(Thr S. 1. litU ICibrara 
 
 North (Taroltua g>tatp Hmorratty 
 
 QH651 
 M4 
 
 iA 
 
 C STATE UNIVERSITY D H^ HIU L BRARY 
 
 S00289467 - 
 
THIS BOOK IS DUE ON THE DATE 
 INDICATED BELOW AND IS SUB- 
 JECT TO AN OVERDUE FINE AS 
 POSTED AT THE CIRCULATION 
 DESK. 
 
 JAN 1 5 1976 
 
 
 ^gg,2^ 
 
 3 
 
 in7(^ 
 
 APR & W78 
 30 198a 
 
 AU 6 1 9 1984 
 JUL 2 7 1986 
 
 tl:Jrt 
 
 NOV 2 1997 
 
 CT ^ -15 
 
 o 
 
 'JUL 2 P 1QQO 
 
 20M/9-74 ■ fvJOC 
 
 ^""APRoazooz 
 
PREFACE 
 
 Parts I and II of this volume consist of an essay for 
 which the Cartwright Prize was awarded by the College 
 of Physicians and Surgeons of Columbia University in 
 1909. The entire volume is the outgrowth of an intensive 
 and extensive study of the processes of orientation in plants 
 and animals, especially those without eyes, i.e., a study of 
 the perplexing and interesting question as to how these or- 
 ganisms regulate their activities so as to bend or move 
 toward or from the source of stimulation. But while the 
 book deals primarily with the question of orientation, it 
 has a broader aspect and may be considered a treatise on 
 the behavior of organisms based on their reactions to light. 
 The generality of the treatment of the subject of actions 
 in organisms, including plants as well as animals, it is hoped 
 will make the work of value to all students of nature, espe- 
 cially to those interested in comparative psychology, 
 zoology, botany and physiology. 
 
 Throughout the work it has been my aim first of all to 
 state precisely what organisms do under different condi- 
 tions of illumination, and then to consider the bearing of 
 the observed reactions on the various theories that have 
 been formulated regarding reactions in general. This aim 
 has made it necessary to present somewhat lengthy and 
 detailed descriptions of methods of stimulation and re- 
 sponses which, it is feared, may be rather tedious to those 
 who are interested only in the general aspect of the problem. 
 To such it will be of particular advantage to consult freely 
 the table of contents and the summaries. 
 
 The historical chapters which are found in Part I deal 
 with the origin and development of theories regarding 
 the activities of organisms, especially those associated with 
 light. No attempt has been made in these chapters to 
 
 av\(o5\ . 
 
 ^\^: ., 26585 
 
 \ 
 
IV PREFACE 
 
 review all the literature on behavior. Only such works 
 are here referred to as appear to have a theoretical bear- 
 ing, but many others are considered elsewhere. 
 
 Part II is dc\oted largely to the description and dis- 
 cussion of experimental observations on orientation made 
 by the author during the past five years. Only a few of 
 these have been previously published. The remaining 
 parts of the book are more general and contain relatively 
 much less original matter. 
 
 A large part of the experimental work connected with 
 this volume was done at Johns Hopkins University dur- 
 ing my residence as Johnston Research Scholar. To this 
 institution I am greatly indebted, not only for the scholar- 
 ship, but also for exceptional facilities placed at my com- 
 mand by the late Professor W. K. Brooks, Director of the 
 Zoological Laboratory during my residence, and for friendly 
 courtesies extended on every hand by other members of 
 the University. I am also under obligation to the Marine 
 Biological Laboratory of Woods Hole, Massachusetts, for 
 research facilities during the summer of 1907, and to the 
 United States Bureau of Fisheries for similar privileges 
 during the following two summers, and especially to the 
 Director of the Laboratory of the Bureau of Fisheries at 
 Woods Hole, Massachusetts, Doctor F. B. Sumner, for 
 generously supplying my needs. It is a pleasure to ac- 
 knowledge my further indebtedness to Professor H. S. 
 Jennings for his enthusiastic interest and support in the 
 work at all times and for critically reading the manuscript; 
 to Professors G. H. Parker and J. B. Watson for valuable 
 suggestions after reading much of the work in manuscript; 
 to Professor R. M. Yerkes for his thorough criticism re- 
 garding both composition and contents; and to my wife, 
 Grace Tennent Mast, for invaluable literary aid and criti- 
 cism. The author however must be held responsible for 
 
 all of the subiect-matter*. c. r\ tv/t 
 
 •' Samuel Ottmar Mast. 
 
 Baltimore, Maryland, 
 February 4, 1910. 
 
TABLE OF CONTENTS 
 
 PART I 
 
 INTRODUCTION AND HISTORICAL REVIEW 
 
 CHAPTER I 
 
 General Introduction 
 
 CHAPTER H 
 
 Historical Review Concerning the Origin and Development of 
 Ideas and Theories Regarding Movements in Plants and Animals 
 with Special Reference to the Question of Tropisms 
 
 PAGE 
 
 1. Early Investigations and Ideas concerning Movement in Or- 
 
 ganisms S 
 
 2. First Attempts at Mechanical Explanation of Life Phenomena 
 
 — Galen, Harvey, Descartes, Borelli, Ray 6 
 
 3. Period of VitaUsm 8 
 
 4. Return to Mechanical Explanations — Johannes Miiller, De 
 
 CandoUe 8 
 
 5. Evolution and its Effect on the Study of Behavior in Plants 
 
 and Animals — Darwin, Bert, Graber, Romanes, Lubbock, 
 Preyer 9 
 
 6. Introduction of the Term " Tropism " and Development of its 
 
 AppHcation to Different Reactions — De Candolle, Knight, 
 Frank, Hofmeister, Darwin 11 
 
 7. Further Analysis of Reactions in Plants to Light — Sachs, 
 
 Strasburger, Engelmann, Darwin 13 
 
 CHAPTER III 
 
 Historical Review Concerning the Origin and Development of 
 Ideas and Theories Regarding Movements in Plants and Animals 
 with Special Reference to the Question of Tropisms (continued) 
 
 I. The Application of the Underlying Principle of Tropisms in 
 the Study of Animal Behavior as opposed to this Study from 
 the Point of View of Comparative Psychology — Loeb, 
 
 Verworn, Davenport, Radl and Others 23 
 
 V 
 
vi TABLE OF CONTEXTS 
 
 2. More Thorough Experimental Analysis showing the Relative page 
 
 Importance of Internal and External Eactors in Behavior — 
 Jennings, Holmes and Others 44 
 
 3. Summary of Historical Review ' 51 
 
 4. Various Definitions of Tropisms 53 
 
 5. Statement of Important Problems in the Study of Reactions 
 
 to Light 57 
 
 PART II 
 
 EXPERIMENTAL OBSERVATIOXS AND DISCUSSIOXS BEAR- 
 ING ON THE QUESTION AS TO HOW ORGANISMS 
 {ESPECIALLY THOSE WITHOUT EYES) BEND OR 
 TURN AND MOVE TOWARD OR FROM A 
 SOURCE OF STIMULATION 
 
 CHAPTER IV 
 
 Processes Involved in the Bending of Different Parts of Higher 
 Plants toward the Source of Light 
 
 I. Observations on Plumules of Indian Corn (Zea mays) and 
 
 Leaves of Tropaeolum 5v 
 
 a. Introduction; h. Apparatus; c. Experiments; d. Re- 
 sults; e. Discussion. 
 
 CHAPTER V 
 
 Observations on Unicellular Forms in the Process of Attaining 
 and Retaining a Definite Axial Position with Reference to the 
 Source of Light 
 
 1. Myxomycetes and Rhizopods 74 
 
 2. Euglena 80 
 
 a. Description; h. Historical account; c. Orientation in 
 light from two sources; d. Material; e. Method of loco- 
 motion; /. Accuracy of orientation; g. Mechanics of 
 orientation in Euglena x in the crawling state; h. Dis- 
 cussion; i. Orientation of Euglena in the swimming 
 state; 7. Threshold or sensitiveness when different sur- 
 faces are exposed to light; k. Function of the eye -spot. 
 
 3. Summary no 
 
TABLE OF CONTENTS 
 
 PAGE 
 
 2. Blowfly Larvae — Musca sp. (?) 175 
 
 a. Introduction; b. Locomotion; c. Accuracy of orienta- 
 tion; d. Orientation in light from two sources; e. Orien- 
 tation and movement — (ij perpendicular to the direc- 
 tion of the rays — (2) toward a source of hght; /. Sen- 
 sitive region; g. Effect of light intensity on rate of 
 locomotion; //. Method; i. Mechanics of orientation; 
 j. Discussion; k. Summary. 
 
 3. Earthworms ig8 
 
 Summary 205 
 
 4. Planaria 206 
 
 Summary 210 
 
 5. Echinoderms 211 
 
 CHAPTER X 
 
 Concerning the Question of Orientation in Mollusks, Arthropods 
 AND Vertebrates, with Special Reference to Circus Movements 
 AND their Bearing on this Question 
 
 1. General Account of Orientation 214 
 
 2. Circus Movements 215 
 
 3. Frogs and Toads 218 
 
 A. Bufo americanus. 
 
 a. Method; b. Orientation in light from two sources; 
 c. Orientation with one eye destroyed. 
 
 4. Caprella 223 
 
 a. Orientation; b. Discussion. 
 
 5. General Summary and Conclusions of Part II 228 
 
 I 
 
 PART III 
 
 GENERAL CONSIDERATION OF REACTIONS TO LIGHT 
 
 CHAPTER XI 
 
 Adaptation, Formation of Aggregations in Regions of a Given Light 
 Intensity and Different Methods of Response in Attaining 
 this Region and Remaining in it. 
 
 1. Introduction showing that Reactions in general are Adaptive. 236 
 
 2. Different Reactions observed in the Process of Collecting in 
 
 Regions having a given Condition of Illumination 239 
 
TABLE OF CONTENTS 
 
 CHAPTER VI ye ^age 
 
 Observations on Unicellular Forms in the Process of Attaini 
 AND Retaining a Definite Axial Position with Reference •' 
 THE Source of Light (continued) 
 
 PAGE 
 
 1. Stentor cocrulcus 1 13 
 
 a. Introduction; b. Orienting reactions; c. Difference in 
 sensitiveness witli different surfaces illuminated; d. Lo- 
 calized stimulation; e. Summary. 
 
 2. OeJ Jgjniura Swarm-spores 1 23 
 
 a. Description; b. Material; c. Locomotion; d. Orienta- 
 tion in light. 
 
 3. Trachelomonas 1 28 
 
 4. Chlamydomonas alboviridis (Stein) 131 
 
 5. Chlorogonium i34 
 
 6. Paramecium i34 
 
 CHAPTER VII 
 
 The Factors Involved in the Process of Orientation in Colonial 
 
 Forms 
 
 1. Volvox globator and minor 136 
 
 2. Pandorina and Eudorina 146 
 
 a. Function of the eye-spots. 
 
 CHAPTER VIII 
 
 Observations on the Responses Involved in the Regulation of 
 Movement toward the Source of Light in Coelenterates 
 
 1. Hydra viridis 149 
 
 a. Historical review; b. Effect of light intensity on ac- 
 tivity; c. Orientation and locomotion; d. Reactions of 
 negative specimens; e. General conclusions. 
 
 2. Eudendrium Planulae I59 
 
 3. Eudendrium Hydranths , 163 
 
 4. Reactions of Medusae 164 
 
 CHAPTER IX 
 
 Regulation in the Direction of Movement with Reference to the 
 Source of Light in Vermes, Fly Larvae, and Echinoderms 
 
 I. Arenicola cristata — Larvae 166 
 
 a. Description; b. Locomotion; c. Orientation; d. Me- 
 chanics of orientation; e. Discussion; /. Orienting 
 stimulation; g. Summary. 
 
TABLE OF CONTENTS ix 
 
 a. Random movements and avoiding reactions; b. Orienta- page 
 tion, change in sense of orientation, and avoiding re- 
 actions; c. Orientation and extent of movement limited 
 by environment; d. Orientation and movement directly 
 toward the place where the organism comes to rest; 
 e. Random movements and coming to rest in a given place. 
 
 CHAPTER XII 
 
 Reactions to Light which do not Result in Aggregation or 
 
 Orientation 
 
 1. Reactions to Shadows — Protective 247 
 
 2. Reactions to Shadows — Procuring Food 249 
 
 3. Reactions to Sudden Increase of Light Intensity 250 
 
 4. Reactions to Light caused by the Effect of Continued Illumi- 
 
 nation 252 
 
 5. Classification of Reactions to Light — Phototropism, Pho- 
 
 topathy 253 
 
 6. Reclassification of Reactions to Light 256 
 
 (i) On the basis of the character of the stimulus. 
 a. Reactions to change of intensity; b. Reactions to con- 
 stant illumination; c. Reactions of questionable cause. 
 (2) On the basis of the fundamental causes of the response. 
 a. Reactions caused by the direct effect of light on the 
 reacting tissue; b. Reactions caused by an indirect effect 
 of light; c. Reactions due, not to any effect of light in 
 itself, but to what a given light condition or configura- 
 tion may represent. 
 
 7. Evolution of the Reactions to Light 262 
 
 CIL\PTER XIII 
 
 Factors Involved in Regulating Reactions to Light — Variability 
 
 and Modifi ability in Behavior 
 
 I. Change in Sense of Reactions 265 
 
 a. Effect of intensity of light; b. Effect of change in tem- 
 perature — Original observations; c. Effect of chemicals 
 — Original observations; d. Effect of concentration of 
 the medium and mechanical stimuli; c. Effect of internal 
 
 changes. 
 
X TABLE OF CONTENTS 
 
 CHAPTER XIV 
 
 Factors Ina'olved in Regulating Reactions to Light — Variability 
 ANT) MoDiFiABiLiTY IN BEHAVIOR (continued) 
 
 PAGE 
 
 1. Changes in Sensitiveness, in the Optimum, and in Various Other 
 
 Features regarding Reactions 288 
 
 2. General Summary of Part III 298 
 
 PART IV 
 
 REACTIONS IN LIGHT OF DIFFERENT WAVE-LENGTHS OR 
 
 COLORS 
 
 CHAPTER XV 
 Energy, Photochemical Reactions, and Brightness 
 
 1. Energy Distribution in the Spectrum 304 
 
 2. Brightness Distribution in the Spectrum 305 
 
 3. Distribution of Actinic Effect in the Spectrum 308 
 
 CHAPTER XVI 
 Effect of Different Rays on the Reactions of Sessile Plants 
 I. Summary 319 
 
 CHAPTER XVII 
 
 The Relative Effect of Different Rays on the Reactions of 
 
 Unicellular Forms 
 
 1. Strasburger's Experiments 321 
 
 2. Engelmann's Experiments 322 
 
 a. Diatoms and Oscillaria with different species of Navicula 
 and Pinnularia as types; b. Ciliates which have chlo- 
 rophyll with Paramecium bursaria as a type; c. Flagel- 
 lates with P3uglena \iridis as a type. 
 
 3. Verworn's Experiments 326 
 
 4. Experiments of Harrington and Leaming on Amoeba 327 
 
 5. Original Observations on Amoeba 328 
 
 a. Experiments with color filters; b. Experiments with the 
 solar spectrum. 
 
TABLE OF CONTENTS xi 
 
 CHAPTER XVIII 
 
 Reactions of Multicellular Animals in Light Consisting of Waves 
 
 Differing in Length 
 
 PAGE 
 
 1. Experiments of Wilson on Hydra 333 
 
 2. Bert's Experiments on Daphnia 336 
 
 3. Lubbock's Experiments on Daphnia 337 
 
 4. Experiments of Yerkes on Simocephalus 341 
 
 5. Experiments of Graber on Various Animals 343 
 
 6. Loeb's Observations 346 
 
 CHAPTER XIX 
 
 Brief Consideration of the Reactions of Multicellular Animals 
 With Well-developed Eyes in Light Differing in Color — With 
 Special Reference to Color Vision. 
 
 1. Ants 348 
 
 2. Bees 352 
 
 3. Higher Crustacea — Experiments of Minkiewicz 355 
 
 4. Fishes 35^ 
 
 5. General Summary and Conclusions of Part IV 360 
 
 CHAPTER XX 
 
 Theoretic Considerations 366 
 
 Bibliography 379 
 
 Index 393 
 
LIGHT AND THE BEHAVIOR 
 
 OF ORGANISMS 
 
 PART I 
 
 INTRODUCTION AND HISTORICAL REVIEW 
 
 CHAPTER I 
 GENERAL INTRODUCTION 
 
 That plants and animals respond to stimulation by 
 light is a matter of common information. It is also well 
 known that many of the motile forms collect in regions 
 of a given intensity of light; that many orient, some moving 
 or turning toward a source of light, others away from it; 
 and that many go toward a source of light under certain 
 conditions and away from it under others. The distribution 
 of the power to respond to stimulation by light in the plant 
 and animal world has likewise been quite fully ascertained,^ 
 and numerous accurate observations concerning the precise 
 methods of response have been recorded. There is how- 
 ever still much contention as to the explanation of these 
 phenomena, and it is this that concerns us chiefly in this 
 work. In what manner and for what reasons do organisms 
 collect in regions of certain light intensity? How do they 
 
 1 See Wiesner, 1879, 1881; Verworn, 1889, pp. 35-61; Nagel, 1896; 
 Davenport, 1897, pp. 182, 195; Radl, 1903, pp. 64-67; Washburn, 1908, 
 pp. 120-147; Congdon, 1908. 
 
 The works of these authors referred to by means of the dates following 
 each name, as well as those of all other authors similarly referred to in the 
 text, will be found in the bibliography. 
 
 I 
 
 j 
 
 N. C State Collegt 
 
2 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 behave in light of different colors? What are the factors 
 involved in orientation, i.e., in attaining a definite axial 
 position with reference to the source of stimulation? How 
 do organisms regulate the direction of movement; how do 
 they remain oriented? What is the cause of reversal in 
 the sense of orientation? What controls variability and 
 modifiability in reactions to light? Are the reactions 
 adaptive? These are the principal problems before us, 
 problems which cannot fail to be of interest to all who are 
 in any way concerned with the activities of organisms. 
 ; \'arious solutions of these problems have been offered 
 by different investigators. Some say that motile plants 
 and animals orient and collect in light of a given intensity 
 because the particular intensity in which they congregate 
 pleases them more than any other, implying that there are 
 psychic phenomena involved in the process and indicating 
 that it is difference of light intensity in the field which 
 controls the direction of movement. Others say the re- 
 actions are not fundamentally adaptive and can be ex- 
 plained mechanically; that the movements of organisms 
 are, with few exceptions, regulated by the direction in 
 which the rays of light penetrate the tissue or by the angle 
 which the rays make with the sensitive surface or by the 
 relative intensity on symmetrical opposite sides. Light is 
 supposed by these investigators to act constantly as a 
 directive stimulus. The organisms are automatically con- 
 trolled by external factors. Still other authors claim that 
 the reactions to light are in general useful to the organism, 
 but that they can be accounted for mechanically and that 
 the essential controlling factor is a change of intensity on 
 the surface of the organism; that the other external factors 
 mentioned are of importance only in so far as they make 
 such a change possible; that light does not act constantly 
 as an orienting stimulus, and that internal physiological 
 processes have much to do with the reactions. Some 
 maintain that only the more refrangible rays of the spec- 
 trum, those toward the violet end, are efficient in stimu- 
 
GENERAL INTRODUCTION 3 
 
 lating the organisms, others appear to be equally positive 
 that all rays are active in this process, and still others say 
 that the stimulating efficiency of different rays varies in 
 different organisms and in the same organism under different 
 conditions. 
 
 Many investigators have apparently not thoroughly 
 analyzed the problems concerning reactions. To them the 
 question regarding orientation, e.g., has been merely: Is it 
 ray-direction or intensity difference that regulates this? 
 And with regard to this question they have failed to see 
 that there may be a vast difference in effect between 
 direction of rays in the field and direction through the 
 organism; between diversity in light intensity in the field 
 and variation on different parts of the surface of the or- 
 ganism. Moreover they have failed to appreciate the 
 importance of difference in sensitiveness of different parts 
 of the reacting organism, and the consequent effect of 
 change in position on stimulation. 
 
 An illustration will serve to emphasize the importance 
 of distinguishing these characteristics. Suppose we have 
 an elongated opaque organism the anterior end of which 
 is more sensitive than the posterior, and suppose that 
 this organism is in a field of direct sunlight without any 
 other obstruction. Now it is evident that under such 
 conditions the intensity in the field is uniform, but the 
 intensity on the illuminated side of the organism may be 
 almost infinitely higher than that on its shaded side, since 
 no light can get through the organism, and if the organism 
 changes its axial relation with reference to the ray direction 
 the intensity on the surface may change just as much as it 
 would if the organism moved about in a field in which the 
 intensity was not uniform. Moreover if the organism takes 
 a position in which the sensitive anterior end is shaded by 
 the rest of the body it is of course in a lower effective inten- 
 sity of light than it would be if this end were illuminated. 
 Here again we see that a change in axial position in a field 
 uniformly illuminated may produce the same effect as 
 
4 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 movement from a region of one intensity to that of another. 
 And all this is dependent upon the direction of the rays in 
 the field whereas ray direction through the organism could 
 have no such effect. It is evident then that the question 
 " intensity difference or ray direction" may mean any one 
 of several things. This loose way of stating the problems 
 has led to much confusion. 
 
 Let us then, first of all, attempt to get a clear under- 
 standing of the questions involved in the reactions to light. 
 With this in view we shall consider the origin and develop- 
 ment of ideas concerning the movements in general of 
 plants and animals, and those induced through stimulation 
 by light in particular. 
 
CHAPTER II 
 
 HISTORICAL REVIEW CONCERNING THE ORIGIN AND DE- 
 VELOPMENT OF IDEAS AND THEORIES REGARDING 
 MOVEMENTS IN PLANTS AND ANIMALS WITH 
 SPECIAL REFERENCE TO THE QUESTION 
 OF TROPISMSi 
 
 I. Early Investigations and Ideas concerning Movement in 
 
 Organisms 
 
 To primitive man motion was the criterion of life. 
 Everything that moved was aUve, not only plants and 
 animals but also various elements in nature, — water, wind, 
 fire, and the heavenly bodies. Motion was thought to 
 be under the control of higher beings, or the result of the 
 action of mind with which all living things were endowed. 
 The philosophers of early civilized races abandoned the 
 idea that all things which move are alive, but they still 
 considered that all physiological processes are due to vital 
 spirits. Aristotle (384-322 B.C.), thought that plants as 
 well as animals had souls. The pith was supposed to be 
 the seat of the soul in plants and all movements and other 
 phenomena characteristic of living things were regarded 
 as due to its activity. During this period, all but a few 
 thinkers seemed to rest content that nothing more could 
 be learned about the cause or sequence of physiological 
 processes, and these few made only feeble attempts from a 
 
 1 The following works are the main sources of information regarding 
 the earlier views on plant and animal activity: History of Botany, by Julius 
 von Sachs (1875), translation revised by I. B. Balfour, Oxford (1890); 
 General Physiology, by Max Verworn (1894), translation second edition 
 by F. S. Lee, New York (1899); Contemporary Psychology, by Guido Villa, 
 translated by H. Manacorda, London, 1903. 
 
 5 
 
6 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 philosophical point of view at further analysis of causa- 
 tion. Not until the work of Galen (131-200 ± A.D.), four 
 hundred years later, was there anything approaching ex- 
 perimental analysis. 
 
 2. First Attempts at Mechanical Explanation of Life 
 
 Phenomena 
 
 Galen studied the structure of animals by direct obser- 
 vation, even practicing vivisection on pigs and monkeys, 
 and thus he sought to learn the functions of the various 
 organs. But others did not continue the experimental 
 work begun by him, and nearly thirteen centuries passed 
 without any progress. It was not until early in the six- 
 teenth century that interest in vital phenomena was again 
 aroused, and it was a century later before Harvey made his 
 important discovery on the circulatory system and pre- 
 sented mechanical explanations for many factors involved 
 in the process of circulation, all of them based on experi- 
 mental evidence. 
 
 A few years later, building on Descartes' idea " that the 
 bodies of animals and men act wholly like machines and 
 move in accordance with purely mechanical laws," Borelli 
 undertook to reduce the movements of the organic motor 
 apparatus to purely physical principles. The work of 
 Borelli formed the foundation of the iatromechanical 
 school, the members of which sought to explain all vital 
 phenomena in animals by the application of physical prin- 
 ciples. Other investigators of this period recognized the 
 importance of chemical reactions in animal activity, and, 
 under the leadership of Sylvius, founded the iatrochemical 
 school, a school which admitted the importance of physical 
 principles in explaining animal activity, but which strongly 
 emphasized the influence of chemical phenomena in vital 
 processes. The seventeenth century, and part of the 
 eighteenth, formed a period in which mechanical explana- 
 
• HISTORICAL REVIEW 7 
 
 tions were offered for practically all reactions and other 
 physiological phenomena in animals, and the same may be 
 said with regard to plants, as will be shown in the following 
 pages. 
 
 Toward the close of the seventeenth century, the striking 
 movements of the sensitive plant, Mimosa, imported from 
 America, attracted considerable attention. Ray described 
 the movements of this plant in his " Historia Plantarum" 
 (1693), and although an apparent believer in the soul of 
 plants as defined by Aristotle, he tried to explain the move- 
 ments mechanically. He thought that they were due not 
 to sensations but to physical causes, — " Planta est corpus 
 vivens non sentiens." The leaves remain erect, he said, 
 because of the constant flow of sap into them. When 
 touched, the tubes which carry the sap to them are par- 
 tially closed, and thus the supply of sap is diminished to 
 such an extent that the leaves are no longer held erect and 
 consequently droop. He was of the opinion, that plants 
 bend toward a window because of difference in rate of 
 growth on opposite sides due to difference in temperature. 
 It was known in a general way that an increase in tempera- 
 ture causes an increase in the rate of growth in plants; 
 and Sharroc had found that the stem on which he was 
 experimenting grew toward that part of a window where 
 air entered through an opening It was from these obser- 
 vations that Ray reached his conclusions. 
 
 At about the same time Dodart came to the conclusion 
 that physical contraction of the fibers on the moister side 
 of roots and their expansion on the moister side of stems 
 caused the former to turn down and the latter up. 
 
 Du Hamel, after studying the effect of light, temperature 
 and moisture on the direction of growth, concluded that 
 the " Richtung der Dampfe " in the vessels and around 
 the plant is of prime importance, and that if heat, light 
 and moisture have any influence on the direction, it is 
 through their effect on the gases. Ridiculous explanations, 
 all of them, in the light of present knowledge! But, even 
 
8 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 so, their importance cannot readily be overestimated, for 
 they formed the foundation of later work which led to 
 most fruitful results. 
 
 3. Period of Vitalism 
 
 It was fully realized before the close of the eighteenth 
 century that the mechanical explanations thus far pre- 
 sented were inadequate to account for many fundamental 
 phenomena at which they were directed. Especially was 
 this true with reference to movements of various kinds, in 
 both plants and animals. It led to the postulation of a 
 controlling principle in living beings, foreign to chemistry 
 and physics, a hypermechanical principle known as vital 
 force. Those who believed in this principle were called 
 vitalists. Some vitalists considered the postulated force 
 inscrutable, and consequently abandoned all hope of gain- 
 ing an insight into vital processes through experimental 
 means. Others, however, among the foremost of whom were 
 the botanist, De Candolle, and the famous physiologist, 
 Johannes Miiller, held the opinion that this force was 
 subject to further experimental analysis. 
 
 The prevalence of the former view was however un- 
 doubtedly the chief cause of stagnation in general physiology'' 
 in its broadest sense, during this period, for there was no 
 corresponding unproductive period in the development of 
 physical sciences. As a matter of fact many who had been 
 prominent investigators in both biological and physical 
 sciences, now abandoned the former, and devoted their 
 entire energies to the latter. 
 
 4. Return to Mechanical Explanations 
 
 Miiller realized the weakness of the iatromechanical 
 school as well as the inadequacy of pure philosophical 
 speculation. On the one hand he recognized the importance 
 
HISTORICAL REVIEW 9 
 
 of speculation in guiding and unifying experimental work; 
 on the other he saw the necessity of founding philosophical 
 speculation on experimental facts. This broad view re- 
 sulted in much comparative work especially In physiology 
 and psychology, work which had a direct bearing on the 
 nature of psychic processes as well as on the nature of 
 physlologrcal activity. Miiller worked on the higher 
 animals almost exclusively. His aim was to analyze the 
 phenomena of life as he found them in these organisms. 
 His followers, Wohler, Llebig, Helmholtz, du Bois-Rey- 
 mond, Lotze, Weber, Fechner and others, perpetuated this 
 aim, but they did not retain his breadth of spirit. Some 
 confined their investigations to the chemical side of physi- 
 ology, others to the physical side, and still others to pure 
 psychology. The question as to the origin and evolution 
 of vital phenomena, especially psychic phenomena, was not 
 yet prominent, if indeed it had been at all considered. The 
 behavior of lower animals had been studied to some extent, 
 but the Cartesian doctrine that there is no resemblance 
 between the mind of man and that of animals was still very 
 generally accepted. 
 
 5. Evolution and its Effect on the Study of Behavior of 
 
 Plants and Animals 
 
 With the establishment of the theory of evolution, there 
 appeared a new incentive in the study of animal behavior. 
 Darwin had demonstrated in a convincing manner the 
 structural interrelationship between various animals, in- 
 cluding man. It seemed clear that the complex anatomical 
 structures found in the higher animals had ^'^^'V origin 
 in the simpler structures found In tl-. !^v%er. Coula u? 
 same be said with reference L^ oehavlor? Did the mental 
 phenomena in mr*- >'-ave their origin in the lower animals? 
 If so, then there must be some evidence of mental activity 
 in the lower animals, the psychic phenomena in these 
 
10 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 organisms must resemble those in man. and the Cartesian 
 doctrine must be wrong. 
 
 The importance of this problem was at once recognized 
 and a number of able investigators undertook its solution. 
 Prominent among these may be mentioned Darwin, Paul 
 Bert, (irabcr, Romanes, Lubbock and Preyer. It should 
 be emphasized that these in\estigators were not primarily 
 interested in explaining behavior either mechanically or 
 otherwise. Their principal aim was to throw light on the 
 origin of mental phenomena in man. Do the lower animals 
 have sensations? Do they have memory? Do they rea- 
 son? were questions which shaped their investigations. 
 These questions they sought to answer by studying the 
 behavior of animals under various conditions. Their 
 results seemed to indicate that the psychic phenomena in 
 animals differ from those in man in quantity rather than 
 in quality. 
 
 With reference to reactions to light they used what is 
 known as the preference method. Experimental condi- 
 tions were so arranged that the animals could get into 
 light of different intensities or different colors. The kind 
 of light in which they collected was supposed to be the kind 
 they preferred. The w^ork was weak in that only end results 
 of the experiments were considered; it was never ascer- 
 tained precisely how the animals got into the region in 
 which they finall>' remained. Variation in the color or 
 in the intensity of light in the field was to these investi- 
 gators the controlling factor in the movement of animals. 
 They failed to consider the possible effects of the direction 
 of the rays, of variation in light intensity on the surface of 
 the animals, and of various internal factors. This led to 
 many erroneous conclusions. Still it must l)e said that 
 whatever one may think as to the point of view of these 
 investigators and the validity of ^heir conclusions in general, 
 one cannot read with unprejudiced rp'*:d the account of 
 their work, especially that of Darwin, Lubbock, and 
 Romanes, without greatly admiring the keenness of their 
 
HISTORICAL REVIEW II 
 
 observations and the ingenuity of their experiments. The 
 point of view of these men dominated the field of animal 
 behavior from the middle of the nineteenth century until 
 the appearance of Verworn and Loeb well on toward 1890. 
 As has been stated, they studied the behavior of animals 
 with the express purpose of demonstrating the evolution 
 of psychic phenomena in man. These investigators were 
 therefore not primarily interested in a physico-chemical 
 explanation of animal behavior. 
 
 The study of behavior in plants during this period was 
 however pursued with a very different aim. The question 
 as to the origin of mental phenomena influenced this study 
 but little, for it was generally conceded that plants were 
 devoid of all traces of psychic activity. There was con- 
 sequently nothing left but to attempt to account for their 
 behavior by means of physico-chemical analysis. Even 
 the vitalists realized that in the attempt of such analysis 
 lay the only hope of progress. 
 
 6. Introduction of the Term *' Tropism'' and Development of 
 its Application to Different Reactions 
 
 In 1806 De Candolle, a vitalist, succeeded in reversing 
 the daily periodic sleep movements of leaves by exposing 
 them to artificial light during the night and to darkness 
 during the day. The same year Knight showed by at- 
 taching developing seedlings to a rapidly revolving wheel 
 that the direction of growth of roots and stems is regu- 
 lated by gravitation. He explained the directive action of 
 gravitation by assuming "that the root, being of a semi- 
 fluid consistence, is bent downwards by its own weight, 
 while the nutrient sap in the stem moves to the underside 
 and causes stronger growth there, until by means of the 
 curvature so produced the stem assumes the upright posi- 
 tion." In 1828 Johnston found that roots in growing 
 downward can overcome considerable resistance and that 
 the direction of growth is therefore not due to their weight 
 
12 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 as Knight had assumed. About the same time Dutrochet 
 a[)i)Hed endosmose and exosmose to explain the movement 
 of plants mechanically. In 1S33 He Candolle proved that 
 it is light which causes plants to grow toward a window and 
 not difference in temperature on opposite sides as Ray had 
 thought one hundred forty years earlier. De Candolle 
 discovered that light retards growth in i)lants and con- 
 cluded that they turn toward a source of light because 
 growth is retarded on the more highly illuminated side. 
 The reaction according to De Candolle is due to difference 
 in intensity of light on opposite sides. 
 
 The turning toward the light was called heliotropism by 
 De Candolle (1835, Vol. 2, p. 609), who was, according to 
 Pfeffer (1906, pp. 154-155), the first to use this term. He 
 used it merely to indicate the exciting agency and not to 
 express the physiological response involved. Hofmeister 
 (1863, p. 86) added the terms positive and negative heli- 
 otropism; Frank (1870), invented the term geotropism; and 
 Darw^in (1881), Rothert (1896) and Massart (1902) intro- 
 duced various special terms. While all these expressions 
 were at first very generally used to designate the relation 
 between the movement of the reacting organism and the 
 source of stimulation, they soon came to be used to desig- 
 nate also the processes underlying the reactions. De 
 Candolle's explanation of the reaction to light assumed a 
 direct effect of the external agent on the tissue involved in 
 the reaction; and the same was true with reference to 
 Knight's explanation of the reactions to gravity. The 
 cells in which the processes producing the curvatures took 
 place were supposed to be stimulated directly. The idea 
 of irritability, of transmission of stimuli, of a differentiation 
 between sensitive and reacting tissue, in plants had not 
 yet been promulgated. The term " tropism" then gradually 
 came to signify not merely turning, but turning due to the 
 direct effect of the stimulating agent on the tissue produc- 
 ing the movement, and this signification it has retained to 
 some extent to the present time. 
 
HISTORICAL REVIEW 
 
 7. Further Analysis of Reactions in Plants to Light n 
 
 Sachs was the first to point out the inadequacy of the 
 explanation brought forward by De Candolle. He and 
 others found negative as well as positive plant structures in 
 which the rate of growth was retarded by increase of inten- 
 sity of light. The bending from the source of light in these 
 structures could therefore not be due to difference in rate 
 of growth on opposite sides induced by difference in illu- 
 mination. Sachs was already of the opinion that gravita- 
 tion does not control the direction of growth in plants by 
 difference in the direct effect on the upper and lower sur- 
 faces of the reacting organ as Knight had assumed. He 
 says (1887, p. 696)/ ** That in geotropic curvatures the 
 important point is only as to the direction in which gravita- 
 tion acts on the part of the plant, and that it is not in any 
 way a matter of a stronger effect on the lower side and a 
 feebler effect on the upper side, requires no proof." He was 
 profoundly impressed by the similarity between the re- 
 actions to light and those to gravity. This together with 
 the inadequacy of the explanations of De Candolle and 
 Knight led him to the conclusion clearly expressed in these 
 words (1887, p. 695): '' It necessarily followed from this 
 that the standpoint assumed by De Candolle must be 
 abandoned, and that the whole subject of heliotropism 
 must be looked at in an entirely different way — a view 
 which impressed me the more, since according to all the 
 facts then known a striking agreement exists between 
 heliotropic and geotropic effects, and at the same time I 
 had even then come to see that geotropism and helio- 
 tropism are to be looked upon as phenomena of irrita- 
 bility. In addition to these reflections, also, I came to the 
 conclusion that in heliotropic curvatures the important 
 point is not at all that the one side of the part of the plant 
 
 ^ The original German edition appeared in 1882. Sachs first announced 
 his views on reactions to light in the preface of a paper by H. JNIuilcr in 
 1876. 
 
JGIIT AND THE BEHAVIOR OF ORGANISMS 
 
 lumlnated more strongly than the other, but that It is 
 ather the direction in whicli the ray of hght passes through 
 the substance of the plaiu ; " (1882, p. 851) . . . "dass es 
 sich bei den heliotropischen Kriimmungen gar nicht darum 
 handle, dass die eine Seite des Pflanzentheils starker als die 
 andere beleuchtet sei, dass es vielmehr nur auf die Richtung 
 ankomme, in welcher der Lichtstrahl die Pflanzensubstanz 
 
 durchsetzt." 
 
 h will thus clearly be seen that the term " ray direction," 
 so frequently used to characterize Sachs' view in ()p{)osition 
 to intensity difference, is confusing. It expresses the truth, 
 but not the whole truth. Sachs did not refer to ray direc- 
 tion in general but to ray direction through the tissue, nor 
 did he oppose intensity difference in general. He had 
 nothing to do w^ith the view of Bert and Graber that varia- 
 tion in illumination of the field regulates reaction to light. 
 He opposed the view of De Candolle who states explicitly 
 that it is difference of intensity on opposite sides of the 
 reacting organ which causes heliotropic curvatures. 
 
 In the study of the reactions of sessile plants to light 
 there is but one phenomenon to consider — the turning of 
 the plant or some of its parts so as to assume a definite 
 position with reference to the source of light, i.e., orienta- 
 tion. In motile forms we have not only to deal with the 
 assumption of a definite axial position and movement but 
 we have also to deal with the phenomenon of aggregation. 
 How and why do certain unicellular organisms, for example, 
 collect in dense masses in certain regions of their environ- 
 ment? How is it that so many sw'arm spores, for instance, 
 collect on the side of the dish toward the source of light? 
 It was generally assumed that this phenomenon is due to 
 difference of intensity in the field, that these organisms in 
 some way select the illumination adapted to their needs 
 and remain there. But Nageli had observed as early as 
 i860 that flagellates and swarm spores collect at the side 
 of a porcelain dish nearest the window although the inten- 
 sity of light at this place is lower than elsewhere owning to 
 
HISTORICAL REVIEW 1 5 
 
 the shadow produced by the side of the dish. This fact led 
 some authors to conclude that these organisms avoid the 
 light, but this did not account for the fact that the swarm 
 spores collect also at the window side of a dish which pro- 
 duces no shadow and in w^iich this part is most highly 
 illuminated. Cohn recognized this difficulty and con- 
 cluded in 1865, eleven years before Sachs announced his 
 ray-direction theory, that it is not difference of intensity 
 in the field but direction of the rays that regulates the 
 direction of movement in these organisms. He does not, 
 however, make it clear whether he means direction of the 
 rays through the tissue or direction in the field. 
 
 Sachs answered the question as to the cause of aggre- 
 gation in unicellular forms in a very simple way. He found 
 (1876, p. 241) that certain inanimate particles suspended 
 in water collect in definite regions when exposed to light 
 owing to currents caused by variation in temperature. 
 He was of the opinion that the movement and aggregation 
 of unicellular forms under similar conditions were largely 
 if not entirely of the same nature. 
 
 For the express purpose of testing this opinion, Stras- 
 burger (1878, p. 552) studied the reactions of swarm spores 
 to light. He repeated the experiments of Sachs and ob- 
 tained confirmatory results, but concluded from detailed 
 microscopic observations on the movements of these organ- 
 isms that the aggregations formed in light under normal 
 conditions are almost entirely due to active swimming of 
 the swarm spores and not to currents in the water. Stras- 
 burger in this paper, however, incidentally supports the 
 general theory of Sachs on heliotropism. He found in 
 agreement with Nageli's observation (i860) that positi\e 
 swarm spores move toward a source of light even if in so 
 doing they pass from regions of higher light intensity into 
 regions of lower, and concluded just as Cohn (1865) had, 
 that this cannot be due to difference of intensity. He does 
 not however consider the fact that under the conditions of 
 his experiments the anterior ends of the spores were con- 
 
1 6 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 tinually more highly ilhiminatcd than the posterior, and 
 that this difference of Hght intensity might determine the 
 direction of movement; he merely states thai this experi- 
 ment sh(nvs that the reactions are due to ray direction 
 without defining precisel)' what he means. Saclis, how- 
 ever, in referring to these experiments says (1887, p. 696), 
 " Even in the case of the infiuence of light on the move- 
 ment of swarm spores, the important point can only be as 
 to the direction of the rays of light, not as to whether the 
 given swarm spore is illuminated more strongly in front 
 or behind." 
 
 The excellent observations of Engelmann (1882-1883) on 
 the reactions of unicellular forms to light have a direct and 
 important bearing on the question of aggregation. Stras- 
 burger (1878) had observed that a sudden reduction of light 
 causes a definite reaction in swarm spores — " zitternde 
 Bewegung " — and others had seen similar responses to 
 sudden changes in the intensity of other stimulating agents. 
 But Engelmann seems to have been the first to point out 
 clearly the relation between such responses and aggre- 
 gation. He made detailed observations on the movements 
 of Paramecium bursaria, Euglena viridis. Bacterium photo- 
 metricum and other similar unicellular forms, in a field on 
 a slide containing a spot more highly illuminated than the 
 surrounding region. The illuminated spot, he says, acts 
 like a trap; the organisms in their random movements swim 
 into it without response, but when they reach the boundary 
 on the way out, they stop suddenly, turn back, and thus 
 remain in the illuminated area, which soon becomes crowded 
 with them. These observations are of such vital impor- 
 tance that it seems wise to emphasize them by quoting 
 directly from the author. Regarding Paramecium bur- 
 saria Engelmann says (1882, p. 393), " Ueberschreiten sie 
 z.B. zufiillig die Granze von Licht und Dunkel, oder tauchen 
 sie auch nur mit dcr vorderen Halfte ihres Leibes eine 
 Strecke weit in das Dunkel ein, so kehren sie sofort um 
 nach dem Licht, wie wenn das Dunkel ihnen unangenehm 
 
HISTORICAL REVIEW 1 7 
 
 ware." Referring to the reaction of Euglena in a drop 
 partially illuminated he says (1882, p. 395), '' Dieses wirkt 
 wie eine Falle, denn einmal hineingekommen, gehen die 
 Euglenen in der Regel nicht wieder heraus. Sie kehren 
 an der Grenze des Dunkels- immer so gleich wieder um ins 
 Helle. Falls sie, was bei schnellem Vorwartsschwimmen 
 wohl einmal geschieht, gans ins Dunkel hineingekommen 
 sind, sistiren sie doch sofort die weitere Vorwartsbewegung, 
 drehen um eine ihres kurzen Axen, probiren — oft unter 
 bedeutenden Gestaltsanderungen — in verschiedenen Rich- 
 tungen fortzukommen bis sie endlich wieder ins Licht 
 gerathen." The effect of sudden reduction of light inten- 
 sity on Bacterium photometricum is described in the follow- 
 ing words (1883, p. no) : " Schwacht man nun plotzlich das 
 Licht ... so sieht man alle bis dahin im Gesichtsfeld 
 schwimmenden Bakterien fast im namlichen Moment eine 
 Strecke weit zuriick schiessen, einige, meist unter leb- 
 haftesten Rotation um ihre Langsaxe, stillstehen und 
 danach wieder die gewohnliche Bewegung aufnehmen. 
 Man erhalt vollstandig den Eindruck eines Erschreckens." 
 According to Engelmann none of the organisms men- 
 tioned above responds to an increase of intensity, nor do any 
 of them respond to a decrease, if it is sufficiently gradual. 
 The response is therefore dependent upon the time rate 
 of change. 
 
 Engelmann's account of aggregation in these organisms, 
 as far as it goes, has stood the test of time. He failed 
 however to grasp the importance of orientation and direct 
 movement toward the optimum. The reactions to sudden 
 changes of intensity described in this account are in all 
 essentials like those discovered by Jennings some fifteen 
 years later in his study of Paramecium. They have been 
 designated Schreckhewegimgen by Pfeffer and motor reflex 
 and avoiding reaction by Jennings. They have much in 
 common with the reactions to shadows in many higher 
 forms, which Loeb (1893) claims are due to Unterschicds- 
 empfindlichkeit and Bohn (1908) says are due to "scfisibi- 
 
iS LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 lite differenticUcy The valuable experiments of Engelmann 
 on the beha\ior of unicellular organisms in microspectra 
 will be considered later (see Part IV). 
 
