much less general irroration of the veins, and the 
 spots stand out as more defined and blacker. These differences 
 vary greatly in degree of emphasis. In meridionalis the later 
 broods are entirely different from the first. Instead of having 
 silvery markings they have the ground colour quite white, with 
 the spots large and a full black. On the under side of the hind 
 wings the usual green veins are almost absent, and I have seen 
 individuals which could scarcely be distinguished from rapae. 
 To these later broods the term napaeae is sometimes applied, 
 but I here use meridionalis for the southern race in general as 
 applicable to all broods. 
 
i 7 6 PROBLEMS OF GENETICS 
 
 The female bryoniae is totally unlike the others. The ground 
 colour is a full yellow, and each nervure is thickly irrorated with 
 a brown pigment often spreading so far as to hide the ground 
 almost entirely in the forewings. The males corresponding with 
 these females are not certainly distinguishable from those of our 
 own napi. Both sexes have the green veining of the underside 
 of the hind wing fully developed, rather more than is usual in the 
 lowland races, but this is not really diagnostic of the variety. 
 The first serious difficulty arises in regard to the second brood 
 of bryoniae. It is stated that there is only one brood, 16 but I feel 
 fairly sure that a second brood is sometimes produced, and that 
 the females with a yellow ground and diminished irroration of the 
 veins, not very uncommon in the Italian Alps in July to August, 
 are generally representatives of it. Such insects would of course 
 be classed with bryoniae in collections. 
 
 My experiments began with eggs of true bryoniae females 
 caught at about 2,500 feet early in July. These emerged in 
 August-September as intermediates with yellow ground and 
 about half as much black on the upper surface as bryoniae. 
 They are exactly like the intermediates usually found in nature 
 and in the light of later experience I regard them as natural Fi 
 forms, and I think the mothers had been fertilised by napi males, 
 though I admit that in view of the rarity of natural intermediates 
 there is a difficulty in this suggestion. Three of these females 
 were mated with males raised from thorough meridionalis 
 females, and three families were produced. Two of them 
 showed distinct evidence of segregation, some being yellow and 
 some white with various intergrades, some being no blacker than 
 meridionalis and some ranging up to a dark intermediate type. 
 Part emerged in the same autumn; and part overwintered, emerg- 
 ing as the spring meridionalis or as the peculiar type which I after- 
 wards learnt to know as the spring Fi form. The distinctions 
 were fairly sharp between the several forms. But the offspring 
 of the third female gave a series practically continuous from 
 
 16 The fact that Weismann by heating pupae obtained only one autumn speci- 
 men seems to me to show rather that a second brood can be produced than that it 
 cannot, which is the inference usually drawn. 
 
CLIMATIC VARIETIES 177 
 
 meridionalis to the Fi type. The work of subsequent years 
 gave results similarly irregular which could only be described 
 adequately at great length. The outcome may however be 
 summed up in the statement that there is evidence that both 
 the yellow ground and the dark veining are due to factors, but 
 that there are several of these and that imperfect segregation 
 is not uncommon, producing various reduction-stages. The 
 yellow ground may be due to one factor, and the several shades 
 may be the result of irregularities in dominance, but the black 
 markings when fully developed cannot I think be the result of 
 less than three factors, one for the basal darkening, one for general 
 irroration, and one for the margins. Probably also the enlarge- 
 ment of the spots is produced by a fourth factor. 
 
 There was not, in my experience any great difficulty in getting 
 the various forms to pair in captivity. Some attempts were 
 made to see whether individuals of either type selected mates 
 of their own type in preference to those of the other, but the 
 results were inconclusive. There were some indications of such 
 a preference; though, from the impossibility of judging how much 
 of this may be due to other circumstances, I could not come to 
 a positive conclusion on the rather meagre evidence. 
 
 Recently Schima 17 has given a careful and detailed account 
 of all the forms found in Lower Austria which he enumerates 
 under 14 distinct varietal names. He gives full references to 
 previous accounts, especially to the beautiful plates lately pub- 
 lished by Roger Verity. 18 Examination of these and of my own 
 specimens strongly suggests that the several forms are due to 
 the recombination of the factors I have named. Among those 
 which I have bred are representatives of most if not all the types 
 enumerated by Schima in addition to other curious forms. For 
 example I have bryoniae markings on a ground practically white ; 
 the dark veins with spots almost obsolete; meridionalis on a 
 yellow ground; the intermediate amount of black on a white 
 ground, etc. The last-named may occur wild and I have one 
 from Macugnaga as well as one given me by Mr. F. Gayner from 
 Lulea (Lapmark). 
 
 17 Schima, K., Verh. Zool. bol. Ges. Wien, LX, 1910, p. 268. 
 
 18 Rhopalocera Palaearctica, Florence, 1905-11, especially PI. XXXII. 
 13 
 
178 PROBLEMS OF GENETICS 
 
 To obtain really exact knowledge of the number of factors 
 and their properties it would be necessary to repeat the work. 
 After the beginning, I made a mistake in using British napi 
 instead of meridionalis and the results were much confused 
 thereby. The contrast between meridionalis and the various 
 dark forms is much greater and classification of the types would 
 have been therefore easier. The British form is presumably 
 meridionalis plus the factor for the basal pigmentation. The 
 problem is greatly complicated by the differentiation of the 
 seasonal forms. The first point to be determined is whether 
 bryoniae is capable of producing a second brood when it is thor- 
 oughly pure-bred, and whether such a second brood is, as I 
 suspect, normally intermediate in character. 
 
 In the Alps generally there is no definitely intermediate 
 population; nor I believe, is any such population met with in 
 the north where the arctic bryoniae meets napi, but as to this 
 I have no precise information. One curious fact, however, must 
 be mentioned, namely that there is a population that can prob- 
 ably be so described with fairness established at Modling near 
 Vienna. This is not in any sense an Alpine locality, and does 
 not, as I am told, differ in any obvious way from the other sub- 
 urbs of Vienna. Dr. H. Przibram was so good as to send me a 
 set taken at this place, representing a second brood, and they 
 were decidedly heterogeneous, ranging from an intermediate 
 form such as bryoniae fertilised by napi usually produces, to a 
 light yellowish second-brood type with little dark pigment. 
 There are also two actual bryoniae. Whether true napi also 
 occur there I do not know, but I have no doubt they do. It 
 would be well worth while to investigate the Modling population 
 statistically, and to breed from the intermediates which might 
 not impossibly prove to be heterozygotes. There are also records 
 of such intermediates being occasionally found in some parts of 
 Ireland, in the north of Scotland, and in south Wales, 19 but I do 
 not know of any regular colony of these forms. W T e can scarcely 
 avoid the inference that one or more of the factors which make 
 up bryoniae may be carried by these intermediates. It is not 
 
 19 See figures in Barrett, G. C Lepidoptera of Brit. Islands, I, pt. 3. P- 25. 
 
CLIMATIC VARIETIES 179 
 
 clear why their interbreeding does not produce actual bryoniae 
 occasionally. If this occurred, the probability is that the fact 
 would be known to collectors, at least in the British localities. 
 The absence of true bryoniae must, I think, be taken to mean 
 that some essential factor is absent from these intermediates. 
 
 To sum up the evidence, the facts that are clear may be thus 
 enumerated : 
 
 1. Napi and bryoniae, or In the Italian Alps, meridionalis and 
 bryoniae frequently meet each other. 
 
 2. They cross without difficulty, producing fertile offspring. 
 
 3. But in the levels at which they overlap there is no inter- 
 mediate population, and only occasional intermediate individuals. 
 
 4. In certain parts of the distribution of napi similar inter- 
 mediates sometimes occur, and at one place (Modling) they are 
 so frequent as apparently to constitute a colony. 
 
 5. As to the genetic relations of the two forms there is no 
 complete certainty. Indications of segregation have been ob- 
 served in some cases, but there are several factors concerned and 
 they are liable to some disintegration. 
 
 Another form in which I tried to investigate the same problem 
 is Coenonympha arcania, which has one Alpine form known as 
 darwiniana, and another, satyrion. In calling satyrion a form of 
 arcania I follow Staudinger and other authorities, but I have 
 never been quite satisfied that it should be so regarded. The 
 differences between arcania and darwiniana are essentially dif- 
 ferences of degree; C. arcania occurs in places where there is 
 cover, and reaches up the valleys usually as high as the mixed 
 woods of deciduous trees, which is about 2,500 feet. The variety 
 darwiniana, on the contrary, is an insect of treeless hillsides, and 
 I regard it as a dwarf and possibly a stunted form. It would not 
 greatly surprise me to find that with the application of good 
 conditions arcania could be raised from darwiniana eggs, or that 
 if arcania larvae were starved they might give rise to darwiniana 
 butterflies. I have been unsuccessful in trying to rear the species, 
 having lost the larvae by disease. Usually one does not catch 
 arcania and darwiniana on the same ground, and as Festuca ovina 
 — a typically hill-side grass — is a common food-plant of dar- 
 
i8o PROBLEMS OF GENETICS 
 
 winiana there can be little doubt that arcania feeds on some other 
 grass, probably woodland species. Colonies of arcania of varying 
 size and brightness are commonly found, and though a sample of 
 arcania, finely grown, from a warm Italian wood, presents a 
 striking contrast with darwiniana from an Alpine pasture, one 
 certainly may get samples which fill all the gradations. Gen- 
 erally the sample from a given locality is fairly homogeneous. 
 
 Of satyrion I have little personal experience. I only twice 
 found it, namely at Zinal, and at Hallstatt in Austria, but it 
 occurs at Zermatt, Arolla, and in several Swiss localities above 
 5,000 feet, and I understand that it is the typical Alpine form in 
 the Engadine. With its darkened colour and reduced size it 
 might well be expected to be a still further stunted form of 
 darwiniana. Yet I have never found the one succeed to the 
 other at the higher levels. If darwiniana appears when Alpine 
 conditions are reached in a valley it will be met with up to the 
 highest level at which such butterflies live. Tutt was of opinion 
 that satyrion is a distinct species. 20 I once, at the top of the Vor- 
 derrheinthal caught a sample of darwiniana a few of which (males) 
 were so dark and had the eye spots so poorly developed that they 
 looked like transitions to satyrion. Otherwise I never found 
 any such transitional forms and they are certainly exceptional. 
 There is further a record 21 of satyrion having been taken flying 
 with arcania. This was near Susa, at about 2,000 feet I infer. 
 Mr. H. E. Page has similar specimens from Caud and from St. 
 Anton (Arlberg). The females, however, both of mine and of 
 Mr. Page's samples are a pale brown, quite unlike the females 
 both of arcania and of the dark Zinal satyrion. The difficulty 
 thus raised has not I think yet been considered by the authorities, 
 and it is possible that the Alpine forms of arcania are in reality 
 three, not two. 
 
 The evidence taken together suggests, I think, that darwin- 
 iana is related to arcania much as so many of the Alpine varieties 
 
 20 Tutt, J. W., Ent. Rec, XVIII, 1905, p. 5- In the same place he states that 
 on the Mendel Pass arcania "runs into" darwiniana and that in the Tyrolean 
 localities the transition is especially evident. Wheeler (ibid., XIII, 1901, p. 121) 
 expresses the contrary opinion, that satyrion does grade to arcania. 
 
 21 H. Rowland-Brown, Ent. Rec, XI, 1899, p. 293. 
 
CLIMATIC VARIETIES 181 
 
 of plants are to the well-developed individuals of the lower 
 levels. I do not anticipate that factorial differences will be 
 found in these insects, and it is by no means impossible that the 
 distinctions between them are the direct consequences of altered 
 conditions. The relations of arcania to satyrion are more doubt- 
 ful, and in that case a factorial difference may at least be sus- 
 pected. 
 
 The species of the genus Setina have Alpine forms which 
 agree in possessing a characteristic extension of the black pigment 
 to form radiating junctions between the spots on the wings. 
 Speyer, who discussed the interrelations of these forms in detail, 22 
 lays stress on the absence of genuine transitional forms between 
 aurita and the variety ramosa. Both are mountain insects but 
 ramosa extends to levels higher than that at which aurita ceases, 
 which is about 4,000 feet. The two forms are often found flying 
 together. Speyer says that his brother searched diligently for 
 transitional forms at the level of overlapping, but found none, 
 so that at least they may be regarded as rare. The variety 
 ramosa is not infrequent at much lower levels (e. g., Chiavenna, 
 1,020 feet; Reussthal, 1,500 feet) and extends as high as the 
 permanent snows. In the British Museum collection, however* 
 I have seen several that I should regard as transitional. Speyer 
 perhaps would have classed as ramosa all in which the spots of 
 the central field were united, and it is by no means unlikely that 
 breeding would prove such individuals to be heterozygous. 23 
 
 22 Speyer, Stettiner Ent. Ztg., XXXI, 1870, p. 63. 
 
 23 In regard to the closely analogous case of Spilosoma lubricipeda, Standfuss 
 makes a similar statement. He bred the type on a large scale with the radiate form 
 which he calls intermedia, and says that in four years of miscellaneous crossing he 
 never obtained really transitional forms. Nevertheless after examining large series, 
 especially those of Mr. W. H. B. Fletcher, I came to the conclusion that several 
 might be so classed, but I am quite prepared to find that such specimens are hetero- 
 zygous. (See Standfuss, Handb. d. Gross-Schmet., 1896, p. 307.) It is by no means 
 unlikely that various dark forms of lubricipeda correspond with a progressive series 
 of factorial additions. Many of the stages have been named, and of these the most 
 definite are the intermedia of Standfuss (probably = eboraci of Tugwell) and the 
 very dark Zatima of Heligoland, in which only the thorax, the nervures and a small 
 field in the forewings remain yellow. A form was bred by Deschange from Zatima 
 in which even the field in the forewing is obliterated. The exact circumstances in 
 which Zatima occurs in Heligoland would be worthy of special investigation, for the 
 
1 82 PROBLEMS OF GENETICS 
 
 There can scarcely be a doubt that the distinction between 
 aurita and ramosa is factorial, the radiate ramosa probably having 
 the factor for striping. In support of this view may be men- 
 tioned the observation of Boisduval, 24 respecting a gynandro- 
 morphous individual, which was aurita male on one side, and 
 ramosa female on the other. Speyer makes another excellent 
 comment. He points out that the simple notion that the radi- 
 ation is a mere extension of pigmentation consequent on the 
 climate of the higher levels, will not fit the facts very easily, 
 because the size of the spots varies greatly in aurita itself at any 
 level, and lowland specimens may actually have more black 
 confined to the spots alone than some ramosa possess on spots and 
 lines combined. 25 
 
 The two Salamanders, S. maculosa and its Alpine form atra, 
 might not improbably furnish evidence bearing on the same 
 problem. The two are of course very distinct, not merely in 
 colour (maculosa being spotted with yellow or orange while atra 
 is entirely black) but also in the mode of reproduction, a feature 
 to which reference will be made in the next chapter. I cannot, 
 however, find any evidence as to the overlapping of the two forms. 
 S. atra occurs from about 3,000 feet or somewhat less, and reaches 
 great elevations in the Eastern Alps, but I do not know if the 
 two forms ever occur in the same localities. Leydig, 26 Boulenger, 27 
 and most modern authorities regard the two types as distinct 
 species, but they are in any case closely allied, and it would be of 
 ^interest to have exact knowledge of their geographical delimi- 
 tations. 
 
 The reader who has considered the cases adduced will ap- 
 preciate the difficulties which must be faced in any attempt to 
 
 normal lubricipeda in also found on the island. For references as to the British 
 occurrences see especially, Hewett, W., Naturalist, 1894, P- 353- As to Zatima see 
 especially Krancher, Soc. Ent., II, 1887-8, p. 26. I am indebted to Dr. Hartlaub 
 for information as to the Heligoland types. 
 
 24 Boisduval, Bull. Soc. Ent. Fr., Ill, 1834, p. 5- 
 
 25 The systematics of Set ina have been much controverted, but no one I believe 
 doubts that aurita and ramosa are forms of one species. See also Chapman, A. T.» 
 Ent. Rec, XIII, 1901, p. 139. 
 
 26 Arch. Naturg., 33, 1867, p. 116. 
 
 & Brit. Mus. Cat., Batrachia Gradientia, 1882. 
 
CLIMATIC VARIETIES 183 
 
 account for the facts in a rational way. As always in a problem 
 of Evolution, two separate questions have to be answered. 
 First how did the form under consideration come into existence, 
 and secondly, how did it succeed in maintaining itself so as to 
 become a race? The evidence from the local forms, though very 
 far from giving complete answers to either of these questions 
 definitely refutes the popular notion that a new race comes into 
 existence by transformation of an older race. If a gradual mass- 
 transformation of this kind took place we should certainly expect 
 that when two types, nearly allied and capable of interbreeding, 
 overlap each other in their geographical distribution, a normally 
 intermediate population would exist. If each type can main- 
 tain itself, and if each came into existence by gradual transfor- 
 mation, then there must have been an intermediate capable of 
 existing and maintaining itself as a population; and if this had 
 ever been, surely in the region of overlapping, that intermediate 
 population should continue. Especially should such a population 
 be found when the two extreme types are adaptational forms and 
 the region of overlap is a region of intermediate conditions. 
 But of the examples we have examined there is only one, that of 
 Pararge egeria and egerides, which can at all be so interpreted, 
 and even in that case it is not impossible that more minute ob- 
 servation would reveal discontinuity between the extremes 
 and the admittedly normal intermediate population. Granting 
 provisionally however that this example, as it stands, is con- 
 sistent with the conventional theory of evolution, I know not 
 where we should look for another case equally good. When the 
 distinctions are produced by direct influence of conditions oper- 
 ating during the lifetime of the individuals, examples of inter- 
 mediate populations occupying the areas of intermediate con- 
 ditions can no doubt be produced. Many turf-like Alpine 
 plants, for instance, if protected from exposure and properly 
 nourished can grow as large as those of the same species found in 
 the valleys, and in the case of such quantitative effects, inter- 
 mediate conditions can doubtless produce intermediate characters. 
 Even these examples however are not very abundant, and 
 often the intermediate locality has not a form intermediate 
 
1 84 PROBLEMS OF GENETICS 
 
 between those of the two extreme localities, but some third 
 form distinct from either. This is the case for instance in the 
 fauna of brackish waters. We are taught to believe that the 
 fresh water fauna was evolved from the marine fauna, which 
 it well may have been; but as students of Crustacea and Mollusca 
 know familiarly, the brackish water forms are not as a rule inter- 
 mediates between fresh water species and sea species, but more 
 usually they are special forms belonging to the brackish waters, 
 with the peculiar property that they can tolerate a great range of 
 conditions, and live without ostensible variation in waters of 
 most various compositions and densities, which very few marine 
 or fresh water species are able to do. 
 
 Sometimes the distinction between local races, as in Rham- 
 phocoelus passerinii and icteronotus may be regarded with con- 
 fidence as due to one simple Mendelian factor possessed by one 
 race and absent from the other, but I think, more often, as in 
 Colaptes or in the varieties of Pieris napi, the existence of several 
 distinct factors is to be inferred. As we have seen, the races 
 of Colaptes show almost beyond doubt that in different areas at 
 least three distinct factorial combinations can be perpetuated 
 as races. 
 
 In the distribution of variability we find, I think, some hint 
 as to the steps by which the phenomena under consideration 
 have come to their present stage, and I am disposed to regard 
 the facts so well attested in the case of our own melanic moths 
 as a true indication of the process. Following this indication 
 we should regard the change in the character of a population 
 as beginning sporadically, by the appearance of varying indi- 
 viduals, possibly only one varying individual, in, it may be, one 
 place only. As to why a variety should increase in numbers we 
 have nothing but mere speculation to offer, and for the present 
 we must simply recognise the fact that it may. That such sur- 
 vival and replacement may reasonably be taken as an indication 
 that the replacing race has some superior power of holding its 
 own I am quite disposed to admit. Nevertheless it seems in 
 the highest degree unlikely that the outward and perceptible 
 character or characters which we recognise as differentiating the 
 
CLIMATIC VARIETIES 185 
 
 race should be the actual features which contribute effectively 
 to that result. 
 
 In discussions of geographical distribution in relation to 
 problems of origin it is generally said that very nearly allied 
 species usually occupy distinct areas, while other competent 
 observers state the exact contrary. Lately, for example, Dr. 
 R. G. Leavitt 28 has published an important collection of evidence 
 upholding the latter proposition, taken chiefly from the botanical 
 side, showing how in numerous genera two or more closely allied 
 species coexist, frequently without intermediates, in the same 
 localities, and may even be thus found in company throughout 
 their distribution. The difference of opinion evidently arises 
 from a confusion as to the sense in which the term "species" 
 is understood and applied. Leavitt, for example, is avowedly 
 following Jordan and, among moderns, Sargent, in applying 
 a close analysis, and denoting as species all forms which are 
 distinct and breed true. Against this use of the term I know 
 no valid objection 29 but it must be obvious that if others follow 
 a different practice confusion may result when observations are 
 summarised in general statements. We will consider this subject 
 again in another place, but here it may be sufficient to say that 
 there can scarcely now be a doubt that numbers of these associ- 
 ated species, such as Jordan discriminated, represent various 
 combinations of the presence and absence of Mendelian factors. 
 This does not in any way weaken the argument which Leavitt 
 founds upon the facts, namely, that the observed distribution 
 of these forms is consistent with the supposition of an evolution 
 largely discontinuous. 
 
 On the other hand, those who have come to the opinion that 
 nearly allied species generally occupy distinct ground are pre- 
 sumably more impressed by the characters differentiating the 
 geographically distinct or adaptational races, seeing that genuine 
 intermediates between them are less commonly found. Those 
 geographical races may no doubt contain various differentiated 
 forms; but when all live together, occasional intermediates are 
 
 28 The Geographical Distribution of nearly related Species. Amer. Nat., XL I . 
 1907, p. 207. 
 
 29 See later, p. 242. 
 
186 PROBLEMS OF GENETICS 
 
 usually to be found even in the case of characters habitually 
 segregating. These segregating forms Jordan would certainly 
 have determined as species, and it must be conceded that no 
 physiological definition has yet been drawn which consistently 
 excludes them. 
 