 Several very important contributions to the knowledge 
 of the reaction of i^lants. both in theory and in fact, were 
 made by Charles Darwin and his son Francis, in their 
 excellent work on "The Power of Movement in Plants" 
 (1880). (i) They made detailed observations on the move- 
 ment of different parts of plants in the absence of definite 
 external stimulations, and found that practically all parts 
 of plants — stems, leaves, roots, flowers, etc. — are constantly 
 performing circumnutation movements. From this they 
 concluded that tropic curvatures are brought about by 
 modification of movements already present, i.e., that tropic 
 stimuli are not the cause of movement but the cause of 
 modification of movement. (2) They studied the reaction 
 to light of plumules with the tips covered w ith small opaque 
 caps; of radicles w^ith the tips cauterized by the application 
 of silver nitrate; and the reactions to gravity of radicles 
 with the tips removed, and found that these structures 
 responded normally after the tips were covered, removed 
 or injured, provided that they had been previously stimu- 
 lated, but that they did not respond if they were not stimu- 
 lated until after the operation. From these results they 
 concluded that plant-organs frequently have a sensitive 
 part separated by some distance from a reacting part which 
 is not sensitive, and that impulses originating in the former 
 are transmitted to the latter. (3) They studied the reac- 
 tions to light of certain plumules with one side covered 
 wnth an opaciue sul)stance, and of others not covered but 
 exposed at intervals, and concluded that the reactions are 
 due to difference in intensity on opposite sides but that the 
 principal factor in producing stimulation is a change of 
 intensity rather than absolute difference of intensity. 
 
 These conclusions are of such fundamental importance 
 that it seems advisable to insert the following quotations 
 from the authors' w^ork cited above, (p. 485)' "All ob- 
 
HISTORICAL REVIEW 1 9 
 
 servers apparently believe that light acts directly on the 
 part which bends, but we have seen with the above described 
 seedlings^ that this is not the case. Their lower halves were 
 brightly illuminated for hours, and yet did not bend in the 
 least towards the light, though this is the part which under 
 ordinary circumstances bends the most." (p. 566), " We 
 believe that this case [referring to an experiment of Wies- 
 ner], as well as our own, may be explained by the excite- 
 ment from light being due not so much to its actual amount, 
 as to the difference in amount from that previously re- 
 ceived; and in our case there were repeated alternations 
 from complete darkness to light. In this, and in several 
 of the above specified respects, light seems to act on the 
 tissues of plants, almost in the same manner as it does on 
 the nervous system of animals." (p. 567), " It is an inter- 
 esting experiment to place caps over the tips of the cotyle- 
 dons of Phalaris, and to allow a very little light to enter 
 through minute orifices on one side of the caps, for the lower 
 part of the cotyledons will then bend to this side, and not 
 to the side which has been brightly illuminated during the 
 whole time." (pp. 568-569), " In the case of the radicles 
 of several, probably of all seedling plants, sensitiveness to 
 gravitation is confined to the tip, which transmits an influ- 
 ence to the adjoining upper part, causing it to bend towards 
 the center of the earth. That there is transmission of this 
 kind was proved in an interesting manner when horizon- 
 tally extended radicles of the bean were exposed to the 
 attraction of gravity for i or I2 h., and their tips were 
 then amputated. Within this time no trace of curvature 
 was exhibited, and the radicles were now placed pointing 
 vertically downwards, but an influence had already been 
 transmitted from the tip to the adjoining part, for it soon 
 became bent to one side, in the same manner as would have 
 occurred had the radicle remained horizontal and been 
 sti'l acted on by geotropism. Radicles thus treated con- 
 tinued to grow out horizontally for two or three days, until 
 ^ The tips of these were covered with opaque caps. 
 
20 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 a new tip was reformed; and this was then acted on by 
 geotropisni, and the radicle became curved perpendicu- 
 larly downwards." ^ {pp. 572-573), " We believe that there 
 is no structure in plants more wonderful, as far as its 
 functions are concerned, than the tip of the radicle. If 
 the tip be lightK' pressed or burnt or cut, it transmits an 
 influence to the upper adjoining part causing it to bend 
 away from the affected side; and, what is more surprising, 
 the tip can distinguish between a slightly harder and softer 
 object, by which it is simultaneously pressed on opposite 
 sides. If, however, the radicle is pressed by a similar 
 object a little above the tip, the pressed part does not 
 transmit an>- influence to the more distant parts, but bends 
 abruptly towards the object. If the tip perceives the air 
 to be moister on one side than on the other, it likewise 
 transmits an inlluence to the upper adjoining part, which 
 bends towards the source of moisture. When the tip is 
 excited by light (though in the case of radicles this was 
 ascertained in only a single instance), the adjoining part 
 bends from the light; but when excited by gravitation the 
 same part bends towards the center of gravity. In almost 
 every case we can clearly perceive the final purpose or 
 advantage of the several movements. Two, or perhaps 
 more, of the exciting causes often act simultaneously on 
 the tip, and one conquers the other, no doubt in accordance 
 with its importance for the life of the plant. The course 
 pursued by the radicle in penetrating the ground must be 
 determined by the tip; hence it has acquired such diverse 
 kinds of sensitiveness. It is hardly an exaggeration to say 
 that the tip of the radicle thus endowed, and having the 
 power of directing the movements of the adjoining parts, 
 acts like the brain of one of the lower animals; the brain 
 being seated within the anterior end of the body, receiving 
 
 ^ This experiment was first performed by Ciesielski (1875). Darwin's 
 interpretation of the results has been questioned. See Francis Darwin's 
 interesting presentation of the controversy concerning this and related sub- 
 jects (1907, PP- 35-42; (■g-7(>)- 
 
HISTORICAL REVIEW 
 
 21 
 
 impressions from the sense-organs, and directing the several 
 movements." 
 
 This work of Darwin seems to have been set aside by 
 some of the most prominent investigators of the day and 
 has even to this time not received recognition in accord 
 with its importance. Loeb does not mention it at all. 
 Sachs refers to it in the following terms (1887, p. 689): 
 '' In such experiments with roots not only is great precau- 
 tion necessary, but also the experience of years and an 
 extensive knowledge of vegetable physiology, to avoid 
 falling into errors, as did Charles Darwin and his son 
 Francis, who, on the basis of experiments which were 
 unskilfully made and improperly explained, came to the 
 conclusion, as wonderful as it was sensational, that the 
 growing-point of the root, like the brain of an animal, 
 dominates the various movements in the root." The very 
 point which Sachs rejects has however been confirmed by 
 Pfeffer (1894), Czapek (1895, p. 244), Rothert (1894, p. 3), 
 and others. Czapek's experiment bearing on this point 
 is ingenious and convincing. He forced the apex of radicles 
 
 I 
 
 n 
 
 Fig. I. I. Seedlings of Lupinus albus (smaller size). The seedling {A) has 
 been removed from the klinostat after the apex is fixed in the plass cap k, and after 
 twenty-four hours has curved so as to place itself parallel with the perpendicular 
 line shown by the arrow. After Czapek, from Pfeffer (1906). 
 
 II. Seedlings of Setaria italica. The roots have been cut away down to the 
 rudiments w, the cotyledon [plumule] fixed in the glass tube a, and the seedling is 
 then placed horizontally. In A the hypocotyl has curved through iSo°, and at 
 B has formed a complete coil. (Twice enlarged). After Darwin, from Pfeffer 
 (1906). 
 
 while being rotated on a clinostat to grow into small bent 
 tubes of glass closed at one end. When the seedlings were 
 
2 2 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 permanently fastened so that the base of the radicle was 
 horizontal and the tip veitical, there was no reaction, 
 but when so fastened that the base was vertical and the 
 tip horizontal, ihey responded by bending in the region 
 above the glass tube until the tip became vertical. (See 
 Fig. I.) 
 
CHAPTER III 
 
 HISTORICAL REVIEW CONCERNING THE ORIGIN AND DEVEL- 
 OPMENT OF IDEAS AND THEORIES REGARDING MOVE- 
 MENTS IN PLANTS AND ANIMALS WITH 
 SPECIAL REFERENCE TO THE QUESTION 
 OF TROPISMS (continued) 
 
 I. The Application of the Underlying Principle of Tropisms in 
 the Study of Animal Behavior as opposed to this Study 
 from the Point of View of Comparative Psychology 
 
 Seven years after the appearance of '' The Power of 
 Movement In Plants," by Darwin, Loeb began his work on 
 behavior of animals, at Wurzburg, in an atmosphere per- 
 vaded by the spirit of Sachs. His first paper on the subject, 
 entitled '' Die Orientierung der Thiere gegen das Licht 
 (thierischer Heliotropismus)," appeared in January, 1888. 
 A far more important and extensive paper bearing the 
 title " Der Heliotropismus der Thiere und seine Ueberein- 
 stimmung mit dem Heliotropismus der Pflanzen," was 
 brought out in pamphlet form the following year. Other 
 shorter papers followed from time to time. Most of these 
 papers, originally published in German, were translated 
 and published in English in Loeb's '' Studies in General 
 Physiology," Chicago, 1905. These translations will be 
 referred to almost exclusively in the following pages. 
 
 Loeb took up the work in animal reactions with the idea 
 of explaining such reactions on chemical and physical 
 bases in opposition to the so-called anthropomorphic 
 explanations current at the time. His object was " to 
 find the agencies which determine unequivocally the direc- 
 tion of motion in animals." He writes (1905, Preface), 
 
 I consider a complete knowledge and control of these 
 agencies the biological solution of the metaphysical problem 
 
 23 
 
24 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 of animal instinct and will." The author assumed that 
 these agencies had been fairl>- definitely ascertained with 
 reference to plants, and it was generally conceded that 
 their movements were not influenced by psychic phenomena. 
 He therefore began his work by attempting to show that 
 the reactions in plants and animals are controlled b\' the 
 same agencies, with the exi)ress purpose of proving that the 
 reactions of animals are not due to subjectixe (anthropo- 
 morphic) sensations as the work of Bert, Graber, Lubbock, 
 Romanes and others might lead one to infer. " I consider 
 it inadvisable," he says (1905, p. 16), "to represent the 
 movements observed in animals as the expression of a 
 'color preference', or a 'color sensation', of a 'pleasurable* 
 or 'unpleasurable sensation', as do most animal physiolo- 
 gists and zoologists who have studied the effects of light in 
 the animal kingdom." (1906, p. 125), " It seemed to me 
 that we had no right to see in this tendency of animals to fly 
 into flame the expression of an emotion, but that this might 
 be a purely mechanical or compulsory effect of the light, 
 identical with the heliotropic curvature observed in plants. 
 I believed that the essential effect of the light upon these 
 animals might consist in a compulsory automatic turning 
 of the head toward the source of light, corresponding to the 
 turning of the head, or the tip, of a plant stem toward the 
 light; and that the process of moving toward the source of 
 light was only a secondary phenomenon. It seemed to me 
 also that if the stem of the plant could suddenly acquire 
 the power of locomotion, it would act exactly like the 
 animals which fly into the flame." 
 
 In his first paper Loeb deals with the reactions of certain 
 insect larvae. He found that positive larvae go toward 
 the light even when conditions are so arranged that in so 
 doing they must go into light of lower intensity. These 
 results lead to the following conclusions (1888, p. 2): " Die 
 Orientirung der Thiere gegen eine Lichtquelle wird bei den 
 Pflanzen (J. v. Sachs) bedingt durch die Richtung, in 
 welcher die Lichtstrahlen die thierischen Gcwebe durchset- 
 
 fmnPTY uhkahy 
 
HISTORICAL REVIEW 25 
 
 zen, und nicht durch die Unterschiedc In der Lichtinten- 
 sitat auf den verschiedenen Seiten dcs Thieres." It is 
 evident from this quotation that Loeb at this time held 
 that the direction of the rays through the tissue is the con- 
 troUing factor in orientation of animals; that is, that orienta- 
 tion in animals takes place just as Sachs had said it does 
 in plants; that it is not due to difference of intensity on 
 different parts of the organism, but to the direction in 
 which the directive rays pass through the tissue. 
 
 The results recorded in the second paper, dated 1889, are 
 in all essentials like those found in the first. The principal 
 points established are (i) that positive animals will pro- 
 ceed toward the window under conditions such that they 
 continually get into weaker light; (2) that only the more 
 refrangible rays are active in causing reactions. From 
 these results Loeb concludes as follows (1905, p. 3; first 
 edition, 1889): " The conditions which control the movements 
 of animals toward light are identical, point for point, with 
 those which have been shown to he of paramount influence in 
 plants.'' Five conditions are considered: (i) ray direction; 
 (2) wave length; (3) constancy of intensity; (4) limits of 
 intensity; (5) temperature. Two of them, the first and 
 the third, are of special interest to us at present. I shall 
 therefore quote Loeb's words with reference to them (1905, 
 p. 2), "So far as the light is concerned, the circumstance 
 which controls the orientation of the animal and the direc- 
 tion of its movements is the direction of the rays falling 
 upon the animal. The condition which is of importance 
 on the part of the animal is the symmetrical shape of the 
 body." It consequently appears that he, at this time, 
 no longer considered the direction in which the rays pass 
 through the tissue of the organism of special Importance 
 but that he still regarded the direction in which they fall 
 upon it of importance. At the same time, however, he 
 accepted Sachs' theory as giving an adequate explanation 
 of orientation In plants and claimed that this theory also 
 holds for animals, for he says (1905, p. 89), " I showed 
 
26 LIGHT AXD THE BEHAVIOR OF ORGAMSMS 
 
 that the law put forward by Sachs for the hcHotropism 
 of plants, namely, that the direction of the rays of light 
 determines the orientation, holds good also for animals." 
 Elsewhere in the same paper he states this law explicitly 
 as follows (1905. I). 5): " Sachs came to the conclusion 
 that the dircctiou in which the rays of light jKMietrate the 
 plant tissue determines the orientation of the plant toward 
 light." This statement of the law is correct, but it should 
 be emphasized that Sachs also said " that in heliotropic 
 curvatures the imj^ortant point is not at all that the one 
 side of the part of the plant is illuminated more strongly 
 than the other." There is evidently much confusion here 
 in the application of Sachs' theory. 
 
 Do Loeb's conclusions in this paper show " that the law 
 put forward by Sachs for heliotropism of plants . . . 
 holds good also for animals"? He writes (1905, p. 28): 
 " From what has been said, no one, I believe, will doubt 
 that the direction of the progressive movements of the 
 caterpillars of Porthesia chrysorrhoea is determined by the 
 direction of the rays of light, and not b>- differences in 
 the intensity of the light in different parts of space. Posi- 
 tively heliotropic animals are compelled to turn their oral 
 pole toward the source of light and to move in the direction 
 of the rays toward this source." And (1905, P- 53). " l^he 
 direction of the rays, and not the distribution of the intensity 
 of the light, in the test-tube, therefore, determines the direction 
 of the progressive movements.'' From these quotations it is 
 evident that Loeb means ray direction in general in opposi- 
 tion to difference in intensity in the field. He proved that 
 under the conditions of his experiments the direction of 
 motion is not governed by the difference of intensity in 
 the field. But this has nothing to do with Sachs' theory, 
 for this theory does not consider the effect of ray direction 
 in the field or " distribution of the intensity in the test- 
 tube." Sachs, as stated above, says very definitely that 
 it is the direction in which the rays pass through the tissue 
 and not difference of light intensity on opposite sides of the 
 
HISTORICAL REVIEW 27 
 
 organism which regulates the movement. Consequently if 
 Loeb's explanation holds for animals and Sachs' for plants, 
 it is clear that the orientation in animals is not necessarily 
 regulated in the same way as in plants. 
 
 Sachs opposed the idea of De Candolle that difference 
 of intensity on opposite sides of the reacting organism con- 
 trols orienting reactions; while Loeb at this time opposed 
 the idea of Bert and Graber that difference of intensity in 
 the field determines the place of aggregation, and that 
 animals are '' untersdiiedsempfindlich." Sachs argued in 
 favor of ray direction through the tissue of the reacting 
 organ, Loeb in favor of ray direction in general. Failure 
 to recognize the difference between these views has led to 
 much confusion. It is on this account that the problem 
 has generally been so loosely stated in the terms "Is it 
 ray direction or intensity difference? " — a question which 
 evidently cannot be answered without an explicit state- 
 ment of the sense in which these terms are used. 
 
 Do Loeb's experimental results prove the absence of 
 sensations as factors in animal behavior as he assumes? 
 The experiments on which he bases his conclusions are 
 similar to those of Strasburger on swarm spores referred 
 to on p. 15. Loeb found that positive animals very gen- 
 erally move toward a source of light even if in so doing 
 they pass into regions of lower light intensity. He con- 
 cluded from this result correctly that this cannot be due 
 to variation in the intensity of light in the space, but 
 incorrectly that this disproves the existence of sensation, 
 for the animals with which he worked are more sensitive 
 to light at the anterior than at the posterior end. If they 
 enjoy light one would expect them to continue to face its 
 source even if the general illumination is decreased, be- 
 cause, if they should turn, the sensitive anterior end would 
 become shaded and this would cause a decrease in the 
 pleasant effect of light. The experimental results just 
 cited, therefore, do not prove the absence of sensation as a 
 controlling factor in the behavior of animals; neither do 
 
2 8 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 they show that it is not difference in li^^ht intensity on the 
 surface of the reacting organism which regiihites orientation. 
 
 Let il be clearly understood thai I am not arguing in 
 favor of psychic phenomena as factors in orientation. 
 Loeb's greatest service to the study of animal beha\ior was 
 his strenuous opposition to this idea, in spite of his failure 
 to demonstrate the absence of sensation as a factor in 
 reactions. 
 
 Let us now turn more directly to Loeb's later views on 
 orientation, or tropisms. These are clearly expressed and 
 explicitly stated in the hallowing quotations. Referring 
 to the analogy between the effect of a constant electric 
 current and light, Loeb says (1897, P- 44o) • " Wir finden 
 hier erstens Wirkungen, die bei constanter Intensitat des 
 Lichtes un\erandert andauern. Das sind die helio- 
 tropischen Wirkungen, die auf dem Einfluss des Lichtes 
 auf die Spannung assoziirter Muskelgruppen beruhen 
 (*das Licht wirkt bei constanter Intensitat daiiernd als 
 heliotropische Reizursache auf die Thiere') . . . Ich glaiibe 
 jetzt, (lass hier erne voUko^nmene Analogic der Licht- und 
 Strofniuirkicngen zu Tage tritt, derart, dass audi, ivie beim 
 Strom, die Licht-intensitdt daiiernd die Spannung der 
 Muskeln beeinfliisst, dass aber die Steilheit der Inteiisi- 
 tdtsschwanknng die Fortleitung der Spanniiytgs'dnderung 
 bestimmt. 
 
 " Aber die Analogic zwischen der Stromwirkung und der 
 Lichtwirkung geht weiter. Als den fiir die heliotropische 
 Orientirung der Thiere wescntlirhen ausseren Umstand 
 wies ich die Richtung der Lichtstrahlen nach, wie das Sachs 
 bereits friiher fiir die Pflanzen gethan hatte. Das Wesen 
 der Orientirung fasste ich dahin auf, dass bei \ollendeter 
 Orientirung Symmetriepunkte der Oberfldche des Thieres 
 unter gleichem \Vi?ikel von den Lichtstrahlen getroffen werden." 
 
 An expla-natory footnote (1905, p. 2), dated 1903, reads 
 as follows: " In these experiments it is presumed that the 
 animals move under the influence of only one source of 
 light. It is explicitly stated in this and the following papers 
 
HISTORICAL REVIEW 29 
 
 that if there are several sources of Hght of unequal iiuen- 
 sity, the hght with the strongest intensity determines the 
 orientation and direction of motion of the animal. Other 
 possible complications are covered by the unequivocal 
 statement, made and emphasized in this and the following 
 papers on the same subject, that the main feature in all 
 phenomena of heliotropism is the fact that symmetrical 
 points of the photosensitive surface of the animal must be 
 struck by the rays of light at the same angle. It is in full 
 harmony with this fact that if two sources of light of equal 
 intensity and distance act simultaneously upon a helio- 
 tropic animal, the animal puts its median plane at right 
 angles to the line connecting the two sources of light. 
 This fact was not only known to me, but had been demon- 
 strated by me on the larvae of flies as early as 1887, in 
 Wtirzburg, and often enough since. These facts seem to 
 have escaped several of my critics." 
 
 In these papers it is clear that the important factors in 
 orientation to light are considered to be: (i) symmetry of 
 the body; (2) the angle between the rays and the sensitive 
 surface on opposite sides; and (3) constant intensity, 
 functioning as it does in case of the electric current. Orien- 
 tation in light is supposed to be controlled by the direct 
 action of the external agent, on the locomotor tissue or 
 through a direct reflex arc. It is controlled unequivocally 
 by the external agent, which acts constantly as a directive 
 stimulus similar to the action of a constant electric current. 
 
 At this time Loeb evidently still placed much dependence 
 upon the assumed effect of the angle which the rays make 
 with the sensitive surface (ray direction), for if he con- 
 sidered merely intensity difference on opposite sides it 
 would be impossible for him to say as he does that when 
 organisms are exposed to light from " several sources . . . 
 of unequal intensity, the light with the strongest intensity 
 determines the orientation and direction of motion of the 
 animal." In a more recent discussion however he uses 
 the following expression (1906, p. 130): '' We started with 
 
30 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 the assumption that the heliotropic reactions are caused by 
 a chemical effect of HglU; in all such reactions time plays 
 a role. W'c assume, furthermore, that if light strikes the 
 two sides of a symmetrical organism with unequal inten- 
 sity, the \elocity or the character of the chemical reactions 
 in the photosensitixe elements of both sides of the body is 
 different." This and the following c}uotation show that 
 he now considers orientation to be controlled by difference 
 of intensity on opposite sides, the very idea which Sachs 
 in his theory opposed. 
 
 In the following quotation he also brings out his idea as 
 to the direct effect of the agent on the reacting tissue with 
 reference to plants. Orientation in animals is supposed to 
 be just like this in principle; in animals, the agent is sup- 
 posed to affect the locomotor organs directly or through a 
 direct reflex arc (1906, p. 118) : " How can light bring about 
 heliotropic curvatures? Let us suppose that light strikes 
 a plant on one side onh', or more strongly on one side than 
 on the opposite side, and that it be absorbed in the super- 
 ficial layers of tissue of that side. In this case we assume 
 that on that side certain chemical reactions occur with 
 greater velocity than on the opposite side. What these 
 reactions are is unknown; we may think provisionally of 
 oxidations. This change in the velocity of chemical re- 
 actions either produces a tendency of the soft elements on 
 that side to contract a little more than on the opposite 
 side, or creates otherwise a greater resistance to those 
 forces which have a tendency to elongate or stretch the 
 plant, e.g., h>drostatic pressure inside the cells, or imbibi- 
 tion of certain tissue elements. The outcome will be that 
 one side of the stem will be stretched more than the oppo- 
 site side, and this will bring about a curvature of the stem. 
 Where the latter is soft at the tip, the bending will occur 
 only, or chiefly, in that region; and as the degree of softness 
 decreases rapidly from the tip downward, the result will 
 be that the tip will bend toward the source of light. This 
 result may possibly be aided by a greater photosensitive- 
 
HISTORICAL REVIEW 
 
 31 
 
 ness of the extreme tip of the stem, although I am not aware 
 that this is an established fact." 
 
 It is strange that such a theory should have been sug- 
 gested to explain heliotropic curvatures in plants twenty- 
 six years after Darwin (see p. 18) proved that only the tips 
 of certain radicles and plumules are sensitive to light and 
 that the region where the curvature takes place is fre- 
 quently not at all sensitive, and several years after Pollock 
 (1900) had shown that traumatic stimuli are in many 
 instances transmitted from the tip of radicles to the motory 
 zone 5 to 8 mm. distant and produce curvatures toward 
 the uninjured side even if the cortex, the conducting tissue, 
 is cut on the side between the point of stimulation and the 
 motory zone. Moreover Loeb's theory fails utterly to 
 account for curvatures in structures having but a single cell 
 cavity as, for example, Vaucheria, the rhizoids of liver- 
 worts, and the hyphae of molds, all of which were known 
 to respond to light long before his theory was formulated. 
 
 Loeb's idea that the movements in plants and animals are 
 unequivocally controlled by external agents is emphasized 
 in the following quotations: (1905, p. 107), "By the help 
 of these causes it is possible to control the ' voluntary * 
 movements of a living animal just as securely and une- 
 quivocally as the engineer has been able to control the 
 movements in inanimate nature "; (1906, p. 128), " It 
 should be observed that the essential feature in these re- 
 actions is the compulsory turning of the head by the light, 
 which leaves the animal no choice, making all the cater- 
 pillars of Porthesia or all the plant lice of the same culture 
 behave exactly alike, just as in the case of a magnet all the 
 pieces of iron are compelled to behave alike"; (1906, 
 p. 124), " The light would turn them automatically until 
 their axis of symmetry was in the direction of the rays of 
 Hght,and theanimal could then move only in this direction." 
 
 Thus we have seen that in 1906 Loeb asserts that orien- 
 tation in light is unequivocally controlled by the relative 
 intensity on symmetrically located sensitive parts of the 
 
32 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 organism; that light stimulates the locomotor organs con- 
 tinuously and directi}' or through a direct reflex arc. When 
 both sides are not equall>- illuminated one moves faster 
 than the other, causing the organism to turn until the light 
 intensity on the two sides is ecjual when they are both 
 equally stimulated and consequentl\- move at the same rate. 
 This \iew he apparently still holds for he affirms it in 
 unquestionable terms in a recent address (1909, pp. 9-15): 
 '' Zwei Faktoren bestimmen die Progressivbewegung der 
 Tiere unter diesen Bedingungen; der eine ist die symme- 
 trische Strukturdes Tieres und der zweite die photochemische 
 Wirkung des Lichtes (p. 9). . . . Wenn nun mehr Licht 
 auf eine Retina fallt als auf die andere, so werden auch die 
 chemischen Reaktionen, Beispielsweise die organischen 
 Oxydationen, in einer Retina mehr beschleunigt als in der 
 andcrn; und dementsprechend werden in dem einen op- 
 tischen Nerven starkere chemische Anderungen auftreten als 
 in dem anderen (p. 11). . . . Diese Ungleichheit der che- 
 mischen Prozesse pflanzt sich von den sensiblen in die 
 motorischen Nerven und schliesslich in die mit denselben 
 verbundenen Muskeln fort. Wir schliessen daraus, dass bei 
 gleicher Beleuchtung der beiden Retinae die symmetrische 
 Muskelgruppe beider Korperhalften in gleicher Weiser 
 chemisch beeinflusst werden und somit in den gleichen 
 Kontractionszustand geraten ; wahrend wenn die Reaktions- 
 geschwindigkeit ungleich ist, die symmetrischen Muskeln 
 auf einer Seite des Korpers in starkere Tatigkeit geraten, 
 als auf der andern Seite. Das Resultat einer solchen un- 
 gleichen Tatigkeit der symmetrischen Muskeln beider 
 Korperhalften ist eine Anderung der Bewegungsrichtung 
 des Tieres " (p. 12). 
 
 In his earlier work Loeb appears to have held that all 
 reactions to light are due to constant intensity, but later 
 (1893, p. 265) he recognizes that some are due to change 
 in intensity. The former he calls heliotropic, the latter 
 photokinetic (unlerscliiedsempfi?idlich). He characterizes the 
 difference betw^een the two thus (1906, p. 135): " Helio- 
 
HISTORICAL REVIEW 
 
 33 
 
 tropism covers only those cases where the turning to the 
 Hght is compulsory and irresistible, and is brought about 
 automatically or mechanically by the light itself. On the 
 other hand, there are compulsory and mechanical reactions 
 to light which are not cases of heliotropism; namely, the 
 reaction to sudden changes in the intensity of light." Orien- 
 tation is therefore, according to Loeb, never due to change 
 in light intensity. "At a constant intensity light acts as 
 a continuous source of stimulation." When animals are 
 not oriented both sides are continuously stimulated but 
 one is stimulated more than the other. This causes one 
 side to move faster than the other ''until symmetrically 
 situated points on the body of the animal are struck at the 
 same angle by equally strong rays of light." 
 
 In a recent paper (1907) Loeb again emphasizes this 
 difference between ''heliotropism" and " Unterschieds- 
 empfindlichkeit.'' It is therefore evident that he was well 
 aware of the fact that certain animals respond to changes 
 in light Intensity. This, however, is an old idea. As a 
 matter of fact it was the fundamental postulate of all who 
 thought that reactions are controlled by psychic phenomena. 
 And in his earlier work Loeb attempted to prove the ab- 
 sence of such phenomena, by showing that aggregation of 
 animals in a given light intensity is not due to difference of 
 intensity, i.e., that the animals are not "unterschiedsemp- 
 findlich." Later, however, he found that planarians collect 
 in regions of lowest intensity because they are " imter- 
 schiedsempfindlich "; (1907), " Both forms of reaction may 
 occur in the same animal {e.g., Spirographis), but this is 
 neither necessary nor the rule." 
 
 Loeb did not study the reactions of unicellular organisms 
 to light and it has been frequently stated that he did not 
 apply his theory to their reactions. Such statements, how- 
 ever, are erroneous as the following quotations will show: 
 (1905, p. 73), " Experiments on infusoria are already suffi- 
 ciently complete to show that Sachs's laws of heliotropism 
 also hold good for them. . . . Trembley's experiments on 
 
34 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 Hydra, however, show that in their case also the relation 
 is the same; at least it seems to me that Trembley's experi- 
 ments cannot be interpreted unless we assume that the 
 progressive moxements of Hydra are determined by the 
 direction of the rays of light." 
 
 I have quoted Loeb rather freely in tr\ing to present his 
 \iews, mainly because he and others have repeatedly main- 
 tained tliat critics have failed to understand his work, 
 particularly that referring to the cause of orientation and 
 aggregation in regions of certain intensity. These quota- 
 tions together with the discussion presented seem to warrant 
 the following summary statements concerning his work on 
 reactions to light. 
 
 (i) His object was to give a mechanical explanation of 
 behavior in opposition to so-called anthropomorphic ex- 
 planations of Bert, Graber and others. 
 
 (2) He proposed to do this by showing that the reactions 
 in animals, especiall}' those due to stimulation by light, are 
 governed by the same law as those in plants. 
 
 (3) He accepted the explanation of orientation in plants 
 given by Sachs and states his theory correctly. Loeb's 
 conclusions however do not support this theory. He 
 confuses ray direction through the tissue with ray direction 
 in the field and difference of intensity on the surface of the 
 organism with diversity of intensity in the field. 
 
 (4) Loeb failed to consider the effect of difference in 
 sensitiveness to light between the posterior and anterior 
 ends of animals and the effect of change in axial position on 
 the relative illumination of these ends. 
 
 (5) His experimental evidence does not prove that the 
 direction of light rays functions in orientation except in so 
 far as it may produce difference of intensity on the surface 
 of the organism ; nor does it prove the absence of sensation 
 in orientation. 
 
 (6) In 1888 Loeb held that orientation in animals is 
 controlled by the direction in which the rays of light pass 
 through the tissue. From 1889 to 1903 he advocated the 
 
HISTORICAL REVIEW 
 
 35 
 
 idea that orientation is controlled by the direction in which 
 the rays strike the surface, or the angle they make with the 
 surface. His statements from 1906 to 1909 indicate that 
 he thinks that orientation is regulated by the relative inten- 
 sity of light on symmetrically located sensitive structures on 
 opposite sides of the orga?tism, a view which Sachs strenuously 
 opposed. 
 
 (7) Loeb's theory of orientation with reference to plants 
 implies that the external agent acts on the motor apparatus 
 directly, and with reference to animals that it acts either on 
 the motor apparatus directly or through a direct refiex arc. 
 
 (8) He thinks that movements in plants and animals are 
 controlled unequivocally by external agents and that they 
 are not fundamentally adaptive. '' Eine 'Auswahl' einer 
 passenden Beleuchtungsintensitat habe ich nie beobachtet" 
 (1909, p. 35). 
 
 (9) Reactions to light may be heliotropic or photokinetic. 
 The former are never due to change in light intensity, they 
 " are a function of the constant intensity; (the latter) a 
 function of the quotient of the change of intensity over 
 time," i.e., rate of change of intensity. There is a perfect 
 analogy between the effect of light and the effect of a 
 constant electric current. 
 
 (10) Aggregation in some forms is due to photokinetic 
 reactions. 
 
 (11) Loeb considers his theory applicable to the reactions 
 of the infusoria as well as to those of higher animals and 
 plants. 
 
 (12) He stands for an objective explanation of the be- 
 havior of animals in all his work, but he cannot be con- 
 sidered as the originator of this idea. Nor was he the first 
 to attempt to put it on an experimental basis. 
 
 Verworn was one of the first investigators in comparative 
 physiology in its broadest sense. He was of the opinion 
 that the fundamental physiological and psychological pro- 
 cesses are common to all animals and that they can be 
 solved in the simple forms more readily than in the more 
 
36 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 complex. In this connection we are interested primarily 
 only in his investigations on the activities of the protozoa. 
 These were taken iij) in 1886, two years before Loeb's first 
 preliminar>' note on the reactions of animals appeared. 
 Verworn was probabl>- the lu'st to attempt an explanation 
 of the beha\ior of animals from a purely objective point of 
 \ic\v. In iiis j-jajx^rs man\- \'ahiahle observations are re- 
 corded on the collection of protozoa in nixen regions, and 
 on the orientation of these creatures when subjected to 
 stimuli of various sorts. Contrary to the idea of Loeb, he 
 concluded that the reactions to light are fundamentally 
 adapti\e (1899, p. 60). His explanation of orientation is 
 of particular interest to us since it has frequently been 
 referred to in works on behavior. This he has presented 
 in his General Physiology^ (1899, p. 499) : "We will examine, 
 first, the forms that possess one flagellum, such as many 
 Bacteria and flagellate Infusoria, and will select as repre- 
 sentative the delicate, green,, flagellate- 
 infusorian Euglena, which, in summer, 
 ^^-s^ .''5 by means of its countless numbers, 
 
 changes the water of standing pools 
 into a deep green. The flagellum of 
 the Flagellata is upon the anterior pole 
 of the body and moves through the 
 water in a screw-like path. For the 
 sake of simplicity its motion may be 
 considered as taking place In a single 
 
 Fig. 2. Scheme of the j^^j^^ j^ j^ ^j^^j^ ^^^^ ^^^^^ J^- og^^jl. 
 contraction of the tla,i,'cl- * . . ,, . . 
 
 lum of a flagellate-infus- latcs about the Straight middle position 
 oriancell. After Verworn rpj ^J l)v means of alternate rhvthmic 
 
 (1899, p. 499)- Sec text. ^ * . ' • i /i' x j 
 
 contractions toward the right (b; and 
 toward the left (bi) ; the swing out of the middle posi- 
 tion (a) into one of the two extreme positions (b or bi) 
 represents the phase of contraction; the return from one 
 
 ^ The first edition of this volume appeared in 1894 at a lime when Loeb 
 was emphasizing the importance of ray direction more strongly than he 
 did later. 
 
HISTORICAL REVIEW 
 
 37 
 
 of the extreme positions into the middle position, the phase 
 of expansion. The flagellum works, therefore, hke an oar 
 that is moved alternately to the right and to the left at 
 the bow of a boat. It is evident that, while undisturbed and 
 having equal conditions upon all sides, the infusorian body 
 must move forward in a straight line, if the fiagellum beats 
 equally strongly toward the right and toward the left, i.e., 
 if contraction and expansion occur with equal rapidity 
 toward the two sides. But if a contractile stimulus acts 
 upon the flagellate suddenly from one side, and if the long 
 axis of the body is not already turned in the direction of 
 the stimulus with the posterior pole toward its source, such 
 a position is assumed by means of a few strokes of the 
 flagellum; for with every oblique or transverse position of 
 the long axis the flagellum is stimulated to contract more 
 strongly upon the side upon which the stimulus falls than 
 upon the opposite side, it makes stronger strokes toward 
 the former than toward the latter side, and the result is 
 that the anterior part of the body is turned away from the 
 source of the stimulus. Exactly the same relations exist 
 here as in a boat moved by a single oar. The bow of the 
 boat also turns toward the opposite side when the boat is 
 propelled more strongly upon one side than the other. 
 The unequal strength of the flagellar stroke in the two 
 directions continues, and the anterior part of the body is 
 turned constantly more away from the source of the 
 stimulus, until the body has placed its long axis in the 
 direction of the incident stimulus. Then both sides of 
 the flagellum become equally stimulated and the protist 
 swims in a straight line, so long as the stimulus continues. 
 Thus, negative chemotaxis, phototaxis, etc., appear in 
 uniflagellated Bacteria and Flagellata as a necessarj^ result 
 of a unilateral excitation of contraction in the flagellum." 
 
 Orientation in forms possessing two flagella and In forms 
 possessing numerous cilia is similarly explained. When an 
 organism of this sort is not oriented it is assumed that the 
 flagella or the cilia are more strongly stimulated on one side 
 
38 LIGHT AXD THE BFJIAVIOR OF ORGAXISMS 
 
 than on the other and that this causes them to beat more 
 or less effectively until the organism becomes directed 
 toward or from the source of stimulation, a direction it must 
 retain. 
 
 By careful readinj^ of X'ervvorn's theory, quoted above, 
 one is led to iiiUr that he considered the liaLiclla or cilia to 
 be stimulated directl>". Tlii^, hu\\e\er, is not an essential 
 part of the theory. The essential point is that there is a 
 difference in the effect of the beat of the cilia on oj^jiosite 
 sides when these sides are dilterentl>' illuminated. It does 
 not matter whether this is caused directl>' \)y the effect of 
 the stimulating agent on the cilia or indirectl}' throtigh 
 impulses transmitted to the cilia from the l)od\- prot()i)lasm. 
 An organism once oriented in accord with this theory- must 
 remain oriented unless it is thrown out of orientation by 
 some other agent than that which has caused the orienta- 
 tion. Orientation according to this theory is direct. Light 
 acts constantly as a directive stimulus. Difference of in- 
 tensity on opposite sides of the organism causes unequal 
 action of the cilia on the two sides. Symmetrical location 
 of organs is essential in the organism. 
 
 It will thus be seen that Verworn's theory of tropisms 
 agrees with the theories of Loeb, especially the more recent, 
 in all essential points. These two authors, however, opposed 
 each other from the beginning. Loeb argued in favor of 
 ray direction, Verworn in favor of intensity difference; 
 neither seems to have known precisely what the other 
 meant. Verworn gives the following statement (1899, 
 p. 450) : " From the preceding consideration and by analogy 
 with the directive effects of other stimuli it is evident that 
 only the difference in the intensity of the light upon differ- 
 ent parts of the body can produce a directive effect; where 
 the stimulus acts upon the surface of the body from all 
 sides with equal intensity, the reason for a defmite axial 
 position disappears, as is to be observed most clearly in the 
 action of chemical stimuli upon all sides. Although this is 
 obvious, some investigators, such as Sachs and Loeb, have 
 
HISTORICAL REVIEW 
 
 39 
 
 believed that the direction of the rays Is more responsiljle 
 for the manifestation of phototactic phenomena than are 
 differences in intensity. It is difficult to conceive this, for, 
 since the assumption of an axial direction Is possible only 
 when differences exist at two different points of the surface 
 of the body, it is wholly mystical how the direction of the 
 rays, which is the same upon all sides of the body, can pro- 
 duce such an effect." Loeb is here classified with Sachs 
 where he claimed to belong. His experimental results and 
 conclusions are, however, from the beginning, more nearly 
 in harmony with the theory of Verworn than they are with 
 that of Sachs. 
 
 Verworn considers his theory applicable to orienting 
 reactions in unicellular forms Induced by stimuli of various 
 kinds. He says (1899, p. 503), " Thus the phenomena of 
 positive and negative chemotaxis, barotaxis, thermotaxis, 
 phototaxis and galvanotaxis which are so highly interesting 
 and Important in all organic life, follow with mechanical 
 necessity as the simple results of differences in biotonus, 
 which are produced by the action of stimuli at two different 
 poles of the free-living cell." 
 
 In 1892 Oltmanns attempted to settle the dispute as to 
 the relative effect of ray direction and intensity difference 
 by studying the reactions of Volvox In an aquarium in 
 which the light became more Intense gradually from one 
 end to the other. Such a distribution of light was pro- 
 duced by placing a hollow prism filled with a mixture of 
 India Ink and glycerine-gelatine between the source of 
 light and the aquarium. The India-Ink mixture of course 
 absorbed only a little light at the thin end of the prism, but 
 gradually more toward the thicker end. Under these con- 
 ditions the Volvox colonies collected in light of a given 
 intensity. Oltmanns says (1892, p. 195) that when the 
 prism was put between the source of light and a vessel 
 containing colonies which had a given direction of motion, 
 they changed their direction of motion almost Instantly 
 and moved toward the region of optimum Intensity. Olt- 
 
40 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 manns and others who used this method of producing Hght 
 of graded intensity assumed that the light rays in the 
 iKjuarium under such concHtions were i)arallel with each 
 other and perpencHcular to tlie side through wliich they 
 entered, and that the change in direction of motion when 
 the prism was i)ut into pkice was due not to the (Hrection 
 of the ra>'s but to difference in light intensity. Oltmanns 
 does not make it clear in what sense he uses these terms. 
 He does not say whether he means difference of intensity in 
 the field or difference on the surface of the organism, ray 
 direction in the field or ray direction through the organism. 
 No matter, however, in which sense these terms were used, 
 the conclusion was not warranted, for it is clear from a 
 theoretical as well as from a practical standpoint, that the 
 rays of light in the aquarium were neither parallel with each 
 other nor perpendicular to the side through which they 
 entered. The India-ink mixture contains numerous solid 
 particles of carbon in suspension, which, together with 
 particles in suspension in the water in the aquarium, 
 unquestionably diffuse the light in such a way that the rays 
 in the aquarium coming from the more highly illuminated 
 end are more numerous than those coming from the other 
 end, and so if the direction of the rays were the control- 
 ling factor one might expect the organisms to go toward 
 either end. 
 
 After reviewing the work of the preceding authors and 
 presenting some original experiments similar in method to 
 those of Strasburger, Davenport (1897) agrees with Loeb 
 in assuming two dissimilar sorts of locomotor responses 
 to light. These he designates phototaxis and photopathy. 
 Phototaxis he defines " as migration in the direction of the 
 light rays, and photopathy as migration toward a region 
 of greater or less intensity of light." He accepts the theory 
 of orientation as outlined by Sachs and formulates another 
 which is in all essentials like that of Loeb. He says (p. 209) : 
 " Let us first think of the way in which light acts on the 
 negatively phototactic (and photopathic?) earthworm. 
 
HISTORICAL REVIEW 4 1 
 
 Represent the worm by an arrow whose head indicates the 
 head end [Fig. 3, ^]. Let solar rays 6'5 fall upon it 
 horizontally and perpendicularly to its axis. Then the 
 
 -s &^ 
 
 Low Light Attunement 
 Low Light Attunement 
 
 Fig. 3 Diagram representing sunlight (SS) falling upon an elongated, bilateral 
 organism (represented by the arrow) whose head is at ^. After Davenport (1897, 
 p. 209). 
 
 impinging ray strikes it laterally, or, in other words, it is 
 illuminated on one side and not on the other. Since, now, 
 the protoplasm of both sides is attuned to an equal intensity 
 of light, that which is the less illuminated is nearer its 
 optimum intensity. Its protoplasm is in a phototonic con- 
 dition. That which is strongly illuminated has lost its 
 phototonic condition. Only the darkened muscles, then, 
 are capable of normal contraction; the brightly illuminated 
 ones are relaxed. Under these conditions the organism 
 curves towards the darker side; and since its head region 
 is the most sensitive, response begins there. Owing to a 
 continuance of the causes, the organism will continue to 
 turn from the light until both sides are equally illumi- 
 nated; i.e. until it is in the light ray. Subsequent 
 locomotion will carry the organism in a straight line, since 
 the muscles of the two sides now act similarly. Thus 
 orientation of the organism is efTected. The same ex- 
 planation . . . will account, miUatis mutandis, for positive 
 phototaxis." 
 
 It is evident that this theory assumes a direct effect of 
 the stimulating agent on the locomotor organs. Da\cn- 
 port thus claims that orientation may be brought about in 
 two ways: " Light acts directly either through difference 
 in intensity on the two sides of the organism, or by the 
 
42 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 course the rays take tlirough the organism " (p. 210). He 
 assumes that changes of light intensity do not result in 
 orientation hut that stimulation caused by such changes 
 may determine the position of organisms in the field in 
 some such way as described by Engelmann. He says 
 (p. 211), "Two kinds of effects are produced b\- light: 
 one by the direction of its ray — phototactic; the other by 
 the difference in illumination of parts of the organism — 
 photopathic." 
 
 Holt and Lee (1901) studied the behavior of Stentor 
 coeruleus in an acjuarium receiving light through a prism 
 similar to the one used by Oltmanns, and found that the 
 animals collected at the darker end. In conclusion they 
 support VerwT)rn's theory; but from the preceding dis- 
 cussion of the effect of the prism on the direction of 
 rays it is evident that the validity of this conclusion is 
 questionable. 
 
 Radl's work on reactions to light was almost entirely 
 confined to the Crustacea and insects. In 1903 after a 
 rather extensive review and criticism of the results and 
 theories of others, and an exposition of his own w^ork, he 
 arrived at two conclusions which are of interest in this 
 connection. One has reference to the mechanics of orien- 
 tation, the other to the explanation of negative reactions. 
 
 His theory of orientation is based on the conception that 
 change in the direction of motion is brought about by 
 unequal stimulation of symmetrical points on the surface 
 of the organism, a conception which lies at the foundation 
 of all the theories thus far i:)resented, excepting that of 
 Sachs and the first one of Loeb. While all of these differ 
 in some respects, they are alike in that they assume the 
 external agent to act through the effect of chemical changes 
 in the organism. Radl proposes to explain orientation as 
 the direct effect of light on the organism. He says (i9«3» 
 p. 151) : " Alle Autoren, welche bisher dieses Thema beruhrt 
 haben, haben an indirekte Wirkungen des Lichtes gedacht, 
 dass namlich durch dasselbe chemischc Veranderungen 
 
HISTORICAL REVIEW 
 
 43 
 
 hervorgerufen werden, welche erst die Reaktionen des 
 Organismus direkt beelnflussen. . . . Gegenuber diesen 
 Anschauungen mochte ich das Problem des Photolropismus 
 als direkte Wirkung des Lichtstrahls auf dan Organismus 
 auffassen. Wenn wir namlich konsequent unsere Auffas- 
 sung der Orientierungserscheinungen durchfiihren wolleii, 
 so miissen wir auch den Phototropismus als Folgeerschein- 
 ung aus dem Spiel zweier Krafte, einer ausseren und einer 
 inneren auffassen — ich bemiihe mich wenigstens umsonst 
 mir vorzustellen, dass die Sache anders sein konnte. Die 
 aussere Kraft ist in diesem Falle der Lichtstrahl ; derselbe 
 muss eine Druckkraft auf den Organismus ausiiben, ich 
 glaube eine ahnliche Druckkraft, wie auf uns etwa der 
 Luftstrom driickt. Diese Vorstellung scheint recht phan- 
 tastisch zu sein, ich sehe jedoch keinen anderen Ausweg. 
 Es ist nicht notig, dass dieser Druck gross sei, er kann sehr 
 fein sein, aber ein Druck, welcher eine Richtung hat, muss 
 es sein, wenn iiberhaupt eine Orientierung, eine Drehung 
 entstehen kann." 
 
 The maximum pressure of direct sunlight having an 
 intensity of 5000 ± candle meters is only 0.4 mg. on one 
 square meter of black surface, and only twice as great on 
 an equal area of reflecting surface. According to this 
 theory then, an organism responding to o.i candle meter 
 would have to be stimulated by light not to exceed 0.000016 
 mg. In view of this fact it is not likely that this theory 
 will ever be seriously considered. It has been presented 
 here merely as a matter of historical interest. 
 
 Radl's view as to the difference between positive and 
 negative reactions is equally untenable. He concludes his 
 discussion on this subject with the following paragrajih 
 (1903, p. 103): '*Ich glaube nun, dass der Unterschied 
 zwischen positivem und negativem Phototropismus iihnlich 
 wie beim Menschen nicht ein Unterschied in der Orien- 
 tierung, sondern nur in der Lokomotion ist; dass das Tier 
 in beiden Fallen gegen die Lichtquelle gleich oricntierl isl, 
 jedoch nicht gleiche Muskeln spannt." It is of course 
 
44 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 well known that contrary to Radl's conclusion, most of 
 the organisms which face the source of stimulation when 
 positive, turn and face in the opposite direction when 
 negative. 
 
 2. More thorough Experimental Analysis SJiowing the Rela- 
 tive Importance of Internal and External Factors in Behavior 
 
 None of the investigators thus far mentioned studied the 
 l)eha\i()r of lower organisms in sufficient detail to be able 
 to tell from direct observation precisely what takes place 
 in the reactions. It was well known from direct observa- 
 tion that many of these organisms form dense aggregations 
 under certain conditions and that they frequently orient 
 when subjected to certain stimuli; but just what takes 
 place during the process of aggregation and orientation was 
 with a few exceptions known only theoretically. 
 
 Jennings was the first to supply this deficiency in obser- 
 vation. He began his investigations on this subject in 
 1897 b>' working out in minutest detail precisely what 
 movements are involved in the formation of the dense 
 aggregations so frequently seen in cultures containing 
 paramecia. His work differs from that of his predecessors 
 in this line largely in that, while they, with the possible 
 exception of Engelmann, studied mass movements and end 
 results, he studied the indixiduals. He was interested 
 not so much in the aggregations as in the process of their 
 formation. How does each indi\'idual get there? and 
 why does it stay there.-' were prominent questions in his 
 mind. 
 
 The observations on the formation of aggregations of 
 paramecia were followed by similar observations on the 
 reactions of representative species of the various groups of 
 protozoa and lower metazoa to various sorts of stimuli. 
 All of this work is characterized by unity of purpose, keen- 
 ness of observation and simplicity of method. 
 
 The results of all of Jennings' work, published in nu- 
 
HISTORICAL REVIEW 45 
 
 merous papers, were brought together and systematized in 
 the well known book on the " Behavior of Lower Organ- 
 isms " (1906). I shall refer to this book almost exclusi\'ely 
 in trying to present his views on the factors involved in the 
 phenomena in which we are especially interested — aggre- 
 gation in regions of given light intensity, orientation and 
 change in sense of reaction. 
 
 Aggregation in a region having a given light intensity 
 may be formed, according to Jennings, in either of two ways, 
 (i) The organisms get into the region just as they would 
 into any other region, merely by swimming about in an 
 aimless manner, without orientation and without direct 
 movement toward the region. When they get to the limit of 
 the region and are about to pass out into light of a different 
 intensity the sudden change to which they are subjected 
 produces a stimulation which causes a definite reaction. 
 This reaction consists chiefly in a sudden turn toward 
 a given side, frequently after backing some distance, and 
 procedure on a new course. They respond with this 
 reaction every time they come to the edge of the region and 
 therefore remain in this region. Other individuals behave 
 In the same way and this results in an aggregation. " Motor 
 reflex " was the first term applied to this method of reaction 
 with Its various modifications; later it was designated 
 " motor reaction," and finally " avoiding reaction." The 
 essential feature In the avoiding reaction Is the fact that 
 the organism always turns toward the same side regardless 
 of the place of application of the stimulus. The side toward 
 which It turns Is determined by internal factors. Thus it 
 is that the direction of turning bears no definite relation to 
 the position of the source of stimulation. The organism 
 may turn directly toward It or away from it or at any 
 angle to it. The method of aggregation thus described by 
 Jennings for Paramecium is in all essentials like that de- 
 scribed by Engelmann In 1882 and 1883 for Paramecium 
 bursaria, Euglena, Bacterium photometrlcum and other 
 organisms. 
 
46 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 (2) In place of getting into regions of a given light in- 
 tensity by mere wandering movements, organisms may 
 orient and move directly toward such regions, and the 
 avoiding reaction may keep them in this region just as 
 described above, or lhe\- may remain because it is illumi- 
 nated b\- liglu ol oplimuni iuLensiU'. ll they get into 
 light of lower intensity they become positive and return 
 to the optimum directl}' after becoming oriented. If they 
 get into light of higher intensity they become negati\e 
 and orient in the opposite direction, which again causes 
 them to return to the optimum intensity. The organ- 
 ism usually tries numerous positions before it becomes 
 oriented. Many errors are made before the successful posi- 
 tion is attained; many directions of motion are tried; one 
 is selected. Jennings has designated this method of orien- 
 tation as orientation by " trial and error," or more recently 
 merely by "trial." Some seem to be of the opinion that the 
 trial movements are haphazard movements, that they are 
 not definitely determined. In answer to this Jennings says 
 (1906a, p. 452): "The behavior may perhaps be most 
 accurately characterized as ' selection from among the 
 conditions produced by varied movements.' In general we 
 find that many organisms are so constituted that internal 
 conditions (permanent or temporary) will produce under 
 stimulation movements that are varied in precisely such 
 a way as to subject the creature to as varied environmental 
 conditions as possible, and thus give it an opportunity to 
 select what is nearest the optimum. Every one of these 
 movements is, of course, as absolutely determined as the 
 most orthodox tropism, onl\- the determining factor is not 
 the localization of the stimulus (or other external factor) 
 alone. 
 
 " Certain recent writers have seemed to imply that there 
 is a contrast between the 'trial and error' method, and 
 behavior that is definitely determined by structural and 
 other internal conditions. It needs to be emphasized, 
 perhaps, that the behavior which I and others have char- 
 
HISTORICAL REVIEW 47 
 
 acterized by this phrase is very precisely determined by 
 structural and other internal conditions; indeed, its dis- 
 tinguishing feature is the fact that it is thus determined by 
 such conditions, rather than exclusively by the external 
 conditions." 
 
 Jennings places particular emphasis on the idea that 
 " activity does not require present external stimulation." 
 This is an idea of which Darwin made much in his work on 
 movement in plants. To explain orientation, Darwin said, 
 we do not need to account for movement; it is only neces- 
 sary to account for change in the direction of movement. 
 Jennings applies this idea to the orientation of animals. 
 The animals are in motion; the question is, how is the 
 direction of motion regulated so as to result in orientation? 
 He says that in many of the infusoria it is regulated by 
 means of the avoiding reaction. ''This reaction" (1906, 
 p. 79) "consists in successively 'trying' not only different 
 directions of locomotion, but also different positions of the 
 body axis. As soon therefore as a position is reached in 
 which the disturbance causing the reaction no longer exists, 
 the reaction of course stops; the animal therefore retains 
 this axial position." 
 
 It will thus be seen that orientation in these forms is, 
 according to Jennings, not brought about by a direct turn- 
 ing of the anterior end of the body toward or away from 
 the source of stimulation. It is not due to unequal stimu- 
 lation of points symmetrically situated on the body; the 
 external agent does not act constantly as a directive stimu- 
 lus. " The position of orientation is not one in which a 
 median plane of symmetry takes up a definite position 
 with reference to the external agent." Not all reactions 
 resulting in orientation are however of this sort. Many 
 organisms have the power of turning directh' toward or 
 away from the side stimulated; in these orientation may 
 take place directly, as Jennings clearly states in the follow- 
 ing words (1906, p. 271), "In the symmetrical Metazoa 
 we of course find many cases in which the animal turns 
 
48 LIGHT AND TWE BEHAVIOR OF ORGANISMS 
 
 directly toward or away from the source of stimulation, 
 without anything in the nature of preliminary trial move- 
 ments." Reactions which show a definite relation to the 
 localization of the stimulus " include perhaps the greater 
 number of the directed movements of the organisms." 
 
 It is evident, judging from these (luotations, that Jen- 
 nings does not hold that all organisms orient by means of 
 avoiding reactions. He does not oppose the idea of direct 
 orientation by means of differential response to localized 
 stimulation. He opposes the view that this is the only 
 method of orientation and the view that orientation is 
 caused by the direct effect of the external agent on the 
 locomotor organs. He holds that the power of differential 
 response to localized stimulation is derived from other 
 methods of reaction, as described in the following quota- 
 tions and abstracts (1906, pp. 306-308): ''First we have 
 the simple phenomenon that when a portion of an organism 
 is stimulated this portion may respond by contraction, 
 extension, or other change of movement." Such local 
 reponses to local stimulation we find in Amoeba, Hydra, 
 Sagartia, flatworms and many other soft-bodied animals, 
 and even in man when the electrode of a battery is applied 
 directly over a muscle. " In many cases we find that the 
 relation of the movement to the source of stimulation is 
 brought about indirectly through selection from among 
 varied movements. The organism tries moving in many 
 directions, till it finds one in which there is no stimulus to 
 further change. ... In still other cases the reaction shows 
 a definite relation to the localization of the stimulus, yet 
 it is not due to local reaction of the part stimulated, 
 nor is it brought about by trial. If an infusorian is stimu- 
 lated at the anterior end it swims backward; stimulated 
 at the posterior end it swims forward. Both these move- 
 ments are reactions of the entire organisms, all the motor 
 organs of the body concurring to produce them; they are 
 not produced by local reactions of the organs at one end 
 or the other. . . . Such behavior apparently represents 
 
HISTORICAL REVIEW 49 
 
 not a primitive condition, but a product of development." 
 " To a change leading away from the optimum (in either 
 plus or minus direction)" the organism responds in such a 
 way as to tend to return to the optimum. " Thus are pro- 
 duced the so-called positive and negative reactions." 
 
 The essential characteristics in behavior, as analyzed 1 •>' 
 Jennings, are clearly set forth in the following quotations 
 (1906, pp. 283-292). Internal factors: "Activity does 
 not require present external stimulation. . . . Activity 
 may change without external cause. . . . Changes in 
 activity depend on changes in physiological states. . . . 
 Reactions to external agents depend on physiological 
 states. . . . The physiological state may be changed by 
 progressive internal processes, particularly those of metabo- 
 lism. . . . The physiological state may be changed by the 
 action of external agents. . . . The physiological state 
 may be changed by the activity of the organism. . . . 
 External agents cause reaction by changing the physio- 
 logical state of the organism. . . . The behavior of the 
 organism at any moment depends upon its physiological 
 state at that moment. . . . Physiological states change in 
 accordance with certain laws. . . . The resolution of one 
 physiological state into another becomes easier and more 
 rapid after it has taken place a number of times." 
 
 Different factors on which behavior depends : "We have 
 seen that the behavior of the organism at a given moment 
 depends on its physiological state, and that it therefore 
 secondarily depends upon all the factors upon which the 
 physiological state depends. Hence we cannot expect the 
 behavior to be determined alone by the present external 
 stimulus, as is sometimes maintained, for this is only one 
 factor in determining the physiological state. The be- 
 havior at a given moment may depend on the following 
 factors, since these all affect the physiological state of the 
 organism: 
 
 " I. The present external stimulus. 
 
 " 2. Former stimuli. 
 
50 LIGHT ASD THE BEHAVIOR OF ORGANISMS 
 
 
 3. Former reactions of the organism. 
 
 4. Progressive internal changes (due to metaboHc pro- 
 cesses, etc.)- 
 
 *' 5. The laws of the resolution of i^h>siological states one 
 into another. 
 
 ** All these factors have been strictly demonstrated by 
 observation and experiment, even in unicellular organisms. 
 Any one of these alone, or any combination of these, may 
 determine the acti\ity at a gi\en moment." 
 
 External factors (p. 299): "We may sum up the external 
 factors that produce or determine reactions as follows: 
 (i) The organism ma\' react to a change, even though neither 
 beneficial nor injurious. (2) Anything that tends to inter- 
 fere with the normal current of life activities produces 
 reactions of a certain sort ('negative'). (3) Any change 
 that tends to restore or favor the normal life processes may 
 produce reactions of a different sort ('positive'). (4) 
 Changes that in themselves neither interfere with nor assist 
 the normal stream of life processes may produce negative 
 or positive reactions, according as they are usualh' followed 
 by changes that are injurious or beneficial. (5) Whether a 
 given change shall produce reaction or not, often depends 
 on the completeness or incompleteness of the performance 
 of the metabolic processes of the organism under the exist- 
 ing conditions. This makes the behavior fundamentally 
 regulatory." 
 
 Reactions and change in the sense of reactions are, 
 therefore, according to Jennings, adaptive ; and if this be 
 true, an explanation of them must be looked for along the 
 same lines as an explanation of any other adaptive charac- 
 teristic in organisms, functional as well as structural. 
 
 Finally we may refer to the "selection of random move- 
 ments" as a factor in orientation, as put forward by 
 Holmes (1905). He studied the reactions to light of earth- 
 worms and blow-fly larvae and found that when these 
 animals are stimulated they turn in many directions, 
 apparently feeling about until they become directed away 
 
HISTORICAL REVIEW 
 
 5T 
 
 from the source of stimulation. From these observations 
 he concluded that "orientation is produced indirectly by 
 following up these chance movements which bring respite 
 from the stimulation." 
 
 This conclusion is in perfect harmony with that of Jen- 
 nings regarding the orientation of protozoa. The only 
 difference between the orienting reactions in the two classes 
 of animals mentioned is that the unicellular forms studied 
 by Jennings turn in different directions by means of the 
 avoiding reaction, i.e., they always turn toward a struc- 
 turally defined side, while the metazoa investigated by 
 Holmes are not thus limited in their direction of turning. 
 Not all protozoa however are limited in the direction of 
 turning. Lacrymaria olar, for example, swings its long 
 anterior proboscis-like appendage about in all directions 
 and there appears to be no limitation set to the direction 
 in which it may turn. 
 
 Holmes contrasts the random movements w^ith forced 
 reflexes, and characterizes the former as '' elements of 
 spontaneous, undirected activity." This statement natu- 
 rally leads to the conclusion that the direction of motion 
 in random movements is not definitely determined. It is 
 however hardly probable that Holmes intends to convey 
 such an idea, for it is undoubtedly true that the direction 
 in random movements is as definitely and absolutely deter- 
 mined as it is in the avoiding reaction or in forced reflexes. 
 The difference is merely that the factors involved are 
 different in the different methods of reaction. 
 
 3. Summary of Historical Review 
 
 (i) During the early periods of civilized man all living 
 things were held to be endowed with a soul which was 
 responsible for all activity. 
 
 (2) Mechanical explanations of activity received l)ut 
 little attention until early in the seventeenth century, the 
 period of Harvey, Descartes and Borclli. 
 
52 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 (3) This period resulted in the origin of the iatromechani- 
 cal and iatrochemical schools. The object of these schools 
 was to explain all \ ital phenomena on purely physical and 
 chemical {principles. 
 
 (4) The failure to accomplish this purpose led to the 
 origin of the doctrine of vital force, during the first years 
 of the eighteenth century. This resulted in a period of 
 stagnation in research in this line which continued until 
 the appearance of Johannes MuUer, De Candolle and many 
 others, early in the nineteenth century. 
 
 (5) The establishment of the doctrine of evolution by 
 Darwin and the consequent interest in the origin of mental 
 phenomena in man led to special activity in the study of 
 behavior of animals from the psychological point of view, 
 and numerous anthropomorphic explanations of their 
 activity. 
 
 (6) In plants activity was studied from the physico- 
 chemical point of view during this period. This study 
 resulted in the development of the idea that the actions 
 are definitely controlled by external agents, e.g., the direc- 
 tion of growth in roots and stems by gravity, moisture, 
 light, etc. The reactions thus definitely controlled were 
 called tropisms. At first the term tropism was used merely 
 to indicate the relation between the direction of bending 
 and the position of the source of stimulation (De Candolle, 
 1832). Tropisms were however in general regarded as 
 reactions unequivocally controlled by external agents. 
 
 (7) The study of animal behavior from the physico- 
 chemical point of view was first taken up by Verworn and 
 Loeb in 1886 and 1887. The activity of the different 
 organs in animals had been studied from this point of view 
 for nearly three centuries, but not the reactions of the 
 animal as a whole. Loeb attempted to show that the 
 behavior in plants and animals is essentially the same, and 
 concluded that the behavior of animals is very largely un- 
 equivocally controlled by external agents. He and his 
 followers therefore described reactions in animals in terms 
 
HISTORICAL REVIEW r^ 
 
 of tropisms in opposition to the anthropomorphic descrip- 
 tions current at that time. Animals go toward a source of 
 Hght neither because it is useful for them to do so nor 
 because they enjoy light or can see, but because they are 
 positively heliotropic. But what is the underlying cause 
 of tropisms? What are the mechanics involved in the 
 processes described by this term? Loeb applied the theo- 
 ries developed by botanists to answer these questions and 
 developed others (see p. 25). Verworn and other in- 
 vestigators added new ones or suggested modifications. 
 Thus it came about that the term tropism came to have 
 a multiplicity of meanings. 
 
 (8) Some of the explanations of behavior offered under 
 the name tropism were founded on the idea that the external 
 agent acts directly or through a direct reflex mechanism on 
 the locomotor organs. This idea together with others 
 assuming unequivocal control of behavior by external 
 factors, Jennings and his followers found to be untenable 
 in their studies on the behavior of the lower organisms. 
 The new features introduced by this school have been 
 clearly set forth above; it will therefore not be necessary to 
 emphasize them here. 
 
 4. Various Definitions of Tropisms 
 
 The term tropism was first used by De Candolle in 1832. 
 He called the bending of plants toward the light helio- 
 tropism, indicating merely the relation between the direc- 
 tion of bending and the source of stimulation. Later the 
 term tropism came to signify not only the bending or orient- 
 ing but also the explanation of the process. Thus for every 
 new explanation the term received a new signification, and 
 this has naturally led to much confusion. Let us point 
 out some of the different meanings which have been applied 
 to the term heliotropism. 
 
 (i) Sachs in 1876 concluded, as stated above, that 
 orientation of plants is due not to difference in light inten- 
 
54 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 sity on the surface as De CandoUe held, but to the direction 
 in which the rays pass through the tissue. HeHotropism, 
 to some of those who agreed with Sachs, meant orientation 
 due to direction of rays through the tissue, to others merely 
 orientation due to ray direction in general. 
 
 (2) Darwin in 1880 said orientation in i)lants is due to 
 modification of circumnutation. It is regulated by differ- 
 ence of intensity on opposite surfaces, probably changes of 
 intensity, and he used the term heliotropism to indicate 
 
 this. 
 
 (3) In 1888 Loeb maintained that orientation in animals 
 is controlled by the direction in which the rays pass through 
 the tissue, that is, in the same way in which Sachs had said 
 it was controlled in plants. In 1889 he still held that light 
 reactions in plants and animals are governed by the same 
 laws. But now he says symmetrically located points on 
 the photosensitive surface must be struck by light at the 
 same angle. " Light automatically puts the plant or the 
 animal into such a position that the axis of symmetry of 
 the body, or organ, falls into the direction of the rays of 
 light." Heliotropism is however used not only to express 
 this explanation of orientation, which differs materially 
 from that of Sachs, but also to indicate movement toward 
 or from the source of light. In his later work, he abandons 
 the idea of the importance of the angle between the sen- 
 siti\'e surface and the light rays and substitutes the idea 
 that it is relative intensity on opposite sides which governs 
 orientation. Thus heliotropism received a new significa- 
 tion. His most recent views are expressed in the following 
 quotations (1906, pp. 135, 138): " Heliotropism covers only 
 those cases where the turning to light is compulsory and 
 irresistible, and is brought about automatically or mechani- 
 cally by the light itself. ... If the current curves of 
 radiating energy, e.,^., light rays, strike an animal on one 
 side only, or on one side more strongly than on the sym- 
 metrical side, the velocity or the kind of chemical reactions 
 in the symmetrical photosensitive points of both sides of 
 
HISTORICAL REVIEW 55 
 
 the body will be different. The consequence will he in a 
 positively heliotropic animal a stronger tension or tendency 
 to contract in the muscles connected with the photosensitive 
 points of the one side of the body than in those connected 
 with the opposite side." This view is affirmed in a recent 
 address (1909). 
 
 (4) It is ordinarily assumed that Verworn considers 
 orientation in the lower forms to be due to the direct effect 
 of the external agent on the locomotor appendages. If, 
 e.g., one side is more highly illuminated than the other the 
 cilia beat more or less effectively on that side and thus 
 produce orientation. This process is termed heliotropism 
 or phototaxis. 
 
 (5) '' Two kinds of effects are produced by light " accord- 
 ing to Davenport (1907, pp. 210, 211), " one by the direc- 
 tion of the rays . . . either through difference of intensity 
 on the two sides of the organism, or by the course the rays 
 take through the organism — phototactic; the other by 
 the difference in illumination of parts of the organism — 
 photopathic." 
 
 (6) Yerkes says (1903, p. 361), "All those reactions in 
 which the direction of movement is determined by an 
 orientation of the organism which is brought about by the 
 light are phototactic; and all those reactions in which the 
 movement, although due to the stimulation of light, is not 
 definitely directed through the orientation of the organism 
 are photopathic." 
 
 (7) To RadI (1903) heliotropism means orientation due 
 to difference In light pressure on unequally illuminated 
 symmetrically located surfaces. 
 
 (8) Holmes (1905) calls orientation by selection of ran- 
 dom movements phototaxis (heliotropism). 
 
 (9) Barrows (1907, p. 530) and Walter (1907, p. 149) 
 suggest "asymmetrical response to asymmetrical stimula- 
 tion" as a criterion of tropisms; and because the organisms 
 worked on respond thus they conclude that their reactions 
 are tropic. According to this criterion it Is of course evident 
 
56 LIGIir AXD THE BEHAVIOR OF ORGAMSMS 
 
 that every differential response to a localized stimulation 
 even in a human being may be a tropic response. 
 
 (10) To Bohn forced orientation constitutes a tropism; 
 (1908, p. 78), " L orientation est directe; I'animal est attire 
 sans quit piiisse rcsistcr: ii y a la un ' tropisme' au sens de 
 Loeb "; (p. 80), " On n'a pas besoin de nier la 'volonte' de 
 ranimal; on pent dire (jue ces impulsions sont plus fortes 
 qu'elle. On ne pent nier les tropismes." 
 
 (11) Parker apparently considers any reaction which 
 carries an animal toward or away from the source of stimu- 
 lation as tropic; he says (1908, p. 426), " Since amphioxus 
 swims awa\' from a source of light, it is negatively photo- 
 tropic." Minkiewicz (1907, p. 47), uses the term tropism 
 in much the same sense, as does also Hadley, wIkj defines 
 it and photopathy as follows (1908, p. 201): " A phototactic 
 reaction [is] one in which the organism tends to place the 
 longitudinal axis of the body parallel to the direction of 
 the rays and to approach or recede from the source of thOvSe 
 rays. ... A photopathic reaction is one in w^hich an or- 
 ganism, without previous assumption of a body-orientation, 
 'selects' regions of optimal light-intensity." 
 
 (12) Washburn (1908, p. 57) refers to tropisms as "the 
 direct motor response of an animal to an external stimulus," 
 and Torrey defines the term similarly but somewhat more 
 definitely. He says (1907, p. 319): "In heliotropism as 
 well as in galvanotropism, the oriented organism is in a 
 condition of physiological stimulation, and . . . the re- 
 sponse to stimulation is local." This definition is in all 
 essentials like those of Verworn and Loeb. 
 
 (13) Driesch (1908, p. 11) says, "A tropism ... is a 
 directed mov^ement of a growing part of a plant or hydroid 
 determined by the direction of a directed agent." 
 
 (14) Wheeler (19 10, p. 515) considers reactions which 
 "involve an adaptive orientation" as tropic. 
 
 (15) Jennings (1909, p. i) suggests the following defini- 
 tion: "The tropism includes those reactions in which the 
 organism takes and maintains a definite orientation — places 
 
HISTORICAL REVIEW 
 
 57 
 
 the axis of its body in a definite position — with relation 
 to some external source of stimulation." 
 
 It is evident from these statements that nearly every 
 reaction in living organisms comes under one or another 
 of the various definitions given to the term tropism. To 
 say that an organism is tropic or not tropic means but little 
 until the sense in which this term is used is defined. Failure 
 to do this has led to serious misunderstanding. I have no 
 objection whatever to the term tropism if used in its 
 original sense, or in any other definite sense. At present, 
 however, it conveys so many different meanings that it 
 inevitably leads to confusion. I shall therefore avoid using 
 it in the following analysis of reactions to light. 
 
 5. Statement of Important Problems in the Study of 
 
 Reactions to Light 
 
 In this analysis we shall ai n to keep in mind the various 
 factors suggested as important in the different tropism 
 theories and other explanations of behavior. We shall ask 
 ourselves: is orientation direct, does the organism turn 
 directly toward or away from the source of stimulation, or 
 does it become oriented after a series of preliminary move- 
 ments? How is the stimulus causing orientation pro- 
 duced: by direction of rays through the organism in accord 
 with the theory of Sachs; by absolute difference of intensity 
 on symmetrically located points on the sensitive surface in 
 accord with the theori33 of Loeb and Verworn; or by changes 
 of intensity on the surface in accord with the ideas of Engcl- 
 mann, Darwin, and Jennings? Does light act constantly 
 as a directive stimulation similar to the action of a constant 
 current of electricity in accord with Loeb's theory of trop- 
 ism, or does it act only when the organism turns out of its 
 course so as to produce changes of intensity, as suggested 
 by Jennings? Is orientation due to the direct effect of 
 light on the locomotor appendages in accord with the 
 theory of Verworn and the analysis of Torrey, to the indi- 
 
58 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 rect effect through a direct reflex arc as suggested by Loeb, 
 or is the whole organism more or less involved in the re- 
 action in accord with the ideas of Jennings and Holmes? 
 If orientation is direct, preciscl\ what movements are 
 invoh'ed in the process? Arc lIic a\oiding reactions due 
 to differential response to localized stimulation, as held by 
 some, or is the direction of turning in stich reactions abso- 
 lutely determined by the structure and physiological state 
 of the organism? Are the reactions to light in general 
 adaptive and modifiable in accord with Jennings' analysis, 
 or are they fixed and forced and uneciuivocally controlled 
 by the external agent in accord with Loeb's ideas? Are 
 the more refrangible rays most active in stimulating all 
 organisms as claimed by Loeb and Davenport, or are some 
 organisms stimulated more by waves of a certain length, 
 and others by waves of a different length as claimed by 
 Verworn and Xagel? These questions and others we shall 
 attempt to answer in the following pages. 
 
PART II 
 
 EXPERIMENTAL OBSERVATIONS AND DISCUS- 
 SIONS BEARING ON THE QUESTION AS TO HOW 
 ORGANISMS' {ESPECIALLY THOSE WITHOUT 
 EYES) BEND OR TURN AND MOVE TOWARD 
 OR FROM A SOURCE OF STIMULATION 
 
 CHAPTER IV 
 
 PROCESSES INVOLVED IN THE BENDING OF DIFFERENT 
 PARTS OF HIGHER PLANTS TOWARD THE 
 SOURCE OF LIGHT 
 
 I. Observations on Plumules of Indian Corn {Zea mays) and 
 Leaves of Nasturtium ( Tropaeolum) 
 
 a. Introduction. — It is well known that many plant 
 structures have a sensitive zone which may be separated 
 by some distance from the motory zone and that impulses 
 are transmitted from the one to the other. Darwin (1880), 
 Pfeffer (1894), Czapek (1900), Pollock (1900), Habcrlandt 
 (1904) and others demonstrated this for leaves and plumules 
 stimulated by light and for radicles stimulated by gravita- 
 tion and injury (cauterization). Newcombe (1902, p. 346) 
 also proved that impulses due to stimulation b\' water 
 currents are transmitted in radicles. In radicles the dis- 
 tance of transmission of impulses is frequently over 10 nun., 
 while in leaves it is often several centimeters. 
 
 Just how the external agent produces the stimulus is not 
 known, although it is generally supposed that it is by caus- 
 ing chemical changes. With regard to light it has been a 
 question as to whether the orienting stimulation is depend- 
 
 59 
 
6o LIGHT AM) THE BEHAVIOR OF ORGANISMS 
 
 ent upon the direclion in which ihe rays pass through the 
 tissue or upon difference of intensity on opposite sides of 
 the reacting organ. Sachs (sec p. 13) originated the former 
 view and Miiller and others suj^portcd it, while Darwin, 
 Wiesner and Oltnianns were prominent champions of the 
 latter. Darwin also emphasized in particular the impor- 
 tance of change in intensity. Pfcffer (1906, p. 228) says 
 that the experimental results and the arguments offered in 
 support of cither \iew are not conclusive. 
 
 Darwin exposed monocot plumules (stems of young 
 seedlings) with one side covered with India ink in front of 
 a window and found that they did not bend straight toward 
 the window, but deflected toward the uncovered side. This 
 result seems to indicate that the curvature is due to differ- 
 ence in light intensity on the surfaces. Pfeft'er (1906, pp. 3, 
 229), however, considers it inconclusive, largely on account 
 of the possible effect of the India ink on transpiration 
 (evaporation). Oltmanns studied the curvature of plants 
 grown behind a hollow prism containing India ink and 
 glycerine gelatine so arranged that the light intensity 
 decreased from right to left, and found that they deflected 
 toward the brighter end of the field. He therefore con- 
 cluded in favor of difference of intensity as the controlling 
 factor in orientation. His results, however, are not con- 
 clusive, owing to the diffusion of light by the particles of 
 India ink in suspension (see p. 40). 
 
 b. Apparatus. — In the following work the objections 
 to the experiments of Darwin and Oltmanns were elimi- 
 nated by the use of an apparatus known as the light grader 
 modified to suit the conditions of the experiments. The 
 important features in the construction of this apparatus 
 will be understood readily by referring to Fig. 4. The 
 walls of the apparatus are all light-proof and dead black 
 inside, so as to prevent reflection. The outline of a cross 
 section at any point is square. The upper portion of the 
 front wall of the vertical part of the apparatus is hung on 
 hinges forming a door. From the bottom of this door is 
 
BENDING OF HIGHER PLANTS TOWARD THE LIGHT 6 1 
 
 Fig. 4. I. A vertical section of the light grader. The lens (a), which is a seg- 
 ment of a cylinder, has its longitudinal axis lying in the plane of the section; b, 
 stage; c, Nernst glower; d, non-reflecting background; f, mirror;/, light rays; g, 
 opaque screens. Distance from glower of lamp to stage, one meter. 
 
 II. Stereographic view of light, lens, and image; a, lens; b, 6cld of light pro- 
 duced by the image of the glower (c); d, opaque screen, which lies flat on lens and 
 contains a triangular opening which causes a gradation in the light intensity of 
 the field (6). 
 
 hung a loose vertical curtain, which can be so opened that 
 observations can be made without admitting light. The 
 source of light is a Nernst glower, which is parallel with the 
 minor axis of the lens. It is mounted in front of a small 
 opening in a light-proof box painted dead black inside, 
 which thus forms a non-reflecting background. The glower 
 
62 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 and stage are at the conjugate focal points of the lens, and 
 therefore at equal distances (50 cm.) from it. The plano- 
 convex c>lindrical lens used is 25 cm. long, 10 cm. wide and 
 has a radius of curvature of 12.5 cm. 
 
 A cylindrical lens will not form a single definite 
 image of an object, but rather a series of images, since by 
 means of it light is focused only in reference to one plane. 
 If, then, the object, e.g., a Nernst glower, is placed at one 
 of the conjugate focal points so that the distance from the 
 lens to the glower is equal to that from the lens to the 
 image, and the glower is so arranged that it is perpendicular 
 to the axis of the lens, the image will not consist of a narrow 
 band of light as large as a glower, which would be true if 
 the segment of a sphere were used as the lens, but it will 
 consist of a comparatively large field of light, the length of 
 which is proportional to the functional length of the lens, 
 while the width is equal to the length of the glower, regard- 
 less of the functional width of the lens (see Fig. 4). But 
 since the amount of light which passes through the lens is 
 directly proportional to the functional width of the lens 
 and the width of the field is constant, it is clear that the 
 intensity of light in the field, if we disregard the amount of 
 light absorbed by the lens, must also be theoreticalh' i:)ro- 
 portional to its functional width. Direct measurements of 
 the light intensity with different functional widths of the 
 lens proved this to be true within the limits of error. If, 
 then, the lens be covered with an opaque screen containing 
 a triangular opening, the base of which is parallel with the 
 minor axis of the lens as represented in Fig. 4, there will 
 result a rectangular field of light in which the intensity 
 gradually diminishes from the end produced by light which 
 passes through the base of the triangular opening to the 
 opposite end, where theoreticalh' it fades into darkness. 
 Practically, however, it was found to be impossible to cut 
 the apex of the triangular opening so as to prevent an 
 apparent line at the end of least intensity. Since the light 
 intensity of the field is proportional to the functional width 
 
BENDING OF HIGHER PLANTS TOWARD THE LIGHT 63 
 
 of the lens, it is evident that the rate of diminution in 
 intensity depends upon the ratio of the altitude uf the 
 triangular opening to the length of its base; i.e., decreasing 
 the altitude or increasing the base causes an increase in 
 the rate of diminution, and vice versa. Yerkes (1903) was 
 the first to make use of a cylindrical lens in stud>ing re- 
 actions to light. 
 
 c. Experiments. — In these experiments the light grader 
 was placed in a horizontal position in such a way that the 
 glower was vertical. The lens was covered with an opacjue 
 screen containing two triangular openings with the apexes 
 facing each other and only a millimeter apart. In this 
 way two parallel horizontal beams of light were produced, 
 the intensity of which gradually diminished from side to 
 side (see Fig. 5). The object of having two beams was to 
 neutralize any possible effect from diffusion of light by the 
 lens. 
 
 A single plumule at a time was exposed in one of these 
 beams of light. In some cases it was allowed to grow up 
 into it from a small pot of sphagnum in which it was ger- 
 minated; in others the seedlings were transferred to the 
 light grader after the plumules were about one centimeter 
 long. 
 
 In former experiments with this apparatus aquatic or- 
 ganisms w^ere used; it was therefore necessary to expose 
 them in an aquarium containing water. Under such con- 
 ditions it is impossible to eliminate light reflected from the 
 glass walls of the aquarium and from particles in suspension 
 in the water. With the plumule growing in air, however, 
 and with only one exposed in the beam of light at a time, 
 it is evident that all such reflections are done away with. 
 Thus the objections to Oltmanns' experiments with the 
 hollow prism have been obviated, and likewise those brought 
 forward against Darwin's w^ork. 
 
 All the following experiments were performed in a large 
 dark room. During the first part of the work the apparatus 
 was situated several meters from a dead black wall uj^on 
 
64 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 which the beams of ht;ht fell and were absorbed. The 
 altitude of the trianguhir openings in the screen over the 
 lens was 7 mm. and the l)ase 50 mm. The beams of light 
 thus produced were 14 mm. wide and 20 mm. high at the 
 focal point in the light grader, the place where the plumules 
 were exposed. Al ihis point the light intensit> in each 
 beam decreased from side to side at the rate of 2 ca. m.^ 
 per mm., it being 100 ca. m. at one side and zero at the 
 other. From these data the intensity at any part could 
 readily be calculated. In order to ascertain the intensity 
 to which the plumules were exposed it was therefore neces- 
 sary only to learn their position in the field ; and to calculate 
 the difference of intensity on opposite sides it was sufficient 
 to know their diameter, the difference in all parts of the 
 field being 2 ca. m. per mm. width. 
 
 During the first part of the work the movements of each 
 plumule were recorded by tracing its shadow cast upon a 
 sheet of paper held in a vertical position a few centimeters 
 back of it. The shadow was thus traced at the beginning 
 of the experiment and again at definite intervals. At first 
 only a few tracings were made in twenty-four hours. It 
 was however soon found that owing to marked circum- 
 nutating movements and to surprisingly indefinite lateral 
 deflections it was necessary to locate the position of the 
 plumules at 30 to 60 minute intervals (see Fig. 5). 
 
 By this method only the lateral and vertical movements 
 of the plumule were recorded. There was no record of the 
 movement toward the source of light; in some of the later 
 experiments however this movement also was recorded. 
 A fine pointer was fastened so that the sharp end was 
 10 cm. above the tip of the plumule. * A glass plate was 
 then fastened in a horizontal position one meter above the 
 pointer. By proper illumination the sharp end of the 
 pointer and the tip of the radicle could clearly be seen 
 through the glass plate, and it was not difficult to fix a 
 
 1 The abbre\nation ca. m. will be used for the term candle meters through- 
 out this volume. 
 
BENDING OF HIGHER PLANTS TOWARD THE LIGHT 65 
 
 dot of ink in line with these on the plate by sighting through 
 a small circular hole in a piece of opacjue paper. The 
 horizontal movements of the tip of the radicle could thus 
 be quite accurately recorded by making dots on the plate 
 in line with the pointer and the tip of the radicle, at any 
 
 ■> 
 
 n 
 
 -> 
 
 ^ . 
 
 13 12 
 
 a a' 
 
 Fig. 5. Tracings of shadow of a plumule of corn showing its reaction in light of 
 graded intensity, three-fourths natural size. I. Cross section of two beams of 
 light as used in the experiment; intensity at a and a', zero; at b and b', 100 ca. m.; 
 I, 2, 3, 4, 5, 6, 7, successive positions of plumule at intervals of 60 minutes, right 
 side more highly illuminated than left; 8, 9, 10, 11, 12, 13, same with left side more 
 highly illuminated than right. It will be seen that the plumules deflect slightly 
 toward the more highly illuminated side under both conditions. 
 
 II. Side view of plumule showing amount of curvature toward source of light at 
 close of experiment, n, direction of light. 
 
 desired intervals, and connecting them with a line. The 
 records thus made represent the movement of the radicle 
 magnified ten times. The direction of the rays was recorded 
 by tracing the edge of a ruler placed on the glass plate in 
 such a position that the edge was in line with the shadow 
 of the plumule cast on a white surface temporarily arranged 
 for the purpose (Fig. 6). 
 
 The intensity of light to which the plumules were ex- 
 posed varied from about 2 to 14 ca. m. In most of the 
 experiments they were exposed to the lowest intensity, the 
 edge of the plumule at the beginning of the experiment 
 being in close contact with that side of the beam of light 
 which had the lowest intensity (see Fig. 5). 
 
66 
 
 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 B 
 
 ^:25 A.M. ,0:.T0 
 
 0:40 A.M. 
 11 :UU 
 
 d 
 
 11.30 ^ 
 
 11:15 
 
 11:15 A.M. 
 
 9:15 A.M. 
 10:00 
 
 Fig. 6. A-E. Courses taken by tips of plumules in bcndinp toward the glower 
 in a graded beam of light; magnified five times. The dots represent the position 
 at time indicated. The large arrows indicate direction of rays; the small ones the 
 direction of movement of plumules; d, side of the beam having the lowest light 
 intensity; /, side having highest intensity. In E the beam was reversed between 
 i.ooand 2.00 p. m. It will be seen that in every case except A the plumules de- 
 flected slightly toward the more highly illuminated side. See text. 
 
 d. Results. — Under these conditions the reactions of 
 36 plumules, 14 of wheat (Triticum \ulgare) and 22 of corn 
 (Zea mays), were studied and recorded with the following 
 results: of the 14 wheat plumules studied 6 deflected toward 
 the more highh' illuminated side, 3 toward the less highly 
 illuminated side, and 5 did not appreciably deflect in either 
 direction. Of the 22 corn plumules 13 deflected toward the 
 more highly illuminated side, 2 toward the less highly 
 
BENDING OF HIGHER PLANTS TOWARD THE LIGHT 67 
 
 illuminated side, and 7 did not definitely deflect toward 
 either side. 
 
 These results seem to indicate that it is difference in light 
 intensity on the organism which regulates the direction of 
 movement. The lateral deflections are, however, as indi- 
 cated in Fig. 6, relatively small. The maximum is scarcely 
 more than 2 mm. in a movement of 10 mm. toward the 
 source of light. Considering the conditions of the experi- 
 ments superficially one would expect a much greater deflec- 
 tion if the direction is regulated by the relation in light 
 intensity on different parts of the surface. A corn plumule 
 frequently has a diameter of over one millimeter at a point 
 not more than one millimeter from the tip, well within the 
 sensitive zone. In such a plumule placed in contact with 
 the edge of the beam of light having the lowest intensity, 
 the difference of intensity betw^een the surface facing the 
 glower and that facing in the opposite direction is appar- 
 ently not as great as the difference of intensity between 
 the two sides. Consequently one might conclude that if 
 the movement is regulated by difference of intensity, the 
 plumule should bend at least as far toward the highly 
 illuminated edge of the beam as toward the glower. 
 
 There are however serious objections to such a conclusion. 
 In the first place it is not known whether or not the sensitive 
 tissue extends to the surface. It may be that it is restricted 
 to the central portion of the plumule and that it is very 
 narrow, so that the intensity difference on opposite sides 
 of this tissue is relatively slight under the conditions of the 
 experiment. In the second place it is evident that light 
 can affect the tissue only by penetrating it, and since the 
 rays strike the surface facing the glower nearly at right 
 angles, and the more highly illuminated side at a very small 
 angle, much more light will penetrate the former than the 
 latter. And in the third place, under the conditions of 
 the experiment, the illumination of the two sides will be 
 equalized by the movement of the plumules much sooner 
 than will that of the two surfaces. 
 
68 LIGHT AXD THE BEHAVIOR OF ORGAMSMS 
 
 However this may be, it must be conceded that while the 
 results of these experiments indicate that orientation is due 
 to diversity of light intensity on the reacting organ, they 
 do not definitely settle the question. 
 
 Much more convincing results were obtained toward the 
 close of the work when it occurred to me that it would be 
 possible to prevent the bending toward the glower entirely, 
 without vitiating the results, by reflecting the beam of 
 light and illuminating the surface directed away from the 
 glower as well as that facing it. A small mirror of finest 
 quality 5 mm. X 2 cm. was therefore supported in the beam 
 of light in a vertical position 3 cm. from the plumule. By 
 careful manipulation and frequent adjustment it was pos- 
 sible to keep the intensity on the surface directed toward 
 the glower and the one opposite nearl>- the same, while the 
 difference of intensity on the right and left sides was nearly 
 twice as great as it was when the beam was not reflected. 
 The reactions of 4 plumules of Zca mays were studied under 
 these conditions. All deflected definitely toward the more 
 highly illuminated side, as represented in Fig. 7. These 
 results seem to prove conclusively that orientation in plu- 
 mules of the gramineae (grasses) is in some way regulated 
 by difference in light intensity on opposite sides, and that the 
 direction in which the rays enter the tissue influences the 
 direction of motion only in so far as this may produce 
 unequal illumination of different parts of the sensitive 
 tissue. 
 
 A number of experiments were made with young nas- 
 turtium (Tropaeolum) leaves in graded light. Different 
 parts of the leaf blades w^ere thus subjected to different 
 intensities. In some experiments one-half of the blade was 
 entirely in the shadow. I was unable to detect any influ- 
 ence of the unequal illumination of the blade on orientation. 
 The leaves turned to\\*ard the source of light just as they 
 did when the blades were entirely illuminated by light of 
 equal intensity throughout. The circumnutation move- 
 ments in these leaves were so great, however, that it would 
 
M Y y f y y y 
 
 10:45 A.M. 
 