CHAPTER IX 
 
 THE EFFECTS OF CHANGED CONDITIONS: ADAPTATION 
 
 In the attempt to conceive a process by which Evolution 
 may have come about, the first phenomenon to be recognized 
 and accounted for is specific difference. With that recognition 
 the outline of the problem is defined. The second prerogative 
 fact is adaptation. Forms of life are on the whole divided into 
 species, and these species on the whole are adapted and fit the 
 places in which they live. To many students of Evolution, 
 adaptation has proved so much more interesting and impressive 
 than specific diversity that they have preferred it to the first 
 place in their considerations. 
 
 Whether this is, as I believe, an inversion of the logical order 
 or not, there is one most serious practical objection to such 
 preference, that whereas specific diversity is a subject which 
 can be investigated both by the study of variation and by the 
 analytical apparatus which modern genetic science has developed, 
 we have no very effectual means of directly attacking the problems 
 of Adaptation. 
 
 The absence of any definite progress in genetics in the last 
 century was in great measure due to the exclusive prominence 
 given to the problem of Adaptation. Almost all debates on 
 heredity centered in that part of the subject. No one disputes 
 that the adaptation of organisms to their surroundings is one 
 of the great problems of nature, but it is not the primary problem 
 of descent. Moreover, until the normal and undisturbed course 
 of descent under uniform conditions is ascertained with some 
 exactness, it is useless to attempt a survey of the consequences 
 of external interference; nor as a rule can it be even possible to 
 decide with much confidence whether such interferences have or 
 have not definite consequences. Those, for example, who de- 
 bated with enthusiasm whether acquired characters are or are 
 not transmitted were constantly engaged in discussing occur- 
 
 187 
 
1 88 PROBLEMS OF GENETICS 
 
 rences which we now know to be ordinary features of descent 
 under uniform conditions, and the origin of variations which 
 were certainly not caused directly by circumstances at all. In 
 the absence of any factorial analysis, or of any conception of what 
 factorial composition means and implies, no one knew what 
 varieties might be expected from given parents. The appearance 
 of any recessive variety was claimed as a consequence of some 
 treatment which might have been applied to the parents. There 
 was no possible standard of evidence or means of controlling it, 
 and thus the discussion was singularly unfruitful. Before we can 
 tell how the course of descent has departed from the normal, we 
 must know what the normal would have been if we had let alone. 
 We are still far from having such knowledge in adequate measure, 
 but it does now exist in some degree, and we are steadily approach- 
 ing a position from which we shall be able to form fairly sound 
 estimates of the true significance of evidence for or against the 
 proposition that environmental treatment can produce positive 
 disturbances in the physiological course of descent. 
 
 Thus described, the field for consideration is very wide. 
 Though the effects of changed conditions were especially studied 
 in the hope of solving the problem of adaptation by direct ob- 
 servation, that, as all are now agreed, is but a part of a more 
 general question. We must ask not only do changed conditions 
 produce an adaptative response on the part of the offspring, but 
 whether they produce any response on the part of the offspring 
 at all. It is not in doubt that by violent means, such as starvation 
 or poisoning of the reproductive cells, effects of a kind, stunting 
 and deformity for instance, can be made evident, just as similar 
 effects may follow similar treatment during embryonic or larval 
 life. Apart from interferences of this class, are there any that 
 may be reasonably invoked as modifying the course of inher- 
 itance? 
 
 No epitome of the older evidence for the inheritance of 
 adaptative changes is here required. That has often been col- 
 lected, especially by Weismann, who exposed its weaknesses so 
 thoroughly as to carry conviction to most minds, and showed 
 that whether the phenomenon occurs or not, no one can yet prove 
 
ADAPTATION 189 
 
 that it does. Belief in these transmissions, after being almost 
 universally held, was with singular unanimity abandoned. This 
 change in opinion, though doing credit to the faith of the scientific 
 community in evidential reasoning, is the more remarkable 
 inasmuch as the strength of the belief was not derived from the 
 minute amounts of supposed facts now demolished. On the 
 contrary, it was really an instinctive deduction from a wide 
 superficial acquaintance with the properties of animals and 
 plants. They can accommodate themselves to circumstances. 
 They do make responses sometimes marvellously appropriate 
 to demands for which they can scarcely have been prepared. 
 What more natural than to suppose that the permanent adapta- 
 tions have been achieved by inherited summation of such re- 
 sponses? No one had actually been driven to believe in the 
 inheritance of adaptative changes because bitches which had 
 been docked had been known to give birth to tailless puppies, 
 or because certain wheat in Norway was alleged to have become 
 acclimatized in a few generations. Evidence of this kind was 
 collected and produced rather as an ornamental appendix to a 
 proposition already accepted and held to be plainly demon- 
 strated by the facts of nature. Looked at indeed in that pre- 
 liminary and uncritical way, the case is simply overwhelming. 
 Those who desire to see how strong it is should turn to Samuel 
 Butler's Life and Habit, and even if in reading they reiterate to 
 themselves that no experimental evidence exists in support of 
 the propositions advanced, the misgiving that none the less they 
 may be true is likely to remain. Making every deduction for 
 the fact that the wonders of adaptation have been grossly ex- 
 aggerated, and that marvels of fitness and correspondence be- 
 tween means and ends have grown out of mere anthropomorphic 
 speculations, there is much more left to be accounted for than 
 can at all comfortably be accepted as the product of happy 
 accidents. So oppressive are these difficulties that we can scarcely 
 blame those who imagine that the study of heredity is primarily 
 directed to the problem of the transmission of acquired characters, 
 a preconception still almost universal among the laity. 
 
 But since the belief in transmission of acquired adaptations 
 
190 PROBLEMS OF GENETICS 
 
 arose from preconception rather than from evidence, it is worth 
 observing that, rightly considered, the probability should surely 
 be the other way. For the adaptations relate to every variety 
 of exigency. To supply themselves with food, to find it, to seize 
 and digest it, to protect themselves from predatory enemies 
 whether by offence or defence, to counter-balance the changes 
 of temperature, or pressure, to provide for mechanical strains, 
 to obtain immunity from poison and from invading organisms, 
 to bring the sexual elements into contact, to ensure the dis- 
 tribution of the type; all these and many more are accomplished 
 by organisms in a thousand most diverse and alternative methods. 
 Those are the things that are hard to imagine as produced by 
 any concatenation of natural events; but the suggestions that 
 organisms had had from the beginning innate in them a power 
 of modifying themselves, their organs and their instincts so as 
 to meet these multifarious requirements does not materially 
 differ from the more overt appeals to supernatural intervention. 
 The conception, originally introduced by Hering and inde- 
 pendently by S. Butler, that adaptation is a consequence or 
 product of accumulated memory was of late revived by Semon 
 and has been received with some approval, especially by F. 
 Darwin. I see nothing fantastic in the notion that memory 
 may be unconsciously preserved with the same continuity that 
 the protoplasmic basis of life possesses. That idea, though 
 purely speculative and, as yet, incapable of proof or disproof 
 contains nothing which our experience of matter or of life at all 
 refutes. On the contrary, we probably do well to retain the 
 suggestion as a clue that may some day be of service. But if 
 adaptation is to be the product of these accumulated experiences, 
 they must in some way be translated into terms of physiological and 
 structural change, a process frankly inconceivable. 
 
 To attempt any representation of heredity as a product of 
 memory is, moreover, to substitute the more obscure for the 
 less. Both are now inscrutable; but while we may not unreason- 
 ably aspire to analyse heredity into simpler components by or- 
 dinary methods of research, the case of memory is altogether 
 different. Memory is a mystery as deep as any that even psy- 
 
ADAPTATION 191 
 
 chology can propound. Philosophers might perhaps encourage 
 themselves to attack the problem of the nature of memory by 
 reflecting that after all the process may in some of its aspects 
 be comparable with that of inheritance, but the student of gen- 
 etics, as long as he can keep in close touch with a profitable basis 
 of material fact, will scarcely be tempted to look for inspiration 
 in psychical analogies. 
 
 For a summary of the recent evidence I may refer the reader 
 to Semon's paper 1 where he will find a collection of these obser- 
 vations described from the standpoint of a convinced believer. 
 At the outset one cannot help being struck by the fact that of 
 the instances alleged, very few, even if authentic, show the trans- 
 mission of acquired modifications which can in any sense be re- 
 garded as adaptative, and many are examples not so much of a 
 transmission of characters produced in the parents as of variation 
 induced in the offspring as a consequence of treatment to which 
 the parents were submitted, the parents themselves remaining 
 apparently unmodified. No one questions the great importance 
 of evidence of this latter class as touching the problem of the 
 causes of variation, but it is not obvious why it is introduced in 
 support of the thesis that acquired characters are inherited. 
 
 It is most difficult to form a clear judgment of the value of 
 the evidence as a whole. To doubt the validity of testimony 
 put forward by reputable authors is to incur a charge of obstinacy 
 or caprice; nevertheless in matters of this kind, where the alleged 
 phenomena are, if genuine, of such exceptional significance, belief 
 should only be extended to evidence after every possible source 
 of doubt has been excluded. We believe such things when we 
 must, but not before. At the very least we are entitled to require 
 that confirmatory evidence should be forthcoming from inde- 
 pendent witnesses. So far as I have seen, this requirement is 
 satisfied in scarcely any of the examples that have been lately 
 published, and until it is, judgment may reasonably be suspended. 
 In some cases, however, the facts are not doubtful. Stand- 
 fuss, by subjecting pupae of Vanessa urticae to cold, produced 
 
 1 Semon, R., Der Stand der Frage nachder Vererbung envorbener Eigenschaften, 
 published in Fortschr. der natarw. Forschung., Bd. n, 1910. 
 
192 PROBLEMS OF GENETICS 
 
 the now well-known temperature-aberrations in which the dark 
 pigment is greatly extended. He put together in a breeding-cage 
 2,2 males and io females showing this modification in various 
 degrees. Two of these females died without leaving young. 
 Seven produced exclusively normal offspring. From the eighth 
 female 43 butterflies were bred, and of these there were four (all 
 males) which to a greater or less extent exhibited the aberrational 
 form. 2 The mother of this family was the most abnormal of the 
 10 females originally put in. 
 
 Fischer's experiment with Arctia caja was on similar lines. 
 From pupae which had been frozen almost all the moths which 
 emerged showed aberrational markings. A pair of these mated 
 and produced 173 young which pupated. Those which emerged 
 early were all normal, but of those which emerged late, 17 had 
 in various degrees abnormal markings like those of the parents. 3 
 In neither of these examples is there any question as to the facts. 
 Both observers have great experience and give full details of their 
 work. 
 
 As regards Vanessa urticae, however, it must be recalled that 
 Fischer himself showed that in Nymphalids somewhat similar 
 aberrations could be produced both by heat and by cold, and 
 even by centrifuging the pupae. Frl. von Linden produced a 
 transitional form of the same aberration in V. urticae by the 
 action of carbonic acid gas. 4 It is highly probable that the ap- 
 pearance is due to a morbid change, perhaps an arrest of develop- 
 ment, which may be brought about by a great diversity of causes. 
 In the experiments the cause probably was a diseased condition 
 of the tissues of the mother herself. She had been subjected to 
 freezing sufficiently severe to prevent the proper development of 
 the pigments and some of the ovarian cells presumably suffered 
 also. It will be observed that the only specimens which were 
 affected were the offspring of the most abnormal female, and of 
 them only four out of forty- three showed any change. 
 
 The same interpretation probably applies to the cases in 
 
 2 Standfuss, M., Denks. Schweiz. naturf. Ges., XXXVI, 1898, p. 32. 
 8 Fischer, E., Allg. Ztschr. f. Entomologie, Bd. VI, 1901. 
 
 4 Out of 12 pupae treated 8 died and of the 4 survivors, one only was affected. 
 See M. v. Linden, Archiv. Rassen. u. Gesells., 1904, I. 
 
ADAPTATION 193 
 
 Arctic caja. In this species the markings are well known to 
 be liable to great variation. As Barrett says, even in nature 
 individuals are rarely quite alike, and an immense number of 
 strange forms occur in collections. 5 These are greatly sought 
 after by some collectors, especially in England, where they fetch 
 high prices at auctions, and it is notorious that most of them come 
 from Lancashire and the West Riding of Yorkshire. It is com- 
 monly supposed that the breeders of that district subject them to 
 abnormal conditions, and especially to unnatural feeding, but 
 I know no clear evidence that this is true. From whatever cause 
 it is certain that the natural pattern is, in some strains at all 
 events, very easily disturbed. 
 
 The elaborate experiments of Schroder with Abraxas grossu- 
 lariata are difficult to follow and are complicated by the fact 
 that the series which was submitted to abnormal temperatures 
 was derived from an abnormal original pair. From the evidence 
 given it is not clear to me whether the temperature had a distinct 
 effect. This insect, like Arctia caja, produces an immense number 
 of variations (especially in the amount of the black pigment) 
 and as most of these are, I believe, reared in domestication for 
 sale, it is highly probable that the species is easily influenced 
 by cultural conditions. 
 
 Schroder describes two other experiments which have been 
 accepted by Semon and other supporters of the view that acquired 
 characters are transmitted. In the first, Phratora vitellinae, a 
 phytophagous beetle living on the undersides of leaves, was used. 
 It naturally feeds on Salix fragilis, a species without a felt, or 
 tomentum, on the underside of the leaves. Larvae were trans- 
 ferred to another willow (near 5. viminalis) which has the under- 
 sides of the leaves felted. The larvae took readily to the new 
 food, pushing the tomentum before them as they gnawed the 
 leaves. They came to maturity and when they were about to 
 lay their eggs they were given a free choice between 5. fragilis 
 and the tomentose species. The greater number of ovipositions, 
 
 6 For illustrations see Oberthur's Etudes d'Enlom., 1896, where many of these 
 curious aberrations are represented; also Barrett, Lepid. Brit. Islands, II, pp. 71 
 and 72. 
 
 14 
 
r 9 4 PROBLEMS OF GENETICS 
 
 £19, took place on fragilis, and there were 127 on the tornentose 
 bush, which we are told was six times as large as the fragilis. 
 The larvae from fragilis were next put on the tornentose species 
 and reared on it. When they became imagines they were simi- 
 larly given their choice, with the result that there were 104 
 ovipositions on the tornentose species and only 83 on fragilis. 
 In the next generations there were 48 ovipositions on the tornen- 
 tose and 11 on fragilis. Finally the fourth generation made 
 15 ovipositions on the tornentose and none on fragilis. 
 
 The difficulty about such experiments is obviously that one 
 has no assurance that the change of instinct, in so far as there 
 is any, may not be a mere consequence of the captivity. It 
 must, besides, be extremely difficult to arrange the experiment so 
 that there is really an equal choice between the two bushes, when 
 one stands beside the other. Przibram, in quoting this case, 
 considers that as the tornentose bush was about six times as 
 large as the fragilis, some indication of the relative attractiveness 
 of the two may be obtained by dividing the ovipositions on the 
 larger bush by six, but I imagine the matter must be much more 
 complex. 
 
 Schroder's second example is not more convincing, in my 
 opinion, though Semon regards it as one of the most important 
 pieces of evidence. It concerns a leaf-rolling moth, Gracilaria 
 stigmatella, the larva of which is said normally to make its house 
 by bending over the tips of the sallow leaves on which it feeds. 
 Schroder placed larvae on leaves from which the tips had been 
 cut, and these larvae made their houses by rolling over the sides 
 of the leaves. Their offspring were again fed on leaves without 
 tips, and as before, they rolled in the leaf-margins either on one 
 side or both. The offspring of this second generation were then 
 fed on entire leaves. There were 19 houses made by these (?I9) 
 larvae, and of them 15 were normal, made by folding down the 
 tips of the leaves, while 4 were abnormal, made by rolling in the 
 leaf-margins. Schroder says that in nature he has only twice 
 seen abnormal houses; but it is clearly essential not only that the 
 frequency of such variability in nature should be thoroughly 
 examined, but also that we should know whether when the species 
 
ADAPTATION 195 
 
 is bred in captivity these irregularities of behaviour do or do not 
 occur when the larvae are fed on uninjured leaves. 
 
 The famous case of Schiibeler's wheat is revived by Semon. 
 The story will be familiar to most readers of the literature of the 
 subject. Briefly it is that annuals, especially wheat and maize, 
 raised from seed in Central Europe take more time in coming 
 to maturity and ripening than similar plants raised in Norway, 
 where the summer days are much longer. The received account 
 is that he imported seed especially of maize and of wheat from 
 Central Europe to Norway and found that in successive years 
 the period of growth and ripening was increasingly reduced. 
 After two generations seed of the accelerated wheat was sent 
 back to Breslau where it was grown, and was found to ripen rather 
 more slowly than in Norway, but much more quickly than the 
 original stock had done. The facts recorded by Schiibeler 6 are 
 that he received seed from Eldena, which is on the Baltic near 
 Greifswald. The variety is described as " 100 tagiger Sommer 
 Weizen," but no more exact record of its behaviour in Germany 
 is given. This wheat, grown at Christiania in 1857, took 103 
 days to harvest. Its seed was again grown in Christiania in 1858, 
 and took 93 days, and sown again in 1859 it took only 75 days, 28 
 days less than in the first year of cultivation in Norway. Seed of 
 the 1858 crop was sent to Breslau, and grown there by Roedelius 
 in 1859; it took 80 days. Evidently before such a record can be 
 used as proving an inheritance of acquired characters numbers of 
 particulars should be forthcoming. The view that Johannsen 
 has taken is that the result was probably due to unconscious 
 selection of the earlier individuals among a population consisting 
 of many types of various compositions. Some effect may no 
 doubt be ascribed to that cause, but I cannot think that alone 
 it would account for the results. My impression is rather that 
 they were produced by differences in the cultivation and especially 
 in the seasons. Research of an elaborate character would be 
 necessary in order to eliminate the various sources of error, and 
 nothing of the kind has been done; nor does Semon allude to these 
 difficulties in prominently adducing Schiibeler's evidence. A 
 
 6 Schiibeler, F. C, Die Culturpflanzen Norwegens, 1S62, especially pp. 24 and 2S.. 
 
196 PROBLEMS OF GENETICS 
 
 difference of even three weeks in time of harvesting may easily 
 be due to variation in the season. It would in any case be dif- 
 ficult to analyse the meteorological conditions, and to decide how 
 much effect in postponing or accelerating the harvest might be 
 due to cold days, to cloudy days, to wet weather, to fluctuations 
 in average temperature, to hot days, and other such incidents 
 occurring at the different periods of growth, even if they were 
 specially watched while the experiments were in progress, and 
 at this distance of time such analysis is practically impossible. 
 Without careful simultaneous control-experiments this evi- 
 dence is almost worthless. The director of the Meteorological 
 Office 7 has, however, kindly sent me some details of the weather 
 at Breslau from 1857 to i860, and I notice that as a matter of 
 fact July, 1859, was an exceptionally hot month, having an average 
 of 2.67 C. above the mean for the twenty years 1 848-1 867. June 
 in that year was slightly (0.3 1° C.) below the mean and May 
 slightly above it (0.18 C). August was also abnormally hot, 
 2. 35 C. above the average. The Breslau wheat was sown on 
 May iq and harvested on August 6. There was a cold spell from 
 May 11 to 14, which this wheat escaped, as it was sown on May 
 19. In the other years the cold spell came much later. These 
 elements of the weather may possibly have done something to 
 iiurry the ripening in 1859. It is unfortunate that we are not 
 told how long similar wheat from Breslau seed took to ripen in 
 that year. 
 
 As regards the Norway cultivations we have the average 
 monthly temperatures recorded by Schiibeler, though he does 
 not discuss them in connection with this special problem. It is 
 quite clear that 1857, in which the period was 103 days, was an 
 •exceptionally cold summer, especially as regards the months of 
 June and July, but though there was, so far as the temperature 
 
 7 1 am obliged to him and to Dr. E. Gold for much trouble taken to answer 
 my questions. Some idea of the kind of weather indicated by an average of 2.67 
 C. above the mean may be got from a comparison with the year 191 1, which most 
 people will remember as one of the hottest summers they have known. The 
 July of that year was in east and southeast England about 4 F. above the mean 
 but 2.67 C. means about 4.8 F. above the mean. At Greenwich July, 1859, was 
 about 6. 5 F. above the average. 
 
ADAPTATION I97 
 
 records go, no great difference between 1858 and 1859, the year 
 1859, in which the period of ripening was the shortest, was some- 
 what colder in Norway than 1858. But we have the further 
 difficulty that there were ten days difference in sowing, for in 
 1858 the sowing was made on May 14, and in 1859 on May 24. 
 With all these possibilities uncontrolled, and indeed unconsidered, 
 I am surprised that Semon should claim these experiments as one 
 of the chief supports for his views. 
 
 Schubeler's other allegations respecting the influence of 
 climate on plants grown in various places and especially at dif- 
 ferent elevations in Norway have been destructively criticised by 
 Wille 8 to whose paper readers interested in the subject should 
 refer. 
 
 Before the appearance of Wille's criticisms Wettstein 9 made 
 a favourable reference to Schubeler's work, accepting his con- 
 clusion. He states also that he has himself made analogous 
 experiments with flax, finding that the length of the period of 
 development and a series of morphological characters show an 
 adaptation to local conditions, and that on transference of seed 
 to other conditions the previous effects are maintained. No 
 details, however, are given, and I do not know if anything more 
 on the subject has appeared since. The other examples cited 
 by Wettstein, such as the observations of Cieslar on forest-trees 
 and those of Jakowatz on gentians seem to me open to all the 
 usual objections applicable to evidence of this kind. Such work, 
 to be of any value for the purpose to which it is applied, must be 
 preceded by a study of the normal heredity and of the variations 
 of the species. 
 
 Most of the recent writers (Semon, Przibram, etc.) on the 
 inheritance of acquired characters accept the story of Brown- 
 Sequard's guinea pigs, which are said to have inherited a liability 
 to peculiar epileptiform attacks induced in their parents by various 
 nervous lesions. 
 
 The question has been often debated and several observers 
 have repeated the experiments with varying results, some failing 
 
 8 Wille, N., Biol. Cbltt., XXV, 1905, p. 521. 
 
 9 Wettstein, R. von, Der Neo-marckismus u. seine Beziehungen zum Darwinis- 
 mus, Jena, 1903. 
 
i 9 8 PROBLEMS OF GENETICS 
 
 to confirm Brown-Sequard, others finding evidence which in 
 various degrees supported his conclusions. Recently a new and 
 especially valuable paper has been published by Mr. T. Graham 
 Brown 10 which goes far towards settling this outstanding question. 
 He states that "the Brown-Sequard phenomenon is nothing more 
 or less than a specific instance of the scratch-reflex," and it is 
 due to a raised excitability of the mechanism of this reflex. This 
 raised excitability is the character acquired as a consequence, 
 for instance, of the removal of part of one great sciatic nerve. 
 The nature of this raised excitability and its causation are dis- 
 cussed and elucidated, but this part of the work is not essential 
 to the present consideration. Mr. Graham Brown in his summary 
 of conclusions remarks that it is very difficult to see how this con- 
 dition of raised excitability can be transmitted to the offspring, 
 and this comment which might be made in reference to any of 
 the alleged cases certainly applies with special cogency to the 
 present example. 
 
 He then calls special attention to three observations: 
 
 1. That guinea pigs which had a " trophic " change in the 
 foot, as a result of division of the great sciatic nerve, have re- 
 peatedly been seen to nibble the feet of other guinea pigs which 
 had this change in the foot from the same causes. 
 
 2. That accidental injury to the toes may be followed by the 
 Brown-Sequard phenomenon in an otherwise normal animal. 
 
 3. That in several instances the young of guinea pigs which 
 exhibited the phenomenon have been noticed to have one or more 
 toes eaten off by the mother. 
 
 Brown-Sequard noticed that almost all his animals in which 
 the great sciatic was divided acquired the " epilepsy " and 
 nibbled those parts of their feet in which sensation had been lost. 
 Of the offspring of such animals he found that a very small pro- 
 portion exhibited a malformation of the feet, and of these some 
 showed the " epilepsy." The proportion which showed the 
 " epilepsy " was one to two per cent, of the offspring. 
 
 Morgan 11 is quoted by Graham Brown as having suggested 
 
 10 T. Graham Brown, Proc. Roy. Soc, 1912, vol. 84, B, p. 555. This paper gives 
 full reference to the previous literature of the subject. 
 
 11 Morgan, T. H., Evolution and Adaptation, New York, 1903- 
 
ADAPTATION 199 
 
 that the loss of toes in the offspring may have been due to mu- 
 tilation by the mother, following his experience in a case in which 
 the tails of mice in succeeding litters were thus devoured, and 
 there can be little doubt that in this suggestion lies the clue to 
 the explanation of the whole mystery. Graham Brown concludes 
 that it may be supposed with every degree of probability that 
 the " transmission " was due to injuries inflicted upon the young 
 by their parents. With this conclusion most people will now be 
 disposed to agree, and we may hope that we shall hear the last 
 of this curious myth — to the elucidation of which a vast quan- 
 tity of research has been devoted. 
 
 The series of experiments made by Kammerer with various 
 Amphibia have attracted much attention and have been ac- 
 claimed by Semon and other believers in the transmission of 
 acquired characters as giving proof of the truth of their views. 
 With respect to these observations the chief comment to be made 
 is that they are as yet unconfirmed. Many of the results that 
 are described, it is scarcely necessary to say, will strike most 
 readers as very improbable; but coming from a man of Dr. 
 Kammerer's wide experience, and accepted as they are by Dr. 
 Przibram, under whose auspices the work was done in the Bi- 
 ologische Versuchsanstalt at Vienna, the published accounts are 
 worthy of the most respectful attention. 
 
 The evidence relates chiefly to three distinct groups of oc- 
 currences : 
 
 1. Modification in Alytes obstetricans, the Midwife Toad, 
 affecting both the structure and the mode of reproduction, in- 
 duced by compulsory change of habits. 
 
 2. Modification in the mode of reproduction of Salamandra 
 atra and maculosa induced by compulsory change of habits. 
 
 3. Modification in the colour of Salamandra maculosa induced 
 by change in the colour of the soil on which the animals were kept. 
 
 1. I will take first the case of Alytes, 12 because it is the most 
 
 12 Kammerer's chief paper on this subject is in Arch. f. Entwm., 1909, XXVIII, 
 p. 447, and it is to this that the paginal references in the present text relate. His 
 previous paper appeared, ibid., 1906, XXII, p. 48. An account of his further ex- 
 periments with Alytes is given in Natur, 1909-10, Heft 6, p. 95. 
 
coo PROBLEMS OF GENETICS 
 
 definite example, and because it is the case which most readily 
 admits of repetition and verification. 
 
 The habits of Alytes obstetricans are well known. The 
 animals copulate on land. As the strings of eggs leave the 
 female they are entangled by the hind legs of the male, and being 
 adhesive they stick to him and undergo their development at- 
 tached to his back and legs. The number of eggs varies from 18 
 to 86, a number much smaller than is usual in toads and frogs 
 which lay their eggs in water. The eggs are large and full of yolk. 
 
 There are two breeding seasons, one about April and the other 
 about September, and a winter hibernation. Not only animals 
 brought in from outside, but their offspring reared in domestica- 
 tion maintain these normal habits in confinement, if the temper- 
 ature does not exceed 17 C. (pp. 499 and 534). 
 
 If, however, the temperature be artificially raised and kept 
 at 25-30 C, the males do not attach the eggs to themselves 
 when spawning occurs on land but let them lie. The adhesion 
 of the eggs is said to be hindered by the comparatively rapid 
 drying of their surfaces. 
 
 More usually in the high temperatures the animals take to 
 the water and copulate there. The eggs are ejected into the 
 water, and as their gelatinous coverings immediately swell up, 
 they do not stick to the males. 
 
 The offspring thus derived from the parents subjected to 
 heat for one breeding-period only, whether they were laid in water 
 or on land, did not show departures from the normal type. 
 
 Kammerer states next, however, that in subsequent breeding- 
 periods the same parents frequently take to the water to breed, 
 though they have become quite accustomed to the heated 
 chamber; and furthermore that if such animals, having thus lost 
 their instinct to brood their young, be transferred to ordinary 
 temperatures they do not readily reassume their normal habits, 
 but for several breeding seasons — at least four — will take to 
 the water. These parents lay from 90 to 115 eggs, which are 
 small and contain little yolk, and the larvae, on hatching, breathe 
 with their embryonic gills until they are absorbed instead of 
 being broken off as normally. 
 
ADAPTATION coi 
 
 The offspring thus abnormally developed when they mature 
 are said never to brood their eggs. If they are derived from 
 the earlier spawnings of their parents, before, that is to say, the 
 parents had been submitted to the changed conditions long enough 
 to transmit their effects, they lay on land ; but if they are derived 
 from the later spawnings, they lay in the water. These changes 
 of habit are manifested without the continued application of 
 the abnormal experimental conditions, and, as I understand the 
 account, in normal conditions of temperature. 
 
 If the abnormal experimental conditions are continued, the 
 toads always lay in water, and their eggs become progressively 
 smaller and more numerous. The larvae in the fourth generation 
 acquire three pairs of gills instead of one pair, and are in other 
 respects also different from the normal form. 
 
 Respecting the Alytes bred in this way Kammerer makes the 
 very striking statement that the males in the third generation 
 (p. 535) have roughened swellings on their thumbs and that in the 
 fourth generation (pp. 516 and 535) these swellings develop black 
 pigment. Together with the appearance of this secondary sexual 
 character there is hypertrophy of the muscles of the fore-arm. 
 To my mind this is the critical observation. If it can be sub- 
 stantiated it would go far towards proving Kammerer's case. 
 Alytes, among toads and frogs, is peculiar in that the males do 
 not develop these lumps in the breeding season, and the fact 
 may no doubt be taken to be correlated with the breeding habits, 
 copulation occurring on land and not in water as is usual with 
 Batrachians. It is to be expressly noticed that these lumps on 
 the thumbs or arms of male toads and frogs are not merely pig- 
 mented swellings, but are pads bearing numerous minute horny 
 black spines, which are used in holding the females in the water. 
 The figures which Kammerer gives (Taf. XVI, figs. 26 and 26a) 
 are quite inadequate, and as they merely indicate a dark patch 
 on the thumbs it is not possible to form any opinion as to the 
 nature of the structure they represent. 
 
 The systematists who have made a special sudy of Batrachia 
 appear to be agreed that Alytes in nature does not have these 
 structures; and when individuals possessing them can be pro- 
 
202 PROBLEMS OF GENETICS 
 
 duced for inspection it will, I think be time to examine the evi- 
 dence for the inheritance of acquired characters more seriously. 
 I wrote to Dr. Kammerer in July, 1910, asking him for the loan 
 of such a specimen 13 and on visiting the Biologische Versuchsan- 
 stalt in September of the same year I made the same request, but 
 hitherto none has been produced. In matters of this kind much 
 generally depends on interpretations made at the time of ob- 
 servation; here, however, is an example which could readily 
 be attested by preserved material. I notice with some surprise 
 that in a later publication on the same subject no reference to the 
 development of these structures is made (see below) . 
 
 The statements here given represent but a small part of 
 Kammerer 's papers on the subject. He gives much further 
 information as to the course of the experiments, especially in 
 regard to the fate of the eggs laid on land and the aberrations 
 induced in them by treatment. The ramifications of the ex- 
 periments are, however, very difficult to follow, and as I am not 
 sure that I have always understood them I must refer the reader 
 to the original. 
 
 More recently Kammerer has published 14 a most curious 
 account of experiments in crossing his modified and abnormal 
 Alytes, derived from the water-eggs, with normal individuals. 
 
 In the first case the cross was made between a normal female 
 and an abnormal male. The offspring were normal in their 
 habits. In the next generation bred from these almost exactly 
 a quarter showed the abnormal instinct. 
 
 The reciprocal cross was made between an abnormal female 
 and a normal male. In this case the offspring were abnormal in 
 their behaviour; but the second generation bred from them 
 showed three quarters abnormal and one quarter normal. 
 
 Certain details as to numbers and sexes of the various families 
 bred in the course of this amazing experiment are given in a 
 
 13 In reply to my letter Dr. Kammerer who was then away from home very 
 kindly replied that he was not quite sure whether he had killed specimens of Alytes 
 with " Brunftschwielen" or whether he only had living males of the fourth genera- 
 tion, but that he would send illustrative material. 
 
 14 Kammerer, P., Natur, 12 December, 1909, Heft 6, p. 95, repeated in 12 
 Flugschrift d. Deutsch Ges. f. Ziichtungskunde, Berlin, 191 0. 
 
ADAPTATION 203 
 
 subsequent publication. 15 This later paper goes somewhat fully 
 into the question of the difference in behaviour between the 
 normal and modified individuals, describing the ways in which 
 the males and females possessing the acquired character could be 
 recognised from the males and females which were normal, but 
 in this account I find no reference to the development of the 
 " Brunftschwielen " — the horny pads on the hands of the males. 
 As these structures would be of special value in such a diagnosis 
 the omission of any allusion to them calls for explanation. 
 Kammerer claims the evidence as proof of Mendelian segregation 
 in regard to an acquired character, the first example recorded. 
 Pending a repetition of the experiments there is no more to be 
 said. 
 
 2. The Mode of Reproduction of Salamandra atra and maculosa. 1 * 
 — Salamandra maculosa, the common lowland form, with yellow 
 bands or spots, deposits its young in water, generally as gill- 
 bearing tadpoles, with a wide, swimming tail, though occasionally 
 they are born still enclosed in the egg-capsule out of which they 
 soon hatch. Spawning extends over a considerable period, 
 often many weeks, and during the season one female may bear 
 more than 50 young. 
 
 5. atra, the black Alpine form, produces its young on land. 
 They are born without gills, ready to breathe air, and with the 
 rounded tail of the adult. These differences may, as Kammerer 
 says, naturally be regarded as adaptations to the Alpine condi- 
 tions. Moreover, the female bears only two young in a season, 
 and this reduction in the number must be taken to be a conse- 
 quence or condition of viviparity. There are many eggs in the 
 ovary, but all except the two which are destined to develop 
 degenerate and form a yolk-material on which these two sur- 
 vivors feed. 
 
 Kammerer gives a long account of the various conditions to 
 which he subjected both species. The treatment was complicated 
 
 16 Festschrift znm Andenken an Gregor Mendel, being vol. XLIX of the Verh. 
 Naturf. Ver. in Briinn, 191 1, p. 98. 
 
 16 Kammerer's chief papers on this subject are Archiv fur En twin., XVII, 
 1904, and ibid., XXV, 1907. An epitome of results is also given by him in 12 
 Flugschrift d. Deutsch. Ges.f. Ziichtungskunde, Berlin, 1910. 
 
eo4 PROBLEMS OF GENETICS 
 
 in many ways, but the essential statements are, as regards 
 S. maculosa, that when no water was provided in which the young 
 might be born, they were dropped on land, larger and in a later 
 stage of development and of a darker colour than is normal; that 
 the larvae so born gradually diminished in number until only 
 two were deposited in each breeding-period; that dissection 
 showed that the other ova degenerated to form a yolk-material. 
 The larvae so produced reached maturity. The summary of 
 results describes their behaviour, stating that they produced: 
 
 (a) In water, either (i) very advanced, large-headed larvae 
 45 mm. long (instead of 25-30 mm.) with gills already reduced, 
 which had awkward, embryo-like movements, and in some few 
 days metamorphosed into small perfect salamanders; or (2) 
 moderately advanced, properly proportioned larvae, 40-41 mm. 
 long, provided with large gills of (at first) intrauterine character, 
 which were reduced during aquatic life. 
 
 (b) On land, small (26 mm. long) larvae with rudimentary 
 gills, having the body rounded instead of being flattened from 
 above downwards, and an elongated narrow head, which were 
 unable to live in deep water. These larvae changed to the sala- 
 mander colour in 10-12 days, and after four weeks metamor- 
 phosed into salamanders 29 mm. long. 
 
 (c) In the foregoing cases the experimental conditions were 
 not continued, or in other words, basins of water were provided 
 in which they could spawn. But if the experimental conditions 
 are continued, these Salamandra maculosa which were born 
 newt-like (viz., not in a larval condition), are themselves newt- 
 bearing from the first time they give birth, using the dry land, 
 and bringing forth only two young, the normal number for the 
 births of S. atra. These young are 40-41 mm. long, and are 
 dark-coloured, resembling greatly the normal new-born S. atra. 
 
 This epitome of the observations illustrating the inheritance 
 of acquired characters has been very widely quoted, and may 
 not unnaturally be taken to summarize a wide experience of 
 the modified animals. Reference to the details given in the 
 same paper shows that, as alleged, each of the four types of be- 
 haviour enumerated was witnessed once only in the case of each 
 
ADAPTATION 205 
 
 of four females, no two agreeing with each other. As to the 
 number of the males or their habits nothing is said. The first 
 female, a (1), bore five young; the second, a (2), bore two, of 
 which one was a partial albino; the third, b, produced four young; 
 and the fourth, c, two as already stated. 
 
 In the case of c the details show that the female gave birth 
 immediately after being transferred from the open-air terrarium 
 to one indoors, which contained no basin of water. This is the 
 example of the consequences which follow on a continuance of 
 the experimental conditions. 17 
 
 As regards S. atra the converse is reported. Various means 
 were used to induce them to eject their young prematurely in 
 water, such as massaging the sides of the mothers, or raising 
 the temperature to 25 or 30 C, with various degrees of success. 
 But afterwards it was found that specimens collected wild at an 
 elevation of about 1,000 metres responded to much simpler 
 treatment, and gave birth prematurely in water when they were 
 kept in a large shallow basin of water not so deep but that they 
 could everywhere touch the bottom with their feet and keep their 
 heads above the surface. With specimens collected at higher 
 elevations this treatment was inoperative, and the suggestion is 
 made that S. atra at the lower confines of its habitat partakes 
 more of the nature of maculosa than do the individuals from 
 greater heights; for Kammerer argues that pools suitable for 
 breeding must be more uncommon at those elevations than they 
 are lower down. 
 
 In the earlier paper 18 Kammerer states that newly caught 
 females of S. atra often give birth in the water, and show an 
 undoubted preference for doing so. He describes also how he 
 once saw several females, wild in their natural habitat, lay their 
 young in a rain-puddle at 1,800 metres elevation, but the larvae 
 thus born were fully formed. 
 
 When the deposition of the young as larvae has become 
 
 17 " Bei Fortdauer der Versuchsbedingungen sind ah Vollmolche geborene Sala- 
 mandra maculosa gleich bei der ersten Geburt abermals voll tnolchgebarend, benutzen 
 zum Geburtsakt das trockene Land, und zwar unter Erreichung der (bei Salatnandra 
 atra normalen) Embryonen-Zweizahl," Kammerer, 1907, p. 49. 
 
 18 1904, p. 56. 
 
206 PROBLEMS OF GENETICS 
 
 " habitual " 19 with S. atra, three to nine larvae may be produced 
 at one spawning period, from 35 to 45 mm. long, with gills at 
 most 8 mm. long, and a tail-fin 2-3 mm. broad. Such larvae 
 are generally coffee-brown, or grey (instead of black), and show 
 other minor differences. 
 
 The summary states that when grown to maturity they be- 
 come in their turn larva-bearing, and go into the water to bring 
 forth. Their young are more than two (3 to 5 being the numbers 
 observed) with a length of 33-40 mm. or of 21-23 mm. at birth. 
 They are light grey, spotted (mottled with lighter and darker 
 colour), have relatively short gills (8 to 9 mm. at most) and a 
 broad tail-fin (3 mm. wide). At metamorphosis they are rela- 
 tively long (44 mm.) and one of them had some yellow pigment. 
 
 Here again this summary is, as a matter of fact, describing 
 the behaviour of two mothers, of which one produced three, and 
 the other five young. 
 
 To my mind these experiments suggest that the reproductive 
 habits of both species, if closely observed, will be found to be 
 subject to considerable variation, and I think it not impossible 
 that each species is, especially in confinement, capable of being a 
 good deal deflected from its normal behaviour. Moreover, there 
 seems to me no great improbability in the idea that there is an 
 interdependence between the number of young and the stage of 
 maturity in which they are born. But, at the same time, the case 
 as told by Kammerer strikes me as proving too much. If each 
 species is so sensitive to conditions that the normal procedure 
 is gravely modified in one generation, and if that modification 
 can reappear in a pronounced form in the next generation without 
 a renewal of the disturbing conditions, it becomes extremely 
 difficult to understand how the regularity which each species is 
 believed to display in nature can be maintained. Surely both 
 species might be expected to be in confusion. From a passage 
 in Kammerer's earlier paper (1904, p. 55) on the subject, I infer 
 that he also would expect considerable irregularity in the natural 
 behaviour, but that he has not investigated the point. 20 
 
 19 Throughout Kammerer's papers this is used almost as a technical term. 
 It means, I presume, that the feature was manifested more than once. 
 
 20 It should be stated that the papers contain a quantity of detail, especially 
 
ADAPTATION 2o; 
 
 3. Modification of the Colour of Salamandra maculosa induced 
 h Change tn the Colour of the Soil on which the Animals were 
 ^.-Kammerer speaks of this as the most convincing of all 
 his experiments on the transmission of acquired characters. So 
 far, however, no full account of them has been published ^ The 
 statement is that when salamanders are kept in yellow surround- 
 ings the yellow markings gradually in the course of years increase 
 in amount relatively to the black ground colour. Conversely by 
 keeping the animals on black garden soil, the yellow may be 
 greatly diminished in quantity until it largely disappears. (The 
 account in Natur adds that very moist conditions also favour the 
 increase of yellow, and that with less moist conditions the yellow 
 diminishes.) From each kind, the (induced) yellower and the 
 (induced) blacker, a second generation was raised, on soil of 
 neutral colour, and each family was later divided into two parts 
 half being put on black and half on yellow ground. 
 
 As regards the offspring of those which had lived on black 
 soil no positive result had'been reached up to the date of publica- 
 tion, but it is stated that these young resembled their parents 
 in having the yellow distributed in irregular spots. 
 
 ^ As regards the offspring of those which had lived on yellow 
 soil the account follows up the story of that part of the offspring 
 which were put on yellow soil again. It is stated that these, though 
 derived from parents with irregular spots, developed the yellow 
 as longitudinal bands. 
 
 This account is given with slight differences of expression in 
 the three places to which I have referred. On returning from 
 Vienna in 1910 I consulted Mr. G. A. Boulenger in reference to 
 the subject, and he very kindly showed me the fine series from 
 many localities in the British Museum, and pointed out that in 
 nature the colour-varieties can be grouped into two distinct types, 
 
 descriptive of the state of the larvae, which I have not attempted to represent but 
 the account here given contains all that seemed essential to an understanding of 
 the more important features of the account. 
 
 * The first appeared in Natur, 1909-10, Heft 6, p. 94; and the second, which 
 contains coloured plates of the animals, in the lecture already referrred to 12 
 Flugschr. d. Dent. Ges.f. Ziichtungkunde, Berlin, 1910, p. 26. In the paper in Mendel 
 Festschrift, 191 1, the subject is continued, but no more is added as to this part of 
 the experiment. " 
 
eo8 PROBLEMS OF GENETICS 
 
 one in which the yellow of the body is irregularly distributed in 
 spots, and one in which this yellow is arranged for the most part 
 in two longitudinal bands which may be continuous or inter- 
 rupted. The spotted form is, as he showed me, an eastern variety, 
 and the striped form belongs to western Europe. Mr. E. G. 
 Boulenger 22 has since published a careful account of the distri- 
 bution of the two forms. The spotted he regards as the typical 
 form, var. typica, and for the striped he uses the name var. 
 taeniata. The typical form occupies eastern Europe in general, 
 including Austria and Italy, extending as far west as parts of 
 eastern France. The var. taeniata is found all over France, 
 excepting parts of the eastern border, Belgium and western 
 Germany, Spain and Portugal. Of the very large series examined 
 there was only one specimen (Lausanne) which could not with 
 confidence be referred to one or other of the two varieties. 
 Mr. E. G. Boulenger points out that both varieties inhabit very 
 large areas, and live on soils of most different colours and com- 
 positions. Both are liable to variations in the amount and the 
 shade of the yellow, but any suggestion that taeniata belongs 
 especially to yellow soils and typica to black soils is altogether 
 inadmissible. He expresses surprise that Kammerer should not 
 allude to these peculiarities in the geographical distribution of 
 the two forms. He suggests further that it is more likely that 
 some mistake occurred in Kammerer's observations than that the 
 east European typica should, in the course of a generation, have 
 been transformed into the west European taeniata by the influ- 
 ence of yellow clay soil. 
 