 5:10 
 
 d 
 
 8:05 A.M. k' 
 
 c 
 
 11:25 P.M. He 
 
 1:10 P.M. 
 
 ^^ 
 
 2:10^\ 
 
 5:30 1 
 
 — ^7.40 
 
 7 5:00 
 4:30 
 
 3:30 
 
 771 
 
 2:55 
 
 
 a 
 
 y t y Y f ' 
 
 d 
 
 m 
 
 1:45 
 
 12:15 P.M. 
 
 1:05 
 9-00-11:80 A.M 
 
 m 
 
 n 
 
 o 
 
 n 
 
 
 (;.;: 
 
 
 •"> -fN 
 
 •;.'■. 
 
 
 " T\ 
 
 .-•.• 
 
 
 i 
 
 % 
 
 '.V- 
 
 V'' 
 
 i 
 
 
 E F 
 
 Fig. 7. ^-£). Courses taken by tips of plumules of Indian corn (Zea mays) as 
 viewed from above in a beam of graded light (a) which was reflected from the 
 mirror m so as to illuminate the two surfaces equally. Magnified five times. The 
 arrows (a) indicate the direction of the rays from the glower, and the other arrows 
 the direction of movement of plumules; d, side of beam of light having lowest 
 intensiti'; /, side having highest intensity (see F below). Movement in the direc- 
 tion of the rays of light was caused by imperfect adjustment of mirror producing 
 unequal illumination from the glower and mirror. 
 
 E, Cross section of beams of light, i, outline of shadow of plumule at the be- 
 ginning of Course C above, 1. 10 P.M.; 2, shadow at 2.10 p.m.; 3, shadow at 11.25 P-M. 
 
 F, I, shadow of plumule at beginning of Course D above, 9.00 a.m.; 2, same at 
 5.15 P.M. The light intensity at was zero; at n, about 200 ca. m. The increase 
 of intensity in the field from side to side was about 14 ca. m. per mm. It will be 
 seen that the plumules deflected strongly toward the side most highly illuminated. 
 
 69 
 
yo LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 have been impossible to detect anything but rather de- 
 cided effects. It is hoped that these experiments may be 
 extended. 
 
 c. Discussion. — The conclusion arrived at above that 
 orientation is regulated by the difference in light intensity 
 on opposite sides of the plumules is in direct opposition to 
 Sachs' theory (see p. 13) of orientation. It opposes that 
 of Loeb in so far as he attaches importance to the idea 
 that symmetrically situated points on the surface must be 
 struck by light at the same angle when the organism is 
 oriented (see p. 28). It neither confirms nor contradicts 
 Loeb's and Vervvorn's idea (see pp. 29, 38) as to the direct 
 effect of the external agent on the motory tissue. Nor does 
 it bear on the (question proposed by Darwin (p. 18) that 
 orientation is due exclusively to modification of circum- 
 nutations. It is entirely possible that the lateral illumi- 
 nation causes an increase as well as a change in the direction 
 of the movement. 
 
 Superficially the evidence seems to indicate clearly that 
 orientation is direct, that there is nothing corresponding 
 to selection of random movements (see p. 50). However, 
 it is impossible to say in how far even very slight circum- 
 nutating changes in position may affect diversity of light 
 intensity within the individual cells in the sensitive zone, 
 and in how far such changes in position may be interpreted 
 as trial movements. Owing to the possibility of such 
 variations in illumination witliin the cells, due to very 
 slight changes in the position of the plumule, it is also 
 impossible to decide whether the stimuli which cause 
 orientation are due to constant intensity or to change of 
 intensity. 
 
 These experiments have no bearing on the question as 
 to how curvature resulting in orientation in the plumules 
 is produced. The experiments of Darwin and others, how- 
 ever, showing that there is a distinct sensory and motory 
 zone in these structures, demonstrate clearly that it is not 
 due to the direct effect of the illumination on the tissues 
 
BENDING OF HIGHER PLANTS TOWARD THE LIGHT 7 1 
 
 which produces the curvature, as Loeb's theory quoted above 
 demands. The mechanism involved is undoubtedly far 
 more complex than this theory indicates. It may be similar 
 to that offered by Pollock to explain the curvatures in roots. 
 He says (1900, p. 59): " The stimulus is transmitted from 
 the sensitive root tip to the curving parts, in the cortical 
 parenchyma. The effect of the stimulus is to increase the 
 normal tension between cortical parenchyma and axial cylin- 
 der on the side that becomes convex, and to decrease or re- 
 verse the normal tension between the cortical parenchyma 
 and the axial cylinder on the side that becomes concave. 
 The change in tension also extends to the different layers of 
 the cortical parenchyma on the concave side, the outer 
 layers becoming negative with respect to the inner ones. 
 So much has been demonstrated. The evidence is in 
 favor of the view that the tensions on the concave side 
 are changed by the protoplasm becoming more permeable 
 to water, some of which passes out into intercellular spaces, 
 possibly to be taken up by the convex cells, which later 
 contain more water than the concave cells. The shorten- 
 ing of the concave side may be masked sometimes by a 
 certain amount of growth." This theory does not account 
 for curvature in structures having but a single cell cavity, 
 like the hyphae of molds, rhizoids of liverworts, and some 
 algae, all of which are known to respond to light by bending 
 toward or from its source. That these reactions cannot be 
 accounted for on the basis of osmotic changes was pointed 
 out by Hofmeister as early as 1867. 
 
 Very little is known concerning the fundamental factors 
 involved in orientation in other plant structures than those 
 mentioned, although much work has been done on them, 
 especially on the leaves. Darwin (1881) was the first to 
 attempt to locate the sensitive structure in the leaf. He 
 found that neither quality nor intensity of reaction is 
 affected by shading the blade, and concluded that the 
 petiole perceives the light. Voechting (1888) came to 
 quite the opposite conclusion in experiments on malva 
 
72 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 and other plants. Krabbe (1889) supporicd Darwin in his 
 conclusion, as did also Rothert (1894) and Czapek. Haber- 
 landt (1904), on the other hand, maintains not only that 
 the blade is functional in h\uht perception, but also that the 
 curved and thickened outer walls of the ei)idermal cells act 
 as lenses and focus the light on the prcjtoplasni within, and 
 that orientation is regulated by responses due to the dis- 
 tribution of the intensity of light within the cells of the 
 epidermis. Kneip (1907) covered the upper surface of the 
 blades of Tropaeoluni with a thin layer of paraltin oil whose 
 index of refraction is about 0.143 greater than the index of 
 cell sap. The oil consequently inverted the lens effect of 
 the curved walls of the epidermal cells and thus caused a 
 dispersal of the rays within the cell. Kneip foimd however 
 that the leaves treated thus responded to light much like 
 those not treated, and concluded (p. 136), " that the lens 
 action is of no importance in the leaves studied." Haber- 
 landt (1909) however does not agree with this conclusion. 
 He claims that the fact that leaves still respond to light 
 after the epidermis is covered in such a way as to neutralize 
 the focusing effect of the curvature of the outer cell walls, 
 merely shows that the effect of these walls can be dispensed 
 with and not that it is useless, and holds that after the lens 
 effect of these walls is neutralized the light intensity is still 
 unequal on the inner surface of the cells, when the light 
 strikes the epidermis oblicjuely, and that this may cause 
 orienting responses, but that the focusing effect of the 
 curved outer walls of the cells enhances the promptness 
 and precision of the orienting responses. 
 
 Various other experiments aside from those mentioned 
 above have been carried out, but the results obtained lead 
 to no definite conclusions concerning the function of the 
 lens action of the epidermal cells, nor do they give any 
 clear notion as to the mechanism of orientation in plants. 
 About all that can be said is that leaves generally take a 
 position such as to facilitate photosynthesis, and that the 
 chloroplasts within the cells likewise assume what may be 
 
BENDING OF HIGHER PLANTS TOWARD THE LIGHT 73 
 
 termed an optimum position. The reactions are adaptive. 
 In some instances if the hght is too intense the chloroplasts 
 are found along the side walls which are more or less nearly 
 parallel with the incident rays. In others the leaves turn 
 so that the edge of the blade faces the light. In all proba- 
 bility both the petiole and the blade are sensitive to light, 
 at least in some leaves, but the method of regulating the 
 movements is still a mystery. 
 
CHAPTER V 
 
 OBSERVATIONS ON UNICELLULAR FORMS IN THE PROCESS 
 
 OF ATTAINING AND RETAINING A DEFINITE AXLAL 
 
 POSITION WITH REFERENCE TO THE 
 
 SOURCE OF LIGHT 
 
 I. Myxomycetes and Rhizopods 
 
 All the Rhizoj)ods and the plasmodia of Myxomycetes 
 that are known to react to light are negative, as was shown 
 by Baranetzsky (1876, pp. 328, 340), Stahl (1884, p. 167), 
 Engelmann (1879, p. 3), Davenport (1897) and others. 
 The contention of Hofmeister (1867, p. 20) that plasmodia 
 are positive in light of very low intensity has not been 
 confirmed. 
 
 Davenport (1897, p. 186) exposed specimens of Amoeba 
 proteus under a compound microscope in a small horizontal 
 beam of direct sunlight with all other light intercepted by 
 means of opaque screens and found that they orient directly. 
 They make no preliminary trial movements in this process. 
 If the direction of the rays is changed they always turn from 
 the source of light at once, never toward it. There is no 
 evidence of selection of random movements in these animals. 
 The same is probably true in case of other Rhizopods and 
 Myxomycetes, although there are no investigations which 
 bear directly on this point. Baranetzsky (1876) found that 
 even a slight increase in illumination causes a distinct 
 retardation in streaming movements of M>'xomycetes. 
 Engelmann (1879) observed that light thrown upon a 
 pseudopod of Pelomyxa palustris causes it to be withdrawn 
 suddenly. Harrington and Leaming (1900) found that a 
 sudden increase in light intensity causes a retardation in 
 the movement of Amoeba. Ewart (1903, p. 69) says that 
 protoplasmic streaming in cells in general is retarded by 
 
 74 
 
 J 
 
OBSERVATIONS ON UNICELLULAR FORMS 75 
 
 increase in light intensity, and Pringsheim (1879, pp. 334, 
 367) maintains that local retardations in streaming move- 
 ment can be produced by local stimulation. Jennings 
 (1904) has shown the same to be true for Amoeba when 
 stimulated mechanically and chemically. 
 
 After completing this part of the manuscript I had the 
 opportunity of observing the orienting reactions in Amoeba 
 proteus in detail, and also the effect of different rays on the 
 reactions. I shall insert a description of the former here; 
 the latter will be discussed in Part IV. 
 
 In studyingorientation numerous specimens were mounted 
 under a large cover glass supported by a ring of vaseline so 
 as to give them ample room for moving about and to pre- 
 vent the solution from drying up. The specimens thus 
 enclosed could be kept in excellent condition for several 
 days. The observations were made under a compound 
 microscope situated in diffuse daylight without any screen 
 around it. Mirrors were so arranged that two horizontal 
 beams of direct sunlight w^ere reflected upon the stage at 
 right angles to each other after passing through 8 cm. of 
 water to eliminate the heat. Specimens exposed in one 
 of these beams without any light from the substage were 
 found to direct their course in a general way from the source 
 of light. In one instance, after a slide had been exposed 
 for fifteen minutes, there were eleven specimens in one field 
 of the low power, all but two of which w^ere moving from 
 the source of light. In another field there were twelve 
 specimens; all but four of these were directed from the 
 source of light. Of these four, two were proceeding at 
 right angles to the rays and tw^o were going toward the light. 
 In still another field containing nine specimens, seven were 
 negatively oriented, one positively and one at right angles 
 to the rays. Orientation, however, was not very precise in 
 any of the specimens. The amoebae usually took a sort 
 of zigzag course. Pseudopods were frequently seen to 
 extend toward one side for some distance, then stop as 
 though they had been checked, after which new ones were 
 
76 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 ordinarily seen to extend on the opposite side for some dis- 
 tance, and stop, etc. 
 
 The details in the process of orientation were observed 
 as follows: a specimen which had oriented in one beam uf 
 light was selected, after wiiich the light in this beam was 
 intercepted and thai in the other simultaneousl}- turned 
 on. The reaction of numerous specimens to a change in 
 the direction of the rays was thus observed and the move- 
 ments in several were recorded by means of camera sketches 
 made at short intervals. A typical record is presented in 
 Fig. 8, although a majority of the specimens observed 
 did not orient as precisely and definitely as did the one 
 represented in this record. By referring to Fig. 8 it will be 
 seen that the amoeba under observation gradually turned 
 from the side most highly illuminated, sending out pseudo- 
 pods only on the shaded side. What is the cause of this? 
 
 If direct sunlight is thrown upon an amoeba which is 
 active in diffuse daylight, all movement stops instantly, 
 but there is ordinarily no immediate contraction of any of 
 the pseudopods. After a few moments of exposure new 
 pseudopods usually appear at the posterior end, and not 
 until these begin to form do the old ones begin to retract. 
 In changing the direction of the rays so that the amoebae 
 become strongly illuminated from the side, as described 
 above, the distribution of the light intensity on the different 
 pseudopods is changed since different surfaces become 
 exposed. Judging from our preceding statement it might 
 be expected that this change of light intensity would 
 inhibit the protoplasmic streaming in the pseudopods on 
 the illuminated side. I could, however, never be quite 
 certain that it did, although it often appeared so. The 
 difficulty in observation here lies in the fact that without 
 any change of illumination the pseudopods form, extend a 
 varying distance, then stop and retract while others form 
 elsewhere. When a pseudopod stops after the direction 
 of the rays is changed it is consequently impossible to be 
 certain that it would not have stopped had the light not 
 
OBSERVATIONS ON UNICELLULAR FORMS 
 
 77 
 
 3:48 P.M. 
 
 Fig. 8. Camera drawing representing diflferent stages in the process of orien- 
 tation of Amoeba proteus. i, Amoeba oriented in light nn before light //is turned 
 on; 2-9 successive positions at the time indicateri on each after Hght // is turned on. 
 Arrows represent the direction of streaming of protoplasm in pseudopods. In 
 those which do not contain arrows there was no noticeable streaming at the time 
 the sketch was made. // and nn, direction of light. 
 
7 8 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 been changed. If light acts directly on the protoplasm 
 it might also be expected that in a pseudopod laterally 
 illuminated, the flow on one side would be retarded, thus 
 causing it to cur\e. Hut no evidence of this could be seen. 
 How then does orientation take place if the pseudopods 
 which are present continue and do not turn from the source 
 of light? There is but one way that I can see, and that is 
 by the inhibition of the formation of new ones on the more 
 highly illuminated side of the organism. 
 
 Since we know that an increase of intensity inhibits 
 streaming in the pseudopods of Amoeba it seems strange 
 that no one has thus far been able to see any reaction in an 
 amoel)a in passing from a region of one intensity to that of 
 another. Davenport (1897, p. 186) studied their move- 
 ments in a field " separated by a sharp line into a light and 
 dark half," but could detect " no effect resulting from the 
 change from light to dark or the reverse." I made ob- 
 servations much like those of Davenport, and found that 
 when the amoebae came in contact with the light area 
 they usually stopped and proceeded in a different direction, 
 as represented in Fig. 9. The light area used in these 
 experiments was about 0.5 mm. square and had very 
 definite edges and a high intensity. It was produced by 
 focusing a limited area of a luminous Welsbach mantle on 
 the slide by means of the mirror and an Abbe condenser. 
 These observations were made in a dark room and no light 
 except the small beam from the Welsbach n antle reached 
 the microscope. 
 
 By referring to Fig. 9 it will be seen that after one 
 pseudopod came in contact with the illumination and was 
 stopped, the amoeba did not at once proceed in the opposite 
 direction so as to avoid the light, but sent out other pseu- 
 dopods at only a slight angle with the first, apparently 
 trying to get around the obstacle in this wa>'. The char- 
 acter of the response did not change after the first pseudopod 
 came in contact with the light, or after the second and the 
 third came in contact with it. But after the fourth became 
 
OBSERVATIONS ON UNICELLULAR FORMS 
 
 79 
 
 exposed the direction of motion was nearly reversed. This 
 indicates that the reaction was modified, that the response 
 to a given stimulus depends upon the preceding experience. 
 
 Fig. g. Sketches representing the reactions of an amoeba proceeding toward 
 an intense area of light the rays of which were perpendicular to the slide. L, field 
 of light formed by focusing a section of a Welsbach mantle on the slide, i-io, 
 successive positions of the amoeba a little less than one-half minute apart. Arrows 
 indicate direction of streaming in pseudopods. 
 
 In view of these facts it is probably true that the orienta- 
 tion of all of the rhizopods in light is due to a local response 
 to a local stimulation, a direct inhibition of the movement 
 of the part most highly illuminated. This would of course 
 result in the prevention of the formation of pseudopods on 
 the more highly illuminated side, and the organism would 
 
8o LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 turn until both sides are equalK' illuminated, and symmet- 
 rically located points on the body equall>' stimulated. 
 
 Such a method of orientation is in harmony with much 
 in Ver\vorn's theory and also with the essentials in Loeb's. 
 It docs not, however, support the idea connected with 
 these theories, that a constant intensity produces a constant 
 directive stimulation. 
 
 Jennings (1904) has shown that certain amoebae roll over 
 and over in their movement. The protoplasm on the 
 underside in relatively low light intensity is constantly 
 coming to the surface into a greater intensity, and moreover 
 the beginning of every laterally directed pseudopod in those 
 forms which do not roll necessarily causes a change in the 
 light intensity of the protoplasm in it. Thus it is clear that 
 the protoplasm is being continuously subjected to changes 
 of intensity. And while the rate of movement in the animal 
 as a whole is no doubt influenced by constant light inten- 
 sity, much as it is by temperature, it may be that orienting 
 reactions are responses solely to changes in light intensity, 
 — in negative organisms to a rather sudden increase of 
 intensity. 
 
 This method of orientation is opposed to the idea of 
 Sachs (see p. 14), that the direction in which the rays 
 penetrate the tissue is of importance in orientation, and 
 also to that of Loeb (see p. 28) with reference to the im- 
 portance of the angle between the rays and the surface. 
 
 2. Euglena 
 
 a. Description. — Euglena is a minute elongated or- 
 ganism. The posterior extremity ends in a spinelike 
 process; the anterior end is rounded off rather bluntly. 
 The different species vary greatly in size; some are not over 
 o.oi mm. long and o.ooi mm. in diameter, while others are 
 nearly fifty times as large. The forms most commonly met 
 with average about o.i mm. In length and 0.015 mm. in 
 diameter. Nearly all are green, having numerous chloro- 
 
OBSERVATIONS ON UNICELLULAR FORMS 
 
 8i 
 
 plasts of various forms. They have a contractile vacuole 
 which opens to the exterior at the anterior end, and a brown 
 pigment spot known as the eye-spot, in close connection with 
 the vacuole. They exist in three states, — free-swimming, 
 crawling and encysted (Fig. lo). In the free-swimming 
 
 '^ ch 
 
 C p 
 
 F 
 
 -0.01 mm.- 
 
 -> 
 
 Fig. io. Sketches of Euglena, showing general structure of different forms. 
 A and C, Euglena x sp. (?) in crawling state; B, probably a form of E. viridis; D, 
 E, E. deses; e, eye-spot; v, contractile vacuole; ch, chloroplasts; space in B limited 
 by dotted lines well filled with small chloroplasts; n, nucleus; c, caudal spine; p, 
 pigment granules which appear to be composed of same substances as eye-spot, 
 — these were found in only a few specimens. E, shows typical curvature toward 
 dorsal surface while swimming in direction indicated by arrow. F, eye-spot highly 
 magnified; s, surface view; a, view from anterior end. The convex surface is 
 directed outward, mm., projected scale. All outlines were made with camera 
 from specimens killed in iodine. Contractile vacuoles and nuclei were sketched 
 free-hand from living specimens. 
 
 state they have a flagellum frequently nearly as long as 
 the body. Wager (1900) found that in E. viridis it passes 
 down through the opening of the contractile vacuole and 
 divides into two branches, each of which is attached to the 
 wall of the vacuole. One of these branches contains an 
 enlargement which lies directly opposite the eye-spot, as 
 
^2 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 represented in Fig. ii. Under certain conditions some 
 forms cast off the tiagellum, sink to the bottom and crawl 
 
 about in a manner to be described in 
 /"".' • detail later. In the encysted state, as 
 
 /^ ijT"^ is well known, they are inactive. 
 
 1 m \''''"^' ^- Historical account. — It has long 
 
 V ^1^^' \ been known that these organisms in 
 
 7 \\ \ their free-swimming state orient and 
 
 _-4— V-ci;. swim toward a source of light, and 
 V y \ Stahl (i88o, p. 410) found that if the 
 
 1 light intensity is high they become 
 
 I negative, i.e., they swim away from 
 the source of light. Engelmann (1882, 
 Fig. II. Side view of p. 396) observed that if Euglenae are 
 
 anterior end of Euglena ^ j ,■ , ^ • • 
 
 viridis. after Wager; ..eye- mounted on a slide contammg a spot 
 spot; /, flagelium; c./., en- of relatively Strong light they collect 
 
 largement in llagellum;c.i'., • i • ^i • . • . 
 
 contractile vacuole. ^^ ^ensc masses m this spot just as 
 
 Paramecia collect in regions contain- 
 ing a little COo. The>^ swim into it without any ap- 
 parent reaction, but when they reach the boundary on 
 the way out they stop suddenly, turn around, and thus 
 remain in the illuminated area. Engelmann called this 
 reaction Schreckbewegung, shock-movement, and Jennings, 
 avoiding reaction. Engelmann also proved that the ante- 
 rior end of E. viridis is more sensitive than the posterior. 
 Jennings (1904) however was the first to demonstrate 
 the connection between the shock-movement, the sudden 
 turning when subjected to a decrease in illumination, 
 and orientation, although the idea expressed in the fol- 
 lowing words shows that Engelmann (1882, p. 395) was 
 also very near the truth in this matter: " Falls sie, was 
 bei schnellem Vorwartsschwimmen w^ohl einmal geschieht, 
 gans ins Dunkel hineingekommen sind, sistiren sie doch 
 so fort die weitere Vorwiirtsbewegung, drehen um eine 
 ihrer kurzen Axen, probiren — oft unter bedeutenden 
 Gestaltsanderungen — in verschiedenen Richtungen fortzu- 
 kommen bis sie endlich wieder ins Licht gerathen." 
 
OBSERVATIONS ON UNICELLULAR FORMS 83 
 
 Jennings found that as Euglena swims on its spiral course 
 it rotates on its long axis so as to keep the side containing 
 the eye-spot constantly facing out, and that when it is 
 stimulated it always turns toward this side, which is desig- 
 nated the dorsal side. The process of orientation is de- 
 scribed as follows (1906, p. 138): "The Euglenae are 
 swimming about at random in a diffuse light, when a 
 stronger light is allowed to fall upon them from one side. 
 Thereupon the forward movement becomes slower and 
 the Euglenae begin to swerve farther than usual toward the 
 dorsal side. Thus the spiral path becomes wider and the 
 anterior end swings about in a larger circle and is pointed 
 successively in many different directions. In some part 
 of its swinging in a circle the anterior end of course be- 
 comes directed more nearly toward the light; thereupon the 
 amount of swinging decreases, so that the Euglena tends 
 to retain a certain position so reached. In other parts 
 of the swinging in a circle the anterior end becomes less 
 exposed to the light; thereupon the swaying increases, so 
 that the organism does not retain this position, but swings 
 to another. The result is that in its spiral course it suc- 
 cessively swerves strongly tow^ard the source of light, then 
 slightly away from it, until by a continuation of this process 
 the anterior end is directed toward the light. In this 
 position it swims forward. The course of Euglena in 
 becoming oriented is shown in " Fig. 12. 
 
 Orientation in Euglena is, therefore, according to Jen- 
 nings, indirect. The stimulus resulting in orientation is 
 due to changes in light intensity on the organism. The 
 direction of the rays functions in orientation only in so far 
 as it makes such changes possible. Changes of intensity 
 on the organism may be due to movement from a region 
 of one intensity to that of another, or to a change in the 
 axial position of the organism with reference to the source 
 of light. There is no evidence that orientation is due to a 
 constantly acting directive stimulus in accord with Loeb's 
 theory of troplsms. Jennings does not deny that the 
 
84 
 
 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 Euglenae are affected by light after they are oriented. He 
 thinks, however, that whatever such effects may be, they 
 are relatively unimportant in the process of orientation. 
 
 / 
 
 Fig. 12. Illustration of the devious path followed by Euglena in becoming 
 oriented when the direction of the light is reversed. From i to 2 the light comes 
 from above; at 2 it is reversed. The amount of wandering (a-h) varies in different 
 cases. After Jennings (igo6, p. 137). 
 
 Torrey (1907, pp. 317, 319) criticizes the analysis pre- 
 sented by Jennings in the following terms: " My analysis of 
 their responses, based upon the figure which Jennings him- 
 
OBSERVATIONS ON UNICELLULAR FORMS 85 
 
 self has drawn, with text description, leads to quite a 
 different conclusion from his. The figure indicates that 
 Euglena is both unterschiedsempfindlich and heliotropic. 
 At a (Fig. 12) the reversal in the direction of the light, 
 which has been coming from the direction in which the 
 creature has been swimming, produces a sudden change in 
 intensity of stimulation, a shock which results in the swerv- 
 ing from the previous course, as indicated between a and c. 
 The organism recovers rapidly, only to be subjected to the 
 constant stimulus of a steady light from one direction to 
 the end of the experiment. The result of the action of the 
 constant stimulus is a path, from c to 5, so perfectly in 
 harmony with the tropic schema, that, in spite of Jennings' 
 descriptions and elucidations, I can only wonder at his 
 running so boldly and so far into the enemy's camp. . . . 
 In heliotropism . . . the oriented organism is in a condi- 
 tion of physiological stimulation, and . . . the response to 
 stimulation is local; finally, . . . the interpretation of the 
 behavior of heliotropic organisms on the basis of general 
 changes concerning the whole organism, not only does not 
 accord with the main facts, but is rather psychical than 
 physiological in character." 
 
 It is thus evident that while Torrey recognizes that 
 Euglena responds to change of light intensity, he considers 
 that orientation is due to the local effect of unequal stimu- 
 lation of symmetrically situated points on the body, and 
 that after the organism is oriented it is held upon its course 
 by constantly acting directive stimulation. He does not, 
 however, explain where the symmetrically located points 
 which are subject to local stimulation aresituated in Euglena. 
 They might be conceived to be in the flagellum or in the 
 body. In the former case it would imply direct action of 
 the point stimulated, in the latter a reaction in harmony 
 with the location of the stimulus, i.e., if the stimulus is 
 applied to the left side of the body the flagellum would 
 strike toward the left; if applied to the right side, it would 
 strike toward the right, etc. 
 
86 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 If Euglenae actually orient by local response to local 
 stimulation, as Torrey assumes, or if light acts constantly 
 as a directive stimulus in accord with Loeb's theory, one 
 should be able to find evidence of it in these organisms in 
 the crawling state. With this in mind, therefore, I took 
 up the sttidy of specimens in this state. 
 
 Before entering on the description of the reactions in 
 Euglena bearing directly on the problem just stated, I shall 
 however refer brielly to the question of orientation in 
 light from several sources, since the experimental results 
 obtained under these conditions throw some light on the 
 idea of Sachs, that the direction of the rays through the 
 organism regulates orientation, and on Loeb's idea that 
 symmetrically located points on the sensitive surface must 
 be struck by rays at the same angle when an organism is 
 oriented. 
 
 c. Orientation in light from two sources. — In studying 
 the movement of Euglenae in light from two sources, 
 Nernst glowers in a dark room were so arranged and 
 screened as to produce two small horizontal beams of light 
 which crossed each other at right angles in the aquarium. 
 One glower was stationary. The other was mounted on a 
 horizontal track so that it could easily be pushed nearer 
 to or farther away from the aquarium. Thus the relative 
 intensity from the two glowers could be changed without 
 any change in the direction of the rays. Several species of 
 Euglena in the free swimming state, and two, Euglena deses 
 and Euglena x in the crawling state, were used in these 
 experiments. The results were the same in all. 
 
 When the light from the two glowers was equal and the 
 Euglenae positive they moved in a general way toward a 
 point very nearly halfway between the glowers. But when 
 it was uneciual, they moved toward a point nearer the 
 source from which the more intense light came. Negative 
 specimens take the same general course but in the opposite 
 direction. This experiment is particularly striking if the 
 glower on the track is gradually moved from a position in 
 
OBSERVATIONS ON UNICELLULAR FORMS 87 
 
 which the light intensity from it is much lower than that 
 from the stationary glower to a position in which it is much 
 higher. Under such conditions one can clearly see these 
 organisms, especially the free-swimming forms, gradually 
 change their direction of motion through an angle of nearly 
 90°. (Just how this change is brought about will be 
 demonstrated later.) By regulating the relative intensity 
 of the light from the two sources, it is thus possible to 
 cause Euglenae to move toward any point between the two 
 sources of light without changing the direction of the rays. 
 It is evident then that the direction of the rays does not 
 absolutely control the direction of motion. These results are 
 in harmony with those which I obtained in experiments on 
 Volvox (1907, p. 134). Identical results were also obtained 
 in light from two sources with Stentor coeruleus, Trachelo- 
 monas, Chlamydomonas, Oedogonium swarm-spores, Eu- 
 dorina, Pandorina, Planulae of Eudendrium, Limulus 
 polyphemus larvae, Musca larvae, Allolobophora foetida, 
 medusae of Bougainvillea superciliaris, trochophores of 
 Hydroides dianthus, Arenicola larvae, zoeae, several forms, 
 and Leptoplana tremellaris. Judging from these results 
 it is highly probable that all individuals without image- 
 forming eyes orient in the same way under like conditions. 
 All of these forms can be induced to change their direc- 
 tion of motion by varying the relative light intensity on 
 opposite sides of the body, or by changing the intensity 
 on the same side, without changing the direction of the rays. 
 It may therefore be concluded that difference in the inten- 
 sity of light on opposite sides, or a change of intensity on 
 the same side of the body of all these creatures, may deter- 
 mine orientation independently of the direction of the 
 rays. The orientation of organisms without image-form- 
 ing eyes can therefore not be explained by the application 
 of Sachs' ray direction theory, nor are the orienting reac- 
 tions in harmony with the statements of Loeb expressed 
 in the following quotations: (1905, p. 2), " It is explicitly 
 stated in this and the following papers that if there are 
 
88 LIGHT AXD THE BEHAVIOR OE ORG AX ISMS 
 
 several sources of light of uncqiuil intensity, the light with 
 the strongest intensity (kUTmines the orientation and 
 direction of motion of the animal. Other possible compli- 
 cations are -covered by the uncciuixocal statement, made 
 and emi)hasizud in this and the following papers on the 
 same subject, that the main feature in all phenomena of 
 heliotropism is the fact that symmetrical points of the 
 photosensitive surface of the animal must be struck by 
 the rays of light at the same angle. It is in full harmony 
 with this fact that if two sources of light of ecjual intensity 
 and distance act simultaneously upon a heliotropic animal, 
 the animal jnits its median plane at right angles to the line 
 connecting the two sources of light. This fact was not 
 onh- known to me but had been demonstrated by me on 
 the larvae of flies as early as 1887, in Wiirzburg, and often 
 enough since. These facts seem to have escaped several 
 of my critics; " (p. 61), " When the diffuse daylight which 
 struck the [Musca] larvae came from two windows, the 
 planes of which were at an angle of 90° with each other, the 
 paths taken by the larvae lay diagonally between the two 
 planes. . . . This experiment was recently published by an 
 American physiologist as a new discovery to prove that I 
 had overlooked the importance of the intensity of light!" 
 (p. 82), '' The direction of the median plane or the direction 
 of the progressive movements of an animal coincides with 
 the direction of the rays of light ... if there is only a 
 single source of light. If there are two sources of light of 
 different intensities, the animal is oriented by the stronger 
 of the two lights. If their intensities be equal, the animal 
 is oriented in such a way as to have symmetrical points of 
 its body struck by the rays at the same angle;" (p. 268), 
 " Attention need scarcely be called to the fact that if rays 
 of light strike the animal [larvae of Limulus polyphemus] 
 simultaneously from various directions, and the animal is 
 able to move freely in all directions, the more intense rays 
 will determine the direction of the progressive movements." 
 Note that this animal is in the list mentioned above (p. 87). 
 
OBSERVATIONS ON UNICELLULAR FORMS 89 
 
 Under the conditions of the experiment described above, 
 the organisms mentioned do not move in a direction parallel 
 with the rays, neither do they necessarily orient so " that 
 symmetrical points of the photosensitive surface [are] 
 struck by the rays of light at the same angle," nor does 
 " the light with the strongest intensity determine the 
 orientation and direction of motion." 
 
 Toads (Bufo americanus) were the only animals with 
 image-forming eyes that -were tested with reference to 
 orientation in light from two sources (see p. 87). If the 
 intensity from the two sources is unequal they usually hop 
 directly toward the stronger light and pay no attention to 
 the w^eaker. This is in accord with Loeb's explanation 
 given above. But if the intensity from the two sources is 
 equal, they go toward either one and not toward a point 
 between the two, as Loeb's explanation demands. In none 
 of the organisms studied are the orienting reactions such 
 as are demanded by Loeb's explanation. These results 
 will be referred to in connection w^ith the discussion of the 
 importance of equal stimulation of symmetrical points on 
 the animal. 
 
 Let us now return to our study of the reactions of Euglena 
 in the crawling state and to the problem suggested by 
 Torrey's criticism of Jennings referred to above. Is orien- 
 tation in Euglena due to light acting constantly as a direc- 
 tive stimulation similar to the effect of a constant electric 
 current, or to an intermittent effect, a response to change 
 of intensity only, in accord with Jennings' explanation? 
 
 d. Material. — During the months of November and 
 December excellent material for this study was discovered 
 in a puddle of water fed by a drain from a dwelling house 
 at Windsor Hills, Baltimore. The bottom of the puddle 
 was covered with a dense green layer composed almost 
 entirely of two species of Euglena, — E. deses and another 
 species which was somewhat like viridis but could not be 
 positively identified. It will be referred to as Euglena x. 
 Most of the E. deses had flagella, but the E. x with very 
 
go LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 few exceptions had none. The latter were considerably 
 smaller than the former. The>' averaged nearly 0.08 mm. 
 in length and somewhat more than 0.015 mm. in diameter. 
 A fairl\- good idea of the form and structure may be obtained 
 by referring to Fig. 10. It will be seen in this figure that 
 the caudal end terminates in a spinelike process, and that 
 the eye-spot, in close contact with the canal leading from 
 the contractile vacuole, forms an angle of about 45° with 
 the long axis of the boch'. The eye-spot has the form of a 
 flattened disk somewhat cur\ed, so as to ht around the 
 canal. 
 
 e. Method of locomotion. — It is frequently stated that 
 Euglenae in this state progress by amoeboid movements, 
 i.e., by streaming movements. I was, however, unable to 
 detect anything resembling streaming movements in any 
 of the several different species studied in the crawling state. 
 Many do change their form very much by contracting in 
 various ways, and some may move slightly by thrusting 
 the anterior end forward and then drawing up the posterior 
 end, but progression in this way is relatively unimportant. 
 
 The process of locomotion without flagella appears to be 
 much the same in all forms observed. It was however 
 studied in detail only in Euglena .v. While in motion these 
 organisms usually are considerably curved, being convex 
 on the ventral surface, the side opposite the eye-spot. 
 They rotate on the long axis either entirely over to the left, 
 as seen from the posterior end, or only halfway, then back 
 again, lying on the dorsal surface during this apparent 
 rocking movement. During either of these rotating move- 
 ments both ends appear to move back and forth. The 
 posterior end however moves laterally much less than the 
 anterior. In many instances it continues forward in nearly 
 a straight path, while the anterior end progresses on a 
 spiral course of considerable relati\e width. 
 
 While thus rotating the organisms appear to slide along, 
 mo\ing forward a little with each turn of the body. They 
 progress at the rate of about 0.3 mm. per minute. Pre- 
 
OBSERVATIONS ON UNICELLULAR FORMS 91 
 
 cisely what factors are involved in causing the forward 
 movement I was not able to ascertain. Only very slight 
 contractions can be seen at any time and no streaming 
 movements at all. 
 
 The posterior end is in much closer contact with the sub- 
 stratum than the anterior. If currents of water are passed 
 back and forth over the Euglcnae it can be seen that the 
 anterior end is free, for it moves with the current. Fre- 
 quently specimens are found attached to the slide with 
 only the tip of the caudal spine in contact with the sur- 
 face. In such specimens the whole body swings about with 
 the current. They are held fast by an adhesive substance 
 which they secrete. The presence of such a substance can 
 be detected by passing a small glass rod across the path of 
 a crawling individual near its posterior end, or by pushing 
 the rod about on a slide containing numerous Euglenae 
 which have been crawling about for a short time. If this 
 is done the end of the rod soon becomes covered with a 
 substance to which cling numerous Euglenae attached 
 usually only at the posterior end. It is however not likely 
 that the extrusion of the secretion forces the Euglenae along, 
 as is supposed to be true in the case of diatoms. The body 
 appears to become alternately more and less curved as they 
 rotate in such a way as to force them forward. The caudal 
 spine appears to be used as a sort of lever in this movement. 
 They can however move without the use of the spine, for 
 moving specimens were repeatedly seen in which the point 
 of the spine was not in contact with the slide at all. This 
 was evident especially in specimens which rotated only 
 partially over and then back again. 
 
 As these creatures crawl along, rotating on the long axis 
 with the anterior end progressing on a spiral course, the 
 dorsal surface, the surface containing the eye-spot, always 
 faces the axis of the spiral. This is just the opposite of 
 Jennings' observations on Euglena viridis in the free- 
 swimming state. I found however that E. acus and a 
 few other species swim with the dorsal side facing the axis 
 
92 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 of the spiral and that E. deses swims with one side facing 
 the axis. 
 
 Euglcnae in the crawHng state, just as in ihe free-swim- 
 ming state, may he either negatixe or positive in their 
 hght reactions. The crawling specimens worked on were 
 howe\'er negative to light (^f surprisingl\- low intensity 
 throughout the entire work. But \er\- few were found 
 which were positive even in diffuse sunlight during the 
 middle of the da\', unless the sk>' was co\ered with very 
 dense clouds. The cause of reversal in the sense of orien- 
 tation will he discussed elsewhere. 
 
 /. Accuracy of orientation. — In the study of their 
 reactions to light, the Kuglenae were exposed either to 
 sunlight direct and diffused, or to light from a Nernst 
 glower, a Welsbach burner or a carbon filament. When 
 exposed to light from a single source, e.g., a Nernst glower 
 so arranged that there is as little reflection as possible, 
 Kuglenae orient and move nearly straight toward or away 
 from the light with little deviation, if they are strongly 
 positive or negative; but if they are not, as is frequently the 
 case, they deviate much. Even under the most favorable 
 conditions there is however little similarity between Kugle- 
 nae moving toward a source of light and iron filings moving 
 toward a magnet, a comparison sometimes met with in the 
 literature on reactions to light. In studying Kuglenae one 
 always finds specimens which do the unexpected thing. 
 Their reactions are very much less dependent upon external 
 conditions than are the reactions of iron filings. To come 
 to a full realization of this, one need only consider the fact 
 that these organisms may be negati\e or positive in almost 
 any light intensity or they may not react at all. To predict 
 with any degree of accuracy what these organisms are going 
 to do under given conditions, it is necessary to know much 
 about the history of their past reactions. 
 
 ^. Mechanics of orientation in Euglena x in the crawling 
 state. — Xernst glowers mounted in front of a non-reflect- 
 ing background (see Fig. 4) and properly screened in a large 
 
OBSERVATIONS ON UNICELLULAR FORMS 93 
 
 dark room were used in all quantitative work, and in all 
 work in which it was desirable to regulate the direction 
 of the rays. I have elsewhere pointed out the advan- 
 tageous features of these glowers for such work (1906, 
 
 p. 363). 
 
 The general movements of Euglenae could readily be 
 
 followed under a Braus-Drtiner binocular, but it was found 
 
 necessary to use a compound microscope in working out 
 
 the details in the reactions owing to the small size of the 
 
 organisms. They progress so slowly however that every 
 
 movement can easily be followed even under the highest 
 
 magnification. They are consequently very favorable for 
 
 the work in hand notwithstanding their minute size. 
 
 In studying the process of orientation the microscope was 
 placed either in front of two windows in the laboratory 
 so situated that the general direction of the light entering 
 them was at right angles on the stage, or in the dark room 
 in a similar relative position in front of two Nernst glowers 
 (see Fig. 13). The two glowers were mounted so that the 
 rays were practically parallel with the plane of the stage. 
 One was stationary; the other was mounted on a track so 
 that the distance between it and the aquarium could readily 
 be varied, and thus the intensity of the light from it on the 
 stage changed without any change in the direction of the 
 rays. Both glowers were of the same kind and both were 
 in the same circuit, so that any fluctuation in the current 
 affected both alike. The relation in light intensity from 
 the two sources could thus be regulated as desired. The 
 glowers were so screened that only a small beam from each 
 reached the stage, and this could readily be cut off from 
 either or both. 
 
 The Euglenae were either mounted on a slide under a 
 cover-glass or exposed in a rectangular glass aquarium made 
 for the purpose by cementing slides together with balsam 
 and linseed oil. After they had oriented in light from one 
 of the two sources, the light from that source was cut off 
 and that from the other turned on simultaneously. In 
 
94 LIGHT AXD THE BFJIAVIOR OF ORGAXISMS 
 
 this way their reactions during the process of reorientation 
 could be studied in detail. The following description of 
 tiiis process refers to E. .v in ilie crawling state. 
 
 If the light in which positive organisnis are oriented is 
 decreased in intensit\' without a change in the direction of 
 tile ra>s, e.g., by pushing back the Xernst glower on the 
 track, they respond in a characteristically definite way. 
 If the decrease is relatively slight the anterior end is merely 
 turned toward the \'entral surface, the whole body becomes 
 more cur\'ed and the spiral course of the anterior end 
 becomes wider. If liowever the decrease is considerable, 
 the\' fre(}uently stop in their forward motion and turn the 
 anterior end toward the \'entral surface to such an extent 
 that the two halves of the organism form a right angle. 
 In tiiis condition they continue to rotate, turning over and 
 over in the same spot, and appear to be squirming and 
 twisting about aimlessly. They soon however straighten 
 again and continue on their way toward the source of light, 
 having apparently become acclimatized to the change in 
 light intensity. If the intensity is increased there is no 
 response in positive Euglenae. Negative indi\iduals, on 
 the contrary, respond precisely as described above if the 
 light intensity is increased, but not at all if it is decreased. 
 If the specimens however are only slightly positive or nega- 
 ti\'e the\' may be caused to respond with this twisting reac- 
 tion either by increasing the intensity or by decreasing it. 
 In order to induce this reaction it is necessary to change 
 the intensity at a certain rate. If the glower is moved back 
 very slowly and steadily, no reaction whatever is seen. 
 A sudden decrease of intensity then without any change in 
 the direction of the rays produces a definite reaction in 
 positive individuals, and a sudden increase of intensity 
 produces the same reaction in negative indi\'iduals. These 
 reactions are in accord with the shock effects of Engelmann 
 and Pfeffer and Unterschiedsempfindlichkeit of Loeb. They 
 are not due to an absolute change of intensity but to the 
 time rate of change of intensity. The amount of change 
 
OBSERVATIONS ON UNICELLULAR FORMS 95 
 
 necessary to induce a reaction will be discussed later 
 (p. 105). 
 
 If the intensity from the two sources of light arranged as 
 described above is equal and the beams which reach the 
 stage of the microscope are alternately cut off with an 
 opaque screen so as to change the direction of the ra>'s 
 suddenly without changing the intensity, it appears as 
 though the Euglenae if positive always turn directly toward 
 the source of light, never away from it no matter in what 
 position they are or which surface becomes illuminated 
 when the ray direction is changed. These results would 
 seem to indicate that there is here a local response to a 
 local stimulation, or at least differential response to local- 
 ized stimulation. I was firmly convinced of the truth of 
 this for several days, as were also other members of the 
 laboratory who observed these reactions. Further work 
 however demonstrated the fallacy of this conclusion. 
 
 By very careful observations under the high power it 
 was found that if the ventral surface, the surface opposite 
 the eye-spot, faces the source of light, after the direction 
 of the rays is changed, there is no immediate reaction. 
 The Euglenae continue on their course as though no change 
 had taken place until the rotation on the long axis carries 
 the dorsal surface over into a position in which it faces the 
 light. As soon as this surface, the surface containing the 
 eye-spot, faces the light there is a definite reaction. The 
 Euglenae turn the anterior end toward the ventral surface 
 more or less sharply, i.e., away from the source of light, but 
 they continue to rotate so that the ventral surface soon 
 faces the light again; but it is evident, owing to the curva- 
 ture in the body, that the anterior end is now directed more 
 nearly toward its source than it was when this surface faced 
 the light during the preceding rotation. While in this 
 position, the body is somewhat straightened, so that the 
 anterior end is not carried back as far during the following 
 rotation, and when the dorsal surface comes to face the 
 light it is directed more nearly toward its source than it 
 
96 
 
 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 was when the organism was in this position before, as 
 represented in l-i;^. 13. This reaction is repeated during 
 each complete rotation. Ever}- time the eye-spot becomes 
 
 f ? f 
 
 < '^ <a c 
 
 -«« 
 
 ■«^ 
 
 -«« 
 
 -*<*: 
 
 -^K 
 
 -<<f« 
 
 -^^ 
 
 -<|S« 
 
 -««C 
 
 n 
 
 ■««« 
 
 I 
 
 Fig. 13. Euglena sp. (?) in crawling state, showing details in process of orien- 
 tation; V, contractile vacuole; es, eye-spot; n, o, direction of light; a-c, positions 
 of Euglena with light from n intercepted; c-m, positions after light from n is turned 
 on and that from o cut off so as to change the direction of the rays. If the ray 
 direction is changed when the Euglena is in position c there is no reaction until it 
 reaches d. Then it suddenly reacts by bending away from the source of light to c, 
 after which it continues to rotate and reaches position/, where it gradually straight- 
 ens to g, and rotates to k, when the eye-spot again faces the light and the organism 
 is again stimulated and bends to i, from which it proceeds to^, etc., to m, where 
 it is practically oriented. If the ray direction is changed when the Euglena is at d, 
 it responds at once and orients as described above. If the intensity from n is 
 lower than that from o the organism may respond at once when the ray direction 
 is changed no matter in which position it is. (Compare with orientation in Stentor, 
 Fig. 14.) 
 
 i 
 
OBSERVATIONS ON UNICELLULAR FORMS 97 
 
 more strongly illuminated the organism responds by bend- 
 ing, and when it becomes shaded the creature gradually 
 straightens out and resumes its normal form again; thus 
 the anterior end becomes directed more and more nearly 
 toward the source of light until the organism reaches an 
 axial position in which the eye-spot is no longer exposed to 
 sufficient changes in illumination during the process of ro- 
 tation to cause a bending reaction. The organism therefore 
 continues in this direction, i.e., more or less nearly toward 
 the source of light. Orientation is frequently brought about 
 in two or three rotations. It is clear that during this pro- 
 cess light does not act continuously as an orienting stimulus. 
 Euglena responds with reactions leading to orientation only 
 when the dorsal side is turned toward the source of illu- 
 mination, not when the ventral side is exposed. And it 
 should be emphasized that the first movement in the re- 
 sponse is a bending away from the source of light, toward 
 which it later becomes oriented. 
 
 It is evident from the above description that turning into 
 such a position that the eye-spot faces the source of light 
 produces a stimulation which results in a definite reaction. 
 In this reaction the organism always bends the anterior 
 end toward the ventral surface. It appears at first thought 
 as though this reaction were due to the illumination of the 
 eye-spot. It will however be demonstrated that this is 
 not true. 
 
 If the light from the two sources arranged as described 
 above is not equal, and the two beams which reach the 
 aquarium are alternately intercepted, it is evident that the 
 organism will be subjected simultaneously to a change in 
 the direction of the light rays and a change of light intensity. 
 If the stronger light is thrown upon the Euglenae after 
 they are oriented in the weaker, they orient just as de- 
 scribed above, but if the weaker is turned on after they are 
 oriented in the stronger there is an immediate reaction, 
 no matter which surface, the ventral or the dorsal, happens 
 to be exposed at the time. If it is the dorsal and the 
 
98 LIGHT AXD THE BEHAVIOR OF ORG AX IS MS 
 
 difference of intensity between the light in the two beams 
 is not too great, orientation takes phice just as described 
 above; 1)UL ii il i.^ ihc \eutral surface which is exposed, it 
 frequently happens that the (organism first becomes directed 
 awa\' from the source of light and then toward it only after 
 repeated reactions. The first step in all these reactions, 
 regardless of how they are induced, is the same. It consists 
 of a bending of the anterior end toward the ventral surface. 
 It has been demonstrated (i) that this reaction can be 
 induced in positive Euglenae by reducing the light intensity 
 of the held without changing the direction of the rays, no 
 matter which surface is illuminated, and (2) that it can be 
 induced without any variation in the light intensity of the 
 field by changing the direction of the rays from one in 
 which the anterior end is illuminated to one in which the 
 dorsal surface is illuminated, or (3) it can be induced by the 
 rotation of the organism on the long axis from a position 
 in which the ventral surface is exposed to one in which the 
 surface containing the eye-spot is exposed. Since the reac- 
 tion under the first condition can be due only to a change of 
 intensity on the whole or some part of the organism it is 
 evident that the reaction under the second and third con- 
 ditions is likewise due to a change of intensity. But since 
 the light intensity of the field is constant under these 
 conditions it is evident that the decrease of intensity must 
 be restricted to a portion of the body and that it must be 
 due to the shading of one part by another owing to the 
 movement of the organisms. Our observations show that 
 a change in the position of the organisms, from one in 
 which the ventral surface is illuminated to one in which 
 the dorsal surface is exposed, causes a reaction. Such a 
 change in position must therefore produce a change of 
 intensity on the sensitive parts of the organism. This may 
 be conceived to be due to the eye-spot's acting as an opaque 
 screen and casting a shadow when it faces the light on some 
 highly sensitive, protoplasmic structure located near it 
 (see Fig. 11), or to the location of the more highly sensitive 
 
OBSERVATIONS ON UNICELLULAR FORMS 99 
 
 material in such a position that it is more strongly affected 
 when the ventral surface is illuminated than it is when the 
 dorsal surface is illuminated. The function of the eye-spot 
 will be referred to again later. 
 
 Orientation in negative specimens takes place precisely 
 as it does in positive specimens. The reactions resulting 
 in orientation however are induced by an increase of inten- 
 sity in place of a decrease, as in the case of positive speci- 
 mens; and a change from a position in which the eye-spot 
 faces the light to one in which the ventral surface is exposed 
 induces the avoiding reaction, while in positive specimens 
 it is a change from the latter to the former which causes this 
 reaction. 
 
 After having thus worked out the details in the orienting 
 reactions in Euglenae in the crawling state, I made obser- 
 vations on specimens in the free-swimming state and found 
 the reactions to be essentially the same. A brief account 
 of these observations will be found below (p. 102). 
 
 h. Discussion. — The orientation of Euglena in the 
 crawling state confirms in general the description of the 
 orientation in the free-swimming state given by Jennings 
 (see p. 83). When the organism is not oriented every 
 change from a position in which the light strikes the ventral 
 surface to one in which it strikes the dorsal, and vice versa, 
 due to rotation on the long axis, may be considered a 
 " trial movement." If such a trial movement results in a 
 decrease of light intensity on the sensitive protoplasm in 
 the organism it responds with a definite reaction, after which 
 it repeats the trial movements. Thus It continues until it 
 becomes so directed in its course that rotation on the long 
 axis no longer produces sufficient change of intensity on 
 the sensitive part to induce a reaction. It Is evident that 
 this condition Is fulfilled when the organism moves toward 
 or away from the general source of light. Orientation can 
 take place in an absolutely constant intensity of light in 
 the field. It is however always Induced by reactions which 
 are due to changes of intensity on some part of the organism. 
 
lOO LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 This is of course due to the successive illuminating and shad- 
 ing of different i)arts of the organism owing to its move- 
 ments. There is no evidence indicating that light, acting 
 constantly as a directive stimulus similar to the action of 
 the electric current, has any influence on orientation of 
 Euglena in accordance with the idea of Loeb supported by 
 Torrey. This however does not mean that light does not 
 act constantly on the organism, for it is probable that it 
 does, much in the manner of temperature. The evidence 
 bearing on the point in question is however not conclusive. 
 Euglena does become more active when the intensity is 
 increased, but it is impossible to sa>- whether this increase 
 in activity is due to absolute intensity or to the change 
 of intensity on certain structures caused by the rotation 
 of the organism. The fact that the movement of the 
 organism does cause changes of intensity on different 
 structures in it makes the problem as to the effect of con- 
 stant light intensity an exceedingly difficult one to reach 
 experimentally. 
 
 Nageli (i860, p. 102) concludes that in swarm spores the 
 rate of movement is independent of the light intensity, and 
 Strasburger (1878, p. 624) comes to the same conclusion. 
 " Die Schnelligkeit der Bewegung wird durch das Licht 
 nicht beeinflusst, doch bewegen sich die Schwarmer je 
 grosser die Lichtintensitat ist, in um so geraderen Bahnen." 
 Pfeffer (1884, p. 375) also is of the opinion that chemical 
 stimulation of fern spermatozoids causes no acceleration of 
 movement. Holmes (1903, P- 323) however says: " It was 
 found that, as the Volvox travelled towards the light, their 
 movement was at first slow, their orientation not precise, 
 and their course crooked. Gradually their path became 
 straighter, the orientation to the light rays more exact and 
 their speed more rapid. After travelling over a few spaces 
 (centimeters), however, their speed became remarkably 
 uniform until the end of the trough was reached." I came 
 to the same conclusion in my study of Volvox (1907, p. 150), 
 but was of the opinion that the increase in rate of movement 
 
OBSERVATIONS ON UNICELLULAR FORMS loi 
 
 Is dependent more upon the time of exposure to light than 
 upon the increase of intensity. 
 
 The experimental difficulty of course lies in the fact 
 pointed out above, that the movement of the organism 
 itself causes change of intensity on different structures 
 in it. The fact however that Volvox, e.g., in swimming 
 toward a source of light into regions of higher intensity 
 without changing its orientation, swims more slowly as it ap- 
 proaches the region of optimum intensity and finally stops 
 altogether, seems to show very clearly that the intensity 
 affects the rate of movement. But the results of Holmes 
 also show that the rate bears no definite relation to the 
 absolute intensity. There is much need of more experi- 
 mental data on this subject. 
 
 Does Euglena always turn toward or from the side 
 stimulated? Is orientation due to differential response to 
 localized stimulation? Or is the organism stimulated as 
 a whole with a reaction dependent more or less upon the 
 structure of the organism? If we are correct in our assump- 
 tion that there is a highly sensitive protoplasmic structure 
 located in the anterior end of Euglena, it is likely that it 
 is always stimulated by light In the same place regardless 
 of the portion of the surface exposed. Judging from this 
 alone one might conclude that the stimulus acts as a local 
 sign. But the fact that Euglena in the crawling state 
 always bends toward the ventral surface when stimulated 
 by light, while specimens in the free-swimming state always 
 turn toward the dorsal surface when stimulated, contra- 
 dicts this conclusion and supports the Idea that the stimulus 
 acts upon the organism as a whole. This is again in direct 
 opposition to Torrey's statement (1907, p. 319), "The 
 interpretation of the behavior of heliotropic organisms on 
 the basis of general changes concerning the whole organism, 
 not only does not accord with the known facts, but is 
 rather psychical than physiological in character." It is 
 difficult to see how the fact that an organism always turns 
 in the same direction when it is stimulated as a whole can 
 
I02 LIGHT AXD THE BFJIAVIOR OF ORGAXISMS 
 
 be considered a criterion of psychic activit>'. For all that 
 is known to the contrary Euglena ma\' be conscious. It 
 may indeed have anthropomorphic sensations accompany- 
 ing each reaction. But surely no one would consider the 
 fact that it always turns toward the same side and does 
 not respond in accordance with the theory of localized 
 stimulatif)n as indicating that it has such sensations. 
 
 /. Orientation of Euglena in the swimming state. — 
 Early in December two species, Euglena deses and a form 
 much like viridis but somewhat larger, were found in the 
 free-swimming state. The\' were howc\'cr not very active; 
 the specimens of E. deses studied swam only at an average 
 rate of approximately 0.25 mm. per minute and rotated 
 only about eleven times per minute; the other species moved 
 somewhat faster. The reactions in both could readily be 
 followed under a magnification of 150 diameters. Their 
 orienting reactions were studied in the same manner as 
 were those of Euglena in the crawling state, and they were 
 found to be practically the same. 
 
 A decrease of the light intensity in the field without a 
 change in the direction of the rays produces definite reac- 
 tions, (i) There is a slight bending of the anterior end 
 toward the dorsal surface. (2) The whole organism turns 
 toward the dorsal surface by the action of the flagellum. 
 (3) Their spiral course becomes wider. If the decrease is 
 considerable they may be thrown out of orientation entirely 
 and turn about several times before they become oriented 
 again. 
 
 If the direction of the rays is changed without the light 
 intensity's being changed, there is usually no reaction, 
 just as m crawling specimens, until in the process of rota- 
 tion the surface containing the eye-spot comes to face the 
 light; then there is a sudden turning toward this surface, 
 i.e., toward the source of light. In many instances the 
 turning is so sharp immediately after the dorsal surface 
 becomes illuminated that it may appropriately be desig- 
 nated as a jerk or a twitch. This reaction is repeated 
 
OBSERVATIONS ON UNICELLULAR FORMS 103 
 
 every time this surface is turned toward the Hght, each 
 reaction resulting in directing the anterior end more nearly 
 toward the source of light, until both surfaces are so nearly 
 equally illuminated throughout the entire rotation that 
 the change of intensity is no longer sufficient to cause a 
 reaction. 
 
 If the light intensity is decreased at the same time that 
 the ray direction is changed, the reaction described above 
 always occurs immediately after the change is made, regard- 
 less of the surface illuminated. All however turn toward 
 the dorsal surface. This results in movement in all direc- 
 tions and apparent confusion. When Euglenae are taken 
 from the culture jars and first exposed in the aquarium they 
 are more sensitive and respond to slighter changes than 
 they do after they have been exposed for some little time. 
 Among these one finds many specimens which always re- 
 spond immediately after the ray direction is changed, even 
 if the intensity remains the same, no matter which surface 
 faces the light after the change is made. This indicates 
 that a change in the direction of the rays from that in 
 which the anterior end is illuminated to one in which the 
 ventral surface is illuminated causes the same response as 
 a decrease of intensity. It may therefore be concluded 
 from this and preceding observations that the organism is 
 most stable^ when the anterior end faces the source of light, 
 less stable when the ventral surface faces it, still less stable 
 when the dorsal surface is exposed, and least stable when 
 the posterior end is directed toward the source of light. I 
 was able to ascertain roughly the amount of reduction in 
 light intensity required to induce the avoiding reaction 
 with each of these different surfaces illuminated excepting 
 that on the posterior end. The results of this work to- 
 gether with a description of methods will be found below. 
 
 The orienting reaction of free-swimming specimens is 
 in all essentials like that of the crawling specimens. It 
 
 ^ It is meant by this that it requires a greater change of Hght intensity 
 in the field to induce a reaction. 
 
I04 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 takes place just as Jennings represents (Fig. 12), with the 
 exception that if the direction of the rays is changed with- 
 out any change of the intensity, orientation may take place 
 without an increase in the diameter of the spiral course 
 represented in Fig. 12, a-c. The organisms may orient by 
 increasing the swerving only toward the source of light 
 after its position is changed, not in the opposite direction. 
 
 The fact that these free-swimming specimens of Euglcna 
 in certain i:)hysiological states do not respond at all after 
 the position of tlie source of light is changed from one in 
 which the anterior end is illuminated to one in which the 
 ventral surface is exposed, until the organism rotates so as 
 to expose the dorsal surface; that as soon as this surface 
 faces the light there is a sudden twitching turn toward the 
 source of light; and that this reaction is repeated every time 
 the surface containing the eye-spot comes to- be illuminated 
 in the course of the rotation on the axis, shows very clearly 
 that orientation in the free-swimming state as well as in 
 the crawling state is not due to a constantly acting stimulus, 
 as Torrey assumes. 
 
 Unequal stimulation of symmetrically located points, as 
 an important factor in causing orientation in accord with 
 the theories of Verworn and Loeb, is of course out of the 
 question in this form. If in heliotropism the results are 
 a function of the constant intensity, as Loeb maintains 
 (i()o6, p. 135), it must be admitted that there is no evidence 
 indicating that Euglena is heliotropic. 
 
 j. Threshold or sensitiveness when different surfaces are 
 exposed to light. — The difference in sensitiveness of the 
 organism with different parts of the surface illuminated 
 was measured in the following way: positive specimens 
 were exposed in the small slide aquarium' to light from the 
 glower on the track. After they had oriented, the intensity 
 was suddenly decreased without any change in the direction 
 of the rays, by sliding the glower away until it could be 
 
 * An aquarium made of glass slides glued together with balsam boiled 
 in linseed oil. 
 
OBSERVATIONS ON UNICELLULAR FORMS 1 05 
 
 clearly seen that a majority responded by definitely in- 
 creasing the width of the spiral The point at which such 
 response was given varied much with different individuals 
 under different conditions and could therefore not be accu- 
 rately ascertained. It was however discovered that if the 
 organisms were oriented in 61 ca. m. the intensity had to 
 be decreased to 17 ca. m. before uncjuestionable response 
 resulted. This shows that under the conditions of the 
 experiment it requires a decrease of 44 ca. m., or over 66 
 per cent of the total intensity, to induce the avoiding 
 reaction when the light strikes the anterior end. 
 
 By changing the position of the movable glower, the re- 
 lation between the intensities of light from the two glowers, 
 here arranged as in many of the preceding experiments, 
 was so adjusted that when the Euglenae were suddenly 
 exposed in the stronger light after they had oriented in the 
 weaker, nearly all responded at once, regardless of the sur- 
 face illuminated. Those in which the ventral surface was 
 exposed turned away from the source of light; those with 
 the dorsal surface illuminated turned toward it, and the 
 rest turned in various other directions. The reaction is 
 very striking under these conditions, although of course it 
 was possible to ascertain only approximately the change 
 of intensity necessary to produce it. It was found after 
 many trials that the least reduction of light intensity with 
 a simultaneous change in the direction of the rays, which 
 caused this reaction in a majority of the specimens, was 
 32 ca. m., the intensity of the light from the stationary 
 glower being 61 ca. m. and that from the movable glower 
 29 ca. m. It will of course be understood that individuals 
 frequently respond to much smaller changes of intensity, 
 depending upon their physiological state. With the an- 
 terior end exposed then, a reduction of 44 ca. m. without a 
 change in the direction of the rays is sufficient to cause the 
 avoiding reaction. With the ventral surface exposed a 
 reduction of 32 ca. m. together with a simultaneous change 
 in the direction of the rays causes the avoiding reaction, 
 
Io6 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 and with the dorsal surface exposed the same reaction is 
 induced b\- a ciiange in the direction of the rays without a 
 decrease of intensit\'. These results lead to the conclusion 
 that a change of the organism from a position in which the 
 anterior end faces the source of light to one in which the 
 dorsal surface faces it, results in a reduction of effective 
 light intensity of approximateK' 44 ca. ni. in a total inten- 
 sity of (H (\i. ni. Since the anterior vnd of Euglena is 
 nearU' transparent, such a relati\el>' large reduction seems 
 possible only if there is a highly sensiti\e bit of i)rotoplasm 
 so situated that the eye-spot casts a shadow on it when the 
 light strikes the dorsal surface. 
 
 k. Function of the eye-spot. — Wager (1900, PI. 32, 
 Fig. 2) observed an enlargement in the flagellum, situated 
 very near the concave surface of the eye-spot (see Fig. 11). 
 It ina\' be that this is highly sensitive to changes in light 
 intensity and that the eye-spot functions as an opaque 
 screen casting a shadow upon the enlargement whene\'er 
 the dorsal surface is exposed. It ma\' however also function 
 in absorbing the rays when the ventral surface or the 
 anterior end is exposed, much as the retinal pigment func- 
 tions in the eye of higher forms, or it may function some- 
 what like the pigment cups in planarians, amphioxus, etc. 
 
 The onh' evidence we have with reference to the function 
 of the eye-spot aside from that presented above is given by 
 Engelmann (1882, j). 3()6). He says, in substance, refer- 
 ring to this structure in Euglena \'iridis, that if a sharp 
 shadow is gradually brought from the posterior end of a 
 swimming Euglena toward the anterior, there is no reac- 
 tion imtil the shadow reaches the colorless anterior portion 
 of the organism which contains the eye-spot. In the case 
 of large individuals moving into a shadow, the reaction 
 could be seen to be given before the eye-spot was in dark- 
 ness. The colorless anterior end is therefore the primary 
 light recipient region, but the eye-spot may still function 
 secondarily, as do the jMgment cells in the retina of higher 
 animals. These observations of Engelmann have been 
 
 i 
 
OBSERVATIONS OX UNICELLULAR FORMS 107 
 
 widely quoted, and it is generally assumed that they prove 
 that the sensitive portion of Euglena is located anteriorly 
 from the eye-spot, and some hold that they even prove 
 that the eye-spot does not function in light reactions at all. 
 My observations on Euglena, however, seem to indicate 
 that the portion most sensitive to light lies in close proximity 
 with the inner surface of the eye-spot, not in front of it. 
 
 I repeated the experiment of Engelmann as follows: An 
 opaque screen containing a rectangular opening 2X3 cm. 
 was placed between the microscope and a Wclsbach burner 
 as near the globe of the burner as possible. A piece of tin 
 was then hung inside the globe of the burner a few milli- 
 meters from the Welsbach mantle and so arranged that 
 one of the straight edges could be seen through the open- 
 ing in the screen. By means of the Abbe condenser that 
 portion of the mantle exposed was focused on a slide under 
 the microscope, containing Euglena deses and E. viridis (?), 
 also E. triqueter and other species. The edge of the tin 
 focused on the slide gave a strikingly sharp edge between 
 the light area and the shadow. The reactions of the 
 Euglenae were studied as they approached this edge. Both 
 low and high power were used in the observations. The 
 relation of intensity between light and shadow could be 
 regulated by manipulating the iris diaphragm connected 
 with the Abbe condenser, and the light area could easily 
 be shifted by turning the mirror. In this way it was pos- 
 sible to move the shadow of the tin over any portion of the 
 specimens while they were in motion. The Euglenae under 
 observation swam about very slowly, E. deses at the rate 
 of approximately 0.3 mm. per minute and viridis (?) not 
 much faster. Every movement and reaction could be 
 distinctly seen. I was however able to confirm Engel- 
 mann's conclusions only in part. The Euglenae generally 
 reacted before the entire body entered the shadow, and no 
 response was observed when the posterior end was shaded 
 until the shadow reached the anterior end, proving in 
 accord with Engelmann's conclusion that the anterior end 
 
lo8 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 is undoubtedly more sensitive than the posterior. Speci- 
 mens were also repeatedly seen to react as soon as the 
 anterior end in front of the eye-spot came into the shadow, 
 but man>' were seen to turn about before the anterior end 
 reached the shadow at all, i)resumabl>' owing to causes 
 other than changes in light intensity. I was therefore 
 at no time certain that those which reacted when only 
 the tip of the anterior end touched the shadow would not 
 have reacted had they not come in contact with the shadow. 
 
 But suppose that those which did react before the eye- 
 spot was shaded were stimulated by the shadow on the 
 anterior end in front of the eye-spot, would this prove that 
 the e\e-spot is not a light recipient organ or that there is 
 no highly sensitive structure back of the part stimulated? 
 It evidently would not, for as soon as the anterior end 
 touches the shadow, the light which is reflected from it 
 onto the structures in the interior of the body before it 
 reaches the shadow is cut off. The light intensity on 
 structures which are not in the shadow at all is therefore 
 reduced as well as that on those which are in the shadow, 
 and it may be that the decrease of intensity on the former 
 causes the reaction. 
 
 The possible effect on structures in Euglena near the eye- 
 spot, due to shading merely the tip of the anterior end, can 
 readily be illustrated by noting the effect if one looks into 
 the mouth of a test tube full of translucent jelly and throws 
 a shadow on the closed end. The reduction of light will 
 of course affect the eye at once, although it may be a con- 
 siderable distance from the shadow. 
 
 If there were a differentiated bit of protoplasm highly 
 sensitive to variation in light intensity, located in close 
 proximity to the eye-spot on the side facing the interior 
 of the body, one might even expect the organism to react 
 before the anterior end reaches the shadow at all, for, since 
 there is no light reflected from the shaded area, it is evident 
 that merely turning the anterior end toward it would result 
 in a decrease of light intensity on the postulated sensitive 
 
OBSERVATIONS ON UNICELLULAR FORMS 109 
 
 Structure, and moving toward the shadow would decrease 
 it still more. 
 
 There is consequently nothing in connection with the 
 observations of Engelmann which contradicts the idea that 
 the eye-spot in Euglena functions as an opaque screen, and 
 that there is a bit of protoplasm which is highly sensitive 
 to changes in light intensity in close contact with it. The 
 hyaline protoplasm at the anterior end condenses the light 
 so that it is most intense in the neighborhood of the eye- 
 spot. This can be seen in Euglena in direct sunlight. It is 
 however much more marked in Chlamydomonas and the 
 zooids of Eudorina and Pandorina (Figs. 17 and 21). If 
 the light is thus actually focused on the most sensitive 
 structure of the organism it is easy to see how changes in 
 the general direction of the rays could produce marked 
 changes of intensity on this structure. Aside from acting 
 as an opaque screen the eye-spot may, of course, as already 
 stated, also function as an absorptive background. 
 
 In Trachelomonas hispida the eye-spot is situated very 
 near the middle of the anterior end (see Fig. 16). If it 
 functions by shading the sensitive portion or by absorbing 
 the rays in this form it is highly probable that the sensitive 
 portion consists of a minute structure situated very near 
 it, perhaps in the hollow of the concave surface. In some 
 of the forms however the location of this structure seems 
 to show that it does not function as a screen. In Vol vox, 
 Pandorina and Eudorina the eye-spots are situated on the 
 outer posterior surface of the zooids. It is difficult to see 
 how they could function as screens in these forms (see Fig. 
 21). The same difficulty is encountered in some forms of 
 Chlamydomonas in w^hich this structure is situated near 
 the posterior end (see Fig. 17). It does however not seem 
 necessary to assume that the eye-spot functions precisely 
 the same in all forms. While it may function both as an 
 opaque screen and as an absorptive background in Euglena, 
 it may possibly function only by absorbing light rays in 
 Volvox and Chlamydomonas. 
 
no LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 3. Siunynary 
 
 (i) Some species of Euglcna exist in three states, — free- 
 swiniining, crawling and encysted. 
 
 (2) While in the crawling state they push themselves 
 along at the rale of about 0.3 mm. ])tT minute by alter- 
 nately cur\'ing and straightening the bod>- ver>' slightly as 
 they rotate on the long axis. There is no evidence of loco- 
 motion b\- means of amoeboid movement. 
 
 (3) In this state they orient fairly accurately in light. 
 They may be either positive or negative. 
 
 (4) When exposed to light from two sources they may 
 move toward or from a point located anywhere between the 
 two sources. The location of this point depends upon 
 the relation in amount of light from the two sources. If 
 the light from one source is stronger than that from the 
 other, this point will lie nearer the source from which the 
 stronger light comes. 
 
 The orientation of sixteen other species in light from two 
 sources was ascertained. All of those without image-form- 
 ing eyes, fifteen in number, oriented just like Euglena. 
 The one with image-forming eyes always moved directly 
 toward one or the other of the sources of light, never 
 toward a point between them. It is therefore evident that 
 Loeb's statement regarding this point will not hold for 
 any of these organisms. 
 
 (5) If the intensity is decreased without any change in 
 the direction of the rays, positive Euglenae in the crawling 
 state always respond by bending the anterior end toward 
 the ventral surface. This may be termed a shock-move- 
 ment, or avoiding reaction, or a bending reaction. 
 
 (6) A change from a position in which the ventral sur- 
 face faces the source of light to one in which the dorsal, i.e., 
 the surface containing the eye-spot, faces it, induces the 
 bending reaction. Such a change in position therefore pro- 
 duces the same result as does a reduction in the light inten- 
 sity. These reactions can consequently be induced either 
 
OBSERVATIONS ON UNICELLULAR FORMS ill 
 
 by changing the direction of the rays or by changing the 
 light intensity of the field. 
 
 (7) The bending reactions are induced wherever the light 
 strikes the dorsal side of Euglena owing to its rotation on 
 the long axis. This reaction is repeated until the organism 
 is oriented and rotation no longer causes a change of illumi- 
 nation on its dorsal and ventral surfaces. It remains ori- 
 ented because, while it proceeds in this direction, there are 
 no stimulations which induce the bending reaction. 
 
 (8) The intensity can be so gradually changed that there 
 is no response. The bending reaction is therefore depend- 
 ent upon a time rate of change, and orientation is conse- 
 quently also due to a time rate of change in the light 
 intensity. 
 
 (9) The results of these experiments support Jennings' 
 conclusion that orientation in Euglena is brought about by 
 selection from trial positions. This of course does not 
 mean conscious selection. 
 
 (10) It is probable that light has a constant effect on the 
 activity of Euglena much as temperature does, but there is 
 no evidence that such activity has anything to do with the 
 process of orientation, as the explanations of Loeb, Ver- 
 worn, and Torrey demand. 
 
 (11) Orientation is not dependent upon the direction in 
 which light rays pass through the tissue, in accordance 
 with Sachs; nor is it dependent upon the angle between 
 the rays and the sensitive surface, or the unequal stimula- 
 tion of symmetrically located points on the surface, as Loeb 
 assumes; nor upon the effect of the stimulating agent upon 
 the locomotor organs directly, or through a direct reflex 
 arc, as the theory of Verworn demands; nor upon light 
 acting constantly as a directive stimulus, in accord with 
 Loeb's idea supported by Torrey. 
 
 (12) The most highly sensitive portion of Euglena is 
 probably situated near the concave surface of the eye-spot, 
 and the eye-spot probably functions in casting a shadow 
 on the highly sensitive substance when the light strikes the 
 
112 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 dorsal surface. The eye-spot may also function in absorb- 
 ing the rays. 
 
 (13) There is no evidence in these experiments bearing 
 on the question of anthropomorphic sensation. The results 
 do not exclude its presence. The reactions are due to 
 changes in light intensity, and every change, for all that is 
 known to the contrary, may cause a sensation. 
 
 I 
 
CHAPTER VI 
 
 OBSERVATIONS ON UNICELLULAR FORMS IN THE PROCESS 
 OF ATTAINING AND RETAINING A DEFINITE AXIAL 
 POSITION WITH REFERENCE TO THE 
 SOURCE OF LIGHT (continued) 
 
 I. Stentor coeruleus 
 
 a. Introduction. — Davenport (1897) and Holt and Lee 
 (1901) worked out the general features in the light reactions 
 of Stentor coeruleus. They found that these animals are 
 negative and that they orient rather accurately. Holt and 
 Lee concluded that orientation takes place in accord with 
 Verworn's theory, that light acts constantly as a directive 
 stimulation. If one side is more highly illuminated than 
 the other the cilia beat more effectively on the illuminated 
 side. This causes the animal to turn directly from the 
 source of light until it is oriented and both sides are equally 
 illuminated. Jennings (1904) found that an increase in the 
 light intensity of the field causes a definite reaction in Sten- 
 tor regardless of the direction of the rays or the surface 
 exposed at the time the change is made. This reaction was 
 designated the avoiding reaction. It consists of turning 
 toward the right aboral side. The organisms may stop 
 and turn very sharply or they may simply swerve farther 
 towards this side as they proceed on their spiral course. 
 By means of this turning the anterior end is directed toward 
 various points in space. Sooner or later it becomes directed 
 away from the source of light and the organism is oriented. 
 This direction is retained because when the anterior end is 
 turned from the light it is not subjected to changes in light 
 intensity as the animal rotates on its axis and continues 
 on its spiral course. 
 
 Orientation, therefore, according to Jennings, Is brought 
 
 113 
 
114 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 about !)>■ reactions which are induced by a change in the 
 effective intensity. This ma>' be due to actual change of 
 the intensit\' of {hv field, lo a niowment from a region of 
 one intensit}' to that of another, or to a change of intensity 
 on the surface of the organism caused by changing the sur- 
 face turned toward the source of light. Direction of rays 
 and difference of intensity in the field are functional in the 
 process of orientation onl}- in so far as ihe>- ma>- influence 
 change of intensity on the organism. Orientation is not 
 induced by a constantly acting directive stimulus; it is the 
 result of a response to a time rate of change of intensity, a 
 shock-effect, Unterschicdscmpfindlichkeit. 
 
 Working independently of Jennings I obtained (1906) 
 results which were in all essentials in harmony with his. 
 Jennings assumed that the anterior end of Stentor is more 
 sensitive than the posterior. I proved that Stentors are 
 more sensitive to light when the anterior end is exposed 
 than they are w^hen any other surface is exposed. The 
 minimum threshold in animals stimulated by rays perpen- 
 dicular to the long axis was found to be 1.2 ca. m., and 
 that in those stimulated by light striking the anterior end 
 only 0.25 ca. m. 
 
 b. Orienting reactions. — I was of the opinion that while 
 the avoiding reactions no doubt play a large part in orien- 
 tation of Stentor, a direct effect of light as a constantly 
 acting stimulus in orientation might be discovered by a 
 careful investigation with this in mind. 
 
 Three methods were used in this investigation: (i) Water 
 was removed from imder the cover glass until the space 
 between it and the slide became so narrow that the Stentors 
 could no longer rotate on their axes. They were then illu- 
 minated so that various surfaces were successively exposed. 
 If light acts constantly as a directive stimulus one might 
 expect the cilia on the illuminated side to strike back more 
 vigorousl}^ than those on the shaded side regardless of the 
 surface exposed. I was however unable to observe any 
 relation between the rate of movement of the cilia and the 
 
OBSERVATIONS ON UNICELLULAR FORMS 1 15 
 
 surface illuminated as indicated by the currents in the 
 water. 
 
 (2) A number of attached Stentors in a small rectangular 
 glass aquarium were repeatedly suddenly exposed in light 
 intensity of 8000 ca. m. produced by a Nernst lamp. When 
 they were thus exposed they were directed toward various 
 points of the compass, so that various parts of the surface 
 faced the source of light in different indi\iduals. A few 
 always contracted immediately after each exposure, others 
 began to swing about the point of attachment, some clock- 
 wise and others counter-clockwise, but all turned toward 
 the ventral surface. The cilia must consequently beat the 
 same in all individuals no matter which surface is exposed 
 to the light. There is therefore no evidence in these results 
 that light acts constantly as a directive stimulus. 
 
 (3) After Stentors had oriented in light from a single 
 Nernst glower, the glower was slightly moved to one side 
 so as to change the direction of the rays slightly, and the 
 method of reorienting was observed. It was found that 
 under such conditions the Stentors merely swerve farther 
 away from the source of light each time after the oral side 
 is directed toward it in the process of rotation. Thus they 
 soon become oriented again. There is no definite avoiding 
 reaction in this process of orientation. The organisms 
 never increase the swerving toward the source of light ; they 
 always increase it in a direction which tends to turn the 
 anterior end from the light. Does light act as a constantly 
 directing stimulation in this process of orientation or does 
 it act by causing repeated successive stimulations due to 
 changes of intensity on some part of the surface of the 
 organism as in Euglena? Is Stentor heliotropic according 
 to Loeb's definition or is it tinterschiedsempfiiuUich? The 
 following experimental observations will furnish answers to 
 these questions. 
 
 Two Nernst glowers were arranged and screened so as 
 to produce two small beams of light which crossed at right 
 angles in a small aquarium containing numerous Stentors. 
 
Ii6 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 The light intensity from each of the two glowers was equal. 
 The direction of the rays could therefore, without any alter- 
 ing of the intensity, be changed by alternately intercepting 
 the light in each of the two beams. If the ray direction is 
 thus changed after the Stentors are oriented in one of the 
 beams of light, one side will of course be directed toward 
 the light. If it chances to be the aboral side and the 
 Stentors are not very strongly negative, they continue on 
 their course just as though the direction of the rays had 
 not been changed, until in the process of rotation the oral 
 side comes to face the light; then the organism responds in 
 one of two ways: it may stop suddenly and sometimes back 
 a little and turn sharply toward the aboral side; that is, it 
 may respond with the avoiding reaction, or it may merely 
 swerve farther from the source of light on its spiral course 
 as represented in Fig. 14. When the oral side again comes 
 to face the light the organism is again stimulated and it 
 again swerves farther from the source of light. This reac- 
 tion is repeated once during each rotation until the oral 
 side is nearly equally exposed to the light throughout the 
 entire rotation. This is evidently true when the anterior 
 end is directed away from the source of light. If the or- 
 ganism responds with the avoiding reaction it turns more 
 directly from the source of light and thus becomes more 
 rapidly oriented, as represented in Fig. 14. 
 
 Why does Stentor respond when the oral side faces the 
 light and not when the aboral side faces it in the same 
 intensity? If Stentors are oriented in light of a given 
 intensity and the intensity is decreased without any change 
 of ray direction there is no response; but if it is increased 
 they respond in one of two different ways, depending upon 
 the amount of increase. If the increase is relatively slight 
 they merely swerve more strongly toward the oral side; and 
 since this side always faces out when the organism swims 
 on its spiral course, the result is that the course is made 
 wider. If the intensity increase is greater the creature 
 stops suddenly, turns toward the aboral side, sometimes 
 
Fig. 14. Stentor coeruleus in the process of orientation. The curved line 
 represents the spiral course; the arrows m and n the direction of light from two 
 sources; a-f, different positions of Stentor on its course; 0, the oral surface; ab, the 
 aboral surface. At a the Stentor is oriented in light from m, n being shaded. If 
 n is exposed and m shaded simultaneously when the Stentor is in position b, there 
 is usually no reaction, if the intensity has not been changed, until it reaches c and 
 the oral side faces the light; then the organism may respond by suddenly stopping, 
 backing and turning sharply toward the aboral side, as indicated by the dotted 
 outline, and become oriented at once; or it may merely swerve more or less toward 
 the aboral side without stopping. At e the oral side is again exposed and the 
 organism is again stimulated and it again swerves from the source of light. This 
 process is continued until the oral side is appro.ximately equally exposed to the 
 light in all positions on the spiral course. If the Stentor is at c when « is cxpo.sed 
 it responds at once and orients as described above. If the light from n is more 
 intense than that from w, or if the organism is very sensitive when n is exposed and 
 m shaded, it responds at once no matter in which position it is. If it is at b it turns 
 toward the source of light, but now repeats the reaction, successively turning in 
 various directions until it becomes oriented. 117 
 
Il8 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 backing slightly at the same time, and goes ahead s\ver\'ing 
 sharply toward the ventral surlace and ihe oral side. This 
 throws the animal out of orientation; the first method of 
 response does not. This shows that the reaction is dtie to 
 an increase of intensit\' on some part or on the whole of 
 the bod\'. It is evident that the intensity of light on the 
 sides of Stentor changes, if it rotates while it is illumi- 
 nated from the side, owing to its own shadow; and since it 
 reacts onl\- when the oral side is carried from a position in 
 which it is shaded to one in which it is illuminated, it is 
 clear that this side must l)e more sensitive than the aboral, 
 or perhaps better, that the animal is more sensitive when 
 the oral side is exposed than it is when the aboral side is 
 exposed. 
 
 If the light intensity is increased at the same time that 
 the direction of the rays is changed as described above, all 
 the organisms respond with the avoiding reaction at once no 
 matter which side faces the light, just as in the case of Eu- 
 glena. In this response some may be seen to turn upward, 
 some downward, and others to the right or left. They all 
 turn toward the aboral side. The direction in which they 
 turn therefore depends upon the position of this side when 
 the change is made. Stentors frequently respond thus when 
 the direction of the rays is changed without varying the 
 intensity. This takes place when the organisms are highly 
 sensitive. 
 
 It is clear from this description that the orienting reac- 
 tions in Stentor and Euglena are the same in j:)rincible. 
 Both organisms can orient in a field of uniform light of 
 constant intensity. The stimuli causing orientation are 
 however due to changes of intensity on the sensitive struc- 
 tures in the body. Such changes of intensity in a field of 
 light of uniform and constant intensity are caused by the 
 shadows produced by one part passing over other parts as 
 the organisms rotate. There is no evidence that the direc- 
 tion of the rays functions in orientation excepting in so 
 far as it may influence changes of intensity; nor is there 
 
OBSERVATIONS ON UNICELLULAR FORMS 119 
 
 any evidence that light acting constantly somewhat like a 
 constant electric current, has any effect on orientation as 
 Loeb's explanation of orientation demands. 
 
 c. Difference in sensitiveness with different surfaces 
 illuminated. — The threshold of reaction in Stentors varies 
 so much in different individuals and in the same individual 
 in different conditions that quantitative results are of little 
 value unless they can be correlated with causes of variation. 
 A few measurements made may however be of interest 
 in showing the relative stability of these organisms with 
 different parts of the surface exposed. 
 
 On February 12, specimens fresh from the culture jar 
 were put into the aquarium with the two Nernst glowers 
 arranged as described above. The intensity from the two 
 glowers was equal; it was 321 ca. m. When the ray direc- 
 tion was changed by intercepting alternately the beams of 
 light after the Stentors had become oriented, practically 
 all of them responded immediately with the avoiding reac- 
 tion regardless of their position wdien the change was made. 
 Some turned toward the light, others away from it, and the 
 remainder turned in various other directions. After these 
 specimens had been experimented upon for about fifteen 
 minutes only those responded immediately in which the 
 oral side faced the light when the direction of the rays 
 was changed. The rest did not respond until after they had 
 rotated sufficiently to expose the oral side. In responding 
 they gave either the avoiding reaction or merely swerved 
 farther from the source of light as they continued on their 
 spiral course. In both methods of reaction they always 
 turned directly from the source of light, never toward it. 
 In casually studying Stentors under these conditions only, 
 one might readily conclude that orientation is always direct 
 and that It Is due to local response to a local stimulation. 
 This however is not the case. 
 
 As soon as the observations described above were com- 
 pleted, I put the Stentors into darkness, left them for a 
 short time and then exposed them to light from the 
 
I20 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 movable glower in an intensity of 150 ca. m. After they 
 had oriented the glower was suddenly pulled toward the 
 aquarium until it could be clearly seen that many of the 
 specimens responded with the avoiding reaction. By re- 
 peating this man\' times it was found that it required an 
 increase from 150 ca. m. to 444 ca. ni. (or 294 ca. m.) to 
 throw them out of orientation. This may then be called 
 the threshold with the posterior end illuminated. 
 
 Frequently during the progress of the preceding experi- 
 ments the ray directi(^n was changed and the light intensity 
 increased simultaneously. It was found that when the 
 intensity was thus increased from 150 ca. m. to 321 ca. m. 
 (or 171 ca. m.), nearly all responded at once regardless of 
 the surface turned toward the source of light after the 
 change was made; and under these conditions they could 
 be seen to turn toward the light as well as from it. But 
 when the intensity was increased from 226 ca. m. to 321 
 ca. m. (or 95 ca. m.), only those responded in which the 
 oral side faced the light after the direction of the rays was 
 changed, and these also responded when the ray direction 
 was changed without an increase in light intensity, as repre- 
 sented in Fig. 14. 
 
 Judging from these results a change in the position of a 
 Stentor from one in which the posterior end faces the source 
 of light to one in which the oral side faces it, is equivalent to 
 increasing the intensity nearly threefold; and a change from 
 a position in which the aboral side is illuminated to one in 
 which the oral side faces the light is equivalent to doubling 
 the intensity. These considerations show clearly how a 
 stimulation in a field of uniform and constant light intensity 
 can be produced by change of intensity. The fact that 
 Stentors are so much more sensitive when the oral surface 
 is illuminated than when the aboral surface or the posterior 
 end is exposed, points toward the presence of a highly sensi- 
 tive region in the neighborhood of the oral opening in Sten- 
 tors. The precise location of this region is a subject for 
 future investigation. The following experiments however 
 
OBSERVATIONS ON UNICELLULAR FORMS 121 
 
 indicate that it is not in the membrancllae or in the ridge 
 from which they project. 
 
 On February 27, 3.30 p.m., a number of Stentors were 
 put into a one-half-normal glycerine solution. In the course 
 of about two minutes the entire ridge with the membrancllae 
 in nearly all the specimens was thrown off, after which the 
 anterior end was rounded and the oral opening was tightly 
 closed. They were then transferred to normal culture fluid, 
 in which they swam about much like normal specimens. 
 They rotated on the long axis, proceeded on a spiral course 
 and responded with the avoiding reaction when they were 
 mechanically stimulated or when suddenly exposed to strong 
 light. The threshold for light reactions was however much 
 greater than normal. At 8.30 p.m. there was no indication 
 that regeneration had begun, but on the following morning 
 nearly all the specimens were normal again. 
 
 d. Localized stimulation. — Are the reactions in Stentor 
 differential responses to localized stimulation ? In Eugiena, 
 it will be remembered, we were obliged to answer this ques- 
 tion in the negative. The results described above seem to 
 show that there is a highly sensitive structure in Stentor 
 at the anterior end near the oral side. It is therefore 
 probable that whenever these animals are stimulated by 
 light they are stimulated in this region regardless of which 
 surface Is exposed; and If this is true It is evident that the 
 fact that these organisms may turn toward the illuminated 
 side or toward the shaded side does not prove that they 
 do not give a differential response to localized stimulation, 
 nor does it prove that they do. We have therefore no 
 conclusive evidence bearing on this question with reference 
 to Stentor. 
 
 Summary 
 
 (i) Stentor coeruleus collects in shaded regions either by 
 orienting and swimming directly toward such regions or by 
 wandering Into them aimlessly. They remain in the shaded 
 region because whenever they come to the edge of it, the 
 
122 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 increase of intensity causes them to respond with the avoid- 
 ing reaction. 
 