 In his last paper on the subject Kammerer states incidentally 23 
 that he has found the striped form recessive to the spotted. No 
 evidence for this statement is given, and I have not found any 
 other reference to crosses effected between the two natural types. 
 If, however, this representation is correct, it is conceivable that 
 the production of taeniata from typica was in fact the re-ap- 
 pearance of a recessive form. The plate which Kammerer gives 
 in illustration of his modified parent figures a single animal at 
 four stages, and though it is certainly more like the spotted than 
 
 22 E. G. Boulenger, Proc. Zool. Soc, 1911, p. 323. 
 
 23 Mendel Festschrift, 1911, p. 84. 
 
ADAPTATION 209 
 
 the striped form, it has a certain suggestion of the striped arrange- 
 ment, such as I can well imagine being produced in the hetero- 
 zygote. 24 
 
 In continuation 25 of the experiments on the colour of 5. macu- 
 losa Kammerer publishes an account of elaborate experiments 
 in grafting ovaries of the various forms, modified and unmodified, 
 into each other, and describes the offspring which foil- 
 Before pursuing this part of the inquiry I am disposed to wait 
 until the earlier steps have been made much more secure than 
 they yet are. 
 
 More recently Kammerer has published similar statements in 
 regard to the inheritance of characters induced in various lizards 
 by keeping them in abnormal temperatures, high and low. The 
 changes induced affected in some species the colours, in others 
 the reproductive habits. Respecting these examples I feel the 
 same scepticism that I have indicated in regard to the others, 
 somewhat heightened by the fact that insufficient evidence is 
 given both regarding the behaviour of these various species in 
 captivity when not subjected to abnormal temperatures, and 
 in the wild state. 
 
 Respecting this part of the evidence Mr. G. A. Boulenger has 
 lately published a criticism 26 from which I extract the following 
 passages. Referring to a previous note 27 on the question of the 
 melanism of the various insular forms of Lacerta mural is he 
 writes: " I also alluded (/. c.) to the theories that have been 
 propounded to explain the melanism of various insular forms. 
 This is a subject which has been lately taken up by Dr. Kam- 
 merer at the Biologische Versuchsanstalt in Vienna, and he claims 
 to have produced nigrinos artificially by a very strong elevation 
 of the temperature, accompanied by extreme dryness. Dr. 
 Werner 28 has already opposed his own experiments to those of 
 Kammerer, artificial melanism having been produced by him in 
 Lacerta oxycephala by keeping two very light specimens from 
 
 24 12 Flugschrift. Deut. Ges. Ziichtungskande, 1910. Fig. 15. P- Rcir.c. 
 26 Mendel Festschrift, 191 1, p. 83. 
 
 26 Field, 1912, 30 March. 
 
 27 Ibid., 1904, p. 863. 
 
 28 Mitth. Naturw. Ver. a. d. Univ. Wien, 1908, p. 53- 
 
 15 
 
2io PROBLEMS OF GENETICS 
 
 Ragusa for a whole summer in very damp conditions. Neither is 
 Kammerer's theory in accordance with the distribution of the 
 black lizards, as pointed out by Werner. Kammerer also finds 
 that those forms which are known to produce melanic races in a 
 state of nature, lend themselves more readily than the others 
 to the success of his experiments. But he shows himself misin- 
 formed when he states that the variety called Lacerta fiumana 
 belongs to the category of those of which black forms are not 
 known. He overlooks the fact, first pointed out by Scherer in 
 1904, and which I can confirm, that the black lizard from Meli- 
 sello near Lissa in the Adriatic is unquestionably derived from 
 the lizard from Lissa, which he correctly regards as not separable 
 from L. fiumana. ..." 
 
 " Another colour modification which Dr. Kammerer states 
 that he obtained by raising the temperature is the assumption by 
 the female of the typical Lacerta muralis of the bright red colour 
 of the lower parts which often distinguishes the male from the 
 female, and which was not shown by the individuals of the latter 
 sex kept by him under normal conditions. He quotes various 
 authorities to show that the lower parts are never red in the 
 females, but he has omitted to consult others who say the con- 
 trary. Thus Bedriaga (1878 and 1879) remarks that a so-called 
 var. rubriventris of the typical wall lizard has the lower parts red 
 in both sexes." 29 
 
 In reading such papers as those of Semon or Kammerer the 
 thought uppermost in my mind is that to multiply illustrations 
 of supposed transmission of acquired characters is of little use 
 until some one example has been thoroughly investigated. If 
 we had certain assurance that even a single unimpeachable case 
 could be repeated at will, the whole matter would assume a more 
 serious aspect. If, for instance, Kammerer were able to show us 
 Alytes males with horny pads on their hands, it would be some- 
 thing tangible; still more, if the experiment were repeated by 
 others until no doubt remained that the offspring of Alytes which 
 had bred in water for some three generations did acquire these 
 
 29 As to the variations of Lacerta muralis in Western Europe and North Africa 
 see Boulenger, G. A., Trans. Zool. Soc, 1905, vol. XVII, p. 351. 
 
ADAPTATION 211 
 
 pads and that they could transmit these novelties to descendants 
 raised in normal conditions. Till evidence of this kind is pub- 
 lished by at least two independent observers investigating similar 
 material, I find it easier to believe that mistakes of observation or 
 of interpretation have been made than that any genuine trans- 
 mission of acquired characters has been witnessed. 
 
 Meanwhile there is no denying that the origin of adaptational 
 features is a very grave difficulty. With the lapse of time since 
 evolutionary conceptions have become a universal subject of Bl udy 
 that difficulty has, so far as I see, been in nowise diminished. 
 But I find nothing in the evidence recently put forward which 
 justifies departure from the agnostic position which most of us 
 have felt obliged to assume. 
 
 Appendix to Chapter IX. 
 
 Professor G. Klebs, as is well known to students of evolution- 
 ary phenomena, has for several years been engaged in investi- 
 gations relating to the inheritance of acquired characters. In 
 his many publications on the subject the issue has always been 
 represented as more or less uncertain. 
 
 Desiring to know how the matter now stands according to 
 Professor Klebs' present judgment I wrote to him asking him to 
 favour me with a brief general statement. This he most kindly 
 sent in a letter dated 8th July, 191 2. 
 
 As such a statement will be read with the greatest interest 
 by all who are watching the progress of these studies I obtained 
 permission to publish it as follows : 
 
 8. Jul] [Qlfl 
 
 Ihre liebenswurdige Anfrage will ich sehr gern beantworten, 
 obwohl ich sie nicht so beantworten kann wie ich erwfinschte. 
 Ihr Skepticismus in der Frage der Uebertragung erworbener 
 Charactere auf die Nachkommen ist nur zu berechtigt. Meine 
 Versuche mit Veronica sind nicht beweisend, da es rair bisher 
 nicht gelungen ist eine einigermasse konstante Varietal mit 
 verlaubten Inflorescenze zu erzeugen. In Bezug auf nu in 
 Sempervivum bin ich allerdings noch hcute der Meinung dass 
 
212 PROBLEMS OF GENETICS 
 
 die starke ktinstliche Veranderung der Bliite einen Einfluss auf 
 einzelnen Nachkommen gehabt hat. Ich habe seither nichts 
 dariiber veroffentlicht : die Mehrzahl der anormalen gefullten 
 Bliiten war leider steril. Von einem weniger veranderten Ex- 
 emplar erhielt ich einige Samlinge, aber sie haben noch nicht 
 gebliiht. Es kann sich in diesem Falle nur urn eine Nachwirkung 
 in der ersten Generation handeln, vergleichbar jenen Fallen in 
 denen Samen von Baumen aus den hohen Alpen in der Ebene 
 gewisse Nachwirkungen zeigen. Aber es ist bisher kein sicherer 
 Fall bekannt in den der kunstliche herbeigefiihrte Charakter 
 mehrere Generationen hindurch unter der gewohnlichen "normalen" 
 Bedingungen iibertragen worden ist. 
 
 Auf der andere Seite sind diese negativen Resultaten nicht 
 entscheidend. Denn wie wenig ist in dieser Beziehung iiber- 
 haupt ernstlich versucht worden! Und zweifellos geht die 
 Sache nicht so einfach. 
 
 Ich versuche es mit anderen Pflanzen weil ich der Meinung 
 bin dass es moglich sein musse wenigstens solche neuen Varie- 
 taten zu erzeugen, wie sie die Gartenvarietaten entsprechen. 
 
 Aber bis jetzt leider sind die Versuche nicht gelungen, weder 
 mir noch irgend einem anderen. 
 
CHAPTER X 
 
 effects of changed conditions continued 
 The Causes of Genetic Variation 
 
 In the last chapter we examined some of the evidence offered 
 in support of the belief that adaptation in highly organised forms 
 is a consequence of the inheritance of adaptative changes induced 
 by the influence of external conditions. The state of knowledge 
 of this whole subject is, as I have said, most unsatisfactory, 
 chiefly for the reason that in none of the cases which are alleged 
 to show a positive result have two observers been over the 
 same ground, or as yet confirmed each other. In the wider 
 consideration respecting the causes of variation at large we find 
 ourselves still in the same difficulty. The study has thus far 
 proved sadly unfruitful. In spite of the considerable efforts 
 lately made by many observers to induce genetic variation in 
 highly organised plants or animals, and though successes have 
 occasionally been announced, I do not know a single case which 
 has been established and confirmed in such a way that we could 
 with confidence expect to witness the alleged phenomena if we 
 were to repeat the experiment. Abundant illustrations are 
 available in which individuals exposed to novel conditions mani- 
 fest considerable changes in characters or properties, but as yet 
 there is no certain means of determining that germ-cells of a aew 
 type shall be formed. 
 
 Of the direct effect of conditions the lower organisms, espe- 
 cially bacteria, offer the best examples, the alterations of virulence 
 which can be produced in so many distinct ways being the most 
 striking and familiar. That attenuation of virulence can be 
 produced by high temperatures or by exposure to chemical 
 agents, and that this diminution in virulence may remain perma- 
 nent is, from our point of view, not surprising; but the fad that 
 in many cases the full virulence can by suitable cultivation be 
 
 213 
 
2i 4 PROBLEMS OF GENETICS 
 
 restored is difficult to understand. Similar variations have been 
 observed in power of pigment production and other properties. 
 
 These phenomena naturally raise the question whether any 
 cases of apparent loss of factors in higher forms may be com- 
 parable. 
 
 The subject of variations in the lower organisms and their 
 dependence on conditions is a highly special one, and I have no 
 knowledge which can justify me in offering any discussion of 
 them, but I understand that hitherto little beyond empirical 
 recognition of the phenomena has been attempted. A useful 
 summary of observations made by many investigators was lately 
 published by Hans Pringsheim, 1 who enumerates the different 
 agencies which have been observed to produce modifications, 
 and the various ways in which these changes are manifested. 
 One of the most comprehensive studies of the subject from the 
 genetic point of view is that made by F. Wolf. 2 In his extensive 
 cultivations of Bacillus prodigiosus, Staphylococcus pyogenes and 
 Myxococcus he succeeded in producing many strains with modi- 
 fied properties. In most of these the modifications arose in 
 consequence of the application of high or low temperatures or of 
 the addition of various chemical substances to the culture-media. 
 Some of the variations, which are for the most part in the powers 
 of pigment-formation, persisted when the strains were returned 
 to normal conditions, and others did not. In reference especially 
 -to the variations witnessed in the Cocci the reader should consult 
 the critical account of variation in that group published by the 
 Winslows, 3 where much information on the subject is to be 
 found. The authors attempted to determine the systematic 
 relationships of the several forms, as far as possible, by the 
 application of statistical methods. The result is interesting as 
 showing that the problem of species in its main features is pre- 
 sented by these organisms in a form identical with that which 
 we know so well in the higher animals and plants, whatever 
 
 1 Pringsheim, H., Die Variability niederer Organistnen, Berlin, 1910. 
 
 2 F. Wolf, Modifikationen u. Mutationen von Bakterien, Zts. F. indukt. Abstain. 
 u. Vererbungslehre, II, 1909, p. 90. 
 
 3 Winslow, C. E. A. and A. R., Systematic Relationships of the Coccaceae* 
 £Jew York. 1909. 
 
CAUSES OF GENETIC VARIATION 215 
 
 properties be selected as the diagnostic characters. There are 
 many types perfectly distinct and others which intergrade. 
 Some of the types change greatly with conditions while others do 
 not. This is exactly what we encounter whenever we study the 
 problem of species on an extended scale among the higher forms 
 of life. 
 
 There is now practically complete agreement among bac- 
 teriologists that the observations made first by Massini on the 
 change in color of Bacterium coli mutabile grown in Endo's 
 medium, associated with the acquisition of the power to ferment 
 lactose, are perfectly reliable and free from possibilities of mis- 
 take. The work has been extended and confirmed by many 
 workers, especially R. Miiller, who finds that this bacterium can 
 similarly acquire and maintain the power to ferment other 
 sugars. A careful account of the whole subject written by Miiller 
 for the information of biologists will be found in Zts.fiir Abstatn- 
 mungsl., VIII, 19 12. After discussing the biological significance 
 of the facts, he concludes with a caution to the effect that bacteria 
 are so different from all other living things that generalizations 
 from their behavior must not be indiscriminately applied to 
 animals and plants. 
 
 In all work with this class of material there is obviously 
 danger of error through foreign infection of the cultures, but 
 there can be no doubt that though some of the "mutations" 
 recorded may be due to this cause, the majority of the instances 
 observed under stringent conditions are genuine. 
 
 Another and equally serious difficulty besetting work with 
 bacteria and fungi cultivated from spores is that the appearance 
 of variation may in reality be due to the selection of a special 
 strain previously living masked among other strains. This 
 possibility must be remembered especially in those instances 
 which are claimed as exemplifying the effects of acclimatisation. 
 Manifestly this consideration can be urged with most force when 
 the strain which gave rise to the novelty was not raised from a 
 single individual spore. Moreover, when once the possibility of 
 spontaneous variation is admitted, it must be difficult to be quite 
 confident that any given variation observed is in reality due to 
 
2i6 PROBLEMS OF GENETICS 
 
 the novel conditions applied, and as I understand the evidence, 
 the appearance of the mutational forms does not with any 
 regularity follow upon the application of the changed conditions. 
 
 Researches into the variation of these lower forms will, no 
 doubt, be continued on a comprehensive scale. So long as the 
 instances recorded are each isolated examples it is impossible to 
 know what value they possess. If they could be coordinated in 
 such a way as to provide some general conception of the types of 
 variation in properties to which bacteria, or any considerable 
 group of them, are habitually liable, the knowledge might 
 greatly advance the elucidation of genetic problems. 
 
 Of mutational changes directly produced with regularity in 
 micro-organisms by treatment, the experiments with trypano- 
 somes provide some of the clearest examples. A summary of 
 the evidence was lately published by Dobell, 4 from which the 
 present account is taken. The most definite fact of this kind 
 established is that certain dyes introduced into the blood of the 
 host have the effect of destroying the small organ known as the 
 "kineto-nucleus" in the trypanosomes. The trypanosomes thus 
 altered continue to breed, and give rise to races destitute of 
 kinetonuclei. This observation was originally made by Wer- 
 bitzki and has been confirmed by several observers. The exact 
 way in which this alteration is effected in the trypanosomes is not 
 quite definitely made out, but there is good reason for supposing 
 that the dyes have a direct and specific action upon the kineto- 
 nucleus itself, and circumstances make it improbable that in 
 some division a daughter-organism without that body is pro- 
 duced, or that any selection of a pre-existing defective variety 
 occurs. 
 
 Ehrlich has suggested with great probability that the dyes 
 which possess this action owe it to the fact that they have the 
 particular chemical linkage which he calls "ortho-quinoid." In 
 outward respects, such as motility and general appearance, the 
 modified organisms are unchanged, but their virulence is di- 
 minished. As regards the possibility of the defective strain re- 
 
 4 C. C. Dobell, Jour. Genetics, 1912, II, p. 201, where full references are given. 
 Still more recently the same author has contributed an excellent summary of 
 the evidence relating to bacteria (ibid., II, 191 3, p. 325). 
 
CAUSES OF GENETIC VARIATION 2I; 
 
 acquiring the kinetonucleus, Werbitzki states that in one cise 
 passage through 5 o animals and treatment with dyes left the 
 strain unaltered; but that in another case at the sixteenth 
 passage 7 per cent, of the trypanosomes were found to have 
 re-acquired the organ, and in subsequent passages the percentage 
 increased, until at the twenty-seventh passage practically all had 
 re-acquired it. Kudicke, however, in similar experiments did not 
 succeed in causing re-acquisition by transplantation. 
 
 By the action of various drugs and antibodies races of try- 
 panosomes resistant to those substances have been obtained. 
 These breed true, at least when kept in the same species of animal 
 in which the resistance was acquired. As to whether change of 
 virulence is produced by passage through certain animals or not, 
 there is as yet no general agreement. 
 
 Other changes, especially in size and some points of structure, 
 are said to occur when certain trypanosomes proper to mammals 
 are passed through cold-blooded vertebrates (Wendelstadt and 
 Fellmer), and it is stated that these changes persist, but the 
 observations have not yet been confirmed. 
 
 Experiments lately conducted by Woltereck with Daphnia 
 are interesting as having given a definite positive result, in so far, 
 at least, as the ova were affected by conditions before leaving the 
 bodies of the parent individuals. The observations relate to 
 the offspring resulting from par theno genetic eggs. Females 
 bearing ephippia (fertilised eggs) were isolated until the ephippia 
 were dropped, and in this way the offspring of fertilisation were 
 excluded. Males, of course, appeared from time to time in the 
 cultures, but as fertilised eggs were rejected, their presence did 
 not disturb the result. The most remarkable observations 
 related to Daphnia longispina. 
 
 This species as found in the lower lake at Lunz had the front 
 end of the body blunt and nearly round in profile; but on 1 
 cultivated in a warm temperature and with abundant nourish- 
 ment the front end of the body became produced into an elon- 
 gated "helmet," as Woltereck calls it. Experiment showed that 
 the change was primarily due to the abundance of food, and owing 
 to temperature in a subordinate degree. 
 
2i 8 PROBLEMS OF GENETICS 
 
 This distinction arose as soon as the species was taken into 
 the hothouse, but when the modified individuals were put back 
 into the original conditions, a lower temperature and scanty 
 food-supply, the next generation returned to their original form. 
 After being cultivated for two years and about 40 generations in 
 the more favourable conditions, when similarly put back into 
 the lower temperature with scanty food the first generation born 
 in these conditions was helmeted like the modified parents. 
 Woltereck is of opinion that the ova were still unformed at the 
 time the parents were put back, and the influence of the favour- 
 able conditions upon the unformed ova he speaks of as a "prae- 
 induction." The effect never extended beyond the one gener- 
 ation, after which the strain returned to its original state. 
 
 The fact that the influence on the offspring was not mani- 
 fested at first led Woltereck to expect that by more prolonged 
 cultivation in the favourable conditions a further extension of 
 this influence would be produced, but this expectation was never 
 fulfilled, though the attempt, was made again and again. 
 
 Similar experiments were made with Hyalodaphnia cucullata, 
 which is far more sensitive to cultural influences, and in nature 
 manifests a considerable elongation of the helmet as a seasonal 
 modification, but the results were essentially the same as in the 
 preceding case, no modification extending beyond the first 
 generation born after the restoration to normal conditions* 
 
 The only criticism of these extremely interesting results which 
 suggests itself is that perhaps the original appearance of the 
 modification was not in reality due to an accumulated effect of 
 the conditions, but to some change in the conditions themselves 
 which was not noticed. It is difficult to see how length of 
 time or even the lapse of several generations could have so specific 
 an effect on the race. It is no doubt often vaguely supposed 
 by many that a long period of time may be necessary for the 
 effect of climate or of other environmental conditions to be 
 produced in an organism which does not thus respond at first. 
 I have never been able to see any reason for this opinion nor how 
 
 6 See Woltereck, Verh. d. Deut. Zool. Ges., 1909, p. no; and 191 1, p. 142. This 
 is a subject which can only be properly appreciated on reference to the original 
 papers. Several complications are involved to which I have not here alluded. 
 
CAUSES OF GENETIC VARIATION 219 
 
 it is to be translated into terms of physiological fact, and I 
 imagine that in those cases in which the lapse of time is really 
 required for the production of an effect, the influence of the 
 prolongation is rather on the conditions than on the organisms. 
 The response of the organisms thus probably indicates not that 
 the creature is at length feeling the effects because of their 
 accumulated action on itself, but that the conditions have at 
 length ripened. 
 
 As this sheet is passing through the press Agar has published 6 
 an abstract of evidence as to another comparable case in a par- 
 thenogenetic strain in the daphnid, Simocephalas vetulus. When 
 fed on certain abnormal foods the shape of the body is changed, 
 the edges of the carapace being rolled backwards so as to expose 
 the appendages. The offspring of animals thus modified showed 
 similar modification in the first, and to a very slight degree, in 
 the second generation, though the original mothers were removed 
 to normal conditions before their eggs were laid. In the third 
 generation there was "a very pronounced reaction in the opposite 
 direction." Agar suggests that the change may be due to some 
 toxin-like substances, carried on passively by the egg into the 
 next generation, against which the protoplasm eventually pro- 
 duces an anti-body. 
 
 The experiments which have been in recent years regarded 
 by evolutionary writers as the most conclusive proof that direct 
 environmental action may produce germinal variation are those 
 of Professor W. L. Tower, of Chicago, on Lcptinotarsa, the 
 potato beetles. This work has attained considerable celebrity 
 and has been generally accepted as making a definite extension 
 of knowledge. After frequently reading Tower's papers and 
 after having been privileged to see some of the experiments in 
 progress (in 1907) I am still in doubt as to the weight which 
 should be assigned to this contribution. 
 
 The work is described in two chief publications, the first of 
 which appeared in 1906. 7 This treatise contains a vast amount 
 of information about numerous species and varieties ot these 
 
 tProc. Roy. Soc, B, Vol. 86, 1913, p. «3- 
 
 7 An Investigation of Evolution in Chrysomelid Beetles of the Genus Leplinotarsa. 
 Carnegie Publications, 1906, No. 48. 
 
220 PROBLEMS OF GENETICS 
 
 beetles which the author has observed and bred in many parts of 
 their distribution throughout the United States, Mexico and 
 Central America. The part of the book which has naturally 
 excited the greatest interest is that in which Tower states that 
 by subjecting the beetles to change in temperature and moisture, 
 he caused them to produce offspring quite unlike themselves, 
 which in several cases bred true. 
 