 (2) An increase in the intensity of Hi;ht in w hich Stentors 
 are oriented, without any variation in the chrertion of the 
 rays, causes them t(j respond either with the axoicHng reac- 
 tion or by simply swerving farther toward the oral side, 
 making the si)iral course wider. 
 
 (3) Orientation in Stent or takes place essentially as it 
 does in Euglena. It is caused by changes in light intensity 
 on the sensitive tissue in tlic organisms. If. without a 
 change in the intensity, the direction of the rays is changed 
 so that the side of the organism instead of the end is exposed, 
 there is no reaction, provided the Stentors are not highly 
 sensitive, except when the oral side comes to face the light 
 in the process of rotation on the long axis. Then tlie reaction 
 may consist either of the avoiding reaction or merely of a 
 greater swerving from the source of light. Both result in 
 orientation. This shows that orientation is brought about 
 by reactions due to changes of intensity and not to light 
 acting constantly as a directive stimulus in accord with 
 the theories of Loeb and Verworn. 
 
 (4) If the animals are highly sensitive or if the light 
 intensity is increased when the direction of the rays is 
 changed, the>' respond no matter which side is exposed 
 after the change is made. In this response they turn in all 
 directions, toward the light as wxll as away from it. 
 
 (5) Stentors may orient in a field of light which is uni- 
 form and constant in intensity; but the orientation even 
 under such conditions is due to a change of intensity. This 
 change is caused by the movement of the animal which 
 results in alternately illuminating and shading different 
 parts of the organism. 
 
 (6) Orientation may be said to be due to selection from 
 trial movements, just as in Euglena, even in those cases 
 where Stentor never errs by turning definitely toward the 
 light, for during every rotation the relati\'cly highly sensi- 
 tive oral side is alternately shaded and illuminated until 
 
OBSERVATIONS ON UNICELLULAR FORMS 123 
 
 the organism becomes directed from the source of Hght. 
 Thus it is that the rotation itself constitutes a trial move- 
 ment. 
 
 (7) There is no evidence that Hght acting continuously 
 has any influence on orientation. These organisms are not 
 heHotropic in accord with Loeb's definition of this term. 
 
 (8) Stentors probably are more active in higher than in 
 lower light intensity. But even here it is impossible to say 
 whether the greater activity is due to stimulations pro- 
 duced by constant intensity or by changes of intensity, 
 since even in a field of absolutely constant light intensity 
 the movements of the organism cause the more sensitive 
 parts to become alternately shaded and illuminated. 
 
 2. CEdogonium Swarm-spores 
 
 The reactions of swarm-spores to light have been studied 
 but little. Most observers have merely recorded the fact 
 that they do respond to stimulation by light and that they 
 may be negative or positive. Strasburger (1878, p. 591) 
 found that if exposed in glass jars they collect near the 
 surface of the water at the side facing the window, but he 
 says that they orient very indefinitely and that he therefore 
 did not attempt to analyze the reactions. 
 
 These organisms are very nearly radially symmetrical. 
 It is in such forms, rather than in asymmetrical forms like 
 Euglena and Stentor, that one might expect to find a defi- 
 nite relation between the direction of turning and the side 
 illuminated. In such forms one might also expect orienta- 
 tion to be the result of light acting constantly as a directive 
 stimulation. I was therefore much interested in working 
 out the details in the reactions of these creatures. 
 
 a. Description. — Q^dogonium swarm-spores are in gen- 
 eral very much like an egg in form (Fig. 15). At the smaller 
 end, the anterior, there is a colorless mound-shaped eleva- 
 tion. The rest of the body is green. At the base of this 
 mound-shaped elevation there is a band of cilia. I was 
 
124 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 unable to find any indication of an eye-spot in living speci- 
 mens although I spent much time in looking for it. Stras- 
 burger however claims to have observed a red pigment 
 spot after treatment with acetic acid. There is great varia- 
 tion in the size of the spores; some of those used in these 
 experiments were several times as large as others. I am 
 not certain, however, that they were all of the same species. 
 In general they are considerably smaller than Paramecia. 
 
 Fig. 15. Oedoponium swarm-spores. In responding with the avoiding reac- 
 tion they always turn toward the same side. This sitie bears no defmite rehition 
 to any visible asymmetry in their structure. A turned toward the more concave 
 side, i^., from the side near which the nucleus was located. B turned toward 
 the more convex side, near which the nucleus was found. C was very nearly sym- 
 metrical in form. It turned toward the side containing the nucleus. D, diagram 
 representing the direction of the stroke of the cilia during the process of turning. 
 This can be very distinctly seen under high magnification. 
 
 b. Material. — Swarm-spores were obtained in great 
 numbers in midwinter by adding fresh water to a jar which 
 contained numerous Q^dogonium filaments, and letting it 
 become slightly stale. In those jars which were in diffuse 
 sunlight the swarm-spores collected near the surface of 
 the water on the side facing the window. In those in 
 direct sunlight they usually collected on the opposite side. 
 They are quite strongly negative in their reactions to 
 gravity; this accounts for the collection near the surface 
 of the water. 
 
 c. Locomotion. — They swim on a spiral course of vary- 
 ing width and rotate on the long axis just as do Euglena 
 and Stentor. The smaller end is usually ahead but they 
 reverse freely, and imder some conditions they swim with 
 the larger end ahead almost constantly. They rotate clock- 
 wise when the smaller end is ahead, but in the opposite 
 
OBSERVATIONS ON UNICELLULAR FORMS 125 
 
 direction when the larger end is ahead. Early in the fore- 
 noon after the jars have been in darkness all night the 
 spores usually all swim with the larger end ahead, but later 
 in the day they proceed with the other end foremost. I 
 was however unable to induce this change by keeping them 
 in darkness during the day. The following quotation from 
 my notebook serves to emphasize this peculiar reversal: 
 On the morning of January 10 all swam with the larger 
 nonciliated end forward rotating counter-clockwise as seen 
 from the posterior end. Most of them swam rather actively 
 in closed curves circling toward the left. In the afternoon 
 
 I was surprised to find all the spores swimming about with 
 the smaller end, the end containing the cilia, ahead. They 
 were very abundant in the jar and quite active. The jar 
 was in strong light all day, part of the time in direct sun- 
 light, and although they were most numerous on the wall 
 of the jar facing the window, they gathered on the side 
 under the cover-glass farthest from the window when exposed 
 to the direct rays of the sun. On the morning of January 
 
 II there were but very few motile specimens, but all that 
 were observed excepting one swam as those found on the 
 preceding morning did, i.e., with the larger end ahead. At 
 2.30 P.M. they w^ere slightly more numerous and nearly all 
 swam with the smaller end ahead. I am, at present, un- 
 able to account for this reversal in locomotion. 
 
 These organisms are so small, move so rapidly, and are so 
 nearly symmetrical that it was impossible to ascertain under 
 normal conditions whether or not they always turn toward 
 the same side in their orienting reactions. They were 
 therefore mounted in a solution of quince-seed jelly. In 
 this solution they swim about very slowly; they stop fre- 
 quently, back some distance, turn toward one side and 
 then proceed on a new course; that is, they respond with 
 the avoiding reaction. If the solution contains consid- 
 erable jelly they frequently swim with the posterior end 
 ahead. 
 
 By focusing attention upon specimens in which the 
 
126 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 nucleus, ordinaril>' located near the surface, was visible, 
 and upon (Others in which opposite sides had a slightly dif- 
 ferent cur\'ature, it could be seen (i) that the same surface 
 continuall\- faces out as they proceed on the spiral course, 
 precisely as in asymmetrical forms; and (2) that a given 
 indi\idual always turns toward the same side in giving the 
 avoiding reaction. The side toward which they turn bears 
 no definite relation to the location of the nucleus or to the 
 curvature of the side. Some turn toward the more convex, 
 others toward the more concave surface. These organisms 
 then, although symmetrical, respond with the avoiding 
 reaction, when mechanically stimulated, just like asym- 
 metrical forms. They stop, usually back quite a distance, 
 turn toward a given side, and then proceed on a new course 
 (Fig. 15). Since there is no known asymmetric structure 
 wliich bears any definite relation to the direction of turning, 
 and since the organism always turns toward the same side 
 no matter which point on the surface comes in contact with 
 a solid, it is evident that as far as the facts are known they 
 indicate that there is no differential response to a localized 
 stimulation. The same is true in case of the symmetrical 
 form Didinium nasutum. During the turning process it 
 was clearly seen in both Didinium and the swarm-spores 
 that the cilia strike forward on one side and backward on 
 the opposite side, showing a remarkable differentiation in 
 fimction. 
 
 d. Orientation in light. — The process of orientation was 
 studied In negative forms in direct sunlight. In the quince- 
 seed jelly solution the spores do not orient definitely enough 
 to make it i)ossible to work out their orientation reactions, 
 and positi\e specimens under normal conditions orient very 
 indefinitely. The method of procedure in this study was in 
 general like that followed in the observations on Euglena 
 and Stentor. 
 
 A small beam of light direct from the sun was allowed to 
 fall on the stage of the microscope, and another beam was 
 reflected at right angles to it with a mirror. The light in 
 
OBSERVATIONS ON UNICELLULAR FORMS 1 27 
 
 each of the two beams was then alternately intercepted, and 
 thus the direction of the rays was changed. It was found 
 that in the process of orientation under such conditions, 
 the swarm-spores always turn away from the source of 
 Hght, never toward it, but in this turning they merely 
 swerve farther in their spiral path every time that the course 
 in the spiral is directed away from the source of light, and 
 not so far when it is directed toward it. Since the same 
 side is always directed out in the spiral it is evident that 
 they do not turn directly from the source of light. They 
 turn from the source of light only when a given side faces 
 the light, not when the opposite side faces it. 
 
 The orientation therefore takes place in these swarm- 
 spores just as it usually does in Euglena and Stentor when 
 the ray direction is changed without a change of intensity 
 of light. This shows that the organisms are more sensitive 
 when one side is illuminated than they are when the op- 
 posite side is exposed, just as was shown to be true in 
 asymmetrical forms. They are however not very readily 
 stimulated by changes of intensity; it was impossible to 
 induce the avoiding reaction in this way. If mounted on a 
 slide containing a bright area in a dark field, the collection 
 in the bright area is not definite, as it is in the case of Euglena 
 under similar conditions; the swarm-spores usually pass out 
 and in without any apparent response. If the light inten- 
 sity is only moderately changed after they are oriented 
 there is but a slight increase in the width of the spiral path. 
 If it is changed much the spores immediately turn and 
 swim up, since they are strongly negative in their reactions 
 to gravity. 
 
 Orientation in these symmetrical forms Is then governed 
 by the same factors as it is in the asymmetrical forms. It 
 is due to changes of light intensity on the organism. These 
 changes are produced in a field of constant intensity by the 
 rotation on the axis. There is no evidence that light acting 
 constantly as a stimulus has any effect on orientation. The 
 swarm-spores are more sensitive when one side is exposed 
 
128 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 than they are when the opposite side is exposed. They 
 always turn toward a given side, wliich, as far as can be 
 seen, is not structurally defined. There is no evidence indi- 
 cating differential response to localized stimulation. 
 
 3. Trachelomonas 
 
 No detailed observations on the light reactions of Trache- 
 lomonas have been recorded. The reactions of this organ- 
 ism are of interest to us here chiefly because it is very 
 nearly radially symmetrical and because it has a very promi- 
 nent eye-spot located very near the middle of the anterior 
 end, a location quite different from that in any other form 
 of which I know. 
 
 Trachelomonas hispida, the species studied most care- 
 fully, is ellipsoidal in form, about 0.02 mm. long and 0.015 
 mm. wide. It is surrounded by a dark brown rough brittle 
 test of considerable relative thickness. A single large flagel- 
 lum, frequently three times as long as the body, projects 
 through a hole in this test at the anterior end. A relatively 
 large contractile vacuole appears to communicate with the 
 exterior through this hole. This suggests that the flagellum 
 may possibly extend into the contractile vacuole as it does 
 in Euglena viridis. The eye-spot is reddish brown in color 
 and has the general form of a thin curved disk (Fig. 16). 
 It is located between the contractile vacuole and the anterior 
 end and appears partially to surround a canal leading from 
 the former to the exterior. The eye-spot is very irregular 
 in outline and appears under an oil immersion lens to con- 
 sist of a number of small granules embedded in a homo- 
 geneous matrix. The granules project in the form of 
 marked knob-like elevations on the convex surface, making 
 it appear very rough. In most specimens similar granules 
 were found lying about loose in the neighborhood of the 
 eye-spot. The test is so nearly opaque in many specimens 
 of hispida that little can be seen through it, while in some 
 other species studied it is actually black, so that nothing 
 
OBSERVATIONS ON UNICELLULAR FORMS 
 
 129 
 
 can be seen of the structure Inside. The tests can however 
 be readily removed. If the cover-glass is allowed to press 
 lightly on the organism it splits open and the cell within 
 with its prominent eye-spot and bright green color escapes. 
 All these forms react definitely to light; they are negative 
 in strong light and positive in weak. They orient quite 
 
 Fig. 16. I. Trachelomonas hispida showing structure; cv, contractile vacuole; 
 t, dark brown test nearly opaque; s, spines on surface; n, nucleus; ch, chloroplast; 
 e, eye-spot. 
 
 II. Different view of same specimen showing that the eye-spot is nearly cen- 
 trally located. Note rough edges of eye-spot and loose granules composed of same 
 material. 
 
 III. End view of eye-spot. 
 
 mm., projected scale. I and II drawn with camera as seen under oil immersion. 
 
 IV. Sketches of specimens seen in the process of leaving the test preparatory 
 to fission. They frequently swim about in the naked elongated state for some 
 time. 
 
 accurately and proceed on a spiral course much like Eu- 
 glena. It is remarkable how sufficient light to cause 
 a stimulation can get through the dense black tests of 
 some specimens. 
 
 In the study of the reactions of these organisms they were 
 mounted in water under a large cover-glass which was sup- 
 ported by a thin ring of vaseline. In this way evapora- 
 tion was prevented. The specimens thus mounted lived 
 
130 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 for over a month and Increased in numbers. When they 
 are about to di\'ide they crawl out throui^h the opening in 
 the test at the base of the flagellum and then they may 
 swim about in ihr naked elongated state (Fig. 16) for some 
 time before they di\ide and form new tests. \Mien the 
 test is new it is nearly transparent. It was in specimens in 
 this condition and in those in the naked state that the reac- 
 tions were obser\ed. 
 
 The orienting reactions were studied just as they were 
 in the forms already described. Trachelomonas was found 
 to orient just like Euglena in the free-swimming state. If 
 the light intensity is moderately decreased without changing 
 the direction of the rays they swim in a wider spiral; if 
 much decreased, they turn sharply in all directions. They 
 always turn toward the convex surface of the eye-spot. If 
 the ray direction is changed without a change of inten- 
 sity only those with the convex surface of the eye-spot 
 directed toward the light react immediately, the rest not 
 until this surface becomes exposed in the process of rotation. 
 The reaction consists in simply swerving farther toward 
 the light each time that the eye-spot faces the source of 
 light; thus they soon become oriented. If the ray direction 
 is changed with a simultaneous decrease in the light inten- 
 sity, all react at once. Under such conditions they turn 
 from the light as well as toward it. 
 
 It is clearly evident that turning the convex surface of 
 the eye-spot toward the source of light produces the same 
 effect as a decrease in the light intensity of the field. There 
 is practically the same amount of protoplasm on all sides 
 around this structure, and as far as can be seen under the 
 best oil immersion lens this protoplasm is the same on all 
 sides. If this is true the shadow of the eye-spot should 
 have the same effect whether illuminated from the concave 
 or the conv^ex surface. The fact that it does not indicates 
 that there is a highly sensitive bit of protoplasm close to 
 the eye-spot on the concave surface. 
 
OBSERVATIONS ON UNICELLULAR FORMS 
 
 131 
 
 4. Chlamydomonas alboviridis (Stein) 
 
 Chlamydomonas is a small green egg-shaped organism 
 usually less than o.oi mm. in length. It has two or four 
 flagella, a contractile vacuole which appears to open to the 
 exterior at the base of the flagella, and a distinct eye-spot 
 located near the surface in various positions on the side of 
 the body, sometimes nearer the posterior than the anterior 
 end (Fig. 17). In Euglena and Trachelomonas the eye- 
 spot is situated near the contractile vacuole and the base 
 
 ■0.03 mm- 
 
 FiG. 17. I. Chlamydomonas alboviridis. II. Chlorogonium, showing struc- 
 ture and form, v, contractile vacuole; n, nucleus; ch, chloroplasts; e, eye-spot; 
 mm., projected scale. 
 
 of the flagellum; and it has been suggested that the eye- 
 spot is nothing more than a collection of waste material 
 deposited by this vacuole. In Chlamydomonas it is, how- 
 ever, so far from the contractile vacuole that there does not 
 appear to be any relation between the two structures. 
 These organisms usually swim with the end containing the 
 flagella ahead, but sometimes they swim for short distances 
 with the opposite end foremost. 
 
 I was interested in this form chiefly because it appeared 
 as though its reactions might have some bearing on the 
 
13 2 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 function of the eye-spot, since in this species it is located 
 well toward the posterior end of the body. 
 
 These creatures react \-ery definitc^ly to light. They are 
 positive in weak and negative in strong light, and swim on 
 a spiral course. The intensity however in which they are 
 positive or negatixe \aries greatly in different indi\iduals 
 and in the same individual under different conditions. It 
 is very difficult to follow their movements since they are 
 so small and swim so rapidly. Jennings (1904, p. 64) found 
 that the\' "react to a decrease in illumination by a sudden 
 turn to one side, by an increase in the width of the 
 spiral, and by a change in the course just as happens 
 in Euglena and Cryptomonas." But he was " unable to 
 determine the relation of its structure to the spiral path 
 and to the direction of turning In the reaction." My 
 observations confirm the conclusions of Jennings as stated 
 above. 
 
 The orienting reactions in these organisms were studied 
 just as they were in Euglena. They were alternately 
 exposed in each of two beams of light which crossed on the 
 stage of the microscope at right angles. If the light from 
 the two beams is equal and the organisms are not very 
 sensitive, all turn toward the source of light, none in the 
 opposite direction, when the direction of the rays is sud- 
 denly changed. This they do by swerving farther in one 
 direction than in the opposite as they proceed on their 
 spiral course, just as do Euglenae and Stentors under similar 
 conditions. But if the light from one source is more 
 intense^ than that from the other, and the intensity is 
 increased at the same time that the direction of the rays is 
 changed, many of them stop and turn sharply, some toward 
 the light, others away from it. This reaction is very strik- 
 ing; there is apparent confusion for some time after the ray 
 direction and the intensity are simultaneously changed, 
 whereas if the ray direction is changed without a change 
 
 ^ The light from one source was 100 ca. m.; that from the other 160 ca. m. 
 in these experiments. 
 
OBSERVATIONS ON UNICELLULAR FORMS 133 
 
 of intensity there appears to be perfect order, all the speci- 
 mens turning gradually toward the light. 
 
 The fact that these organisms turn in various directions 
 when the intensity is decreased regardless of the side illu- 
 minated indicates that they always turn toward a struc- 
 turally defined side. I was however unable to follow the 
 reactions under these conditions so as to see if there was 
 a definite relation between the location of the eye-spot 
 and the direction of turning. But by carefully studying 
 specimens swimming about slowly in an optimum light 
 intensity, I saw that with very few exceptions they turn 
 toward the side on which the eye-spot is situated. A 
 few specimens which were loosely entangled in debris 
 were seen to turn in the opposite direction. This was 
 probably due to some interference with the movement 
 of the flagella. It may therefore be concluded that 
 Chlamydomonas always turns toward the side contain- 
 ing the eye-spot. 
 
 The fact that positive specimens turn toward the source 
 of light when the ray direction is changed without a change 
 of intensity, and that they turn toward the side containing 
 the eye-spot, indicates that they are most sensitive when 
 the side without the eye-spot is illuminated, for it is a de- 
 crease of intensity which causes a reaction in these organ- 
 isms when they are positive. It therefore follows that a 
 change from a position in w^hich the eye-spot is on the 
 shaded side to one in which it is on the illuminated side has 
 the same effect as a decrease of intensity. Is this decrease 
 due to the shadow cast by the eye-spot? In Euglena it 
 seems likely that it is. In some specimens of Chlamydo- 
 monas, however, this structure is situated so near the 
 posterior end that it is difficult to see how it could 
 function in this way. The long axis of one of the speci- 
 mens represented in Fig. 17 would have to be at an 
 angle of nearly 45° with the direction of the light before 
 the eye-spot would cast any shadow on structures in the 
 body. 
 
134 LIGHT AND THE BEHAVIOR OF ORGAXISMS 
 
 5. Chlorogonium 
 
 Chlorogonium is a green spindle-shaped organism with 
 two flagella and a very prominent bright reddish eye-spot 
 located very near the surface, only a short distance from 
 the anterior end (Fig. 17). This organism is frequently 
 found in cultures containing Chlamydomonas. 
 
 The reactions in Chlorogonium are essentialh' like those 
 in Chlamydomonas. They were studied in much the same 
 way in both forms, but it was much easier to follow these 
 reactions in the former than in the latter form. 
 
 The eye-spot in Chlorogonium is favorably situated to 
 function by shading the interior. 
 
 6. Paramecium 
 
 The assumption held by some investigators that the 
 power to react to light is common to all protoplasm is 
 probably wrong. It is well known that Paramecia and 
 many other protozoa do not respond to light of ordinary 
 intensity. If all protoplasm can be stimulated by light, 
 one would certainly expect these forms to show some evi- 
 dence of response when suddenly subjected to powerful 
 illumination. 
 
 At noon on a perfectly clear day in July I arranged a 
 double convex lens 10 cm. in diameter so as to focus the 
 direct rays from the sun on a slide under the mic oscope. 
 The light was passed through distilled water in order to cut 
 out the heat rays. The light at the focal point was at least 
 500,000 ca. m. in intensity. This extremely intense light 
 was repeatedly flashed upon the Paramecia as they swam 
 about under the microscope, but there was no evidence of 
 any response whatever. It is altogether probable then 
 that the power to respond to light is not common to all 
 protoplasm. 
 
 The fact that Paramecia do respond to ultra-violet rays 
 as shown by Hertel (1904) has no bearing on this question. 
 
OBSERVATIONS ON UNICELLULAR FORMS 135 
 
 The wave length of the rays used in Hertel's experiment 
 was only 280 fi/i, while the length of the shortest wave of 
 the visible spectrum is a little over 400 /jLfi. Paramecia 
 continuously exposed to the ultra-violet rays are injured 
 almost immediately; their movements become uncoordi- 
 nated and they die in from 10 to 50 seconds. It seems 
 probable, then, that these rays stimulate Paramecia because 
 of their injurious effect. 
 
CHAPTER \'1I 
 
 THE FACTORS INVOLVED IN THE PROCESS OF ORIEN- 
 TATION IN COLONIAL FORMS 
 
 I. Volvox globator and minor 
 
 Many interesting observations have been made on the 
 light reactions of Volvox since Leeuwcnhoek discovered 
 this organism over two hundred years ago. The details in 
 the reactions have however only recently been worked 
 out. In 1907 I published an extensive paper on this sub- 
 ject, and the following account is based largely on this 
 paper. 
 
 Volvox is an organism somewhat like a hollow sphere 
 slightly elongated. The largest colonies are nearl>- i mm. 
 in diameter and the smallest can readily be seen with the 
 naked eye. Each colony is composed of from 200 to 22,000 
 individuals and each individual consists of a single cell 
 known as a zooid. The zooids, interconnected with proto- 
 plasmic strands, are arranged side by side so as to form a 
 wall inclosing a cavity. They are very much like Chlamy- 
 domonas in structure and color. Each one contains two 
 flagella and an eye-spot which is situated on the outer pos- 
 terior surface. The eye-spots at the anterior end of the 
 colonies are from eight to ten times as large as those at 
 the posterior end much as in Pandorina represented in 
 
 Fig. 21. 
 
 The colonies usually rotate counter-clockwise on the long 
 axis, like Euglena, but they seldom swim on a spiral course. 
 They orient and swim toward a source of light or away 
 from it in a general way; they do not however orient very 
 accurately. Colonies swimming horizontally toward a com- 
 pact source of light usually deflect either to the right or to 
 the left or up or down. The more strongly positive the 
 
 136 
 
ORIENTATION IN COLONIAL FORMS 137 
 
 colonies are the more nearly parallel with the rays they 
 swim. If the position of the source of light is changed 
 without a change of intensity, they change their direction 
 of motion until the course bears to the light rays the same 
 relation that it had before. In thus changing their direction 
 of motion they always turn directly toward the source 
 of light without any preliminary movement. No matter 
 which surface is illuminated there is an apparent differential 
 response to localized stimulation. There is no evidence of 
 trial movements in the colonies taken as a whole. They 
 never turn in the wrong direction as Euglena and Stentor 
 frequently do, even if the intensity and the ray direction 
 are changed simultaneously. There is no evidence that the 
 sensitiveness of the colonies depends upon the surface 
 exposed as was found to be true in many unicellular forms, 
 nor is there any indication of an avoiding reaction when 
 the intensity is changed. If the intensity is much decreased 
 they merely stop forward progress and, because of the effect 
 of gravity, the anterior end turns up. They do not aggre- 
 gate extensively in highly illuminated areas in a dark field 
 as Euglena and various other forms do. They pass from 
 darkness into light and vice versa without any apparent 
 reaction. 
 
 There is no evidence that the direction of rays through 
 the organism, in accordance witli Sachs' theory, or that the 
 angle between the rays and the sensitive surface as suggested 
 by Loeb controls orientation. This process is regulated by 
 the relative intensity of light on opposite sides of the colony. 
 The following facts prove this to be true: i. If exposed to 
 light from two sources they swim toward any point between 
 them. The location of this point depends upon the relative 
 amount of light from the two sources as indicated in Fig. 18, 
 2. In the light grader (see Fig. 4) so arranged as to produce 
 a field of light which consists of parallel rays, but in which 
 the intensity gradually diminishes from side to side so that 
 one side of a colony, swimming toward the source of light, 
 is more strongly illuminated than the other, it deflects 
 
138 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 toward the more strongly illuminated side as represented in 
 Fig. 19. 
 
 Difference in light intensity on opposite sides of the 
 
 ۥ- 
 
 Fig. 18. Representation of the movement of V'olvox when subjected to light 
 from two sources, a, plate ^lass aquarium 8 cm. long and 8 cm. wide; b, 222-volt 
 Nernst glower, 66 cm. from aquarium (distance from aquarium constant); c, iio- 
 volt glower (distance from aquarium variable); d, screen; e, point of introduction 
 of Volvox;/, direction of light rays; i, 2, 3, 4, courses of Volvox exposed to light 
 from both glowers: i, with iio-volt glower igg cm. from aquarium; 2, with iio- 
 volt glower qq cm. from aquarium; 3, with no- volt glower 40 cm. from aquarium; 
 4, with 1 10- volt glower 24 cm. from aquarium; x-y, course of Volvox when exposed 
 to light from glower b only; y-z, course when exposed to light from glower c only. 
 
 colonies, then, causes them to turn until the two sides are 
 equally illuminated. "The turning^ may be conceived to 
 be due to an increase in the backward phase of the stroke 
 
 1 Mast, 1907, pp. 151-154. 
 
ORIENTATION IN COLONIAL FORMS 
 
 139 
 
 on the shaded side, or a decrease in the same phase on the 
 illuminated side or a decrease in the forward phase on the 
 shaded side, or an increase in this phase on the illuminated 
 side. Can it be ascertained which of these is the cause of 
 the difference between the effect of the stroke of the flagella 
 on the shaded sides and that of those on the illuminated 
 side of the colonies? 
 
 a 
 
 d 
 
 lO.i'i 
 
 / 
 
 Fig. 19. Graphic representation of the total average difference in deflection 
 due to difference in Hght intensity on opposite sides of Volvox colonies, compiled 
 from numerous experimental records, a, plate-glass aquarium 8 cm. wide and 
 15 cm. long; b, light rays; c, c' , points where the colonies were introduced; d, aver- 
 age course with the region of highest light intensity to left; e, average course with 
 strongest illumination to the right. Light intensity at (/) the middle of field 57.12 
 candle meters. From the middle the intensity gradually increased toward either 
 end where it was 442.68 candle meters. Intensity at c, 327 candle meters, at c' , 
 263 candle meters. 
 
 "If the light intensity of the field is suddenly decreased 
 while colonies of Volvox are swimming horizontally toward 
 it, they stop forward motion, the longitudinal axes take a 
 vertical position due to the effect of gravity, and then the 
 colonies swim slowly upward. It is not at all difficult to 
 find specimens in which this upward swimming is just suffi- 
 
140 LIGHT AXD THE BEHAVIOR OF ORGAMSMS 
 
 cient to overcome the effect of gravity, and under such con- 
 ditions they appear to be hanging in the water motionless. 
 They are, however, rotating on their longitucHnal axes. If 
 now the Hght intensity, to which these apparently motion- 
 less organisms are exposed, is increased they soon begin to 
 turn toward its source; but in so doing they swim upward, 
 as represented in the accompanying diagram (Fig. 20). 
 
 Fig. 20. Diagram representing the reaction of a Volvox colony when the light 
 intensity is suddenly changed, a, outline of colony; b, longitudinal axis; c, light 
 rays; d, point in the course where the light is suddenly decreased; e, point where" 
 it is suddenly increased; /, course taken by colony. In continuing from e, the 
 side of the colony facing the source of light travels over a shorter distance than 
 the shaded side. Consequently the backward stroke of the flagella on the latter 
 side must be more effective than that of those on the former. 
 
 "In thus swimming upward and horizontally toward the 
 source of light, it is clear that the effect of the backward 
 stroke of the flagella increases both on the shaded side and 
 on the illuminated side, for both sides move forward. But 
 the shaded side moves farther than the illuminated side, 
 consequently the increase in the effect of the backward 
 stroke must be greater on the former than on the latter. 
 The difference in the effect of the stroke of the flagella on op- 
 posite sides, which results in orientation of positive Volvox 
 colonies, is, therefore, due to a greater increase in the back- 
 ward stroke of the flagella on the shaded side than of those 
 on the illuminated side. 
 
 "If the light thrown upon apparently motionless colonies 
 is quite intense, they frequently may be seen to sink 4 or 
 
ORIENTATION IN COLONIAL FORMS 141 
 
 5 mm. immediately after the light is turned on, but while 
 they are sinking this short distance, they apparently become 
 acclimated and soon turn toward the light, and at the same 
 time swim upward, just as described above. During the 
 time in which these colonies sink they continue to rotate 
 in the same direction as before. The sinking must then 
 be due to a decrease in the effect of the backward stroke 
 of the flagella on all sides, and this decrease is due to an 
 increase in light intensity. But when the colonies turn 
 toward the source of light, and at the same time swim 
 upward, it is evident that the increase in light intensity 
 must cause an increase in the backward phase of the stroke 
 of the flagella on all sides, for if this were not true there 
 could be no upward motion. The side nearest the source 
 of light, however, passes over a shorter distance than the 
 opposite side, as will readily be seen by referring to the dia- 
 gram, and therefore the increase in the effect of the back- 
 ward phase must be greater on the latter than on the 
 former. But the light intensity is greater on the former 
 than on the latter (a paradox). When the light intensity 
 in the field is increased the effect of the backward phase of 
 the stroke of the flagella may be increased or decreased on 
 all sides. If it is increased the effect is most marked on the 
 side in lowest light intensity. Furthermore, if the light is 
 strong the colonies turn toward its source more rapidly and 
 do not swim upward so far and thus make a sharper curve 
 than when it is weak; but the stronger the light the greater 
 the difference between the intensity on the shaded and that 
 on the illuminated side. It therefore follows that the 
 greater the difference in intensity on these sides, the greater 
 the difference in effect of the backward phase of the stroke 
 of the flagella, the effect being greatest on the side least 
 illuminated. These considerations support the conclusion 
 arrived at above, i.e., that the factors which regulate the 
 activity of the colonies, as a whole, are different from those 
 which regulate the direction of motion. 
 
 "We have thus demonstrated that while orientation is 
 
142 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 due to difference in light intensity on opposite sides of the 
 colonies, it is brought about in positi\c specimens by the 
 flagella striking backward with greater effect on the side 
 in lowest light intensity than elsewhere. I suggest the fol- 
 lowing explanation of this: 
 
 " First, it must be remembered that the organism con- 
 stant 1\- rotates on its longitudinal axis. If then a colony 
 is so situated that one side is more highly illuminated than 
 the opposite, it is clear that the zooids will constantly be 
 carried from a region of higher to a region of lower light 
 intensity, and vice versa. They are thus subjected to con- 
 stant changes in strength of illumination. As stated above, 
 the flagella strike backward with greater vigor on the shaded 
 side than on the opposite one and, therefore, it is evident 
 that as the zooids reach the region of lower light intensity, 
 in other words when the light intensity to which they are 
 subjected decreases, they increase the effect of the back- 
 ward stroke of the flagella, i.e., they attempt to turn toward 
 a structurally defined side (the side facing the anterior end 
 of the colony). This is precisely what Euglena does when 
 it passes from a region of higher to one of lower light inten- 
 sity, i.e., it turns toward a structurally defined side, the 
 larger lip. The individuals in a colony then respond with a 
 motor reaction induced by change in light intensity; they 
 react on the same basis as do Euglena, Paramecium, Stentor 
 and other unicclltilar forms, in their trial and error reactions, 
 but (J wing to the way in which they are interrelated, and 
 to the rotation of the colony on the longitudinal axis, this 
 reaction of the zooids causes orientation in the colony as a 
 whole, without error. 
 
 " This ex{)lanation of orientation in entire colonies holds 
 also for orientation in segments. As previously stated, only 
 those segments orient which have such a form that they 
 can rotate. As they rotate the cut surface constantly faces 
 the center of the spiral, so that if the axis of the spiral is 
 not directed toward the source of light, the outer surface 
 where the zooids are situated is alternately turned toward 
 
ORIENTATION IN COLONIAL FORMS 143 
 
 the light and away from it. Thus the zooids are carried 
 from regions of higher to regions of lower light intensity 
 and vice versa, and the motor reaction is induced just as it 
 is in entire colonies. 
 
 " Orientation in negative colonies can be explained in 
 precisely the same way as that in positive ones, assuming 
 merely that in this condition the zooids respond with the 
 motor reaction when they pass from lower to higher light 
 intensity instead of when they pass from higher to lower 
 (as is true when the organisms are positive) . The backward 
 stroke then becomes most effective on the side most highly 
 illuminated." 
 
 It is altogether likely that the orientation in Vol vox is 
 not entirely due to the change of light intensity on the 
 zooids caused by difference of intensity on the colonies as a 
 whole and rotation on the long axis as described above; but 
 that as in Euglena and other forms discussed above, the 
 sensitiveness of the zooids depends upon the surface exposed. 
 When the zooids are carried from the illuminated to the 
 shaded side it is clear that they are not only transferred 
 from a region of higher to a region of lower light intensity, 
 but the surface turned toward the light is also changed. 
 This change in itself may, as in various unicellular forms 
 studied, cause a reduction of intensity on certain structures 
 within the zooids by the movement of shadows of other 
 structures, and consequently an orienting stimulus. The 
 orientation of segments indicates that this factor plays a 
 very important part in the orientation of the colonies. 
 
 Take for example a segment formed by cutting a colony 
 in half lengthwise. The zooids in this segment lie side by 
 side and are some distance apart, being connected by thin 
 strands of protoplasm. Most of the substance in the cavity 
 runs out when the colony is cut, and what remains is trans- 
 parent. It is therefore evident that there is no more sub- 
 stance to shade the zooids when the inner surface of the 
 segment faces the source of light than there is when the 
 outer faces it. If the segments are in a field of uniform 
 
144 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 intensity the zooids are not carried from regions of higher 
 to regions of lower light intensity and vice versa as described 
 above. Under such conditions then the orienting reactions 
 must be due entirely to the shading of certain structures 
 within the zooids by other structures also witliin them, as 
 the surfaces turned toward the source of light change. 
 
 Orientation in V^olvox is, according to this account, due 
 to changes in light intensity on the zooids as a whole 
 together with changes of intensity on certain structures in 
 the zooids, made possible by difference of intensity on the 
 surface of the colony and rotation on the long axis. The 
 change of intensity is caused by the movement of shadows 
 of certain structures in the organism cast upon other struc- 
 tures. These shadows are present in a field of uniform 
 intensity. There is no evidence that the direction of the 
 rays in the field or through the body, or that the angle the 
 rays make with the surface of the body, is of importance 
 in orientation excepting in so far as it may affect difference 
 of intensity in the individual zooids or the colony as a 
 whole. Nor is the symmetry of the organism of prime 
 importance, for, as was stated above, asymmetrical seg- 
 ments of various forms orient nearly as accurately as entire 
 colonies. Orientation may take place in constant light 
 intensity quite as well as in a field having various intensities. 
 Difference of intensity in the field does however determine 
 the distribution of the colonies. They are negative in light 
 of high intensities, positive in that of low and neutral in 
 that of optimum intensity. Orientation then, whether posi- 
 tive or negative, tends to direct the colonies to the area of 
 optimum illumination. Light acts as a constant directive 
 stimulus on the colonies as a whole, but there is no evidence 
 that there is a directive stimulus without change of inten- 
 sity, for the reacting elements, the zooids, and especially 
 the structures within the zooids, are not subjected to con- 
 stant intensity. 
 
 It is probable however that constant intensity affects the 
 activity of these organisms somewhat as temperature does. 
 
ORIENTATION IN COLONIAL FORMS 1 45 
 
 They become negative in high Intensity and appear to be 
 more active in some intensities than In others, but since it 
 is practically impossible to subject the different parts of the 
 colonies to constant intensity owing to the shadow of one 
 part on another and the movement of the organism, it Is 
 Impossible to say whether or not they would become nega- 
 tive or more active if there were no such change of light 
 intensity. This subject was dealt with more in detail under 
 Euglena. 
 
 Bancroft (1907, p. 163) Intimates that orientation of 
 Volvox in light takes place in the same way as it does in a 
 constant electric current; that It is not due to changes of 
 intensity but to constant Intensity; and that it is therefore 
 a tropic reaction In accord with Loeb's definition. He says 
 (p. 162): '' It has been shown that the galvanotropic orien- 
 tation of Volvox Is brought about by a cessation or great 
 diminution In the stroke of the flagella at one pole of the 
 organism. This diminution in activity of the flagella 
 appears to be the only way In which Volvox Is capable of 
 responding to stimuli. Nothing In the nature of a motor 
 reflex has ever been observed In this organism so far as I 
 know. The flagella always strike most strongly backward. 
 We have then the simplest possible kind of a mechanism 
 for bringing about galvanotropic orientation. The current 
 diminishes the activity of the flagella at one pole of the 
 colony and consequently the activity of the flagella at the 
 other pole causes the organism to turn In that direction. 
 We have here a tropism reduced to its lowest terms. There 
 is nothing of the nature of trial and error present at all." 
 
 We have clearly demonstrated above that orientation of 
 Volvox In light Is not due to a '' cessation or great diminu- 
 tion of the stroke of the flagella at one pole of the organ- 
 ism " as it is in a constant electric current. It Is due to 
 an acceleration in the backward stroke of the flagella on 
 the shaded side. This Is caused by a response to a decrease 
 in the light Intensity on the zoolds as a whole together with 
 a decrease in Intensity on certain structures within the 
 
146 
 
 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 zooids, owing to the rotation of the colonies on the long 
 axis and tlie consequent transfer of the zooids from the 
 illuminated to the shaded side. This response is similar to 
 the avoiding reaction in Euglena, Trachelomonas, Chlamy- 
 domonas and other unicellular forms. 
 
 If Bancroft is correct in his description it is evident that 
 the factors involved in orientation in a galvanic current 
 and in light are not the same, and that the orienting reac- 
 tions in light are not tropic according to Loeb's definition. 
 
 2. Pandorina and Eudorina 
 
 These organisms are much like Volvox in structure. 
 They are however very much smaller and contain only 
 
 -0 1 mm— 
 
 M 
 
 Fig. 21. I. Eudorina; II, Pandorina, showing structure and form; a, anterior 
 end; z, zooids; ch., chloroplasts, all the zooids are well filled with them; e, eye- 
 spots — note difference in size at opposite poles, and location on outer posterior 
 surface of zooids. Each zooid has two llaRcUa, — only a few of them are repre- 
 sented. Eudorina is surrounded by a hyaline layer the outline of which is repre- 
 sented by a dotted line. Outlines made with camera; mm., projected scale. 
 
 III. Eye-spot greatly magnified; n, surface view; m, side view. The flat surface 
 is directed outward and slightly posteriorly. 
 
 from 32 to 64 zooids (Fig. 21). By means of methods 
 similar to those used in studying Volvox it was found that 
 
ORIENTATION IN COLONIAL FORMS 147 
 
 the process of locomotion and the reactions in both Pan- 
 dorina and Eudorina are in all essentials like those in Vol- 
 vox. They are negative in strong and positive in weak 
 illumination, but the degree of intensity in which they are 
 positive or negative varies greatly. They always swim 
 with the end containing the larger eye-spots ahead. They 
 usually rotate counter-clockwise on the long axis and pro- 
 ceed on a straight path. Only a very few colonies were 
 found to swim in a spiral course. If the light intensity is 
 decreased or increased with or without a change in the 
 direction of the rays there is no shock effect, nothing 
 resembling an avoiding reaction. If the general direction 
 of the rays is changed, positive specimens turn directly 
 toward the source of light without any preliminary move- 
 ments whatever, and negative specimens always turn in the 
 opposite direction. If exposed to light from two sources 
 so situated that the rays cross at right angles they swim 
 toward or from a point situated between the two sources. 
 The location of this point depends upon the relative inten- 
 sity of light from these sources. Orientation in these forms 
 takes place just as it does in Vol vox. 
 
 a. Function of the eye-spots. — The eye-spots in both 
 Eudorina and Pandorina are located on the outer posterior 
 surface of the zooids just as in Vol vox. They have the 
 form of a segment of a sphere. The flat surface which is 
 slightly concave faces out. Those at the anterior end of 
 the colonies are much larger than those at the posterior, as 
 represented in Fig. 21. The former are nearly 2.5 /x in sur- 
 face diameter, and 0.9 11 thick; the latter are only approxi- 
 mately 0.6 iJL in dic*meter, but relatively thicker than the 
 former. They are reddish brown in color and stand out 
 boldly in strong illumination from below, showing that they 
 are comparatively opaque. In direct sunlight they become 
 luminous, giving off a greenish blue light, and as the colo- 
 nies rotate they sparkle and glitter, presenting a wonderfully 
 beautif il spectacle. After having been in direct sunlight 
 for some time they are also luminous in diffused sunlight, 
 
148 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 but not so in darkness. No evidence of this property was 
 seen in the eye-spot of Euglena. It therefore appears that 
 the eye-spots in these two forms cHlfL-r in composition, and 
 it may be that they function differently. At any rate, 
 judging from their location I am unable to see how they 
 could function in Volvox, Pandorina or Eudorina by shading 
 structures in the interior, as they appear to in Euglena, 
 unless they function onl>' when the colonies are negative. 
 The fact however that the eye-spots at the anterior are 
 much larger than those at the posterior end is strong evi- 
 dence in opposition to such a view. If these structures func- 
 tion in light reactions in these forms at all, they must 
 function either as an absorptive background somewhat like 
 the retinal pigment in the eye or as direct light recipient 
 organs as I suggested (1907, p. 112). 
 
 In our work on Euglena it was pointed out that the 
 hyaline protoplasm at the anterior end condenses the light 
 and brings it to a focus in the neighborhood of the eye-spot 
 when the organism is oriented, possibly on the structure 
 most sensitive to light. In Pandorina and Eudorina each 
 zooid acts as a condensing lens. In direct sunlight a highly 
 illuminated spot can be clearly seen at the surface directed 
 away from the source of light in each zooid, even in those 
 well filled with chloroplasts. The focusing of the light is 
 much more definite in these forms than it is in Euglena. 
 It is evident that every lateral movement of the organism 
 causes a change in the location of the point on, which the 
 light is focused in the zooids, and this of course produces 
 definite and marked changes in light '. tensity. It may be 
 that the eye-spots, located near the posterior end of the 
 zooids as they are, function in some w^ay in connection with 
 such changes of the focal point. 
 
CHAPTER VIII 
 
 OBSERVATIONS ON THE RESPONSES INVOLVED IN THE 
 
 REGULATION OF MOVEMENT TOWARD THE 
 
 SOURCE OF LIGHT IN COELENTERATES 
 
 • 
 
 Only a few of the animals belonging to this group orient 
 In Hght. Many do not respond to Hght at all. In others 
 the general activity depends upon the light intensity. Some 
 get into regions of optimum light intensity by means of 
 orientation, others by random wandering movements. In 
 both cases they come to rest in this region. The former 
 method is much more effective than the latter. If animals 
 have the power to orient they can move directly toward 
 the region of optimum intensity and consequently get there 
 much more quickly than do those which reach such regions 
 by random movements. 
 
 In this paper we shall consider only a few species, all of 
 which show some evidence of orientation. 
 
 I. Hydra viridis 
 
 a. Historical review. — Trembley (1744) seems to have 
 been the first to record experimental observations on the 
 effect of light on the movements of Hydra. He exposed 
 the animals in a glass jar covered with an opaque case con- 
 taining an opening on one side, and found that they migrated 
 toward the opening. He did not however record the details 
 in the method of migration. Loeb (1905, p. 73) referring 
 to these experiments says, " Trembley 's experiments on 
 Hydra, however, show that in their case also the relation 
 is the same," i.e., "that Sachs's Maws of heliotropism . . . 
 
 ^ Sachs, it will be remembered, claimed that orientation is controlled by 
 the direction in which the rays pass through the organism. 
 
 149 
 
I50 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 hold good. ... It seems to me that Trembley's experi- 
 ments cannot be interpreted unless we assume that the 
 progressive movements of Hydra arc determined by the 
 direction of the rays of light." 
 
 Wilson (1891) found that while Hydra viridis usually 
 collects on the bright side of the dish, it collects in shaded 
 regions if the light is very intense; that it aggregates more 
 freely in a blue field than in a yellow or white field ; and that 
 it collects more abundantly in the blue field even if it con- 
 tains no more blue rays than the white field and is therefore 
 of a much lower intensity. He found that a change from 
 light to darkness or from blue or white light to light of 
 other colors causes the animals to wander about more. 
 When they pass into blue or white light they tend to come 
 to rest. This explains their collection in blue and white 
 light. Wilson thinks that the Hydras may also go toward 
 the source of light directly; that the collection at the more 
 highly illuminated side of the aquarium is not entirely due 
 to random wandering. Washburn (1908, p. 123) however 
 says, "Hydra shows no response to light other than a tend- 
 ency to come to rest in the more illuminated parts of the 
 vessel containing it." 
 
 The following experiments were undertaken with these 
 questions in mind: i. Do Hydras wander about more in 
 darkness than in light? 2. Do they move directly toward 
 a source of light? 3. Do they orient? 4. What factors 
 are involved in orientation ? 
 
 b. Effect of light intensity on activity. — Experimental 
 results recorded in literature show that H>-dra is in general 
 more active in sub and supra optimum intensities than in 
 optimum intensity. The following observation shows, how- 
 ever, that total darkness seems to inhibit movement. On 
 April II at 10 a.m. six green Hydras were taken from the 
 culture which was in strong diffuse light, put into some 
 water in a small rectangular acjuarium and placed in total 
 darkness without the temperature's being changed. They 
 soon became attached to the bottom of the vessel in posi- 
 
MOVEMENT TOWARD LIGHT IN COELENTERATES 151 
 
 tions represented by dots (a) In the accompanying diagram 
 (Fig. 22). At 12 M. all were still in the positions where 
 
 
 .-•^ 
 
 i 
 
 
 O.Xy 
 
 *..v 
 
 \ 
 
 
 J 
 
 
 
 OjX 
 
 
 »,a; 
 
 
 \ 
 
 
 !« 
 
 o,y, 
 
 ^ 
 
 •s^ 
 
 >. 
 
 ^ 
 
 Fig. 22 
 
 (>,i/ 
 
 K^iiii lU:Oo 
 
 IhOu A.M 
 
 11:00 
 
 8:00 A.M. 
 
 11:00 
 
 Fig. 23 
 
 11:00 
 
 4:55 P.M. 
 
 12:30 P.M 
 
 3:40 
 
 3:45 
 Q /4:05 jr3:12 
 
 12:30— A-^—2iEM3;10 
 
 12:30 ^X 12:30 
 
 <- 
 
 <- 
 
 n 
 
 Fig. 24 
 
 Fig. 22. Movement of Hydra viridis in total darkness, a, position at begin- 
 ning of course, 8 a.m., yj; x, position at 8 a.m., y\; o, 9 a.m., y\; y, 8 a.m., y\. 
 Fig. 23. Movement of the same specimens after exposure to light. 8 to 
 
 11 a.m., y\. Note that they have become much more active. 
 
 Fig. 24. Movement of H. viridis toward source of light, n; intensity 
 
 12 ± ca. m. All the looping movements of each specimen are represented from 
 three o'clock until the close of the experiment. The dots represent points of 
 attachment. The animals extended in various directions from each point but 
 usually traveled only toward source of light. 
 
 they first became attached. They were well expanded and 
 the anterior ends were variously directed. At 2.15 p.m. 
 one had moved about 3 mm., the rest not at all. At 8 a.m. 
 the following morning four had moved to positions desig- 
 
152 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 nated x. At 9 a.m. the next morning two had taken posi- 
 tions represented by 0. During the ne.xt 24 hours three 
 moved to positions marked y; all were well contracted and 
 motionless. They were now exposed to diffuse sunlight 
 without being moved or jarred. They began lo expand 
 almost immedialely and soon began to move about. The 
 paths takrn arc indicated in iMg. 23. After 11 a.m. they 
 did not again change their positions until the close of the 
 experiment 24 hours later. 
 
 This shows that darkness inhibits movement in Hydra 
 viridis; that they become exceptionally active when exposed 
 to light after having been in darkness for some time, and 
 that they apparenth' become acclimatized readily, since 
 they come to rest in the same intensity after having been 
 exposed a few hours. The inhibition of movement by 
 darkness may however not be due to absence of direct 
 stimulation by light, but to the effect of darkness on photo- 
 synthesis. 
 
 c. Orientation and locomotion. — In the study of the 
 movements of Hydras, they were put into water from the 
 culture jar, i cm. deep, in a rectangular aquarium 2X5X8 
 cm. made by cementing slides together. The aquarium 
 was exposed in the dark room to light from a 50-candle- 
 power Nernst glower situated 2 meters from the end of it. 
 The glower was so arranged that the rays were parallel with 
 the bottom and sides of the aquarium. The end of the 
 aquarium was covered with an opaque screen containing 
 an opening such that the sides and surface of the water 
 were in darkness, so that reflection from various surfaces 
 might be reduced as much as possible. All the light except 
 that in the beam which fell on the end of the aquarium was 
 absorbed by screens. 
 
 At 10 A.M., April II. six green Hydras were put into the 
 aquarium near the end opposite the glower. They became 
 attached very soon and stretched out in various directions, 
 some toward the source of light others away from it. 
 Twenty-four hours later they were near the middle of the 
 
MOVEMENT TOWARD LIGHT IN COELENTERATES 153 
 
 aquarium, and the following morning (April 13) they were 
 all at or near the end facing the liglit. It therefore took 
 them nearly 48 hours to move a distance of 8 cm. There 
 were now nine specimens; three buds had ])een set free. 
 The aquarium was turned end for end so that the Hydras 
 were again at the end farthest from the glower. During 
 the following 24 hours several of the specimens traveled 
 the entire length of the aquarium, 8 cm. They therefore 
 moved as far during the 24 hours as they had during the 
 preceding 48 hours. This shows that they l)ecame much 
 more active after they had been exposed in a given light 
 intensity 48 hours than they were at first. There was at 
 no time any indication of orientation. The specimens 
 appeared to face in all directions equally, but they appeared 
 to move quite directly toward the source of light. 
 
 In the following experiment more detailed observations 
 on movement were made. Several specimens were put into 
 the aquarium near the end farthest from the light at 9 a.m., 
 April 16. They became attached almost at once. At 12.30 
 all were removed but the five which had been most active 
 during the preceding hours. The courses taken by these five 
 specimens are recorded in Fig. 24. Two of the specimens, 
 A and B, were attached to the surface film. One remained 
 there during the entire experiment, but the other, B, went 
 to the bottom after moving a short distance. The surface 
 of the water was in darkness, but the Hydras hanging from 
 it extended into the light. The animals moved from place 
 to place by stretching out the body, attaching the tentacles 
 to the substratum, and then pulling up the foot and fasten- 
 ing it again near the end bearing the tentacles, sometimes 
 on the same side it had been and sometimes on the oppo- 
 site side. They progress by what may be called the loop- 
 ing method. From 3 o'clock to the close of the experiment 
 every progressive change in position of the specimens ,1 
 and B, and most of those of the other three specimens, were 
 recorded. During the periods between the looping move- 
 ments the specimens contracted and expanded from time 
 
154 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 to time and bent in various directions, remaining in any- 
 given position only a few minutes, so that during the periods 
 in which the foot was fixed, the oral end was directed toward 
 various points of the compass. But it will be seen by refer- 
 rimr to the fiirure that the looping movements were in 
 general directed toward the source of light. The only 
 movements in the opposite direction occurred in specimens 
 Cand E. The former moved in this direction but once, the 
 last looping move it made during the experiment; the latter 
 was not seen in the act of moving. It did however reach 
 the end of the aquarium farthest from the source of light 
 where it came to the surface and remained to the end of 
 the experiment. 
 
 d. Reactions of negative specimens. — At 11.35 a.m., 
 April 17, two specimens were exposed in the small rectangu- 
 lar aquarium to direct sunlight. They became very active 
 at once and bent from side to side, expanding and con- 
 tracting frequently. One changed its position by looping 
 five times in a little more than 15 minutes, the other by 
 looping three times. Both proceeded from the source of 
 light nearly as directly as the positive specimens studied 
 moved toward it. There was no apparent relation between 
 the direction of bending and the source of light, just as 
 was found to be true in positive specimens. The anterior 
 end appeared to be directed toward the source of light as 
 much of the time as away from it, but locomotion occurred 
 only when this end was directed from the light. After 
 having been exposed a little over 15 minutes they lost their 
 attachment to the bottom and became perfectly quiet, 
 apparently having been injured by the intense light. 
 
 As already stated the anterior end of Hydra is successively 
 directed toward various points of the compass. After re- 
 maining in a given position a few minutesthe animals usually 
 contract, turn toward one side and expand again. Some- 
 times however they bend and turn so as to change their 
 position without contracting. There is no definite relation 
 between the direction of bending and the source of light. 
 
MOVEMENT TOWARD LIGHT IN COELEN T EFLiT ES 155 
 
 There is however in positive specimens a tendency to retain 
 the position in which the oral end is most highly illumi- 
 nated. This was demonstrated as follows: 
 
 Ten specimens were exposed in the rectangular aquarium 
 in the dark room. As they traveled toward the light the 
 direction in which they faced was recorded at intervals. 
 These records appear in the table given below. 
 
 TABLE I. 
 
 Time of 
 
 observation 
 
 Anterior end directed *■ 
 
 Toward 
 source of light 
 
 From source 
 of light 
 
 Perpendicular 
 
 to direction 
 
 of light 
 
 3-45 
 4. 10 
 4.20 
 
 4-35 
 5.00 
 
 5-3° 
 6. 10 
 
 6.30 
 
 7-15 
 8.00 
 
 9.00 
 
 9-30 
 10.00 
 
 10.30 
 
 7 
 
 5 
 6 
 
 4 
 6 
 
 3 
 
 5 
 8 
 
 3 
 4 
 7 
 
 5 
 6 
 
 5 
 
 3 
 2 
 
 2 
 
 2 
 
 2 
 
 2 
 
 3 
 
 
 3 
 2 
 
 I 
 
 3 
 
 I 
 
 I 
 
 
 
 3 
 2 
 
 4 
 2 
 
 5 
 2 
 
 2 
 
 4 
 4 
 2 
 2 
 
 3 
 
 4 
 
 Total 
 
 74 
 
 27 
 
 39 
 
 1 In making the table all those specimens in which the oral end was near a plane passing 
 through the foot and perpendicular to the direction of the rays were recorded in column 4; 
 all in which this end was definitely to the right, i.e., toward the light, in column 2; and those 
 to the left, in column 3. 
 
 The table shows very clearly that the oral end of the ten 
 specimens studied was directed approximately toward the 
 source of light nearly three times as much as from it. If 
 the direction of locomotion depended merely upon the 
 direction in which the oral end points, one would expect 
 these organisms in the positive state to loop from the source 
 of light more than one-third as often as toward it. This 
 
156 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 however is not true; movement from the source of Hght is 
 ordinarily relatively rare. There must therefore be a 
 greater tendency to travel when the oral end faces the 
 light than when it faces in any other direction. 
 
 It may then be stated that Hydra tends to orient with 
 the anterior end directed either toward or away from the 
 source of light depending upon whether the specimens are 
 positive or negative, and that it tends to travel in the direc- 
 tion in which it orients. There are evidently two appar- 
 ently independent phenomena involved here, orientation 
 and locomotion. How can these phenomena be explained ? 
 Let us first consider orientation. 
 
 Since Hydras tend to expose the anterior end to light 
 when they are positive and to shade it when they are nega- 
 tive, it is probable that the oral end in this organism, as in 
 Euglena and Stentor, is more sensitive to light than other 
 parts of the body. If this is true it may be that orientation 
 is, as Jennings suggests (1906, p. 213), " due to the fact 
 that when it turns this end away, the change to relative 
 obscurity at the anterior end causes further movement, till 
 the light again falls on the anterior end." This explana- 
 tion fits the facts, as far as they are known, fairly well. It 
 is however difficult to see, since Hydra frequently retains a 
 position in which the anterior end is shaded for more than 
 two minutes, how " the change to relative obscurity at the 
 anterior end could cause further movement." There is 
 certainly no reaction in these animals comparable to the 
 avoiding reaction or shock movements in the lower forms, 
 for sudden changes of intensity even if extremely great 
 produce no immediate reactions. If it is assumed that the 
 organism tends to become oriented by random movements 
 and tends to remain oriented because of inhibition due to the 
 illumination of the anterior end, this difficulty is obviated. 
 
 As already pointed out, locomotion ordinarily occurs only 
 when Hydra is approximately oriented. Positive speci- 
 mens travel only when the oral end is illuminated, not when 
 it is shaded. It is therefore evident that the light itself 
 
MOVEMENT TOWARD LIGHT IN COELENTERATES 157 
 
 has something to do with this movement. It is not due 
 entirely to internal changes. The organism must be af- 
 fected differently when the anterior end is illuminated than 
 it is when this end is shaded. A decrease in illumination 
 ordinarily causes increase in activity, but this fact cannot 
 be the cause of locomotion after orientation, for if it were, 
 we should expect the greatest tendency to move when the 
 anterior end is directed from the source of light in place of 
 toward it. 
 
 One thing is clear from our results stated above, that is, 
 that the light condition which tends to inhibit turning in 
 various directions also tends to cause locomotion. From 
 this it may be concluded that orientation and locomotion 
 are phenomena which are regulated by different processes. 
 It may be that the former is dependent largely on difference 
 of intensity on opposite sides of the organism and the latter 
 upon an action of light similar to that of heat. 
 
 e. General conclusions. — It can be definitely stated 
 then that Hydra in the positive state reaches a position in 
 which the anterior end faces the light by random move- 
 ments; that it remains in this position longer than in any 
 other, and that it ordinarily starts to travel only when it is 
 in this position. The last two statements prove that light 
 affects it differently when the oral end is exposed than when 
 it is shaded. 
 
 It is impossible to say whether Hydra tends to retain 
 the position in which the light strikes the oral end because 
 of an inhibition due to an increase in effective light inten- 
 sity, when the body is turned from a position in which the 
 oral end is shaded to one in which it Is Illuminated; or 
 whether It tends to remain oriented because of increase in 
 motion due to a reduction In effective intensity when the 
 anterior end Is turned away from the light; or whether the 
 tendency to retain the oriented position is due not to a 
 change of Intensity, but to the fact that In this position 
 the anterior end is most highly illuminated and that the 
 inhibition is due to the effect of constant intensity. 
 
158 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 While the tendency in Hydra to remain oriented may 
 then be the result of stimulation by constant intensity, 
 there is no evidence whatever that light acts constantly as 
 a directive stimulus. Light may however affect locomotion 
 by acting constantly, much as temperature does; but even 
 in this case it is impossible to be certain that such effects 
 are not due to changes of intensity, for the shadows of 
 some parts of the body move almost constantly over other 
 parts, owing to the fact that this animal is quiet only for 
 short periods. 
 
 n 
 
 -% 
 
 Fig. 25. I. Reaction of an attached Hydra to a constant electric current of 
 moderate intensity. 1-5, successive stages in the reaction. 
 
 II. Successive stages in the reaction of a Hydra to the electric current when 
 the foot is unattached. The foot becomes directed toward the anode. After 
 Pearl (1901). 
 
 The fact that there is no definite relation between the 
 direction of turning and the side illuminated shows that 
 neither the symmetry of the body, nor the angle the rays 
 make with the surface, nor the direction of the rays through 
 the body, nor local response to local stimulation, nor dif- 
 ferential response to localized stimulation, can be of special 
 importance in orientation in light. 
 
 The difference between the orienting reaction of Hydra 
 in light and in a constant electric current is striking. As 
 
MOVEMENT TOWARD LIGHT IN COELENTERATES 159 
 
 stated above, in light there is no evidence whatever of direct 
 bending toward or from the source of stimulation. In a 
 constant electric current however it bends directly until 
 the long axis is in line with the direction of the current, as 
 represented in Fig. 25. The electric current in the process 
 of orientation acts constantly as a directive stimulation; 
 the reaction is tropic, according to Loeb's dcfmition. Light 
 does not act constantly as a directive stimulation; orienta- 
 tion is the result of " selection of. random movements; " the 
 reactions are not in accord with Loeb's definition of 
 tropism. 
 
 2. Eudendriiim Planulae 
 
 The planulae of Eudendrium are set free during the latter 
 part of July and the first part of August. In the labora-- 
 tory they are usually liberated early in the forenoon, after 
 which they immediately begin to travel toward the light. 
 These organisms are cone-shaped, about i mm. in length 
 and about 0.2 mm. in diameter at the larger end when 
 expanded; when contracted they are shorter and consider- 
 ably wider at the larger end. They are light reddish in 
 color and consist of numerous similar cells so arranged as 
 to inclose a cavity. All the cells are well covered with 
 comparatively short cilia on the outer surface. 
 
 Eudendrium planulae are always in contact with the 
 substratum. They move along something like planaria. 
 Locomotion seems to be due entirely to the action of the 
 cilia, but the planulae are never found swimming freely 
 through the water like infusoria. There is no evidence of 
 constant rotation on the long axis. It may be, however, 
 since all sides appear the same, that they move with dif- 
 ferent sides in contact with the substratum at different 
 times. Locomotion is very slow, the average rate being 
 only about I cm. in 15 minutes. Every movement of this 
 creature can therefore be easily followed even under high 
 magnification. The larger end is always ahead, and this 
 end is constantly turned from side to side very slowly, and 
 
l6o LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 raised slightly from time to time during the process of 
 locomotion. As the organism proceeds on its course it 
 secretes a mucous substance in the form of a delicate fiber 
 which can be readlK' detected by pushing a needle across 
 the path a short distance from the posterior end. The 
 planula and the needle usually adhere so firmly to the 
 mucous fiber that the former can be lifted to the surface of 
 the water. 
 
 Hargitt (1904, p. 272) referring to the reactions of Euden- 
 drium planulae says, " At the height of the breeding season 
 their numbers were large and they promptly swam directly 
 toward the strongest light with great uniformity. By inter- 
 posing a dark screen between this source of light and allow- 
 ing another from the opposite side to operate upon the 
 aquarium, there was an almost instantaneous response, the 
 entire number almost without exception facing directly 
 about, like a body of soldiers at command, and moving 
 without deviation in the opposite direction, that is, toward 
 the second source of light." When studied en masse in 
 light from a window they do appear to orient \'ery accu- 
 rately; but if attention is focused on individuals it soon 
 becomes evident that there is considerable variation in the 
 direction of motion. This becomes still more evident if the 
 reactions of individuals are studied in light from a single 
 compact source. Thus several active specimens which ap- 
 peared to be moving directly toward the window were 
 selected and exposed one at a time in light of about the 
 same intensity from a Nernst glower so arranged as to 
 eliminate practically all refraction and reflection from the 
 different substances in the aquarium, the glass walls, 
 the surface of the water, and particles in suspension in the 
 water. The angle between their direction of motion and the 
 direction of the rays was frequently measured, and it was 
 found that it varied all the way from o to 10° and even 
 more in a few individuals. If a number of specimens are 
 put into the aquarium at the same time and at the same 
 point, it is found that as they proceed toward the source 
 
MOVEMENT TOWARD LIGHT IN COELENTERATES i6i 
 
 of light, some deflect to the right, others to the left, so that 
 the group gradually becomes wider and wider. \\'hen 
 exposed to light from two sources they may travel toward 
 any point between, as stated under Euglena. 
 
 They are positive in strong as well as in weak light, but 
 if the intensity is very high, as, e.g., direct sunlight, or 
 quite low, they do not orient so accurately as they do in 
 light of moderate intensity, and the lateral movements of 
 the anterior end are more pronounced. In general the more 
 strongly positive the planulae are, the more accurately they 
 orient and the less they swing the anterior end from side 
 to side. From time to time the anterior end also becomes 
 flatter and broader. The lateral movements of the anterior 
 end, as w^ell as the process of becoming. broader, are due to 
 internal contractions. If the ray direction is but slightly 
 changed after the planulae are oriented, they do not turn 
 directly toward the source of light in its new position, but 
 merely swing the anterior end a little farther toward it 
 each time. In the meantime the body gradually turns so 
 as to become oriented again. If however the direction of 
 the rays is changed to such an extent that the sides of the 
 organism become fully exposed, they with very few excep- 
 tions appear to turn toward the light at once. In this 
 process they swing the anterior end laterally until it nearly 
 if not quite faces the source of light. It is thus frequently 
 bent at right angles to the posterior end. The anterior 
 end often swings back after turning but never so far as it 
 was before. The lateral turning is a slow^ steady movement 
 due, no doubt, to contraction of the tissue in the planulae 
 and not to the action of the cilia, for it is much more rapid 
 than the forward movement, which is entirely due to the 
 action of the cilia, and moreover no currents indicating 
 unequal ciliary action on opposite sides could be detected. 
 
 There is no definite reaction if the intensity is suddenly 
 decreased or increased, nothing similar to an avoiding reac- 
 tion or a shock movement. I have seen the planulae pass 
 from darkness into strong light, consisting of rays perpen- 
 
1 62 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 dicularto the bottom of the aquarium, without any reaction 
 excepting perhaps a very shght increase in the lateral move- 
 ment of the anterior end. 
 
 The fact that these organisms turn directly toward the 
 light when the side is illuminated apparently shows that 
 they have the power of differential response to localized 
 stimulation, that they can orient without preliminary trial 
 movements. The fact that the anterior end is constantly 
 being turned from side to side, and that orientation may 
 take place if the ray direction is only slightly changed by 
 merely swinging this end a little farther toward the source 
 of light each time that it^ turns in that direction in the 
 regular process of lateral movement, shows that under cer- 
 tain conditions orientation takes place by the trial method. 
 The fact that the planulae can move toward any point 
 between two sources of light shows that neither the direc- 
 tion of rays through the body nor the angle the rays make 
 with the surface is of importance in orientation. If this 
 be true, the orienting stimulus must be due to difference of 
 intensity or a change of intensity on opposite sides of the 
 body, especially on the anterior end. The lateral swinging 
 movements serve to magnify the difference or change of 
 intensity on this end, and tiius make it possible for the 
 animal to orient more accurately than it otherwise could. 
 
 The turning of the anterior end appears to serve in direct- 
 ing the course much as a cane serves a blind man in keeping 
 him on the path. The man may go directly toward his 
 goal without deviation and still it is evident that he orients 
 and keeps on his course by the trial method. Every move- 
 ment of the cane is a trial movement. Likewise every 
 lateral movement of the anterior end in the planulae and 
 many other organisms, as well as separate movements of 
 the antennae, eyes, and other special organs in various 
 forms may be trial movements. It is therefore clear that 
 the mere fact that an organism moves directly toward a 
 source of stimulation is not sufficient evidence to show that 
 its orientation and direction of movement are not regulated 
 
MOVEMENT TOWARD LIGHT IN COELENTEILiTES 163 
 
 by the trial method, as has been assumed by some investi- 
 gators. 
 
 While the planulae of Eudendrium may undoubtedly 
 orient by differential response to localized stimulation it 
 is at present impossible to say whether such stimulation is 
 due to changes of light intensity or to constant intensity. 
 The question as to the importance of lateral movements of 
 the anterior end in orientation, and the cause of stimulation 
 will be referred to more in detail under the reactions of fly 
 larvae and earthworms. 
 
 3. Eudendrium Hydranths 
 
 After the planulae are a few days old the anterior end 
 becomes attached to the substratum and they soon develop 
 into hydranths, which bend toward the source of light as 
 they grow. In order to study this process of bending 
 toward the light, I placed on the stage of the compound 
 microscope, a small aquarium containing hydranths which 
 had bent so that the distal end was nearly horizontal, and 
 turned it so that one side of the organisms faced the light, 
 and then projected a selected specimen with a camera 
 lucida. This same individual was projected later from 
 time to time, and in this way its movements were definitely 
 recorded. It was found that the hydranths turn directly 
 toward the source of light. There was no indication of cir- 
 cumnutation movements. The bending takes place only 
 in the region of growth. All sides elongate but the shaded 
 side elongates more than the illuminated side. The organ- 
 isms bend toward the source of light very slowly. In all 
 the individuals studied it required 48 hours or more to turn 
 through 90°. 
 
 In the orientation of this organism it seems probable that 
 light acts as a constant directive stimulation. But the 
 knowledge we have concerning the process hardly warrants 
 even a suggestion as to the probable mechanism involved. 
 The bending may possibly be due to contraction as sug- 
 
164 LIGHT AXD THE BEHAVIOR OF ORGAXIS.\fS 
 
 gested by Loeb (1906, p. 121): " Tlie hellotropic curwiture 
 consists here In the stem undergoing a stronger contraction 
 or shortening on the more strongly illuminated side of the 
 polyp than on the opposite side." The fact however that 
 the stem elongates on all sides does not favor this view, 
 although gnjwth might possibly mask shortening due to 
 contraction. Unequal rate in growth ma>' have something 
 to do with the bending, since it takes place only in the region 
 of elongation and is an exceedingly slow process. 
 
 Loeb thinks the orienting reactions in Eudendrium are 
 the same as those in plants. He says (1906, p. 120), " The 
 same phenomena of heliotropism which we find in i)lants 
 we find also in sessile animals; and the identity of the 
 heliotropic reactions in these two groups of organisms is so 
 complete that it would be at any time possible to demon- 
 strate the phenomena and laws of plant heliotropism in such 
 animals, and vice versa.'* The identity Loeb maintains 
 exists here is in all probability extremely superficial. 
 
 4. Reactions of Medusae 
 
 Many medusae do not react to light at all ; others respond 
 to changes of intensity by contracting; and still others 
 become more active with change in the illumination. Only 
 a few are known to orient. Both Ycrkes and Morse have 
 shown that Gonionemus murbachii orients under certain 
 conditions, although very indefinitel>'. It apparentl)' turns 
 directly toward or away from the source of light without 
 any preliminary movements. These organisms appear to 
 have the power of differential response to localized stimula- 
 tion. As to how light produces the orienting stimulation 
 nothing is known. Many of the light reactions of this form 
 are clearly due directly to change of intensity, while others 
 appear to be due to the effect of constant light intensity. 
 
 The medusae of Bougainvillea superciliaris orient far more 
 accurately than does Gonionemus or any other medusa of 
 which I know, but even in these orientation is not accurate 
 
MOVEMENT TOWARD LIGHT IN COELENTERATES 165 
 
 enough to warrant definite conclusions regarding the me- 
 chanics involved in this process. 
 
 These creatures are only about i mm. in diameter. They 
 have four prominent reddish brown spots symmetrically 
 situated on the margin of the bell. From the tissue sur- 
 rounding each of these spots there project three short 
 tentacles which are much contracted when the medusae 
 swim. The medusae are negative in their reactions to 
 gravity, and positive to light of intensities ranging from 
 weak diffused sunlight to intense direct sunlight. This 
 causes them to collect at the surface of the water in the sea 
 and to swim toward regions of highest light intensity. 
 
 In swimming toward the source of light they frequently 
 turn to the right or left rather sharply so as to produce a 
 zigzag course. The turning from side to side indicates that 
 light does not act as a constant directive stimulation. If 
 however the ray direction is changed through 90° the 
 medusae turn directly toward the source of light without 
 any preliminary trial movements. It may be then that 
 they are stimulated only when they turn a certain amount 
 and that, owing to the power of differential response to 
 localized stimulation, they always turn toward the light 
 after such stimulation and consequently remain oriented, 
 in a general way. If orientation is due to differential 
 response to localized stimulation the stimulation may be 
 caused by an increase of intensity on the illuminated side 
 or a decrease on the shaded side. We have however no 
 evidence bearing on this question. 
 
 If exposed to light from two sources they swim toward a 
 point between them. The location of this point depends 
 upon the relative intensity of light from the two sources. 
 
CHAPTER IX 
 
 REGULATION IN THE DIRECTION OF MOVEMENT WITH 
 
 REFERENCE TO THE SOURCE OF LIGHT IN VERMES, 
 
 FLY LARVAE, AND ECHINODERMS 
 
 Many of the organisms belonging to these groups respond 
 very definitely to stimulation by light. In some, the 
 response results in orientation, in others it consists merely 
 of an increase or decrease in rate of movement, and in 
 still others it consists chiefiy of a sudden contraction. We 
 shall concern ourselves here primarily with forms which 
 orient, emphasizing particularly the orienting reactions. 
 The reactions of the blowfly larvae will be discussed in 
 this section owing to their worm-like structure and 
 method of locomotion. 
 
 I. Arenicola cristata — Larvae 
 
 a. Description. — Arenicola deposits its eggs in great 
 numbers in masses of jelly-like substance. In the course 
 of a few day^ the eggs develop into finger-shaped free-swim- 
 ming larvae about 0.3 mm. long. These organisms are 
 strongly positive in their reactions to light and negativ^e in 
 their reactions to gravity. They contain two ciliary rings, 
 one near the anterior and the other near the posterior end; 
 these are connected by a median ventral band of cilia. On 
 either side near the anterior end is an eye-spot. Lillie 
 (1903, p. 345) says, " Each [eye-spot] consists of a compact 
 clump of pigment or excretory granules on the surface of 
 the brain." I studied the eye-spots in living specimens 
 slightly flattened with the cover-glass, under an oil immer- 
 sion lens and found that they consist of a brownish granular 
 cup-shaped portion which partially surrounds an ellipsoidal 
 
 166 
 
VERMES, FLY LARVAE, AND ECHINODERMS 167 
 
 hyaline portion and that this is directed dorso-anterio-later- 
 ally. The hyaUne structure in the eye-spot is probably highly 
 sensitive to light, while the pigmented portion appears to 
 serve in shading the inner surface so as to admit light only 
 from in front and from the side on which the eye-spot is 
 located. 
 
 b. Locomotion. — The larvae have two methods of loco- 
 motion. They swim by means of the cilia fur a few days 
 then settle to the bottom and crawl. The crawling move- 
 ment is brought about by means of muscular contraction 
 much as in many other annelids. During this method of 
 progression they are slightly negative and there is scarcely 
 any indication of orientation. In the free-swimming posi- 
 tive state however they orient quite accurately. Both 
 methods of reaction are adaptive. The positive reaction 
 in the free-swimming state serves to keep the larvae at the 
 surface of the water and causes them to scatter widely. 
 The negative reaction serves to keep them at the bottom 
 and to guide them into the mud where most of their future 
 days are to be spent. 
 
 In swimming they proceed much like the ciliates. They 
 rotate counter-clockwise on the long axis and travel on a 
 spiral course. The ventral surface, the surface containing 
 the median band of cilia, constantly faces out in the spiral, 
 contrary to what might be expected if these cilia are 
 functional. The organism is slightly curved, the ventral 
 surface being concave. It may be that this causes the 
 constant swerving toward this surface, which together with 
 rotation on the long axis results in the spiral course. 
 
 c. Orientation. — Orientation is not so accurate as is 
 generally assumed. If casually observed it is true that the 
 larvae do appear to move directly toward the source of 
 light, but if the course of a given individual exposed to light 
 from a single compact source is carefully followed it is found 
 that there are frequent deviations. As the organisms pro- 
 ceed they frequently turn the anterior end slightly but 
 suddenly toward either side by means of muscular contrac- 
 
1 68 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 tion. This causes the spiral course to become very irregu- 
 hir and makes it appear as though tliey were constantly 
 being thrown out of orientation and reorienting. Their 
 general course is toward the source of light but it is a very 
 irregular course. 
 
 d. Mechanics of orientation. — If tiie direction of tlie 
 rays of light is ciianged after the larvae are oriented they 
 all appear to turn directly toward the source of light in its 
 new position without preliminar>- trial movements. What 
 is tlie mechanism involved in this apparent direct orienta- 
 tion? If one edge of a cover-glass is supported so that it is 
 a little higher than the other, and if the larvae mounted in 
 water on the slide are forced to travel toward the lower 
 edge, they soon reach a place where the cover-glass is so 
 near the slide that they no longer rotate. Under such con- 
 ditions the animals lie usually on the ventral surface, but 
 some specimens are found on either side or on the dorsal 
 surface. No definite movement is seen in those on either 
 side, excepting occasionally a slight forward motion. But 
 in those on either surface, the anterior end is constantly 
 seen to move from side to side with a slight jerky motion. 
 This lateral movement of the anterior end is undoubtedly 
 due to muscular contraction. If one of the specimens with 
 the dorsal surface up is selected and light thrown upon it 
 from such a direction that the rays strike its side at right 
 angles, the lateral movement toward the side illuminated 
 is at once much increased and the organism turns in that 
 direction. By using two sources of light so situated that 
 the rays cross at right angles in the region where the speci- 
 men is located, and then alternately intercepting the light 
 from each of the two sources, it can be seen clearly that the 
 larva, by muscular movement, turns the anterior end toward 
 the source of light directly. There is no trial reaction in 
 this process. It is an asymmetrical response to an asym- 
 metrical stimulation. This does not however mean that 
 both sides of the organism are stimulated in accord with 
 the theories of orientation of Loeb and Verworn. The reac- 
 
VERMES, FLY LARVAE, AND ECIIINODERMS 169 
 
 tion may be due to an increase in illumination on one side 
 or a decrease on the other. The stimulation may be local 
 and the reaction a differential response. We shall refer to 
 this problem again later. 
 
 The larvae are so small and move so rapidly in the free- 
 swimming state that it is exceedingly difficult to follow 
 their movements in detail during the process of orientation. 
 By carefully observing this process, however, in a low tem- 
 perature by means of which the rate of movement is much 
 reduced, it was found that it takes place somewhat as fol- 
 lows: If the source of light is changed in its position after 
 a free-swimming specimen is oriented, reactions occur im- 
 mediately only if either eye-spot is fully exposed after the 
 change is made. If the ventral or the dorsal surface is 
 directed toward the source of light after the ray direction 
 is changed, there is no reaction until the organism has 
 rotated through 90° as it proceeds on its spiral course, and 
 one of the sides comes to be illuminated. Then the anterior 
 end is turned sharply toward the source of light, frequently 
 to such an extent as to form a right angle with the posterior 
 end. This causes rapid swerving on the spiral toward the 
 light and speedy orientation (see Fig. 26). I was unable 
 to detect any change in the course due to ciliary action. 
 
 e. Discussion. — The method of orientation in Arenicola 
 larvae has some features in common with that of Euglena 
 in the free-swimming state. In both forms there is a defi- 
 nite reaction whenever an eye-spot comes to face the source 
 of light as they proceed on their spiral courses. This reac- 
 tion consists of a turning toward the side containing the 
 eye-spot and a swerving on the spiral course in the same 
 direction, and this results in orientation. The larvae how- 
 ever, having two eye-spots, can turn toward the source of 
 light in two positions in the spiral, whereas the Euglcnae 
 can turn toward it in only one. If one were to imagine two 
 Euglenae united so as to form an organism with two eye- 
 spots facing in opposite directions, it would not be difficult 
 to conceive the organism thus formed capable of turning 
 
lyo LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 f $ i^ I 
 
 t; r. 
 
 Ill 
 
 \f u 
 
 I n 
 
 Fig. 26. I. Arenicola lar\-a in the frce-swimminR state, proceeding; on a spiral 
 course, m, n, directions of light; a-h, diflerent positions on the spiral; b, dorsal 
 surface up, right eye-spot toward n; d, ventral surface up, left eye-spot toward n. 
 If the ray direction is changed by simultaneously exposing n and shading m when 
 the larva is in position a or c, no reaction takes place until it reaches h or d, then 
 it bends the head sharply toward the source of light and turns in its course. In 
 the former position it turns toward the right side of the body, in the latter toward 
 the left. This indicates that the larvae have the power of differential response 
 to localized stimulation, and that the orienting stimulus may be due to a change 
 of light intensity. 
 
 II. Much enlarged sketch showing the general structure and position of the 
 eye-spots as seen under an oil immersion objective. The eye-spots are com- 
 posed of a dark brownish caplike portion, y, which partially surrounds a colorless 
 portion, x, directed slightly dorso-latcrally; z, band of cilia. 
 
VERMES, FLY LARVAE, AND ECHINODERMS 1 71 
 
 toward the source of light in two positions on the spiral 
 course just as Arenicola larvae do. If in place of a union 
 of two individuals we should have a union of three, it would 
 result in an organism that could turn toward the source of 
 light in three positions on the spiral. And if a sufficient 
 number were united it is clear that the organism could turn 
 toward the source of light in all positions on its course. 
 Such organisms we have in the colonial forms Volvox, 
 Eudorina, and Pandorina. All of these consist of numerous 
 individuals united, and all can turn toward the source of 
 light directly no matter which side is illuminated. It is 
 however probable that this analogy is, with reference to 
 Arenicola larvae, merely superficial. 
 
 /. Orienting stimulation. — In positive Euglenae it was 
 found that orientation is due to a reaction caused by a 
 reduction in effective light intensity due either to a change 
 in the intensity of the field or to rotation of the organism 
 owing to the fact that it is more sensitive when the ventral 
 surface is illuminated than when the dorsal surface is. In 
 Volvox the orienting stimulus is likewise due to a reduction 
 of effective intensity. To what is it due in Arenicola lar- 
 vae? Is it due to a decrease of intensity caused by the 
 shadow of the pigment on the hyaline portion of the eye- 
 spot on the side turned from the light, or to an increase of 
 intensity on this structure in the eye-spot turned toward 
 the light, or to an absolute difference of intensity on the 
 two sides in accord with the theories of Verworn and Loeb? 
 
 Two methods were used in attempting to answer these 
 questions. In both the larvae were mounted under a large 
 cover-glass supported by means of a ring of vaseline. The 
 cover was then pressed down until the space became so 
 narrow that the larvae could not rotate, (i) A piece of 
 sheet metal containing an opening i cm. square was hung 
 about 3 mm. from a Welsbach mantle. From the middle 
 of one side of the opening there projected nearly to the 
 center a spinelike process. The incandescent mantle was 
 focused on the slide by means of the plane mirror and Abbe 
 
172 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 condenser where it produced a sharply defined rectangular 
 area of intense light with a narrow triangular shadow pro- 
 jecting from one side. By mani{nilating the mirror I was 
 able to change the position of this area of light so as to 
 illuminate or shade any part of a larva fairly accurately in 
 spite of its microscopic size. All light except that from 
 the opening in front of the mantle was thoroughly elimi- 
 nated by niL^ans of suitable screens. Without going into 
 detail regarding the numerous observations made at dif- 
 ferent times, the reactions may be summarized as follows: 
 (a) If the anterior end is suddenly illuminated the larva 
 bends from side to side vigorously, (b) If the light inten- 
 sity on one eye-spot is increased without changing that on 
 the other, it bends both ends rather sharply toward the 
 illuminated side, (c) If the intensity on either eye-spot is 
 decreased it also bends toward the illu ninated side, (d) If 
 any portion back of the eye-spots is shaded or illuminated 
 there are no definite reactions. This shows that the ante- 
 rior end (probably the eye-spots) is the most sensitive part 
 of the larvae if it is not the only sensitive part, and that if 
 the light intensity is increased or decreased on either side 
 regardless of the direction of the rays, the larvae turn 
 toward the m )re highly illu ninated side. 
 
 (2) Two sources of light were so arranged and screened 
 as to produce small horizontal beams which crossed at right 
 angles on the stage. The larvae exposed in the light from 
 these two beams oriented toward a point approximately 
 halfway between the two sources. If the light from one 
 source was now intercepted they turned directly toward the 
 other, and when it was again exposed they returned to their 
 former position. These reactions do not occur in all in- 
 stances nor is the orientation always precise and definite, 
 especially if the larvae are not in prime condition. They 
 were however seen in so many cases that there can be no 
 doubt concerning the conclusions stated above. 
 
 What bearing have these conclusions on the problem in 
 hand? It is evident that when the light in one beam is 
 
VERMES, FLY LARVAE, AND ECIIINODERMS 173 
 
 intercepted after the larvae are directed toward a point 
 between the two, the intensity on the side facing this beam 
 is decreased more than that on the opposite side, and when 
 this light is turned on again after the larvae are directed 
 toward the other, the intensity on the same side is increased 
 more than it is on the opposite side. Under both condi- 
 tions however we find that the larvae turn toward the side 
 most highly illuminated. Under one therefore they turn 
 toward the side on which the intensity is increased, under 
 the other from the side on which it is decreased. It is 
 evident then that if the orienting stimulus is due to change 
 of intensity, it may be due to an increase as well as to a 
 decrease of intensity. And if this is true the organism must 
 in some way perceive the difference between a stimulus due 
 to an increase and one due to a decrease of illumination, 
 for in response to the former it turns toward the point of 
 stimulation whereas in response to the latter it turns from 
 this point. 
 
 It can be definitely stated then that orientation in Areni- 
 cola larvae is due to difference of intensity on opposite sides. 
 Whether it is the result of light acting constantly as a 
 directive stimulation like a constant electric current, or 
 whether it is the result of reactions due to changes of inten- 
 sity on the sensitive structures in the organisms brought 
 about largely by its movements, is a question concerning 
 which our evidence does not warrant a definite conclusion. 
 The facts that the organisms are frequently thrown out of 
 orientation as they proceed toward the source of light, and 
 that the anterior end is almost constantly turned from side 
 to side speak in favor of the latter. The organism is not 
 held definitely on its course as one would expect in case of 
 light acting constantly as a directive stimulation in accord 
 with the definitions of tropisms of Loeb and Verworn. It 
 must however be remembered that even if the organism is 
 under the influence of a constantly acting directive stimu- 
 lus it might turn from side to side frequently owing to the 
 effect of internal processes or other external stimuli. 
 
174 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 Summary 
 
 (i) Arenicola lar\ae arc positive to light in their free- 
 swimming state. They rotate on the long axis and swim 
 on an irregular spiral course owing to frequent sharp lateral 
 movements of th(^ head. 
 
 (2) If lu'ld under a cover-glass so that they cannot 
 rotate, it is found that the head is suddenly turned directly 
 toward the source of light when either of the two sides 
 is illuminated, frequently to such an extent that the 
 anterior end of the body is at right angles to the posterior 
 end. 
 
 (3) The larvae have two prominent eye-spots, which con- 
 sist of a hyaline portion partly surrounded by an opaque 
 caplike structure. The hyaline portion, which is probably 
 sensitive to light, is directed dorso-anterio-laterally. In 
 the free-swimming state orientation takes place by a greater 
 swerving toward the source of light on the spiral course 
 every time an eye-spot faces the light just as in Euglena, 
 In Arenicola, since it has two eye-spots, the increase in 
 swerving takes place in two different positions on the spiral, 
 i.e., twice during a complete rotation on the long axis. In 
 Euglena, since there is but one eye-spot, it takes place only 
 in one position on the spiral, or once during a complete 
 rotation. Euglena however responds when the eye-spot 
 faces the source of light because when it is in this position 
 the eye-spot shades the tissue. It responds onh' to a 
 decrease of intensity while it is positive. Arenicola larvae, 
 on the contrary, respond to either an increase or a de- 
 crease of intensity on the sensitive tissue on either side, 
 but they always turn toward the more highly illuminated 
 side. 
 
 (4) The stimulus causing this reaction, a reaction by 
 means of which the organism orients, is probably due either 
 to a decrease or to an increase of intensity on either side. 
 The orienting reaction is probably a differential response 
 to a localized stimulus. Our evidence however does not 
 
VERMES, FLY LARVAE, AND ECIIINODERMS 175 
 
 warrant a definite conclusion on this point. Orientation 
 may be the result of a response regulated by the absolute 
 difference of intensity on opposite sides, that is, it may be 
 due to the action of light owing to continued intensity 
 rather than to its action owing to change of intensity. 
 
 2. Blowfly Larvae — Musca sp.{?) 
 
 a. Introduction. — The reactions of the blowfly larvae 
 to light were described by Loeb in 1890. He exposed the 
 larvae in front of a window in diffused and direct sunlight 
 and found them to be negative and to orient very accurately. 
 He says (1905, p. 57), " They crept with mathematical pre- 
 cision in the direction of the rays. When a shadow was 
 thrown on the board by a penholder, it could be noticed 
 that the animals moved away from the light in a direction 
 exactly parallel to the edge of the shadow. . . . They 
 acted as though they w^ere impaled on the ray of light which 
 passed through their median plane. When I turned the 
 board around, the animals immediately turned about also, 
 and again placed their median planes in the direction of 
 the rays." Loeb, assuming that the method of orientation 
 in this form is the same as it is in others, concluded that it 
 is controlled by the same factors. In this form as in others 
 light acts constantly as a directive stimulus and " the main 
 feature ... is the fact that symmetrical points of the 
 photosensitive surface of the animal must be struck by the 
 rays of light at the same angle." (1897, p. 440), '' Ich 
 glaube jetzt, dass hier eine vollkommene Analogie der Licht- 
 und Stromwirkungen zu Tage tritt, derart, dass aiich, wie beim 
 Strom, die Lichtintensitdt dauernd die Spannung der Muskeln 
 beeinflusst, dass aber die Steilheit der Intensitdtsschwankimg 
 
 die Fortleitung der Spannimgsdnderung bestimmt 
 
 Das Wesen der Orientirung fasste ich dahin auf, dass bei 
 
 •vollendeter Orientirung Symmetriepunkte der Oberflciche dcs 
 
 Thieres unter gleichem Winkel von den Lichtstrahlen getroffen 
 
 werdeny According to Loeb then, if the position of the 
 
176 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 source of light is changed, the larvae turn immediately and 
 directly from the light in its new position until both sides 
 are again struck by the rays at the same angle. 
 