 It is much to be regretted that the author did not happen to 
 become acquainted with Mendelian analysis at an earlier stage 
 in the investigation. The evidence might then have been 
 handled in a much more orderly and comprehensive way, and a 
 watch would have been kept for several possibilities of error. 
 
 The headquarters of the genus is evidently as Tower states, 
 in Mexico and the adjoining countries. In this region there is 
 a great profusion of forms, some very local, some as for instance 
 the well-known decemlineata, 8 more widely spread. The dis- 
 tinctions are almost all found in peculiarities of colour and 
 pattern, and the limits of species are even more indefinable than 
 is usual in multiform animals. Tower arranges the various types 
 into seven groups of which the one most studied is that which 
 he calls the lineata group. To this group belong all the forms 
 to which reference is here made, and, as I understand, they differ 
 among themselves entirely in size, colour and pattern. There 
 is no suggestion of infertility in the crosses made between the 
 several forms of the lineata group; in fact they present, like many 
 Chrysomelidae, a good example of what most of us would now 
 call a polymorphic species, consisting of many types, some found 
 existing in the same locality, others being geographically isolated. 
 
 A series of experiments was devoted to the attempt to fix 
 strains corresponding to the extremes of continuous variations. 
 For example, those with most black pigment and those with 
 least black taken from a population continuously varying in this 
 respect, were separately bred; but almost always the selection 
 led to no sensible change in the position of the mean of the popu- 
 
 8 This is the famous Colorado beetle or potato-bug, which has caused such serious 
 destruction in potato crops. There seems to be no doubt that this insect, formerly 
 unknown in the eastern States, made its way east along the mining trails when the 
 west was opened up. 
 
CAUSES OF GENETIC VARIATION 221 
 
 lation. The variations in these cases were evidently fluctu- 
 ational. In some instances, however, real genetic differences 
 were met with, and strains exhibiting them were, as usual, 
 rapidly fixed. 
 
 Tower points out that several of the varieties (or species, as 
 he prefers to call them) were obviously recessive to decemlineata. 
 This is most clearly demonstrated in the case of the form called 
 pallida, which is a pale depauperated-looking creature, with the 
 orange of the thorax almost white and the eyes devoid of pig- 
 ment. 9 This form behaved as an ordinary Mendelian recessive, 
 breeding true whenever it appeared in the cultures, or when 
 individuals found wild were studied in captivity. A black form 
 which Tower names melanicum was similarly shown to be a 
 Mendelian recessive. Wild specimens of this variety of opposite 
 sexes were not found simultaneously in nature, and there was thus 
 no opportunity of breeding them together, but the hereditary 
 behaviour was seen in the F 2 generation from a melanicum found 
 coupled with decemlineata. Experiments also occurred giving 
 indication that a variety with the stripes anastomosing in pairs 
 {tortuosa), was another recessive, and that a variety — called 
 " rubri-vittata" — gave an intermediate Fi with subsequent 
 segregation. All these are forms of decemlineata Stal. 
 
 Similar observations were made, regarding forms recessive to 
 multitaeniata Stal. Of these two were thrown by multitacniata 
 itself, namely a form named by Stal melanothorax, and regarded 
 by him as a species, and one which Tower names rubicunda n. sp. 
 The facts proving the recessive behaviour of their several forms 
 will be found in the following places in Tower's book : 
 
 pallida, pp. 273-278. 
 
 melanicum, p. 279. 
 
 tortuosa, p. 280. 
 
 rubrivittata, pp. 280-281. 
 
 melanothorax and rubicunda, pp. 283-285. 
 
 Following this evidence of recessive nature of the six forms 
 
 9 This is indicated in the coloured plate, but I have not found any explicit state- 
 ment to this effect in the text, and am not sure if the absence of pigment was re- 
 garded as complete. 
 
222 PROBLEMS OF GENETICS 
 
 enumerated, Tower describes experiments showing, as he believes, 
 that some of them may be caused to appear by applying special 
 treatment to the parents during the " growth and fertilisation" 
 (p. 287) of the eggs. The most striking example is that in which 
 4 males and 4 females of decemlineata were kept very hot (average 
 35 C.) and dry, and at low atmospheric pressure (19-21 inches). 
 The eggs laid were restored to natural conditions. These gave 
 506 larvae, from which emerged 14 normal, 82 pallida and 2 
 " imrnaculo-thorax" viz., without pigment on the pronotum. 
 The account of the rest of the experiment is somewhat involved, 
 but I understand that the pallida, of which two only survived, 
 behaved as normal recessives when bred to the type: also that 
 the parents, after having laid the eggs whose history has been 
 given, were restored to normal conditions and laid 319 eggs which 
 gave 61 normals. 
 
 In another case normal parents laid 409 eggs in the hot and 
 dry conditions, and on restoration to normal conditions, the 
 same parents laid 840 eggs. The 409 eggs gave 64 adults as 
 follows : 
 
 Males Females 
 
 decemlineata 12 8 
 
 pallida 10 13 
 
 immaculothorax 2 3 
 
 albida _9 _7 
 
 33 31 
 
 The 840 eggs laid in normal conditions gave 123 normal decem- 
 lineata. 
 
 Similar experiments were made with multitaeniata and gave 
 comparable results, the two recessives (melanothorax, rubicunda) 
 being produced in large numbers when the parents were subjected 
 to heat, but in this case the atmosphere was kept saturated with 
 moisture, instead of dry, as in the previous instance. The same 
 parents transferred to normal conditions gave normals only. 
 
 Lastly the form undecimlineata was exposed "to an extreme 
 stimulus of high temperature, io° C. above the average," and a 
 dry atmosphere, with the result that from 190 eggs there emerged 
 11 beetles, all of the form angustovittata Jacoby, which subse- 
 quently bred true to that type (see p. 295). 
 
CAUSES OF GENETIC VARIATION 223 
 
 In the results of these experiments, as described, there is one 
 feature which I regard as quite unaccountable. Tower mal, 
 comment upon it. Indeed, from the general tenour of the paper, 
 I infer, not only that he does not perceive that he is recounting 
 anything contrary to usual experience, but rather that he regards 
 the result as conforming to expectations previously formed. 
 The point in question is the genetic behaviour of the dominant 
 normals produced under the abnormal conditions. These 
 normals were the result of the breeding of parents declared to be 
 at the same time giving off many recessive gametes. Some of 
 these normals must be expected therefore to be heterozygous 
 unless some selective fertilisation occurs. Nevertheless in every 
 case they and their offspring are reported to have continually 
 bred true. I allude especially to the tables given on pp. 288, 289, 
 292, and 293. Tower does not mention any misgiving about 
 this result, and I think he regards himself as recounting phe- 
 nomena in general harmony with the ideas of mutation expressed 
 by De Vries. This they may be; but to anyone familiar with 
 analytical breeding the course of these experiments must seem so 
 surprising as to call for most careful, independent confirmation. 
 
 In 1910 10 Tower published an account of further experiments 
 with Leptinotarsa. The work described related to two subjects. 
 Crosses were made between three forms, undecimlineata Stal, 
 signaticollis Stal and "diverse" named by Tower as a new 
 species. The distinctions between these three depend partly on 
 characters of the adults and partly on those of the larvae. The 
 adults of undecimlineata and diver sa have the elytra striped, but 
 the elytra of signaticollis are unstriped. The larvae of sig- 
 naticollis and of diversa are yellow, but those of undecimlineata 
 are white. 11 Moreover, in signaticollis and diversa the black in- 
 creases in the third stage of the larvae to form transverse bands 
 which are absent in undecimlineata. The general course of the 
 experiments shows that these differences may be approximately 
 
 10 Biol. Bull., XVIII, 1910, p. 285. 
 
 11 This description does not quite agree with the representation of the !. 
 
 in PI. 17 of the book Evolution in the Genus Leptinotarsa for there the larva oi 
 undecimlineata is shown as white in the second stage, but yellowish in the third 
 stage; perhaps there is an error in printing. 
 
224 PROBLEMS OF GENETICS 
 
 represented as due to the action of three factors, any of which 
 may be independently present or absent. The stripings of the 
 elytra and of the larvae are each due to a separate factor. As 
 regards the distinction between the yellow and the white larvae 
 the evidence does not prove that there is decided dominance of 
 either colour and I infer that the heterozygotes are often inter- 
 mediate. 
 
 The chief contribution which this new paper claims to make 
 relates to differences in the results which ensue from crosses 
 effected between these three types at different average tempera- 
 tures. 
 
 We are first concerned with four experiments which I number 
 (i), (2), (3), (4): 
 
 i. Signaticollis 9 X diver sa cf bred at an average tempera- 
 ture of 8o° F. by day and 75 F. by night, gave two groups in 
 about equal numbers. The first (49) was pure signaticollis and 
 bred true. The second (53) was of an intermediate type, which 
 on being bred together gave the typical Mendelian result — 1 sig.: 
 2 intermediate: 1 div. 
 
 2. Next, as the account originally stood in the published 
 paper, we are told that sig 9 X div & bred together at a day- 
 temp, average 75 F. and night average 50 F. gave an inter- 
 mediate only, which subsequently produced a normal 1:2:1 
 ratio. The two crosses were repeated eleven times with identical 
 results. 
 
 In a further experiment (3) signaticollis 9 X diversa o" were 
 bred under the same conditions as those used in expt. (1). They 
 again gave sig. and intermediates as before in fairly equal numbers. 
 The sig. as before bred true, and the intermediate gave 1 : 2 : 1, 
 all exactly as in expt. (1). 
 
 In expt. (4) the same parents used in (3) were again mated 
 under conditions of expt. (2) at the lower temperature, and this 
 time gave signaticollis exclusively, which bred true for four 
 generations. This experiment was repeated seven times with 
 uniform results. 
 
 Diagrams are given representing all these histories in graphic 
 fashion. 
 
CAUSES OF GENETIC VARIATION 225 
 
 From these observations, Tower concludes that the determi- 
 nation of dominance, and the ensuing type of behaviour, is clearly 
 a function of the conditions incident upon the combining 
 plasms. 
 
 It will be observed that expts. (1) and (3) gave identical 
 results but (2) and (4), though much the same conditions 
 applied, are at variance, for (2) gave all intermediates, while 
 (4) gave all signaticollis . In Amer. Nat., XLIV, 1910, p. 747, 
 Professor T. D. A. Cockerell commented on this paper of Tower's 
 and pointed out that there must be an error somewhere, for when 
 he discusses these experiments Tower speaks of (2) and (4) as 
 confirming each other. To this Tower replied 12 that there had 
 been a mistake. He states that in preparing the paper "cei tain 
 minor experiments were taken from a larger series and combined 
 to illustrate a general point in the behaviour of alternative 
 characters in inheritance," and that expt. (2) was introduced 
 inadvertently in place of another which he desires to substitute. 
 In this, which I number (5), signaticollis 9 X diver sa o* from 
 exactly the same stocks as those used in (1), were mated at the 
 lower temperatures specified for (2), day average 75 F., night 
 average 50 F. These gave all of the signaticollis type with a 
 narrow range of variability, which bred true, in some cases to F«. 
 Tower says he has repeated this experiment six times with identi- 
 cal results. 
 
 Nevertheless he proceeds to say that the description of expt. 
 (2), which was repeated eleven times with identical results, was 
 correct "as far as given." That experiment was " from a second 
 series of cultures parallel to the one given, but in which there are 
 other factors involved, which in H. 410 [my (2)] are productive 
 of a typical Mendelian behaviour." He adds he does" not 
 at this time to make any statement of what these fa< 
 nor of their relations to the behaviours given in the 1 1. 400. 1 1 . 4 1 1 , 
 H. 409/n series [my (1), (5) and (3H4)] which are the sin 
 and most easily presented series obtained in the crossing of 
 signaticollis and diver sa." 
 
 Professor Cockerell's intervention has thus elicited the fact 
 
 12 Biol. Bull., XX, 1910, p. 67. 
 16 
 
226 PROBLEMS OF GENETICS 
 
 that we have as yet only a small selected part of the evidence 
 before us, even as concerning the effect of temperature on the 
 cross between signaticollis 9 X diver sa d\ We learn that at 
 the lower temperatures the result was eleven times the expected 
 one, and six times an unexpected one; further, that we owe it to 
 the author's inadvertence that we have come to hear of the 
 expected result at all, and that though he knows the factors 
 which determine the discrepancy, he declines for the present to 
 name them. In these circumstances we can scarcely venture as 
 yet to estimate the significance of these records. 
 
 The paper goes on to recount somewhat comparable, but more 
 complex instances in which the descent of the colour of adults and 
 of larvae was affected by temperature in crosses between un- 
 decimlineata and signaticollis. As they stand the results are 
 very striking and unexpected, but I think, in view of what has 
 been admitted respecting the former part of the paper, full dis- 
 cussion may be postponed till confirmation is forthcoming. 
 
 One feature, however, calls for remark. This second paper is 
 written apparently without any reference to the discoveries 
 related by Tower in his previous book, to which no allusion is 
 made. This is most noticeable in the case of an experiment in 
 which ( p. 296, H. 700A ) undecimlineata 9 (the dominant) 
 was mated to signaticollis o 71 with the result that all the offspring 
 were undecimlineata and bred true to that type (Parthenogenesis 
 was tested for, but never found to occur). This experiment was 
 made at a temperature averaging 95 F. ± 3.5 by day and 89 F. 
 =*= 4. 8° by night, and in a humidity given as 84 per cent, by day 
 and 100 per cent, by night; but in the previous book (p. 294) we 
 are told that pure undecimlineata bred together "under an ex- 
 treme stimulus of high temperature, io° C. above the average" 
 and a relative humidity of 40 per cent, gave 11 beetles only, 
 all angustovittata. But reference to the Plate 16, Fig. 2, shows 
 that angustovittata must be exceedingly like signaticollis, having, 
 like it, the elytral stripes obsolete, and if there is any marked 
 difference at all, it can only be in the larvae. It seems strange 
 that if undecimlineata really gives off ova of this recessive type at 
 high temperatures, the fact should not be alluded to in connection 
 
CAUSES OF GENETIC VARIATION 227 
 
 with expt. H. 700A, where, as the father was sig?iaticollis, having 
 the same recessive character, their appearance might have been 
 expected not to pass unobserved. The temperature in the 
 older experiment is, of course, not given with the great accuracy 
 used in the second, and it may have been higher still. The humid- 
 ity also was widely different. Still, in discussing the phenomena 
 we should expect some reference to the very remarkable and 
 closely cognate discovery which Tower himself had previously 
 reported in regard to the same species. 13 
 
 The hesitation which I had come to feel respecting these two 
 publications of Tower's has been, I confess, increased by the 
 appearance of a destructive criticism by Gortner 14 who has ex- 
 amined the parts of Chapter III of Tower's book, in which he 
 discusses at some length the chemistry of the pigments in Lep- 
 tinotarsa and other animals. As Gortner has shown, this dis- 
 cussion, though offered with every show of confidence, exhibits 
 such elementary ignorance, both of the special subject and of 
 chemistry in general, that it cannot be taken into serious con- 
 sideration. 
 
 Some observations made by Dr. D. T. MacDougal 15 have 
 also been interpreted as showing the actual causation of genetic 
 variation by chemical treatment. Of these perhaps the least open 
 to objection were the experiments with Raimannia odorata, a 
 Patagonian plant closely allied to Oenothera. The ovaries were 
 injected with various substances and from some of the seeds 
 which subsequently formed in them a remarkable new variety 
 was raised. This varying or mutational form was strikingly 
 different from the parental type, with which it was not con- 
 nected by any intergradational forms, and it bred true. It made 
 
 " As to the interrelations of these three forms, Tower states (1906, p. 18) that 
 angusiovittata, which he reared from undecimlineala, is intermediate between it 
 and signaticollis. Compare Stal, "Monogr. des Chrysomelides," 1S62, p. 163; and 
 Jacoby, Biol. Centr. Amer. Celeopt., vi, Pt. 1, p. 234, PI. xiii, fig. 20; Tab. 41. fig. 
 15; ibid., Suppl., p. 253. All these forms are evidently very closely related, and 
 the delimitation of species is quite arbitrary. Jacoby indeed suggests that unde- 
 cimlineata may be a variety of decemlineata. 
 
 14 Gortner, Amer. Nat., Dec, ion, XLV, p. 743. 
 
 16 Mutations, Variations, and Relationships of the Oenotheras, Carnegie Institu- 
 tion Publication No. 81, 1907. PP- 61-64. 
 
228 PROBLEMS OF GENETICS 
 
 no rosette, growing to a much smaller size than the parent, and 
 was totally glabrous instead of being very hairy as the parental 
 type is. I was shown specimens of these plants by the kindness 
 of Dr. Britton in the Bronx Park Botanic Garden in 1907 and 
 can testify to their very remarkable peculiarities. They had a 
 somewhat weakly look, and might at first sight be thought to be 
 a pathological product, but they had bred true for several 
 generations. From the evidence, however, I am by no means 
 satisfied that their original appearance was a consequence of 
 the treatment applied. This treatment was of a most miscel- 
 laneous description. Two of the mutants came from an ovary 
 which had been treated with a ten per cent, sugar solution. Ten 
 came from one into which a 0.1 per cent, solution of calcium 
 nitrate had been injected. One was from a capsule which "had 
 been exposed to the action of a radium pencil." MacDougal 
 speaks of these results as decisive, but clearly before such evidence 
 can be admitted even for consideration it must be shown by con- 
 trol experiments that the individual plants which threw the 
 mutant were themselves breeding true in ordinary circumstances. 
 Nothing is more likely than that the mutant was an ordinary 
 recessive. I may add that Mr. R. H. Compton made a number 
 of experiments with Raimannia odorata, raised from seeds kindly 
 given me by Dr. Britton, injecting the ovaries with a variety of 
 substances, including those named by MacDougal; but though a 
 numerous progeny was raised from the ovaries treated, all were 
 inormal. MacDougal relates also that some mutational forms 
 came from ovaries of Oenothera Lamar ckiana exposed to radium 
 pencils, and also from Oenothera biennis injected with zinc sul- 
 phate a peculiar mutant was raised, but taking into account the 
 frequency of these occurrences in those species, he very properly 
 regarded this evidence as of doubtful application. In a later 
 paper, 16 however, he has returned to the subject and affirms his 
 conviction that the appearance of a mutant among seedlings 
 raised from an ovary of Oenothera biennis treated with zinc 
 sulphate was really a consequence of the injection, saying that 
 
 16 MacDougal, D. T., "Alterations in Heredity induced by Ovarial Treat- 
 ments," Bot. Gaz., vol. 51, 191 1, p. 241. 
 
CAUSES OF GENETIC VARIATION 229 
 
 the variation previously observed in the species was afterwards 
 shown to be due to fungoid disease. The circumstances to which 
 he mainly points in support of his view is that the mutation bred 
 true, but this is only evidence of its genetic distinctness, which 
 may, of course, be admitted by those who remain unconvinced 
 as to the original cause of its appearance. He adds that he is 
 making similar experiments with some twenty genera; but what 
 is more urgently needed is repeated confirmation of the original 
 observation. When it has been shown that this mutation can 
 be produced with any regularity from a plant which does not 
 otherwise produce it on normal self-fertilisation, the enquiry 
 may be profitably extended to other plants. 
 
 A curious and novel experiment, which however, led ulti- 
 mately to a negative result, was made by F. Payne. Many dis- 
 cussions have been held respecting the blindness of cave animals. 
 The phenomenon is one of the well-known difficulties, and most 
 of us would admit that the theory of evolution by the natural 
 selection of small differences does not offer a really satisfying ac- 
 count of it. Those who believe in the causation of such modifica- 
 tions by environmental influences and in their hereditary trans- 
 mission make, of course, the simple suggestion that the darkness 
 is the cause of the loss of sight, and that disuse has led to the 
 reduction of the visual organs. Payne bred Drosophila am pel 0- 
 phila, the pomace-fly (which is easy to keep in confinement, fed 
 on fermenting bananas) , for sixty-nine generations in darkness. 
 At the end of that period there was no perceptible change in the 
 structure of the eyes, or in any other respect. The number of 
 generations may possibly be regarded as insufficient to prove 
 anything, but comparing them, as he does, with the generations 
 of mankind, we see that they correspond with a period of about 
 two thousand years, an interval far longer than those which 
 many writers in particular cases have deemed sufficient. 
 
 In his first paper Payne states that, though no structural 
 difference could be perceived, the flies which had been bred in 
 the dark reacted less readily to light than those which had been 
 reared under normal conditions, and he inclined to think that 
 the treatment had thus produced a definite effect. After more 
 
230 PROBLEMS OF GENETICS 
 
 careful tests, however, he withdrew this opinion. It proved 
 that both individual flies and individual groups of flies, both of 
 those bred in the light and of those bred in the dark, differed 
 greatly in their reactions, which were measured by counting the 
 time that it took for a fly to travel to the light end of a covered 
 tube, various sources of error being eliminated. He found further 
 that these differences of behaviour were not inherited in any 
 simple way, but he is disposed to attribute them to accidental 
 differences in the nature of the food, an account which seems 
 probable enough. 17 
 
 In several recent publications Blaringhem 18 has described 
 the origin of many abnormal forms of plants, especially of maize, 
 which he attributes to various mutilations practised upon the 
 parents. Respecting these the same difficulty which has been 
 expressed in other cases reappears, that before drawing any 
 conclusion as to the value of such evidence we require to know 
 that the plants treated belong to a really pure line, which if 
 left to nature in the ordinary circumstances of its life in that 
 locality would have had normal offspring. Abnormalities abound 
 in the experience of everyone who examines pans of seedlings 
 of almost any species of plant, and in maize they are well known 
 to be exceptionally common. Some of those which we meet 
 with when we attempt to ripen maize in this country are very 
 similar to those which Blaringhem describes, consisting in ir- 
 regularities in the distribution of the sexes, in the shapes of the 
 panicles, etc. Many of these are doubtless imperfections of 
 development, due to the dullness of our climate, but others are 
 presumably genetic and would recur in the offspring however 
 treated. If some one working in a climate where maize could 
 be raised in perfection would repeat these experiments, and show 
 that a strain which was thoroughly reliable and normal in its 
 genetic behaviour did, after mutilation, throw the miscellaneous 
 types observed by Blaringhem, that would be evidence at least 
 that the development of the seed could be so influenced by 
 injury to the parental tissues that its properties were changed. 
 
 17 Payne, Fernandus, Biol. Bull, XVIII, 1910, p. 188, and ibid., XXI, 1911, 
 p. 297. 
 
 18 See especially, Mutation et Traumatismes, Paris, Felix Alcan, 1908. 
 
CAUSES OF GENETIC VARIATION 231 
 
 Such evidence could be used for what it is worth ; but pending an 
 inquiry of this kind I am disposed to regard these observations of 
 variation following on parental injury as suggestive rather than 
 convincing. 
 
 Some evidence of a remarkably interesting kind has been 
 collected by J. H. Powers 19 respecting the structure and habits 
 of Ambly stoma tigrinum, which led him to the conclusion that 
 striking differences in the form, anatomy, and developmental 
 processes could be effected directly by change in the conditions 
 of life. It is well known that a profusion of forms, distinct in 
 various degrees, is grouped round Ambly stoma tigrinum. Some 
 of these are believed to be geographically isolated, others occur 
 together in the same waters, and, as usual, authorities have dif- 
 fered greatly as to the number of names to be given. These forms 
 were studied in detail by Cope who described them in the Ba- 
 trachia of North America. The view which he inclined to take 
 was that the individual variations of Ambly stoma tigrinum re- 
 sulted from variations in the time and completeness of the 
 metamorphosis, and these were regarded as due to external 
 causes, such as differences in season, temperature, and geo- 
 graphical conditions. Powers, however, states that collecting 
 within a radius of six or eight miles he found almost if not quite 
 the whole "gamut of recorded variation in this species." Some, 
 however, as he states, occurred rarely except under experimental 
 conditions, but considerable differences in temperature were not 
 found necessary in producing them. Every year, he says, he 
 has been able to add to the number of peculiar types found in 
 the same small area in nature, until the amount of natural 
 variation at least equals that seen by Cope in the collections of 
 the National Museum and those of the Philadelphia Academy. 
 
 Powers states that his observations by no means confirm 
 Cope's view that these differences are in the main referable to 
 variation in the completeness of metamorphosis, and on the 
 contrary, he regards metamorphosis as on the whole a levelling 
 process, tending to obliterate diversity. The enormous dif- 
 
 19 J. H. Powers, "Morphological Variation and its Causes in Amblystoma 
 tigrinum." Studies from the Zoological Laboratory. The University of Nebraska, 
 No. 71, 1907. 
 
e 3 2 PROBLEMS OF GENETICS 
 
 ferences in size and proportions which he describes can only be 
 appreciated by reference to his figures. They affect almost all 
 features of bodily organisation. These striking differences he 
 looks upon as brought about by differences in nutrition, " diversi- 
 ties in habitual locomotion," and diversity in the age at which 
 metamorphosis occurs, and to sexual difference. Apart from 
 sexual difference he regards the chief distinctions, in brief, as 
 " acquired variations of the larva." 
 
 As an example he gives the great elongation of some of the 
 forms as "due first to slow growth, second to the free-swimming 
 habit, third to the prolongation of larval life, and finally to the 
 assumption of sexual maturity as males," either in the branchiate 
 or non-branchiate condition. He describes the rapid growth of 
 some and the slow growth of others. A larva of intermediate 
 type may grow about a centimeter a month, but a rapidly growing 
 specimen may grow more than four times as much. The slower 
 rate of growth may, he says, be induced by winter feeding, and 
 other treatment. 20 
 
 When, however, he goes on to describe the influences which 
 he regards as exerted by the habit of freely swimming, I am led 
 to wonder whether after all in most of these illustrations, the 
 primary distinctions are not in reality genetic. " Specimens 
 raised in the same aquarium or in similar aquaria, side by side 
 with all conditions as uniform as it is possible to make them, 
 seldom fail to furnish striking examples of broad -headed, short- 
 bodied, and short-tailed types which are habitually found at the 
 bottom, while others, slender and elongated, are free swimmers, 
 and maintain themselves in almost as continual suspension and 
 motion as does a gold fish." Later, again, he writes, " Yet despite 
 the uniformity of these favourable conditions, the larvae soon 
 began to split up into two noticeably distinct groups, the one of 
 
 20 In connexion with this case I would refer the reader to some remarkable 
 observations of Dr. T. A. Chapman on various types of larvae which he reared 
 from the moth Arctia caja (Ent. Rec, IV, 1893, p. 265, and following parts). From 
 a single mother he raised a great diversity of forms, some which fed up rapidly 
 and passed through their development without assuming certain stages, and others 
 which were, as he called them, "laggards," moulting more times than their brethren 
 and developing at a much slower rate. It is greatly to be hoped that such a case 
 may be critically investigated by analytical breeding. 
 
CAUSES OF GENETIC VARIATION 233 
 
 unusually compact proportions, the other of uniform intermediate 
 build, such as is most commonly met with." It is to my mind 
 scarcely possible to resist the inference that, though there may be 
 definite responses to certain conditions, yet the chief distinct i< >ns 
 are genetic, and that it is these distinctions which confer the 
 power to respond. The parts respectively played by cause and 
 effect are always difficult to assign; but when it is stated that 
 "a weak-limbed, long-bodied and long-tailed animal becomes 
 well nigh perforce an undulatory swimmer, while the strong- 
 limbed, short-tailed, heavy -bodied specimen, when these charac- 
 teristics are rapidly forced upon it, is, under certain circum- 
 stances, just as forcibly induced to become a crawler," we feel 
 how erroneous any estimates of causation are likely to be. 
 
 One of the most remarkable and interesting sections of 
 Powers' paper is that in which he describes the differences in 
 bodily structure and habits which he attributes to cannibalism, 
 and the whole account of the phenomena should be read in the 
 original. It appears that there are two extremely distinct 
 types of larvae, those with narrow heads and slender bodies 
 which live for the most part on small Crustacea such asDapJuiias, 
 and those with huge mouths and very wide heads, which dis- 
 regard such small animals altogether and live on amphibian 
 larvae, whether of their own or other species. As the illustra- 
 tions show, the differences between these two types are very 
 great, and the differences in instinct and behaviour are no less. 
 The cannibals take no heed of the pelagic Crustacea, lying slug- 
 gishly at the bottom, rousing themselves immediately to a 
 violent attack on the larger living things which approach them. 
 Nothing but the most incontrovertible evidence based on abun- 
 dant control experiments should convince us that such differences 
 are not primarily genetic, and in the present state of knowledge 
 I incline to think that the families really consist of individuals 
 which are ready to assume the cannibal habit if opportunity 
 offers, and others which are congenitally incapable of it. It may 
 readily be that if all chance of cannibal diet be excluded, the 
 full development of the wide head and mouth, or the other 
 peculiarities, would never become pronounced, but I doubt whether 
 such change could be induced in any individual taken at random. 
 
CHAPTER XL 
 Sterility of Hybrids. Concluding Remarks. 
 
 When we consider the bearing of recent discoveries on those 
 comprehensive schemes of evolution with which we were formerly 
 satisfied, we find that certain details of the process are more easy 
 to imagine. We readily now understand how varieties once 
 formed, can persist, but at the same time difficulties hitherto 
 faced with complacency become formidable in the light of the 
 new knowledge. So generally is this admitted by those familiar 
 with modern genetic research that most are rightly inclined to 
 postpone the discussion. The premisses, indeed, on which such 
 a discussion must be based are almost wholly wanting. 
 
 The difficulties to which I chiefly refer are not those created 
 by the phenomena of adaptation, though they are serious enough. 
 In treating of that subject I have felt obliged to express scepti- 
 cism as to the validity of nearly all the new evidence for the 
 transmission of acquired characters. At the present time the 
 utmost we are bound to accept is the proof that (i) in some 
 parthenogenetic forms variations, or perhaps we may say mal- 
 formations, produced in response to special conditions, recur in 
 one or perhaps two generations asexually produced after removal 
 to other conditions. (2) That violent maltreatment may in rare 
 instances so affect the germ-cells contained in the parents as to 
 cause the individuals resulting from the fertilisation of those 
 cells to exhibit an arrest of development similar to that which 
 their parents underwent. 
 
 I do not doubt that evidence of this type will be greatly 
 extended. As a contribution to genetic physiology these facts 
 are very important and interesting, but I cannot think that any 
 one, on reflexion, will feel encouraged by such indications to 
 revive old beliefs in the direct origin of adaptations. 
 
 In these respects we are simply left where we were. The 
 force of objections based upon the existence of adaptative 
 
 234 
 
STERILITY OF HYBRIDS 235 
 
 mechanisms is no greater than it has always been. On the con- 
 trary the fact that variations can now so generally be recognized 
 as definite is some alleviation of the difficulty. We can moreover 
 disabuse ourselves of the notion that for all characters which are 
 definite or fixed, some utilitarian rationale may be presumed. 
 Upon that point the study of variation has provided a perfectly 
 clear answer. 
 
 In frankly recognizing that the fixity of characters in general 
 need not connote usefulness to their possessors we deliver our- 
 selves of a distracting pre-occupation and prepare our minds for 
 an investigation of the properties of living organisms in the 
 same spirit as that in which the chemist and the physicist 
 examine the properties of unorganized materials. The creature 
 persists not merely by virtue of its characteristics but in spite 
 of them, and the fact of its persistence proves no more than 
 that on the whole the balance of its properties leaves something 
 in its favour. 
 
 It may be noted by the way that the fact that the structures 
 of living things are on the whole adaptative was not always 
 obvious. Though to naturalists of this generation it is a truism, 
 we have only to turn to Buffon to find that in his philosophy of 
 nature it played no essential part. The passage in which Buffon 
 describes what he regards as the forlorn and degraded condition 
 of the Woodpecker is well known. We have come to think of 
 the Woodpecker as a capital example of adaptation to the mode 
 of life; but Buffon after enumerating the hard features of the 
 bird's existence, forced to earn its living by piercing the bark of 
 trees in an attitude of perpetual constraint, remarks 1 " Tel est 
 l'instinct etroit et grossier d'un oiseau borne a une vie triste et 
 chetive. II a recu de la Nature des organes et des instrumcns 
 appropries a cette destinee ou plutot il tient cette destinSe memo 
 des organes avec lesquels il est ne " (my italics). His reflexions 
 on the Stilt (Hitnantopus) read even more strangely to us, 
 accustomed as we are to see in the prodigious length and thinness 
 of the shanks and in the other features of its organisation pal- 
 pable adaptations to a wading life. For Buffon, however, this 
 
 1 Buffon, Hist. Nat., Oiseaux, 1780, VII, p. 3- 
 
236 PROBLEMS OF GENETICS 
 
 curious bird seemed a poor, neglected production, extravagant 
 in its disproportions, one of the misfits of creation, left as a 
 shadow in the picture composed of nature's more successful 
 efforts. 2 This theme he develops at some length, being evidently 
 well pleased with the idea. 
 
 Our way of regarding these things is doubtless sounder and 
 more fruitful than Buffon's, but it is well to remember that what 
 seems so obvious to us looked quite differently to other excellent 
 observers; and stupid as it may have been to have overlooked 
 plain examples of adaptation, it is a far worse mistake to see 
 adaptation everywhere. I do not seek to minimise the real 
 and permanent difficulty which the existence of adaptations 
 creates, but by the suggestion that all normal specific differences 
 are adaptational that difficulty was quite gratuitously increased. 
 
 In these respects it may be claimed that progress has been 
 made, even if that progress seem outwardly of small account. 
 
 But all constructive theories of evolution have been built on 
 the understanding that what we know of the relation of varieties 
 to species justifies the assumption that the one phenomenon is a 
 phase of the other, and that each species arises or has arisen 
 from another species either by one or several genetic steps. In 
 the varieties we have accustomed ourselves to think that we see 
 those steps. We still know little enough of the mode of occur- 
 rence of variation, but we do begin to know something, and if we 
 ask ourselves whether our knowledge, such as it is, conforms at 
 all readily with our former expectations, we cannot with any 
 confidence assert that it does. Among the plants and animals 
 genetically investigated are many illustrations of very striking 
 and distinct varieties. Many of these might readily enough be 
 accepted as species by even the most exacting systematists, and 
 not a few have been so treated in classification; but when we 
 have examined their relationship to each other we feel not merely 
 that they are not species in any strict sense but that the dis- 
 tinctions they present cannot be regarded as stages in the direc- 
 tion of specific difference. Complete fertility of the results of 
 inter-crossing is and I think must rightly be regarded as incon- 
 
 2 Ibid., VIII, p. us. 
 
STERILITY OF HYBRIDS 
 
 237 
 
 sistent with actual specific difference; and of variations leading 
 to that consequence no clear indication has yet been found. 
 As an example of possible exceptions mention should perhaps be 
 made of the case of a giant form of Primula sinensis investigated 
 by Keeble.3 It arose from a " Star " Primula of normal size, 
 and though fertile with its own pollen all attempts to fertilise 
 it with the pollen of other forms failed. Miss Pellew, who did 
 these fertilisations, tells me that very extensive trials were made, 
 and repeated in several seasons. Ultimately two plants were 
 raised from it fertilised with a plant of the strain from which it 
 sprang, and these proved sterile. In the light of modern expe^ 
 rience the significance of such isolated instances is doubtful. 
 All the strains known as " Giants " are, as Messrs. Sutton 
 have always found, more or less sterile, and their sterility is 
 presumably due to some negative defect. 
 
 In regard to the fertility of Primula species there are several 
 paradoxes. For example the long-styled varieties, apart from 
 giants, are fertile with their own pollen, and for many years 
 short-styled plants have not been used in most strains. Auriculas 
 and Polyanthuses, on the contrary, are generally if not always 
 bred from short-styled plants, as the florists have decided that 
 the long-styled are inadmissible. Mr. R. P. Gregory tells me 
 that, though most strains of P. sinensis give seed enough 
 when only long-styled plants are used, he finds nevertheless 
 that when a " legitimate " union is made the amount of seed 
 usually increases much as Darwin observed. Darwin's state- 
 ment that plants of " illegitimate " origin are less fertile than the 
 " legitimately " raised plants is also in general confirmed by his 
 experience. To this rule there were some marked except ions in 
 strains derived from long-styled plants, which thougfi illegitimate 
 showed a high degree of fertility, but illegitimate unions betw< 
 short-styled plants always produced comparatively sterile off- 
 spring. I have no records of the behavior of Auriculas and 
 Polyanthuses. It would be interesting to know whether among 
 them pure strains of short-styled plants (dominants) have 
 appeared, and, if so, how their fertility is affected. Without 
 
 8 Keeble, Jour. Gen., 1912, II, p. 173. 
 
238 PROBLEMS OF GENETICS 
 
 much more critical data I suppose no one would nowadays be 
 inclined to follow Darwin in instituting a comparison between 
 the sterility of hybrids and that of illegitimately raised plants of 
 heterostyle species. 4 It is even difficult to imagine any essential 
 resemblance between these two phenomena, nor has evidence 
 ever been produced to show that illegitimately raised plants 
 have bad pollen grains, which is the usual symptom of sterility 
 in hybrid plants and the consequence, as we believe, of failure 
 of some essential division in the process of maturation. 
 
 The difficulty that we have no knowledge of the contemporary 
 origin of forms, from a common stock, which when crossed together 
 give a sterile product, is one of the objections constantly and 
 prominently adduced from the time of the first promulgation of 
 evolutionary ideas. In the light of recent work the objection 
 has gathered strength. Why, if we are able to produce instances 
 of variation colourably simulating specific difference in almost 
 all other respects, do we never find an original appearance of this 
 most widely spread of all specific characteristics? No doubt all 
 breeders know that sterile animals and plants occasionally appear 
 in their cultures, but it is more in accordance with probability 
 that the sterility in these sporadic instances should be regarded 
 as due to defect than that it should be thought comparable 
 with that of the sterile hybrids. For their sterility must, by all 
 analogy with results elsewhere seen, be attributed not to the 
 absence of something, but to the presence and operation of 
 complementary factors leading to the production of inhibition 
 of division; and consistently with that interpretation, we find 
 that when from a partially sterile hybrid comparatively fertile 
 offspring can be raised, their comparative fertility continues in 
 the posterity generally if not always without diminution. The 
 distinction between these several kinds of sterility was of course 
 not understood in Darwin's time. The comparison, for example, 
 which he instituted 5 between the sterility of " contabescent " 
 anthers and that of hybrids no longer holds, for at least in those 
 cases in which the nature of contabescent anthers have been 
 genetically investigated (Sweet Pea, Tropaeolum) they proved 
 
 4 Animals and Plants, ed. I, 1868, II, pp. 180-5. 
 
 5 Animals and Plants, ed. 1, 1868, II, p. 165. 
 
STERILITY OF HYBRIDS 239 
 
 to be a simple recessive character. Nor can we now easily 
 suppose that the attempt there made by Darwin to si: 
 resemblance between the sterility produced by unnatural condi- 
 tions and that of hybrids has any physiological justification. 
 
 In regarding the power to produce a sterile or partially 
 sterile hybrid as a distinction in kind, of a nature other than 
 those which we perceive among our varieties, I am aware that 
 I am laying stress on an impression which may hereafter prove 
 false. The distinction nevertheless is so striking and so con- 
 tinually before the eyes of a practical breeder that he can 
 scarcely avoid the inference that when he meets a considerable 
 degree of sterility in a cross-bred he is dealing with something 
 belonging to a distinct category, and not merely a varietal feature 
 of an exceptional kind. 
 
 Besides the sterility of hybrids appeal has often been made to 
 the phenomenon of incompatibility, in its several stages of 
 completeness, as distinguishing species. No one doubts that in- 
 compatibility may arise from a variety of causes of most diverse 
 degrees of importance, but though sometimes referred to as an 
 extreme case of interspecific sterility, it is really a very different 
 matter. In regard to one phase of this incompatibility, that 
 associated with self-sterility, some progress has been made, and 
 we are not wholly without experimental evidence of its being 
 within the range of contemporary variation. 
 
 Given the outline of Mendelian teaching as to gametic dif- 
 ferentiation and the classification of individuals in a mixed 
 population, it seemed highly probable that what we call self- 
 sterility must mean that the species really consisted of classes, 
 some of which are capable of interbreeding with others while 
 others are not. According to the received account every indi- 
 vidual, though incapable of fertilising itself, was supposed to I e 
 able both to fertilise and to be fertilised by any other individual. 
 This notion has always seemed to me a self-evident absurdity, 
 for it would imply that there can be as many categori* 
 individuals. Such experiments, however, as I made did cer- 
 tainly give results consistent with that belief. I first tried 
 Cinerarias, which are usually self-sterile, but I found no in- 
 
240 PROBLEMS OF GENETICS 
 
 compatible pairs of plants. Whether I was deceived by the 
 consequences of apogamy, or whether the pollen of certain plants 
 may belong to more than one class I do not know. The results 
 were confused in various ways. Usually the self-fertilised plants 
 set little or nothing, and cross-fertilised they set fully with such 
 uniformity that the few failures could plausibly be attributed to 
 mistakes in manipulation or to other extraneous causes. Later 
 de Vries announced 6 (without giving particulars) that he had 
 proved the existence of such classes in Linaria vulgaris; but on 
 making experiments with that species I again got no positive 
 results, and I came to the conclusion that in spite of inherent 
 improbability the conventional belief must be substantial y true. 
 At last, however, the work of Correns, lately published, 7 does 
 definitely show that in one species, Cardamine pratensis, classes of 
 individuals exist such that individuals of the same class are 
 incapable of fertilising themselves or each other, but fertilisation 
 made between the classes is usually completely effective. Many 
 complications were encountered and some contradictory evidence 
 is recorded, but the general bearing of the results was positive 
 and indubitable. 
 
 We know far too little of this phenomenon as yet to be able 
 to understand its significance, but I suppose we may anticipate 
 with some confidence that it will be found to be a manifestation 
 of dissimilarity between the male and female gametes of the 
 same individual, comparable with that first seen in the Stocks 
 (Matihiola) which throw doubles — a state of things in all likeli- 
 hood to be found widely spread among hermaphrodite organisms. 
 Whether the incompatibility between species is to be associated 
 with that of the self-steriles also cannot be positively asserted, 
 though it seems not unreasonable to expect that such an associa- 
 tion will be discovered. 
 
 The case of the apple and the pear is an impressive illustration 
 of this possibility. The two species are of course exceedingly 
 alike in all outward respects, but nevertheless the pollen of each 
 is entirely without effect on the other. Presumably we should 
 
 6 Species and Varieties, 1905, p. 471. 
 
 7 Correns, Festschr. med.-nat. Ges. zur 84 Versamml. Deutsch. Naturf. u. Aertze. 
 MiXnster i. W„ 191 2. 
 
STERILITY OF HYBRIDS 241 
 
 interpret this fact as meaning not so much that the apple and 
 the pear are in reality very wide apart, but rather that either, 
 each is lacking in one of two complementary elements, or that 
 each possesses a factor with an inhibitory effect. Their incom- 
 patibility may well be of the same nature as that of the classes 
 in Cardamine pratensis. 
 
 Returning now to the problem of inter-specific sterility; we 
 note, as I have said, the absence of contemporary evidence that 
 variation can confer on a variety the power to form a sterile 
 hybrid with the parent species. The considerations based on 
 this want of evidence have for a long while been familiar to all 
 who have discussed evolutionary theories, and it is worth observ- 
 ing the exact reason why the difficulty strikes us now with a new 
 and special force. In pre-Mendelian times all that was known 
 was that some forms could freely interbreed without diminution 
 of fertility in the product, while others could not. But now we 
 find that, by virtue of segregation, from one and the same pair 
 of parents, or even, in the case of hermaphrodites, from one and 
 the same individual, offspring commonly arise showing among 
 themselves exactly such differences as distinguish species — and 
 very good species too. This we see happening again and again. 
 But to forms capable of arising as brethren in one family the 
 title species has never been meant to apply, and if we are going 
 to use the term in application to fraternal groups we must 
 definitely recognise that by " specific " difference is to be under- 
 stood simply difference, without any immediate or even ulterior 
 physiological limitation whatever. Naturally, therefore, we begin 
 to think of the appearance of sterility in crosses as something 
 apart, and as a manifestation which distinguishes certain kinds 
 of unions in a very special way. 
 
 I am perfectly aware that there are gradations in the sterility 
 of hybrids as in every other characteristic upon which it has been 
 proposed to base specific definitions; but, as also so often happens 
 in the matter of defining intergrading categories, the difficulty 
 in practice is not often such as to lead to actual ambiguity. I 
 am speaking of course of those examples which are amenable to 
 genetic experiment. As to the rest there is complete and perma- 
 
 17 
 
e4 2 PROBLEMS OF GENETICS 
 
 nent uncertainty. But the experience of the practical breeder 
 does, I think, on the whole, support the contention to which 
 systematists have so steadily clung under all the assaults of 
 evolutionary philosophers, that, though we cannot strictly define 
 species, they yet have properties which varieties have not, and 
 that the distinction is not merely a matter of degree. 
 
 The first step is to discover the nature of the factors which by 
 their complementary action inhibit the critical divisions and so 
 cause the sterility of the hybrid. Thus expressed, we see the 
 problem of inter-specific sterility in its right place; and the 
 question why we do not now find contemporary instances of 
 varieties lately arisen in domestication, which when crossed back 
 with their parents, or with their coderivatives, can produce 
 sterile products, is perceived to be only a special case of a problem 
 which in its more general form is that of the origin of new and 
 additional factors. 
 
 For the requisite evidence no comprehensive search has been 
 made, but perhaps it will yet be found. All that we can say at 
 the present time is that the incidence both of hybrid sterility, 
 and of incompatibility also, is most capricious; and provided 
 that two forms have such features in common that a cross between 
 them seems not altogether out of the question, no one can predict 
 without experiment whether such a cross is feasible, and if 
 feasible whether the product will be fertile, or sterile more or 
 less completely. For instance, though probably all the British 
 and some Foreign Finches (Fringillidae) have been crossed 
 together, and some of these crosses, as for instance, the various 
 Canary-mules have been made in thousands, I believe no quite 
 clear example of a fertile hybrid can be produced. Many species 
 of Anatidae cross readily and produce fertile hybrids : others give 
 results uniformly sterile. Though most of the Equidae can be 
 crossed and some of the hybrids are among the commonest of 
 domesticated animals there is no certain record of a fertile mule. 
 Among the Canidae the dogs, wolves and jackals all give fertile 
 hybrids, but there is no clearly authenticated instance of a cross 
 between any of these forms and the European fox. In spite of 
 their close anatomical resemblance it is doubtful if the rabbit 
 
STERILITY OF HYBRIDS 2 ^ 
 
 and the hare have ever interbred. Many of the wild species of 
 Bo* r have been crossed and recrossed both with each other and 
 with many domesticated races, but I understand that no cross 
 with the Indian buffalo (Bos bubalus) has yet been successful 
 even in producing a live calf." In the genus Primula many 
 hybrids are known and several of them occur in nature but 
 hitherto no certain hybrid between P. sinensis and any other 
 species has been made, in spite of repeated attempts. 
 
 In Nicotiana many— doubtless all— the various forms of Y 
 tabacum can be crossed together without diminution of fertilitv' 
 though some are very distinct in appearance, but crosses between 
 tabacum and sylvestris are highly sterile (in my experience totally 
 sterile 9 ), though the distinctions between them are not to outward 
 observation nearly so great as those which can be found between 
 the various races of Primula sinensis. 
 
 Recently some remarkable experiments bearing closely on 
 these questions have been published by F. Rosen. 10 They con- 
 cern the forms of Erophila (Draba) verna, celebrated in the history 
 of evolutionary theory as the plants especially chosen by Alexis 
 Jordan for the exposition of his views on these subjects. 
 
 The "■ species " contains a profusion of forms dissimilar in 
 many structural characters, such as the size and shape of leaved 
 flowers, fruits, etc. Of these forms many grow in association! 
 Jordan found, on experiment, that each, to the number of some 
 two hundred, bred true, and that therefore, the conventional 
 assumption that polymorphism of this kind must mean great 
 contemporary variability had no foundation in fact. So far 
 
 8 This is a case of a somewhat different order and I mention it partly for that 
 reason as an illustration of the complexity which such negative instances may 
 present. The difficulty is that though the buffalo and the zebu can breed together, 
 the foetus is too large to be born alive. (See Ackermann Bcr. J. \\r. f Watmrk ' 
 Kassel, 1898, p. 69. Prof. S. Nathusius, of Halle, who has great experience In 
 crossing Bovidae, tells me that he has always failed to cross the buffalo with 
 other species.) 
 
 9 In a paper to be published in the Report of the Genetic Conference, Paris. 
 191 1. Bellair states that he obtained some partially fertile hybrids in the a 
 N. sylvestris X tabacum. As to the various degrees of sterility in hybrid* bet* 
 Nicotiana species see Lock, R. H., Ann. Roy. Bot. Gardens. Peradeniya, IV 1909 
 P- 195- 
 
 10 Beilrage zur Biol, der PJlanzen., X, 191 1, p. 379. 
 
244 PROBLEMS OF GENETICS 
 
 indeed is the evidence from favouring the belief that such forms 
 are in any way transitional or indeterminate, that, as is well 
 known, Jordan used it with every plausibility to support the 
 doctrine of the fixity of species. To certain aspects of Jordan's 
 work we will return later in this chapter, but the matter is in 
 the present connection of especial interest for the reason that 
 Rosen has lately found by experiment that some of these pre- 
 sumably very closely allied forms, crossed together, gave hybrids 
 more or less sterile. In the case of the offspring of one pair of 
 forms only {E. cochleata and stricta) was the fertility undi- 
 minished, and the various degrees of sterility found in the 
 other crosses ranged up to the extreme infertility of the hybrids 
 between E. stricta X elata. From this cross ten plants were 
 bred. Of these the four strongest were chosen to breed from, 
 but two of the four proved totally sterile; one had only bad seeds; 
 and from the fourth a single seedling was raised which in its 
 turn proved to be sterile. From the less sterile hybrids F2 
 families were raised, with the usual experience that in this and 
 subsequent generations the sterility diminished among extracted 
 forms, new and true-breeding types with complete fertility being 
 thus derived from the original cross. 11 
 
 The production of sterility as a consequence of crossing 
 plants so nearly approaching each other as these Erophila 
 " species " do is not a little interesting, and the fact well ex- 
 emplifies the futility of the various attempts to frame general 
 expressions as to specific properties or behaviour. Commenting 
 on his results Rosen argues that the polymorphic group commonly 
 called by systematists Erophila (Draba) verna may now be 
 regarded as having arisen by crossing, as did his own types men- 
 tioned above. The question, however, what species were the 
 original progenitors of the group cannot be answered. Rosen 
 considers that no form which he knows satisfies the requirements, 
 
 11 One very peculiar feature was observed, namely, that all the new forms in F2 
 which were bred from came true. As I understand, this statement applied to five 
 such new types, and they were represented by 76 individuals in F 3 , but further 
 details on this point are desirable. Another curious fact was observed, namely 
 that one of the Fi forms (cochleata X radiata) when fertilised by cochleata gave a 
 highly polymorphic family, but fertilised by radiata the resulting offspring were 
 almost uniform. 
 
STERILITY OF HYBRIDS 245 
 
 and that it or they must be supposed to be lost. This conclusion 
 will recall the similar problem raised by the Oenothera mutants 
 (Chap. V); and unsatisfactory as it may be to nave n 
 to such hypotheses we must remember the possibility thai 
 consequence of hybridisation, subsequent segregation and re- 
 combination of factors, species may have thus actually, as we 
 may say, exploded, and left nothing but a polymorphic group of 
 miscellaneous types to represent them in posterity. If this way 
 of regarding the phenomena be a true one, the sterility now 
 when some of the group are re-crossed, becomes analogous to 
 that " reversion on crossing " which we now so well understand 
 to be a consequence of the recombination of characters separated 
 at some previous point in the history of descent. In the partial 
 sterility of the contemporary hybrid we see this character re- 
 appearing, formed now as it was on the occasion of the original 
 cross, by the meeting of complementary factors. 
 
 Another case that may be mentioned in this connection is 
 that of the crosses between various culinary peas (Pisnm sativum) 
 and a peculiar form found by Mr. Arthur Sutton growing os- 
 tensibly in a wild state in Palestine. This Palestine Pea is low 
 growing, rarely reaching 18 inches. It is in general appearance 
 like a small and poorly grown field pea. The stems are thin and 
 rather hard. The most obvious differences which distinguish 
 this from other field peas are the marked serration of the stipules, 
 and the development of pith in the pods. Such pith is often 
 present in the pods of peas more or less, but in the Palestines it 
 is so strongly developed as almost to form a lomentum. Curi- 
 ously enough, though the flowers are purple much as those of 
 ordinary field peas, there is no coloured spot in the axils. On 
 the other hand, the stems have coloured stripes running up 
 from the axils. Though this plant differs so little from domes- 
 ticated peas, all crosses with them either failed, or produced 
 hybrids quite or almost quite sterile. This was Mr. Sutton's 
 experience, and on repeating the experiments with material 
 kindly given by him I found the same result. 12 
 
 In a large series of crosses some seeds died or gave rise to 
 
 u I also had a few Fi seeds given me by Mr. R. H. Lock. 
 
246 PROBLEMS OF GENETICS 
 
 feeble plants. Of the plants which lived, few gave any seed. 
 The seed, however, that was obtained from Fi plants grew well 
 enough, and the F 2 plants proved, as often in such cases, fertile. 
 In these, indeed, no sign of sterility was noticeable. The experi- 
 ment is being repeated in various ways, for, as the genetic 
 behaviour of peas is comparatively well known, the subject is an 
 exceptionally favourable one for these investigations. 
 
 Such an example shows the confusion produced the moment 
 we attempt to harmonize conceptions of specific difference with 
 results attained by experimental methods. It has been usual 
 to regard the field pea (P. arvense) as a species distinct from the 
 edible pea (P. sativum). De Candolle and others regard the 
 field pea as derived from a form wild in Italy, but the origin of 
 the edible pea is considered to be unknown. From breeding 
 experiments we find no sterility whatever in the crosses between 
 the various arvense and sativum types, nor in the crosses made 
 between them and several other peculiar types from various 
 countries; whereas this Palestine Pea, which only differs from a 
 small arvense in what might have been thought trivial characters, 13 
 either fails to cross altogether or gives a sterile or partially 
 sterile product, whatever type be chosen as the other parent. 
 
 Examples of this kind have at least the merit that they lead 
 to more precise delimitations of the problem. We are confronted 
 with two distinct alternatives. 
 
 I. We may apply the term Species promiscuously to all 
 distinct forms. If we do so it must be clearly understood that 
 we cannot even rule out the several combinations of " presences 
 and absences " represented by the various types whether wild or 
 domesticated. For we may feel perfectly assured that at least 
 all the arvense and all the sativum types yet subjected to experi- 
 mental tests are on precisely the same level in this respect. There 
 is no distinction, logical or physiological, to be drawn between 
 them. Some contain more factors, and others contain fewer. 
 In some the re-combinations have been brought about by natural 
 variation or crossing, while the same consequences in the others 
 have resulted from man's interference. 
 
 18 In a paper about to appear in Jour Linn. Soc. Mr. A. W. Sutton identifies 
 this Palestine pea as Pisum humile of Boissier and Noe. 
 
STERILITY OF HYBRIDS 247 
 
 2. We may follow the conventions of systematise and dis- 
 tinguish the outstanding or conspicuous forms such as arvense, 
 quadratum, sativum and perhaps a few more as species, and leave 
 the rest unheeded. If this course is followed it must be clearly 
 understood and permitted as a piece of pure pragmatism, deliber- 
 ately adopted for the convenience of cataloguers and colic 
 without regard to any natural fact or system whatsoever. 
 
 But while following either the one plan or the other we shall 
 be still awaiting the answer, which only genetic experiment 
 can provide, to the question whether among the various types 
 there are some which differ from the rest in a peculiar way: 
 whether by having groups of characters linked together in 
 especially durable combinations, or by possessing ingredients 
 which cause greater or less disturbance in the processes of cell- 
 division, and especially in the processes of gametic maturation, 
 when they are united by fertilisation with complementary 
 ingredients. 
 
 Before any but the vaguest ideas regarding the nature and 
 significance of inter-specific sterility can be formed, a vast 
 amount of detailed work must be done. Sterility as a result of 
 crossing, as well as that which is alleged sometimes to arise in 
 consequence of changed conditions, is at best a negative charac- 
 teristic, and there are endless opportunities for mistake and mis- 
 interpretation in studying features of this kind. No one, I 
 suppose, would now feel any great confidence in most of the data 
 which from time to time are resuscitated for the purpose of such 
 discussions. Even the best collections of evidence, such as those 
 given by Darwin in Forms of Flowers, cannot be regarded as 
 critical when judged by present-day standards. Nothing short 
 of the most familiar acquaintance with the habitual behaviour 
 of individuals, and of strains kept under constant scrutiny tor 
 several years would enable the experimenter to form reliable 
 judgments as to the value to be attached to observations of 
 this class. 
 
 The admission must, however, be faced that nothing in recent 
 work materially tends to diminish the surprise which has always 
 been felt at the absence of sterility in the crosses between co- 
 
248 PROBLEMS OF GENETICS 
 
 derivatives. We should expect such groups of forms to behave 
 like the Erophila types, and frequently to produce sterile products 
 on crossing. Whatever be the explanation, the fact remains 
 that such evidence is wanting almost completely. In spite of 
 all that we know of variability nothing readily comparable with 
 the power to produce a sterile hybrid on crossing with a near 
 ally, has yet been observed spontaneously arising, though that 
 characteristic of specificity is one of the most widely distributed 
 in nature. It may be that the lacuna in our evidence is due 
 merely to want of attention to this special aspect of genetic 
 inquiry, and on the whole that is the most acceptable view which 
 can be proposed. But seeing that naturalists are more and 
 more driven to believe the domesticated animals and plants to be 
 poly-phyletic in origin — the descendants, that is to say, of several 
 wild forms — the difficulty is proportionately greater than it was 
 formerly, when variation spontaneously occurring was regarded 
 as a sufficient account of their diversity. 
 
 Concluding Remarks. 
 
 The many converging lines of evidence point so clearly to 
 "the central fact of the origin of the 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 mode, 
 by which specific diversity has become what we perceive it to 
 be, we have to confess an ignorance nearly total. The trans- 
 formation of masses of population by imperceptible steps guided 
 by selection, is, as most of us now see, so inapplicable to the 
 facts, whether of variation or of specificity, that we can only 
 marvel both at the want of penetration displayed by the advo- 
 cates of such a proposition, and at the forensic skill by which it 
 was made to appear acceptable even for a time. 
 
 In place of this doctrine we have little teaching of a positive 
 kind to offer. We have direct perception that new forms of life 
 may arise sporadically, and that they differ from their progenitors 
 quite sufficiently to pass for species. By the success and main- 
 tenance of such sporadically arising forms, moreover, there is 
 no reasonable doubt that innumerable strains, whether in isola- 
 
CONCLUDING REMARKS 249 
 
 tion or in community with their co-derivatives, have as a fact 
 arisen, which now pass in the lists of systematists as B ] m icies. For 
 an excellent account of typical illustrations I would refer the 
 reader to the book lately published by R. E. Lloyd 14 on the rat- 
 population of India. The observations there recorded arc typical 
 of the state of things disclosed whenever the variations of large 
 numbers of individuals are closely investigated, whether in 
 domestication or in natural conditions. 
 
 Guided by such clues we may get a good way into the prob- 
 lem. We see the origin of colourable species in abundance. 
 Then, however, doubt arises whether though these new forms 
 are as good species as many which are accepted as such by even 
 cautious systematists, there may not be a stricter physiological 
 sense in which the term species can be consistently used, which 
 would exclude the whole mass of these petites especes. 
 
 If further we find that we have, with certain somewhat 
 doubtful exceptions, never seen the contemporary origin of a 
 dominant factor, or of inter-racial sterility between indubitable 
 co-derivatives, it needs no elaboration of argument to show that 
 the root of the matter has not been reached. 
 
 Examination of the inter-relations of unquestionably distinct 
 species nearly allied, such as the two common species of Lychnis, 
 leads to the same disquieting conclusion, and the best suggestion 
 we can make as to their origin is that conceivably they may have 
 arisen as two re-combinations of factors brought together by the 
 crossing of parent species, one or both of which must be supposed 
 to be lost. 
 
 All this is, as need hardly be said, an unsatisfying conch: 
 To those permanently engaged in systematics it may well bring 
 despair. The best course for them is once for all to reo 
 that whether or no specific distinction may prove hereafter to 
 have any actual physiological meaning, it is impossible tor the 
 systematist with the means at his disposal to form a judgment oi 
 value in any given case. Their business is purely that of the 
 cataloguer, and beyond that they cannot go. They will serve 
 science best by giving names freely and by describing everything 
 
 "Lloyd, R. E., The Growth of Groups in the Animal Kingdom. London, 1912- 
 
250 PROBLEMS OF GENETICS 
 
 to which their successors may possibly want to refer, and gen- 
 erally by subdividing their material into as many species as 
 they can induce any responsible society or journal to publish. 
 Between Jordan with his 200 odd species for Erophila, and 
 Grenier and Godron with one, there is no hesitation possible. 
 Jordan's view, as he again and again declares with vehemence, is 
 at least a view of natural facts, whereas the collective species is a 
 mere abstraction, convenient indeed for librarians and beginners, 
 but an insidious misrepresentation of natural truth, perhaps 
 more than any other the source of the plausible fallacies regarding 
 evolution that have so long obstructed progress. 
 
 Nevertheless though we have been compelled to retreat from 
 the speculative position to which scientific opinion had rashly 
 advanced, the prospect of permanent progress is greatly better 
 than it was. With the development of genetic research clear 
 conceptions have at length been formed of the kind of knowledge 
 required and of the methods by which it is to be attained. If 
 we no longer see how varieties give rise to species, we may feel 
 confident that a minute study of genetic physiology of varieties 
 and species is the necessary beginning of any critical perception 
 of their inter-relations. It is little more than a century since 
 no valid distinction between a mechanical mixture and a chemical 
 combination could be perceived, and in regard to the forms of 
 life we may well be in a somewhat similar confusion. 
 
 As yet the genetic behaviour of animals and plants has only 
 been sampled. When the work has been done on a scale so 
 large as to provide generalisations, we may be in a position to 
 declare whether specific difference is or is not a physiological 
 reality. 
 
INDEX OF SUBJECTS 
 
 PAGE 
 
 Abraxas grossulariata 105, 193 
 
 Aceras hircina, local variability. . . 123 
 
 Achatinellidae, local forms of 133 
 
 Acquired characters, inheritance of, 
 
 188, et seq., 217, 234 
 
 Acronycta psi, melanic 138 
 
 Adaptation, problem of 187, 235 
 
 Agelaius, local forms 120 
 
 Agrotis, fixed and variable species. 25 
 
 Alkaptonuria 83 
 
 Alpine Plants, growing larger, if 
 
 protected 183 
 
 Alpine Varieties 165 
 
 Alytes obstetricans, Kammerer's 
 
 experiments on 199, 210 
 
 Amblystoma, races of 231 
 
 Amphidasys betularia, melanic 
 
 form 136, 138 
 
 dimorphic larvae 141 
 
 Anodonta, polymorphism of 130 
 
 Antirrhinum, striped 57 
 
 species-hybrids 99 
 
 albinos no 
 
 Apple, will not cross with pear. . . . 240 
 
 Arctia caja, effects of temperature. 192 
 
 larval variation in 232 
 
 Arctic varieties 165 
 
 Argynnis paphia and valesina in 
 
 Italy 121 
 
 Armadillo, polyembryony 42 
 
 Artistic faculty 89 
 
 Arum, rights and lefts 58 
 
 Auriculas, short-styled selected. . . . 237 
 
 Axis of symmetry in hand and foot 48 
 
 Axolotl, alleged effect of conditions 231 
 
 Azalea, bud-sports 55 
 
 Bacillus anthracis, unsegmented 
 
 form 71 
 
 Bacillus prodigiosus, variation in. . 214 
 
 Bacteria, variation in 213 
 
 Bacterium coli, variation in 215 
 
 Baeolophus, geographical races of. 159 
 
 Barley, right and left-handed 58 
 
 Basilarchia, geographical races of. 161 
 
 Begonia phyllomaniaca 50 
 
 hybrids 51 
 
 Bizarre Carnation, genetics of 54 
 
 Black, as a variation from red .... 148 
 
 2.= 
 
 PACB 
 
 Blackbird, varying 1 ^ 
 
 Black Cock, fixity of 
 
 Boarmia repandata, melanic form. 136 
 
 rhomboidaria 1 .; 
 
 Botrytis susceptibility to 
 
 Bovidae, hybrid 243 
 
 Brachydactyly 89, 05 
 
 Bradypus, vertebral variation. . . 
 Bud-sports geometrically irregular 54-57 
 Buffalo, attempts to hybridize. 
 
 Bullfinch, gynandromorph 
 
 Bulimus detritus, local variation of 126 
 
 Canary, asymmetrical markings in 154 
 
 Canidae, hybrid 
 
 Capsella 100 
 
 Cardamine pratensis 240 
 
 Cat, Polydactylism 53 
 
 Carnation, Picotees and bizarres 
 
 compared 54, 58 
 
 Cataract, hereditary 89 
 
 Certhiola, melanic 142 
 
 Chladni figures 60 
 
 Choloepus, vertebral variation in. . 68 
 
 local variation in 119 
 
 Cinerarias, self-sterility in 239 
 
 Cistudo, local variation in 119 
 
 Climatic varieties 164 
 
 Coccaceae, variation in 2 1 4 
 
 Coenonympha arcania, climatic 
 
 forms of 1 70 
 
 satyrion 
 
 Coereba, melanic 142 
 
 Colaptes, geographical races. 147. - 
 
 chrysoides 154 
 
 Colloids, growth in 
 
 Colorado beetles, exjn riments on. . 219 
 
 Colour blindness in twins 44 
 
 Continuous variation, p< 
 
 ample of 173 
 
 Coracias, geographical races of. . . 
 
 Cotton, genetics of <. 
 
 Coupling 1 10 
 
 Crab, extra claws 74 
 
 Crustacean appenda ial 
 
 Homology 
 
 Crystals, analogy with 7^ 
 
 Cydopian monsters, artificial 50 
 
 Daphnia, changed by environment 217 
 
252 
 
 INDEX OF SUBJECTS 
 
 Dasypus, polyembryony 4 2 
 
 Dianthoecia, fixed and variable 
 
 species 2 S 
 
 Disease-resistance 87 
 
 Division, power of, a fundamental 
 
 attribute of living things 38 
 
 Genetics of 4°. 50 
 
 Dogger Bank, large varieties on. . . 125 
 
 Dogs, hybrid 242 
 
 Dominance, nature of 95 
 
 Dominants, origin of new ... 88, 90, 95 
 
 Double monsters 4 2 
 
 Draba, experiments with 243 
 
 Drosophila 82, 91 
 
 Payne's experiments on 229 
 
 Earthworm, regeneration 77 
 
 Elephant, tusk segmented 38 
 
 Entelechy 80 
 
 Environmental treatment, effects 
 
 of 188, et seq. 
 
 Enzymes and genetic factors 86 
 
 Epilepsy, inheritance of traumatic. 197 
 
 Equidae, sterility of hybrid 242 
 
 Erophila, experiments with 243 
 
 species 250 
 
 Exacum, right and left 57 
 
 Euphonia elegantissima , local forms 120 
 Eupithecia rectangulata, melanic 
 
 form 137 
 
 Factors, new 88 
 
 loss of 96 
 
 Factorial representation of varieties 
 
 158, 165 
 
 Falcons, geographical races 147 
 
 Fasciation 49 
 
 Ferments, Boyle on 54 
 
 Finger-prints of twins 44 
 
 Fixity and Variability in species. . 25 
 
 Flax, climatic experiments 197 
 
 Fowl, Silky 84 
 
 Leghorn 85, 90 
 
 Dominant white 94 
 
 Wyandotte 97 
 
 Rumpless 4° 
 
 Foxes, incompatibility with dogs. . 242 
 
 Free-martin 44 
 
 Fringillidae, sterility of hybrid 242 
 
 Fundulus, cyclopian 50 
 
 Gallus, invariability of wild species 13 
 and origin of poultry 90, 97 
 
 Genitalia, a basis for classification 
 in insects ... 13 
 
 Gentians, climatic experiments. . . . 197 
 
 Geometrical structure and differen- 
 tiation 54. 5° 
 
 Geometrical distinction between 
 
 germ-cells and somatic cells 58 
 
 Gladiolus, right and left 57 
 
 Gnophus obscurata, protective col- 
 ouring 141 
 
 Goldfinch, geographical races 147 
 
 Gonioctena variabilis, variation in 
 
 sexes of I21 
 
 Gouldian Finch, polymorphism, 148, 149 
 Giacilaria stigmatella, experiments 
 
 on 193 
 
 Grantia, large varieties of 125 
 
 Ground-Squirrels, local forms of . . . 132 
 
 Grouse, red, variation 29 
 
 Guillemot, Ringed 150 
 
 Guinea-pig, Brown-Sequard's ex- 
 periments on 198 
 
 Gynandromorphs 45 
 
 Heliconius erato, forms of . . . . 122, 164 
 
 Helix lapicida, local variation of . . 126 
 
 striata 127 
 
 Heripensis 127 
 
 Caespitum 127 
 
 trochoides 127 
 
 nemoralis and hortensis 128 
 
 Helminthophila, geographical races 
 
 of 157 
 
 Hemerophila abruptaria, melanic. . 142 
 
 Hepialus humuli, in Shetland 119 
 
 Heterostyle plants 237 
 
 Hieracium 9 
 
 Himantopus 235 
 
 Homocosis °8 
 
 Hybernia progemmaria 139 
 
 Hybrids, sterility of 234 et seq. 
 
 Incompatibility between certain 
 
 allied species 240 
 
 Individual, geometrical indepen- 
 dence of 58 
 
 Inhibiting Factors 95 
 
 Intermediates, nature of 131. 135 
 
 Isolation, consequences of 118 
 
 Lacerta muralis, Kammerer's ex- 
 periments on 209 
 
 fiumana 210 
 
 Leptinotarsa, Tower's experiments 
 
 on 219 
 
 Limbs, extra, in pairs 72, 82 
 
 Limnaea, sinistral 134 
 
 Linaria vulgaris, self-sterility 240 
 
 Loasa fruits, right and left 57 
 
 Lobster, extra claws 76 
 
 Locality, variation connected with 
 
 14, 118, 146, et seq., 209 
 Lumbricus, regeneration 77 
 
INDEX OF SUBJECTS 
 
 
 Lychnis dioica and vespertina, in- 
 ter-relations of jS 
 
 macrocarpa, possibly a common 
 
 parent of. 
 
 ig 
 
 Blaringhem's experiments 
 
 28 
 
 230 
 
 Machetes pugnax, polymorphism of 
 
 male 
 
 Maize 
 on. 
 
 Maize, cumulative factors in 116 
 
 Malformations, dominants, arising 
 
 de novo g g 
 
 Manx Cat, heredity 4 6 
 
 Matthiola 84, 104, 113 
 
 Melanic varieties 135, et seq. 
 
 Memory, analogy with heredity. . . 190 
 
 Meristic variation 69, 83, 86 
 
 Mirabilis, striped ry 
 
 Models of segmentation 59, 60 
 
 "Modes," Coutagne's conception of 126 
 Modling, peculiar race of Pieris 
 
 napi at j yg 
 
 Mole, albino 27, 28 
 
 Mule, Linnaeus on 8 
 
 Mutation, Matthioli on 4 
 
 in Mercurialis c 
 
 in Kales 5 
 
 alleged in bulbs 5 
 
 Theory , oy 
 
 periods of ! I4 
 
 in Bacteria 215 
 
 Mutilation, consequences of. . . 71, 82 
 
 alleged effect of, on offspring 230 
 
 Myxococcus, variation in 214 
 
 Narwhal, asymmetry of tusks 44 
 
 Nemesia strumosa 9! 
 
 Neuration, a basis for classification 13 
 
 Nicotiana, sterility of hybrid 243 
 
 Nightjars, varying I50 
 
 Noctuidae, fixity and variability. . 
 Noctua, polymorphic and fixed 
 
 species 
 
 Noctua castanea, local forms of . . . 
 
 Nomenclature, future of 94, 
 
 Notonecta, variations of 
 
 -5 
 
 25 
 122 
 246 
 130 
 
 bidentata, melanic 
 
 137 
 40 
 
 Odontoptera 
 
 form 
 
 Oedipodidae, protectively coloured 
 
 Oenothera, new dominant in 92 
 
 rubricalyx and rubrinervis. . . .92, 95 
 
 Lamarckiana 92, 101 
 
 origin of I0 2, 245 
 
 has bad pollen-grains 102 
 
 factorial analysis of 103 
 
 pollen and egg-cells genetically 
 dissimilar 104 
 
 1 1 1 
 " 1 
 114 
 
 228 
 
 Oenothera, "twin hybrid" 
 
 laeta and velutina 
 
 ""ill < rOMCI in 
 
 possihle coupling in 
 
 dwarfs " n \ 
 
 "Triple hybrids" 
 
 alleged variation due I 
 
 riK'nt 
 
 Ophrys, local variability ]]- 
 
 Oranj e, polyembryony ' ; - 
 
 Osmotic growth [[ () . 
 
 Overlapping forms , 
 
 Papilio, geographical races of 169 
 
 Papilio turnus, variation of j . j 
 
 Pararge egeria, geographical 1 
 
 Parthenogenesis - Q 
 
 Partula, local forms of j j 3 
 
 Passer domesticus and montanu* 
 
 distinctions ._, 
 
 Pea, round and wrinkled 
 
 Pear, will not cro>s with a; 
 
 Pelargonium, variegated 
 
 bud-sports 
 
 Periodic phenomena in structure 
 Peronea, fixed and variable 
 
 " Petites especes " 249 
 
 Petunia, double 
 
 Phalanger maculatus, local varia- 
 tion IIQ 
 
 Pheasant, fixity of 
 
 Phigalia pilosaria, melanic. 
 Phratora vitellinae, experiment 
 
 Phyllotaxis 
 
 Pied varieties common in Fa 
 domesticus unknown in n 
 
 tanus 
 
 Pieris napi and bryoniae. . 
 
 Pig, mule-footed 
 
 Pigeon, web-footed 
 
 Indian Rock, a rece- 98 
 
 Pigments, nature of 83 
 
 Pisum huniile, hybrids with culi- 
 nary peas 
 
 species 
 
 Planarian, regeneration of 71, 77 
 
 Plotheia frontalis, polymorphic. I 
 Plusia, fixity and variation In 
 Poephila gouldiae, variation 
 
 Polai ity of individual 
 
 Polia chi, melanic 
 
 Polyanthus, Bhort-styl 
 
 Polydaetylism in Cat 
 
 Polyembryony 
 
 Potato, variation in 91 
 
 Poultry, evolution of 90 
 
 Prae-induction 
 
 69 
 
254 
 
 INDEX OF SUBJECTS 
 
 Primula obconica 91 
 
 sinensis, flaked 57 
 
 Leaf -shapes 70 
 
 new dominant in 92 
 
 sterility in 237 
 
 "Giants" 237 
 
 Primula, species-hybrids 243 
 
 Protective colouration 140 
 
 Pyrrhulagra, local forms 120 
 
 Python, twin- vertebrae 60 
 
 Quiscalus, geographical races of . . . 156 
 
 Rabbit, Angora 46 
 
 colours of 93 
 
 Incompatibility with hare 243 
 
 Raimannia odorata, MacDougal's 
 
 experiments on 227 
 
 Rats, Variation in 249 
 
 Recessives, origin of 90 
 
 Reciprocal crosses, giving distinct 
 
 results 105, et seq. 
 
 Regeneration 70 
 
 Repulsion no 
 
 Reversal on Regeneration 77 
 
 Rhamphocoelus, geographical forms, 
 
 159. 184 
 
 Rhinoptera, variation in jaws of . . 38 
 
 Rhythm in repetition 69 
 
 Ribs, variation of 68 
 
 Rights and Lefts 57~58 
 
 Ripples, analogous to segments, 
 
 60, 66, 67 
 
 regeneration of 79 
 
 Rollers, geographical races of 160 
 
 Ruff, polymorphism of male 28 
 
 Salamandra, maculosa and atra 
 
 182, 199, 203 
 
 spotted and striped 207 
 
 geographical variation of 208 
 
 Segmentation, nature of 63 
 
 simulated mechanically 64 
 
 compared with rippling 65 
 
 analogies with 68 
 
 Segmentation of normally unseg- 
 
 mented structures 38 
 
 Selection, Natural, an insufficient 
 cause of definiteness of types, 
 
 17, 134, 142 
 
 Sempervivu'm 211 
 
 Serial Homology, the true nature 
 
 of 62, 66 
 
 Setina, Alpine varieties 181 
 
 Sex of Twins 44 
 
 Sex-factors, possible coupling of. . . in 
 Sexual characters, variation in, 
 
 119 et seq. 
 
 Siamese twins 44 
 
 Silky Fowl 84, 85 
 
 Simocephalus, changed by environ- 
 ment • 219 
 
 Sinistral forms I33~4 
 
 Situs transversus 43 
 
 Skate's jaws, variation in 38 
 
 Sloths, vertebral variation 68 
 
 Species, conceptions of, 
 
 3, 94, 99, 241, 246 
 
 allied, distribution of 185 
 
 alternative uses of the term. . . . 246 
 Specific difference, universality of . . 12 
 of organisms compared with 
 those of inorganic materials, 15 
 failure of theory of Selection to 
 
 explain 18, 134, 248 
 
 Sphyropicus varius 149, 156 
 
 Spilosoma lubricipeda, varieties of 181 
 
 Zatima, Heligoland form 181 
 
 Spinal nerves, segmentation of . . . . 67 
 
 Sporadic variation 131, 134, 249 
 
 Squashes, polymorphism of 100 
 
 Staphylococcus pyogenes, variation 
 
 in 214 
 
 Sterility of hybrids, in general .... 234 
 
 in Lychnis hybrids 20 et seq. 
 
 in crossing forms of Draba 244 
 
 Significance of 245 
 
 Self 239 
 
 Stilt 235 
 
 Stocks 84, 104, 113 
 
 Striped varieties 57 
 
 Substantive variation 84 
 
 Subtraction-stages 93 
 
 Supernumerary limbs 72-76 
 
 Sweet pea, variation of 91 
 
 sterile anthers in. 238 
 
 Symmetry compared with heredity 41 
 Symmetry of body approximate. . 78 
 
 Syndactyly 47 
 
 in foot 48 
 
 Synthetic formulae, in nomencla- 
 ture 94 
 
 Taeniocampa, fixed and variable 
 
 species ... 25 
 
 Tamias, local forms of 132 
 
 Tanagers, geographical races of . 159 
 
 Teeth, variation in 67, 39 
 
 Tephrosia consortaria and con- 
 
 sonaria 137. 139. 140 
 
 Tephrosia species, separated by 
 
 season 119 
 
 Terminal members, variation of. . . 68 
 
 Thais rumina, local variation in . . 27 
 Tolerance, persistence of diversity 
 
 due to . .17. 134 
 
INDEX OF SUBJECTS 
 
 255 
 
 Tomato, number of cells in fruit. . 46 
 
 Transitional populations, rarity of, 165 
 
 an example 178 
 
 Tropaeolum, sterile anthers in. . . . 238 
 
 Trypanosomes, variation in 216 
 
 Tusk, of Elephant, segmented. ... 38 
 
 of Narwhal 44 
 
 Twinning 41, 44, 71 
 
 heredity of 45 
 
 in organs 46 
 
 Uria troile, variety of 150 
 
 Vanessa urticae, effects of tempera- 
 ture 191 
 
 Variation, a medley of phenomena, 
 
 14. IS 
 
 sporadic 131, 134 
 
 and locality 118 
 
 Variation, Causes of gi 
 
 -7. I31i »X3 
 
 Substantive and meristic 83 
 
 Veronica, specific difference in. . . . 10 
 rimental modification 211 
 
 intermedi. <ies... 17 
 Vertebuu-. dh ision in | 
 
 homologies of 
 
 Vespa, specific difference in 23 
 
 Vortex, living organism compared 
 
 with 40 
 
 Wave-motion compared with n 
 tition of parts 62, I 
 
 Wheat, cumulative factors in 116 
 
 climatic experiments on 195 
 
 Woodpecker 235 
 
 Zebra, pattern of stripes compared 
 with ripples 38 
 
INDEX OF PERSONS 
 
 PAGE 
 
 Ackermann 243 
 
 Agar 219 
 
 Allen, J. A 132, 147, 159 
 
 Annandale 47 
 
 Arrigoni degli Oddi 167 
 
 Backhouse 50 
 
 Baker, G. T 166 
 
 Bangs, Outram 120, 142, 155 
 
 Barrett 26, 136, 167, 173, 178, 193 
 
 Baur, E 55. 99 
 
 Baur, G 119 
 
 Beneden, van 75 
 
 Bentham, on species of Veronica. . 16 
 
 Lychnis 21 
 
 Primula 22 
 
 Bernadin 42 
 
 Bishop, L. B 153. 157 
 
 Blaringhera 230 
 
 Bobart 5 
 
 Boisduval 182 
 
 Boissier 19 
 
 Borradaile 74» 75 
 
 Boulenger, E. G 208 
 
 Boulenger, G. A 182, 207, 209 
 
 Boyle 5. 54 
 
 Brewster, W 149. 150 
 
 Britton 228 
 
 Brown, T. Graham 198 
 
 Brown-Sequard 197 et seq. 
 
 Bruant, P 51 
 
 Buffon 235 
 
 Butler, S 189, 190 
 
 Buysson, R. du 24 
 
 Candolle, de 246 
 
 Carpenter, J. H 172 
 
 Chapman, F. M 148, 156, 157, 158 
 
 Chapman, T. A 13, 167, 182, 232 
 
 Church, A. H 69 
 
 Cieslar 197 
 
 Clark, Austin 142, 144 
 
 Cockayne, E. A 43 
 
 Cockerell, T. D. A 225 
 
 Compton, R. H 50, 58, 228 
 
 Cope 231 
 
 Cory 142 
 
 Correns 240 
 
 Coutagne 125, et seq. 
 
 256 
 
 PAGE 
 
 Darwin, on Variation 1, 2 
 
 Systematics 10 
 
 Selection 134, 139 
 
 Heterostyle plants 237, 238 
 
 Darwin, F 190 
 
 Darwin, Sir G 41 
 
 Davenport 46 
 
 Davis, H. M 102 
 
 Delcourt 130 
 
 Deschange 181 
 
 Dobell 216 
 
 Doncaster 105, 121, 136 
 
 Driesch 80, 81 
 
 Duchartre 51 
 
 East 91, 116 
 
 Edwards, W. H 162 
 
 Ehrlich 216 
 
 Fellmer 216 
 
 Field, W. L. W 161 
 
 Fischer, E 192 
 
 Fleck 171, 174 
 
 Fletcher, W. H. B 26, 181 
 
 Foster, Sir M 39 
 
 Galle 123 
 
 Garrod 83 
 
 Gates 92, 95, 102 
 
 Gayner, F 177 
 
 Godron 250 
 
 Gold, E 196 
 
 Goldschmidt 116 
 
 Goodwin, E 137 
 
 Gortner. , 227 
 
 Greene, E. L 8 
 
 Gregory, R. P 92, 100, 237 
 
 Grenier 250 
 
 Grover 173 
 
 Gruber 48 
 
 Gulick 119, 133 
 
 Hamling 142 
 
 Hampson, Sir G 26 
 
 Harris 142 
 
 Hartlaub 182 
 
 Herbst 42 
 
 Heribert-Nilsson 116 
 
 Hewett 182 
 
INDEX OF PERSONS 
 
 257 
 
 Hoge 82 
 
 Honing 105 
 
 Hunter, John 44 
 
 Jakowatz 197 
 
 Janet 24 
 
 Jeans 41 
 
 Jenkinson 40 
 
 Jentink 120 
 
 Johannsen 195 
 
 Jordan 185, 243, 250 
 
 Kammerer 199, et seq. 
 
 Keeble 237 
 
 Klebs 211 
 
 Krancher 182 
 
 Kiichenmeister 44 
 
 Kudicke 216 
 
 Lamarck 9 
 
 Lang, A 128 
 
 Lawrence, W. N 142, 145 
 
 Leake, H. Martin 98, 100 
 
 Leavitt 185 
 
 Lecoq 99 
 
 Lederer 167 
 
 Leduc 64, 65, 80 
 
 Leydig 182 
 
 Linden, M. von 192 
 
 Linnaeus 6, 7, 8 
 
 Lloyd, R. E 248 
 
 Locard 130 
 
 Lock, R. H 243, 245 
 
 Loeb 42, 45. SO, 71, 77 
 
 Lotsy 99 
 
 Lowe, P. R 143 
 
 MacDougal, D. T 102, 227 
 
 Marchant 7 
 
 Mathew 171 
 
 Matthioli 4 
 
 Mayer, A. G 133 
 
 Mendel, Rediscovery of 2 
 
 On Fasciation 49 
 
 Merrifield 169, 172 
 
 Miller, W. D 120, 149 
 
 Morgan 71, 77, 91, 198 
 
 Moggridge 125 
 
 Nathusius, S 243 
 
 Nettleship 44 
 
 Newman, H. H 42 
 
 Newsholme 48 
 
 Nilsson-Ehle 116, 169 
 
 Norman, A. M 125, 156 
 
 Ober 142 
 
 Oberthur 168, 170, 193 
 
 Oliver, J 45 
 
 Page, H. E 167, 180 
 
 Patterson, J. T 42 
 
 Payne, F 279 
 
 Pellew 237 
 
 Poll 45 
 
 Porritt 136 
 
 Poulton 1 1 ,- 
 
 Powers, J. H 231 
 
 Pringsheim, H 214 
 
 Przibram. . .72, 78, 178, 194, 197, 199 
 Punnett no 
 
 R ay 4. 5 
 
 Raynor 105 
 
 Renner 117 
 
 Ridgway 10, 120 
 
 Roedelius 195 
 
 Rolfe 20 
 
 Rosen, F 243 
 
 Rosner 42 
 
 Rowland- Brown, H 167, 180 
 
 Sargent 185 
 
 Saunders, E. R 84, 104, 112 
 
 Schima 177 
 
 Schroder 193, 194 
 
 Schiibeler 195 
 
 Semon, R 190, et seq. 
 
 Sharrock 5 
 
 Shull 100 
 
 Speyer, A 166, 170, 181 
 
 Spillman 47 
 
 Standfuss 135. 181, 191 
 
 Staples-Browne 49. 98 
 
 Staudinger 170, 179 
 
 Stockard 50. 71 
 
 Sutton 237, 345 
 
 Tornier 7- 
 
 Tower, W. L 219-227 
 
 Trechmann 133 
 
 Tugwell 1S1 
 
 Tutt, J. W. On Definiteness of 
 
 Species 13 
 
 On Plusia interrogationis 26 
 
 On Tephrosia 
 
 On N. castanea IM 
 
 On Pararge egeria 167 el 
 
 Verity. R ';>■ ^ 77 
 
 Virchow 90 
 
 Vries, H. de 101-115. 2- 
 
 Walker, G 19 
 
 Weir, Jenner 
 
 Weismann 1 ?6. 188 
 
2 5 8 
 
 Wendelstadt. 
 Werbitzki . . . 
 
 Werner 
 
 Wettstein . . 
 
 INDEX OF PERSONS 
 
 Wheeler, G l68 « 
 
 Wheldale 
 
 Wilder 
 
 Wille 
 
 216 
 216 
 
 210 
 197 
 
 171 
 83 
 44 
 
 197 
 
 Williams, H 
 
 Windle, B. C. A... 
 
 167, 
 
 172 
 43 
 
 Winslow 2I4 
 
 Wolf, F 2I4 
 
 Woodforde I2 J 
 
 Woltereck 2l6 
 
 Zeijlstra , 
 
 114 
 
 H*«* 
 
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