 Holmes' observations of the orienting reactions of fly 
 larvae do not support Loeb's theory. He found among 
 other things (1905, p. 105), " If a strong light is thrown 
 upon a larva from one side it may swing the head either 
 towards or away from the light ... In the animals here 
 described there is, so far as I can discover, no forced orien- 
 tation brought about by the unequal stimulation of the 
 two sides of the body, but an orientation is produced 
 indirectly by following up those chance movements which 
 bring respite from the stimulus. I do not deny that there 
 may be an orienting tendency of the usual kind, but if 
 there is it plays only a subordinate role in directing the 
 movements of the animal. The orientation of these forms 
 is essentially a selection of favorable chance variations of 
 action and following them up." 
 
 h. Locomotion. — The blowfly larvae move from place 
 to place entirely by means of muscular contraction. They 
 proceed somewhat as follows: the anterior end is raised, 
 thrust forward toward one side, fastened to the substratum, 
 and then the posterior end is pulled forward, after which 
 the anterior end is again raised and thrust forward, now 
 toward the opposite side, fastened, and the posterior end 
 again drawn up. The anterior end is thus turned alter- 
 nately toward the right and left quite regularly during the 
 process of locomotion. The extent of this lateral move- 
 ment varies much, but it is usually great enough so that 
 the extremity of the anterior end is nearly at right angles 
 to the direction of locomotion (see Fig. 31). 
 
 In drawing forward the posterior end the whole body 
 contracts, but the contraction is greater on the ventral than 
 on the dorsal surface, forming an arch, in which the extreme 
 anterior end is nearly vertical and the sensitive tip (Fig. 30) 
 well drawn under so as to be hidden from view. Thus the 
 tip of the anterior end becomes alternately thrust out and 
 
VERMES, FLY LARVAE, AND ECHINODERMS 177 
 
 exposed, retracted and concealed. This is of importance 
 in the orienting reactions as will be seen later (Fig. 31). 
 
 c. Accuracy of orientation. — The orientation of organ- 
 isms is generally supposed to be far more accurate than 
 it really is. This is no doubt due to the fact that many of 
 the observations on light reactions have been made in light 
 which is more or less diffused and the direction of which 
 is not thoroughly under control. In testing the accuracy 
 of orientation in the blowfly larvae they were exposed on a 
 piece of smooth moist black paper on a glass plate in a 
 small horizontal beam of light from a Nernst glower. The 
 course taken by the larvae was traced on the paper in white 
 ink with a small pen held about i cm. in front of the end 
 of the larvae. In this way the movements could be quite 
 accurately traced. Neither the presence of the pen nor 
 the ink on the paper made any appreciable difference in the 
 course taken. The courses of four different individuals are 
 given in Fig. 2^. All of the specimens tested deviated con- 
 siderably; those used in Fig. 27, B and C, deviated toward 
 the left in all the trials; the one used in Fig. 27, A, to the 
 right, and that in Fig. 2^], D, to the right in some trials 
 and to the left in others. In Fig. 27, A, the head move- 
 ments are represented more in detail than they are in the 
 others. It will be seen that the lateral movements to the 
 right and the left alternate quite regularly. The posterior 
 end takes a much more regular course than the anterior. 
 In direct sunlight orientation is somewhat more accurate 
 and the lateral movements are not so pronounced. But 
 I failed to find any specimens which " crept with mathe- 
 matical precision in the direction of the rays, . . . exactly 
 parallel to the edge of a shadow," or which " acted as 
 though they were impaled on the ray of light which passed 
 through their median plane," as Loeb states. 
 
 d. Orientation in light from two sources. — In the study 
 of orientation in light from two sources the larvae were 
 exposed on moist black paper just as in the preceding experi- 
 ment, in a field of light composed of two small horizontal 
 
lyS 
 
 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 beams, one from each of two Nernst glowers so situated 
 that the beams crossed at right angles. One of the glowers 
 was stationary and tlie light from it constant. The other 
 was mounted on a track so that the light from it could be 
 
 Fig. 27. The lines i, 2, etc., \n A, B, C, D, represent courses taken by four 
 different blow-fly larvae in light of 78 ca. m. intensity, n, direction of horizontal 
 rays from single Nernst glower. Orientation is not as accurate as one would ex- 
 F)ect if light acts constantly as an orienting stimulus in accord with the theories of 
 Sachs, Loeb, and Verworn. See text. 
 
 varied. The paths taken by the larvae under the different 
 conditions are represented in Fig. 28. 
 
 It will be seen by referring to the figure that the larvae 
 can move in a direction leading from any point between the 
 two sources of light just like all the other lower organisms 
 tested under these conditions. Loeb says (1905, p. 61), 
 
VEILUES, FLY LARVAE, AND ECHINODERMS 179 
 
 *' When the diffuse daylight which struck the [fly] larvae 
 came from two windows the planes of which were at an 
 angle of 90° with each other, the paths taken by the larvae 
 
 Fig. 28. Direction of movement of fly larvae in light from two sources; n, tn, 
 direction of rays; i, course in light from n and m 50 and 5.5 ca. m. respectively; 
 2, course in light from n and w, 50 and 15 ca. m. respectively; 3, course in light 
 from n and m, 50 and 60 ca. m. respectively. Lin^s 1, 2, and 3 represent the aver- 
 age direction of several courses taken by each of three larvae. 
 
 lay diagonally between the two planes; " and (p. 2), " It is 
 explicitly stated in this and the following papers that if 
 there are several sources of light of unequal intensity, the 
 light with the strongest intensity determines the orientation 
 
I So LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 and direction of motion of the animal. Other possible com- 
 plications are covered by the unequivocal statement, made 
 and emphasized in this and the following papers on the 
 same subject, that the main feature in all phenomena of 
 heliotropism is the- fact that symmetrical points of the 
 photosensitiv^e surface of the animal must be struck by the 
 rays of light at the same angle. It is in full harmony with 
 this fact that if two sources of light of equal intensity and 
 distance act simultaneously upon a heliotropic animal, the 
 animal puts its median plane at right angles to the line 
 connecting the two sources of light. This fact w^as not 
 only known to me, but had been demonstrated by me on 
 the larvae of flies as early as 1887 in Wurzburg, and often 
 enough since. These facts seem to have escaped several of 
 my critics." 
 
 It is evident without further discussion that the reac- 
 tions of fly larvae in light from two sources are not in accord 
 with Loeb's conclusions. When exposed in light from two 
 sources of different intensity the stronger does not deter- 
 mine the orientation and direction of motion, nor are sym- 
 metrical points on the photosensitive surface struck by the 
 rays of light at the same angle. 
 
 e. Orientation and movement — (i) perpendicular to the 
 direction of the rays — (2) toward a source of light. — The 
 following experiments bring out clearly the importance of 
 intensity in the orientation of fly larvae. A small horizontal 
 beam of light from a single Nernst glower w^as thrown on the 
 black paper used in the preceding experiments. In this 
 beam a small vertical post was erected so as to produce a 
 well-defined narrow shadow. By means of a mirror this 
 shadow was illuminated with rays of light either perpendic- 
 ular to or parallel with its edges, as represented in Fig. 29. 
 The intensity of light in the shadow could be regulated by 
 changing the position of the mirror. It was always con- 
 siderably lower than that in the field on either side. 
 
 If a specimen taken from darkness is placed on the plate 
 in the shadow with its anterior end directed toward the 
 
VERMES, FLY LARVAE, AND ECHINODERMS i8l 
 
 mirror, i.e., toward the source of light, it soon begins to 
 crawl and turn so as to direct the anterior end away from 
 the mirror, but in attempting this the anterior end extends 
 into the direct light owing to the narrowness of the shadow. 
 This produces a stimulus which causes it to withdraw and 
 swing in the opposite direction, where it soon comes into 
 the intense direct sunlight again. It continues this swing- 
 ing and crawling from one side of the shadow to the other 
 
 Fig. 29. Representation of arrangement of apparatus used to produce light 
 conditions in which negative fly larvae crawl toward the source of light or perpen- 
 dicular to the rays, a, glass plate; b, b', beams of light; c, shadow cast by the 
 opaque standard d; e, fly larva; g, Nernst glower; s, opaque screen; m, m' , mirrors, 
 relatively much farther from the glass plate than represented. The larva is placed 
 in the shadow, c, which is illuminated by light reflected from either the mirror m' 
 or m. The intense illumination on the anterior end whenever it projects beyond 
 the shadow prevents the larva from turning around and shows that under the 
 conditions of the experiment it is the change of light intensity on the anterior end, 
 and not the direction of the rays or the relation of intensity on symmetrically 
 located sensitive points, which regulates the direction of movement. 
 
 for a short time, but soon comes to travel more nearly 
 parallel with the edges of the shadow and consequently 
 extends into the light much less frequently. If the rays 
 from the mirror are perpendicular to the edges the larva 
 also remains in the shadow, but usually it crawls along near 
 the edge farthest from the mirror. The negative larvae can 
 
l82 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 thus be forced to move toward a source of light or at any 
 angle with the rays. 
 
 If exposed in a narrow shadow in a field of light con- 
 sisting of rays perpendicular lo the substratum they crawl 
 along in the shadow. If the anterior end chances to {pro- 
 ject out into the light it is stimulated and tuiiu'd in the 
 opposite direction. The direction of the ra\'s here is how- 
 ever perpendicular to the substratum; under the j)receding 
 conditions it was parallel with the substratum, yet the reac- 
 tion is the same under both conditions. It is of course due 
 to a change of intensity and is not primarily dependent 
 upon the ray direction. In a narrow shadow in the field 
 consisting of vertical rays the larvae can also be made to 
 move toward or perpendicular to light rays in the shadow 
 just as under the conditions described above. Cole (1907) 
 obtained similar results in experiments on Bipaliumkewense. 
 
 These results indicate that in their movements the larvae 
 attempt to keep the sensitive anterior end in the lowest 
 possible light intensity regardless of the direction of the 
 rays or the angle between them and the sensitive surface. 
 Loeb claims that the larvae follow the direction of the rays 
 even if in so doing they go from regions of lower into regions 
 of higher light intensity. He says (1905, p. 58), " I put 
 the almost fully grown larvae into a test-tube and placed it 
 horizontally on the table, with its longitudinal axis perpen- 
 dicular to the plane of the window. The sun's rays made 
 a small angle with the window. By means of a screen I 
 arranged the test-tube so that only diffuse light fell through 
 the window upon the half turned toward the window, while 
 direct sunlight fell on the half turned toward the room. 
 At the beginning of the experiment the animals were all 
 on the window side of the test-tube. They immediately 
 moved from the shaded part into the direct sunlight on the room \ 
 
 side, and remained there.'' Do these results prove Loeb's 
 conclusions? Can the reactions described in the cjuota- 
 tion be explained on the assumption that orientation reac- 
 tions are due to difference of intensity? 
 
VERMES, FLY LARVAE, AND ECIIINODERMS 183 
 
 Loeb says (p. 58), " When the animals crossed the bound- 
 ary from diffuse Hght into direct sunlight, the reaction 
 caused by the increase in the intensity of the light did 
 not take place until a half or a third of the body was in the 
 sunlight (because in all phenomena of stimulation some time 
 elapses between the application of the stimulus and the re- 
 action to it). The animal checked its movement and turned 
 its head through an angle of 90°- 130° from side to side. If 
 in so doing the head again came into the shade the animal 
 returned into the shade; but if this did not happen, as was 
 more usually the case, the animal continued its movement 
 into the sunlight." Under the conditions of the experiment 
 quoted above there was therefore no cause for turning until 
 one-half or one-third of the body was in direct sunlight. 
 It is evident that after the anterior end is in the sunlight, 
 it is in lowest light intensity when it is directed from the 
 source of light. Consequently if the larvae did start to 
 turn around so as to get back into the shaded region, the 
 effective intensity would be increased and this would at 
 once cause them to turn the head back again to the position 
 in which it is shaded. Moreover Loeb says, as quoted 
 above, that if the head came into the shadow in turning, 
 the animal returned into the shade. It is therefore evident 
 that there is nothing in the observations of Loeb inconsist- 
 ent with the idea that the orienting reactions are due to 
 difference or change of intensity on the surface of the organ- 
 ism. Nor do these observations show that these reactions 
 are not accompanied by anthropomorphic sensations as 
 Loeb intimates, since every reaction shows that the larvae 
 assume positions such that there is a minimum exposure of 
 the sensitive anterior end. 
 
 /. Sensitive region. — Both Loeb and Holmes assume 
 the anterior end to be the most sensitive part of the fly 
 larvae with reference to stimulation by light. The follow- 
 ing experiments on the effect of intensity on the rate of 
 locomotion indicate that this is not only the most sensitive 
 region, but that it is the only region sensitive to light. At 
 
i84 
 
 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 the anterior end there are two minute cone-shaped pro- 
 tuberances not over 0.5 mm. ai3art (Fig. 30). These pro- 
 tuberances can barely be seen with the naked eye when 
 the anterior end is extended, and not ai all when it is con- 
 tracted. Judging from their connection with the nervous 
 system, I am inclined to believe that they are light recipient 
 organs. 
 
 a.sp 
 
 D 
 
 I I I 
 
 — 5 mm- 
 
 FiG. 30. Musca larva. A, side view anterior end expanded; a.sp., anterior 
 spiracular process showing seven spiracular papillae; o.t, optic tubercle. After 
 Hewitt (1908, PI. 30, Fig. g). B, camera outline, dorsal view showing anterior 
 end expanded. C, same showing anterior end contracted, and optic tubercle 
 withdrawn and turned under as it is during the process of looping. D, dorsal 
 view of entire animal. 
 
 g. Effect of Ught intensity on rate of locomotion. — The 
 effect of light intensity on the rate of locomotion in fly 
 larvae was tested under three conditions: (i) with the entire 
 larva exposed; (2) with the posterior tliird exposed; and 
 (3) with the posterior three-fourths exposed. 
 
 h. Method. — A glass plate 25 cm. square was covered 
 with two sheets of filter paper over which was placed a 
 sheet of smooth black paper. This was then thoroughly 
 
VERMES, FLY LARVAE, AND ECIIINODERMS 185 
 
 soaked in water and the plate so arranged that the edges 
 of the filter paper which projected over the edge of the 
 glass plate extended into water kepi in a vessel below. In 
 this way a smooth surface containing a constant amount 
 of moisture was produced. It was found that such condi- 
 tions are very essential in quantitative work with the larvae, 
 especially that of constant moisture, since their rate of loco- 
 motion depends much upon the amount of moisture in the 
 surface upon which they crawl; either too much or Um Hi tie 
 causes marked retardation. Two fine white threads were 
 placed parallel with each other on the paper 15 cni.^ apart, 
 so as to form a definitely limited course upon which to try 
 the speed of the larvae under different light conditions. A 
 horizontal beam of light 4 cm. wide was projected from a 
 Nernst glower upon the glass plate perpendicular to the 
 threads. The light in this beam halfway between the 
 threads was 7 ca. m. in intensity. The time it required a 
 given larva taken from the culture jar kept in total dark- 
 ness to travel the distance between the threads was accu- 
 rately ascertained by means of a stop watch. At the end 
 of the course the larva was allowed to crawl onto a piece 
 of black paper supported on a section lifter and then trans- 
 ferred to the starting point without changing its orientation, 
 and allowed to continue on its course with the least disturb- 
 ance possible. After having ascertained the time required 
 to crawl 15 cm. in the beam of 7 ca. m. intensity, the plate 
 was turned through an angle of 90° so as to expose the 
 larva in a similar beam of light but one of a much higher 
 intensity. The intensity of this second beam of light in 
 the middle of the course was 3888 ca. m. It was produced 
 by a group of three Nernst glowers so arranged that a cross 
 section formed a small triangle. When the current was on, 
 the three glowers appeared much like a highly illuminated 
 
 ^ Through some oversight I failed to record the distance between the 
 threads. I am not quite positive whether it was 15 cm. or 10 cm. This 
 however does not invalidate the results recorded in the following table since 
 they are comparative in every case. 
 
i86 
 
 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 solid rod several times as large as a single glower. It there- 
 fore cast a sharp shadow, a point of Importance In the 
 following experiments. The time required to travel the dis- 
 tance was recorded just as under the preceding conditions. 
 The rate of movement was now obtained alternately under 
 the two conditions. The results appear in Table II. This 
 table shows that It required on an average 46.4 seconds to 
 
 TAHLI-: II 
 
 Distance 
 
 Time in seconds 
 
 Lifiht intensity, 
 3888 ca. m. 
 
 Light intensity, 
 7 ca. m. 
 
 15 cm. 
 
 (< (( 
 
 (( (( 
 
 (i K 
 (( (( 
 
 44.4 
 43-2 
 42.4 
 44.8 
 
 42. 
 
 47.2 
 46.8 
 
 44. 
 46.6 
 
 47-4 
 
 Total Average 
 
 43-36 
 
 46.4 
 
 travel 15 cm. In an Intensity of 7 ca. m., and 43.36 seconds 
 to travel the same distance In an Intensity of 3888 ca. m. 
 Under the former conditions the larvae therefore crawled 
 at the rate of 0.321 cm. per second, and under the latter 
 at the rate of 0.345 cm. per second, a difference of only 
 0.024 cm. per second, due to a difference of 3881 ca. m. of 
 
 light. 
 
 In studying the effect on the rate of locomotion of expos- 
 ing the posterior third and three-fourths of the larvae, the 
 apparatus was arranged just as described above. The time 
 required to travel 15 cm. in 7 ca. m. Intensity was first 
 ascertained with a given larva, then the larva was trans- 
 ferred to the starting point, and after it had crossed the 
 thread a small beam of light 3888 ca. m. in Intensity from 
 the three glowers was thrown on the posterior end and held 
 there by means of moving along by the side of the larva a 
 screen containing a small rectangular opening. The time 
 
VERMES, FLY LARVAE, AND ECIIINODERMS 187 
 
 required to complete the course was thus alternately 
 obtained under each of the two conditions. The results 
 obtained with one-third exposed are recorded in Table III; 
 
 TABLE III 
 
 
 
 istance 
 
 Time in 
 
 seconds 
 
 Date D 
 
 
 
 
 
 Posterior J; in 
 
 Entire larva 
 
 
 
 3888 ca. m. 
 
 in 7 ca. m. 
 
 Jan, 29 I 
 
 5 cm. 
 
 36.95 
 
 37-35 
 
 
 " 30 
 
 
 50.64 
 
 51-40 
 
 
 [( (( ( 
 
 
 36.77 
 
 36.67 
 
 
 i( (< 1 
 
 
 35-38 
 
 35-i8 
 
 
 
 
 41 . 10 
 
 41.75 
 
 
 Hit i 
 
 
 43-66 
 
 43-58 
 
 
 ' 31 
 
 
 69.45 
 
 71-5 
 
 
 ( i< < 
 
 
 42.64 
 
 42.64 
 
 (( <( ( 
 
 
 42.01 
 
 41.96 
 
 Feb. I 
 
 
 40.3 
 
 40.4 
 
 Total average 
 
 
 43-89 
 
 44.24 
 
 those with three-fourths exposed in Table IV. Each figure 
 in columns three and four in the tables represents the aver- 
 
 TABLE IV 
 
 
 
 Distance 
 
 Time in 
 
 seconds 
 
 Date 
 
 Posterior f in 
 
 Entire larva 
 
 
 
 3888 ca. m. 
 
 in 7 ca. m. 
 
 J^ 
 
 in. 29 
 
 ' 30 
 
 15 tm. 
 
 38. 94 
 36.56 
 47-84 
 
 40.73 
 38.12 
 
 48.18 
 
 
 ( u 
 
 
 39-05 
 
 40.01 
 
 
 i n 
 ' 31 
 
 
 53-23 
 42.18 
 
 46.60 
 
 54.30 
 42.26 
 46.80 
 
 Feb. I 
 
 
 41.78 
 42.9 
 
 43.01 
 42.8 
 
 Total average 
 
 
 43-23 
 
 44.02 
 
1 88 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 age of ten trips across the course made by dilTerent indi- 
 viduals. The total average as seen in Table III, with the 
 entire larva exposed in 7 ca. m. light intensity, is 44.24 
 seconds, and that in 7 ca. ni. intensity with the posterior 
 third of the body in S^SS ca. m. intensity, is 43.89 seconds, 
 a difference of only 0.35 seconds in traveling 15 cm. In 
 Table IV the total average in 7 ca. m. intensity is 44.02 
 seconds; and in 7 ca. ni. with ij of the body exposed in 3888 
 ca. m. intensity, it is 43.23 seconds, a difference of 0.79 
 seconds in traveling 15 cm. By comparing these results 
 with those recorded in Table II it will be seen that there 
 is very little difference in rate of locomotion under the 
 different conditions of illumination, i.e. lar\'ae entirely ex- 
 posed in ;^SSS or 7 ca. m. or the posterior one-third or 
 three-fourths exposed in 3888 ca. m. This seems to indi- 
 cate that the tissue sensitive to light is restricted to the 
 anterior tip of the body. The difference in rate of loco- 
 motion under the different conditions can be accounted 
 for by assuming it to be caused by the light reflected from 
 the highly illuminated posterior end of the body upon the 
 sensitive anterior end. 
 
 The exposure of the side of the body to the very Intense 
 light from the three glowers has apparently no effect what- 
 ever on orientation. The larvae continue as directly on 
 their course as though they were exposed only to light of 
 7 ca. m. intensity from the single glower. As a matter of 
 fact all but the very tip of the anterior end can be illumi- 
 nated by this intense lateral light without causing any 
 noticeable deviation in the direction of motion. If how- 
 ever the tip is exposed there is a sudden sharp turning 
 of the anterior end either toward or from the source of 
 light. 
 
 The results recorded in the last two tables indicate either 
 that the tissue sensitive to light in fly larvae is confined to 
 the extreme anterior end, or that light of constant intensity 
 has no effect on the rate of locomotion, the increase in rate 
 due to increase in light intensity when the entire organism 
 
VERMES, FLY LARVAE, AND ECIIINODERMS 189 
 
 is exposed being due to change of intensity caused by the 
 extension and contraction of the anterior end. 
 
 i. Mechanics of orientation. — Holmes (1905, p. 105) 
 says, " If a strong light is thrown upon a larva from one 
 side it may swing the head either towards or away from 
 the light," intimating that it is turned in one direction as 
 often as in the other. I exposed various individuals to sud- 
 den lateral illumination by direct sunlight, or light of nearly 
 equal intensity from the three glowers, at different times 
 and recorded the direction in which the anterior end turned. 
 The results appear in Table V. It will be seen by referring 
 
 TABLE V 
 
 Number of times anterior end is turned 
 
 r 
 
 From source of 
 
 Toward source of 
 
 light 
 
 light 
 
 6 
 
 6 
 
 13 
 
 8 
 
 9 
 8 
 
 13 
 26 
 
 7 
 
 26 
 
 26 
 
 24 
 
 19 
 28 
 
 31 
 
 22 
 
 3 
 
 8 
 
 3 
 
 8 
 
 27 
 
 21 
 
 Total 177 
 
 165 
 
 to this table that in all there were 177 turns from the light 
 to 165 toward it, i.e., nearly the same number in both 
 directions. Later however I obtained results very different 
 from these. They are recorded in Table VI. The results 
 recorded in Table VI show that when a larva is first exposed 
 to intense unilateral illumination, it turns toward the source 
 of light practically as frequently as from it, and orientation 
 is indirect, but that after being exposed for some time it 
 turns considerably more often from the source of light than 
 
I go 
 
 LIGHT AND THE BEHAVIOR OF ORGAXISMS 
 
 toward it, and if then exposed to lateral illumination of a 
 lower intensity it seldom turns toward the source of light 
 at all, and orientation is direct.' 
 
 TABLE VI 
 
 
 Number of times anterior end is turned 
 
 Date 
 
 From source of 
 light 
 
 Toward source 
 of light 
 
 Feb. 6 Direct sunlight 
 
 25 
 34 
 
 38 
 
 45 
 
 2^ 
 
 ,< ^ ( Same individual / 
 ) Direct sunlight ( 
 
 ,, ^ ( Same intiividual | 
 ( Direct sunlight ) 
 
 ,1 ^ { Same individual 1 
 
 16 
 12 
 
 5 
 
 ( Diffused light ) 
 
 Feb. 7 New individual 
 
 12.30 P.M. Direct sunlight 
 
 29 
 3i 
 48 
 
 21 
 
 ( Same individual 1 
 
 ^•°° ^•^- i Direct sunlight \ 
 
 _ ,, ( Same individual ) 
 ^-^°^-^-i Diffused light } 
 
 2 
 
 How are these results to be explained? It is ordinarily- 
 supposed that the higher the illumination the more direct 
 the orientation in such organisms as fly larvae. The results 
 above indicate the opposite to be true. Have these crea- 
 tures the power of differential response to localized stimu- 
 lation, as the final results recorded in the table seem to 
 indicate ? 
 
 During the normal process of locomotion, as already 
 stated, the larvae alternately swing the anterior end slightly 
 to the right and left at the same time that they thrust this 
 
 ^ The variable results recorded in Table VI show very clearly the im- 
 portance of studying reactions under different conditions, and also that 
 statistical results in the study of reactions may be very misleading. This is 
 particularly true in case of organisms which readily become acclimatized, 
 as the blowfly larvae do. If the larvae are exposed to direct sunlight half 
 an hour or so they frequently lose all power of response to lower intensities 
 and sometimes respond no longer even in direct sunlight. 
 
VERMES, FLY LARVAE, AND ECHINODERMS 191 
 
 end forward. When the anterior end is thus extended the 
 two cone-shaped elevations at the very tip (Fig. 30) become 
 fully exposed; and these, owing to the lateral movements 
 of the anterior end, face alternately to the right and the 
 left. When the animal fastens the anterior end to the sub- 
 stratum and pulls up the posterior end, the cone-shaped 
 structures cannot be seen. They appear to be drawn in, 
 and the whole anterior end is turned under somewhat as 
 the arch is formed in the looping process. This causes the 
 tip to be thoroughly concealed and shaded. 
 
 When the larvae are first exposed to sudden lateral illu- 
 mination in direct sunlight, they respond immediately by 
 throwing the anterior end toward one side violently, no 
 matter in what position this end chances to be. If it hap- 
 pens to be directed from the source of light when the sun- 
 light is flashed upon the organism, it turns toward the source 
 of light, and if the sunlight is immediately intercepted after 
 the larva turns, It will continue in the direction toward 
 which the anterior end points; if it is not intercepted, the 
 anterior end is thrown in the opposite direction, and then 
 the larva may follow this turn and become oriented imme- 
 diately, or it may swing the end back and forth a few times 
 before becoming oriented. If the anterior end faces the 
 light when it is exposed to the sun it is first thrown in the 
 opposite direction and orientation takes place just as de- 
 scribed above. The anterior end is thus turned in the direc- 
 tion opposite to that in which it is when the exposure is 
 made. It is therefore evident that under these conditions 
 the larvae will turn toward a strong unilateral illumination 
 about as often as from it. 
 
 According to the tables, however, turning toward the 
 source of light becomes less frequent after the organism is 
 exposed for a time and much less frequent if the intensity is 
 decreased. What Is the cause of this? If the larvae are 
 carefully observed when they are suddenly exposed to lat- 
 eral illumination, by diffuse light, it is found that they 
 respond immediately only if the anterior end is turned 
 
192 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 toward the source of light when the exposure is made (Fig. 
 31). If this end is in any other position, there is no reac- 
 tion whate\er until \hv organism, in its normal {process of 
 locomotion, extends it toward the source of light. Then 
 it is at once turned from the light to such an extent that it 
 frequently makes a right angle with the posterior end. 
 Later it is swung hack, but only part way. The tip is 
 however exposed and so the animal may be stimulated 
 again, after which it again turns sharj)l>' from the source of 
 light. This process is repeated until the organism has 
 turned to such an extent that the anterior end is practically 
 as much exposed when it turns in one direction as it is when 
 it turns in the other. The great preponderance of lateral 
 movements from the source of light and direct orientation in 
 diffuse light therefore do not indicate that fly lar\ae have 
 the power of differential response to localized stimulation. 
 
 But why does the organism turn toward the light if the 
 lateral illumination is very intense? Whenever the larva 
 is stimulated, it turns the anterior end in a direction oppo- 
 site to that in which this end is when it receives the stimu- 
 lus. The tip of the anterior end is relatively very sensitive; 
 in diffuse light the larvae are stimulated only when this end 
 is extended and fully exposed, but in \er\' intense light, 
 owing to the transluc-ency of the surrounding tissue, it is 
 stimulated no matter in what position the anterior end is; 
 consequently if this end is turned from the source of light 
 when the organism is exposed it is at once turned sharply 
 in the opposite direction, i.e., toward the light. 
 
 j. Discussion. — It has already been demonstrated that 
 neither the direction of the rays through the organism, in 
 accord with Sachs' theory, nor the angle between the ra>'s 
 and the sensitive surface, in accord with Loeb's explana- 
 tion, is of importaiice in explaining the orienting reactions 
 of the fly larvae. Nor is the direction of the rays in the 
 field of importance except in so far as it may produce 
 difference of intensity on the body. How then are the 
 orienting stimulations produced ? Are they due to light 
 
n 
 
 n 
 
 1)1 
 
 Fig. 31. The process of locomotion and orientation in blow-fly larvae, a-j, 
 different positions taken during the process; w, n, direction of light rays. The 
 anterior end of the larvae is quite regularly turned from right to left during the 
 process of locomotion. If n is exposed and m shaded simultaneously when a larva 
 is at d, it turns sharply to e, then loops to /, turns and expands to ,?, where the 
 sensitive anterior end becomes fully exposed and consequently stimulated. This 
 causes the larva to turn sharply at once to h, where it becomes attached and loops 
 to i, expands and turns to^ and is again stimulated, after which it repeats its former 
 response, etc., until it is oriented and the oral end is no longer subjected to marked 
 changes of intensity, as it swings back and forth in the process of locomotion. If 
 n is exposed when the larva is in position b, no reaction takes place until it expands 
 and turns to c, then it responds as described above. If the light from n is much 
 more intense than that from w, or if the larva is in a very sensitive state it responds 
 at once when n is exposed no matter in which position it is. If it is at a or 6 it throws 
 the anterior end sharply toward «, then in the opposite direction, after which it 
 orients as described above. It may however wave the anterior end back and 
 forth several times before it orients. igj 
 
194 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 acting constantly as a directive stimulus similar to stimula- 
 tion by a constant electric current, or to absolute difference 
 of intensity on symmetrically located points in the sensitive 
 surface, or to changes of intensity? 
 
 According to the idea of Loeb that light acts constantly 
 as a directive stimulation, the organism is continuously 
 stimulated by light on both sides. When one side is more 
 highU- illuminated than the other, that side becomes stimu- 
 lated more than the other and causes a more rapid move- 
 ment of the lucuniutor organs connected with the sense 
 organs of that side. This of course causes the organism to 
 turn until both sides are equally stimulated. We have 
 demonstrated that in a light intensity of 3888 ca. m. the 
 rate of locomotion is only 0.024 mm. per second greater than 
 in an intensity of 7 ca. m. (Table II). If then the rate of 
 motion of the two sides of the organism under discussion is 
 due to the absolute intensity on the two sides in accord 
 with Loeb's theory, and if one side were exposed to an 
 intensity of 3888 ca. m., while the other is exposed to an 
 intensity of 7 ca. m., the former would move only 0.024 mm. 
 per second faster than the latter. It would therefore require 
 several seconds for a fly larva to become oriented even with 
 a difference of intensity on opposite sides amounting to 
 nearly 4000 ca. m. ; whereas it actually requires only a frac- 
 tion of a second for the larvae to orient under conditions in 
 which the greatest difference of intensity could not possibly 
 be more than 5-10 ca. m. The theory that orientation is 
 due to light acting constantly as a directive stimulation is 
 therefore inadequate to account for the orientation of fly 
 larvae. Moreover the fact that the larvae, when exposed 
 to moderate light intensity, respond only when the anterior 
 end comes to be fully exposed to the light in the process of 
 locomotion, shows clearly that the orienting stimulation is 
 not acting constantly. 
 
 The symmetry of the body with reference to the location 
 of the sensitive surface seems to be of no special importance 
 as far as orientation is concerned in this organism. I was 
 
VERMES, FLY LARVAE, AND ECIIINODERMS 195 
 
 unable to obtain any evidence of the power of differential 
 response to localized stimulation. The orienting reactions 
 could readily be explained by assuming the area sensitive 
 to light to be restricted to a small mass of substance located 
 in the middle of the very tip of the anterior end. 
 
 The idea that the stimulations leading to orientation are 
 due to changes of intensity (in fly larvae an increase only) 
 on the sensitive surface seems to fit the facts as far as known. 
 Stimulations thus produced cause an increase in the lateral 
 head movements somewhat similar to the avoiding reac- 
 tions and shock movements in the lower forms. Owing to 
 the difference in exposure of the anterior end, the move- 
 ments from the source of light are increased more than 
 those toward the light. This continues until the organism 
 is directed away from the source of light and the change of 
 intensity on the anterior end is no longer sufficient to cause 
 a response. 
 
 The process of orientation in the fly larva is strikingly 
 similar in principle to that in Euglena and Stentor. Sten- 
 tor, e.g., is most sensitive when the oral side is exposed; the 
 fly larvae when the anterior end is exposed. When Stentor is 
 not oriented the highly sensitive oral side is alternately fully 
 illuminated and shaded by means of rotation on the long 
 axis. In the fly larvae the alternate illuminating and shad- 
 ing of the sensitive anterior end is brought about by the 
 swinging of the head from side to side. If the intensity is 
 not high, Stentor never turns toward the light; it responds 
 only after the oral side is turned toward the light. This 
 response consists in a rapid swerving from the source of 
 light and eventually results in orientation. Likewise the 
 fly larva under similar conditions responds only after the 
 anterior end is exposed, and the response consists in sharp 
 turning from the source of light, which on repetition results 
 in orientation. Stentor makes no mistakes in the pro- 
 cess of orientation under these conditions. It never turns 
 toward the source of light, but still there are constant trial 
 movements during the process of orientation. The same 
 
196 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 is true with regard to the fly larvae. In Stentor every rota- 
 tion on the spiral course may be considered a trial move- 
 ment. If the organism is not oriented it swerves a little 
 farther from the source of light in each rotation after the 
 oral side is turned toward the light, until this side is equally 
 exposed throughout the entire rotation. Just so every lat- 
 eral movement of the fly larvae may be considered a trial 
 movement. It the organism is not oriented the anterior 
 end becomes much more fully exposed when ii is turned 
 toward the light than when it faces in the opposite direc- 
 tion. This produces a stimulation and causes it to be 
 turned farther than usual in the opposite direction, but it 
 is swung back again, receives another stimulation, and is 
 turned still farther from the light. Thus the organism may 
 be considered to try difi"erent positions by swinging the 
 anterior end back and forth. This trial process does not 
 cease after the organism is oriented; the anterior end con- 
 tinues to swing from side to side. If it is subjected to but 
 little difference of light intensity as it swings from side to 
 side, there is no response and the organism continues as it 
 is directed, but if it is subjected to considerable difference 
 of intensity it responds and turns as described above. 
 
 The fly larva presents an excellent example of an organ- 
 ism guided fairly directly on its course by successive trial 
 movements, and shows again that the mere fact of accurate 
 orientation is not a satisfactory criterion of direct orienta- 
 tion. Of course it is not necessary to assume that this 
 organism consciously tries different positions in the process 
 of orientation. 
 
 In how far do the reactions of the fly larvae agree with 
 the explanation Holmes presented with reference to them ? 
 Is "orientation in these forms . . . essentially a selection of 
 favorable chance variations of action and following them 
 up" ? The answer to this question depends entirely upon 
 what is meant by chance variations. It is therefore evi- 
 dent that a statement with reference to it would add little 
 or nothing to our analysis. 
 
VERMES, FLY LARVAE, AND ECIIINODERMS 197 
 
 Summary 
 
 (i) Fly larvae are negative in their light reactions in all 
 intensities to which they respond. They become acclima- 
 tized very readily so that after they have been exposed in a 
 given intensity for about half an hour they fail to respond 
 unless the intensity is increased. 
 
 (2) The tip of the anterior end is the only sensitive region 
 on the larvae. On this tip there are two cone-shaped struc- 
 tures which probably are light recipient organs. 
 
 (3) In locomotion the larvae turn the anterior end slightl> 
 from side to side with considerable regularity; but if sud- 
 denly exposed to high intensity they throw the anterior end 
 from side to side violently. 
 
 (4) Absolute difference in light intensity has but little 
 effect on the rate of movement. In 7 ca. m. it was found 
 to be 0.321 cm. per second; in 3888 ca. m., 0.345 cm. per 
 second. 
 
 (5) Unilateral illumination of the posterior third or three- 
 fourths of the body has practically no effect on the rate of 
 locomotion. 
 
 (6) The process of orientation in the fly larva is similar 
 in principle to that in Euglena and Stentor. It is brought 
 about by reactions which are similar to the avoiding reac- 
 tions or shock movements of thejower organisms. These 
 reactions are due to changes of light intensity on the sensi- 
 tive anterior end; and the changes of intensity are due 
 largely to the lateral movements of this end. 
 
 (7) Orientation is the result of trial movements, but it is 
 doubtful whether it could be considered as the result of selec- 
 tion of random movements as defined by Holmes (1905). 
 
 (8) In light from two sources they may take a path 
 extending from any point between them. The location of 
 this point depends upon the relation in intensity of light 
 from the two sources. 
 
 (9) There is no evidence indicating differential response 
 to localized stimulation. If the fly larva has the power of 
 
igS LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 such response at all. it is but little developed and is of very 
 little importance in the general reactions to light. 
 
 (lo) Neither the direction of the rays through the organ- 
 ism, nor the angle with the surface, nor the symmetry of the 
 sensitive surface, nor al)S()lute differeuce of intensity on the 
 body, is of importance in orientation excepting in so far as 
 they ma\' intluence change of intensity on the anterior end. 
 
 (ii) There is no evidence indicating that the orienting 
 reactions in fly larvae are tropic in accord with Loeb's 
 definition of this term. 
 
 3. Earthivorms 
 
 The light reactions of various earthworms have been 
 studied by a number of investigators, several of whom 
 directed special attention to the process of orientation. 
 Parker and Arkin (1901), Miss Smith (1902), and Adams 
 (1903) made observations on the direction of movement of 
 the anterior end when illuminated from one side, and found 
 that it turned from the light more often than toward it, 
 indicating, since these organisms are ordinarily negative, 
 that orientation is direct. Holmes, however (1905), is of 
 the opinion that the animals actually start to turn toward 
 the light just as often as from it, but that the movements 
 toward the light are inhibited owing to the greater exposure 
 as the end expands. This causes marked movements only 
 in the direction from the source of light. He believes that 
 Parker and Arkin and others may have failed to take into 
 consideration the slight preliminary movement which occurs 
 before the actual extension takes place, and that this may 
 account for the preponderance of negative turning recorded 
 by these investigators. Holmes, taking account of all the 
 minute preliminary movements, says (p. loi): "In the two 
 specimens employed the first detectable turn was away from 
 the light 27 times and towards the light 23 times. After a 
 few extensions the worm in nearly all cases soon turned 
 and crawled away from the light. The first detectable 
 
VERMES, FLY LARVAE, AND ECHINODERMS 199 
 
 movement of the earthworm seems, therefore, to be nearly 
 as likely to be towards the light as away from it. The 
 slight preponderance of negative turns may be due to the 
 fact that some of the smaller trial movements were over- 
 looked, to a slight direct orienting effect of the rays, or 
 merely to chance." 
 
 Harper (1905), working on Perichaeta bermudensis and a 
 species of Lumbricus in various light intensities, concluded 
 that in light of comparatively low intensity orientation is 
 indirect and that there are numerous random movements, 
 but in direct sunlight, especially if the worms have previ- 
 ously been kept in darkness, orientation is direct and ran- 
 dom movements are almost entirely eliminated. 
 
 I undertook the study of the reactions of Allolobophora 
 foetida with the express purpose of ascertaining the effect 
 of constant light intensity on the rate of movement with 
 different portions of the animal highly illuminated, thinking 
 that it might be possible thus to demonstrate the difference 
 between the action of light as an orienting stimulus, and 
 a stimulus affecting the general activity of the organism. 
 The rate of movement in this form however is so irregular 
 that I found it impossible to obtain results worthy of con- 
 sideration. I therefore turned my attention to direct obser- 
 vation of the process of orientation. 
 
 In locomotion the earthworm usually swings its anterior 
 end from side to side, but not nearly so regularly as do blow- 
 fly larvae. If after a specimen is oriented in a beam of 
 light, the ray direction is suddenly changed so as to illumi- 
 nate the side, one of four different kinds of movements may 
 result: (i) a contraction of the anterior end; (2) an exten- 
 sion of the anterior end; (3) sudden raising of the anterior 
 end frequently accompanied by swinging from side to side, 
 or (4) direct turning either toward or from the source of 
 light in the plane of the substratum. If the animal is 
 active, the lateral movements of the anterior end are more 
 pronounced and regular during its locomotion than if it is 
 sluggish. When exposed to unilateral illumination in such 
 
200 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 a condition the anterior end is simply turned sharply in the 
 direction opposite to that in which it is when it receives 
 the stimulus, just as in the case of blowtly larvae. Thus 
 it is turned toward the source of light about as often as 
 from it, regardless of the light intensity. 1 found this to be 
 true in direct sunlight, contrary to Harper's conclusion, as 
 well as in light of lower intensities. 
 
 If the animal however is rather sluggish so that there is 
 little lateral movement of the anterior end it turns from the 
 source of light with \ery few exceptions. The direction in 
 which the anterior end started to move after exposure to 
 lateral illumination in six such specimens is recorded in 
 Table \T I . These specimens were allowed to orient in light 
 of 15 ca. m. intensity, after which they were suddenly ex- 
 posed to a horizontal beam of light, ordinarily of higher 
 intensity, from one side. There was but little lateral move- 
 ment of the anterior end after the specimens were oriented 
 in the lower intensity, and they moved so slowly that the 
 direction in which the anterior end started to turn after 
 one side was illuminated could be clearly seen. 
 
 TABLE VII 
 
 Intensity of 
 
 Number of times anterior end 
 turned 
 
 Condition of 
 specimens used 
 
 
 Lateral Illumi- 
 nation 
 
 From source 
 of light 
 
 Toward source 
 of light 
 
 Time 
 
 200 ca. m. 
 
 200 ca. m. 
 
 50 ca. m. 
 200 ca. m. 
 
 12 ca. m. 
 200 ca. m. 
 
 21 
 
 22 
 
 23 
 25 
 24 
 
 25 
 
 4 
 
 3 
 2 
 
 
 
 I 
 
 
 Fresh specimen 
 
 taken from 
 
 darkness 
 
 Same specimen 
 
 Fresh 
 
 Same 
 
 <( (( 
 
 ? 
 
 ? 
 12 .00 
 12. 15 
 
 12.55 
 
 3 50 
 
 In a few other sluggish specimens the exposure to unilat- 
 eral illumination was not made until after they had come 
 to rest in the light of 15 ca. m. Under such conditions the 
 animals did not react at all until a few moments after 
 
VERMES, FLY LARVAE, AND ECIIINODRRMS 201 
 
 the exposure, then they very slowly extended and turned 
 the anterior end from the source of light every time. The 
 movements were so slow that they could be readily fol- 
 lowed in detail under a hand lens. There was no evidence 
 of even the slightest preliminary turning toward the source 
 of light. It must therefore be concluded that these animals 
 have the power of differential response to localized stimula- 
 tion by light. This conclusion is in harmony with the 
 results of Parker and Arkin, Miss Smith, Adams, and 
 Harper. 
 
 Parker and Arkin also found that if only the middle or 
 the posterior third of the body is exposed there are more 
 negative head movements than positive. I was unable to 
 confirm these results. Specimens were repeatedly allowed 
 to orient in a horizontal beam of light of 15 ca. m. intensity 
 on smooth moist black paper supported by a glass plate as 
 described above; and after they had started to crawl away 
 from this source of light a portion of the body w^as exposed 
 in an intensity of 200 ca. m. to lateral illumination from a 
 Nernst glower. The glower was mounted vertically so that 
 it cast a well-defined sharp shadow. By means of an opaque 
 screen containing a rectangular opening a beam of light 
 could be thrown upon any portion of the body and held 
 there by moving the screen in harmony with the movement 
 of the animal. The orientation of specimens was thus fre- 
 quently studied and the direction of movement carefully 
 noted while they crawled across the field, a distanceof 20 cm., 
 alternately with and without some portion of the body 
 exposed to unilateral illumination. The exposure of any 
 portion back of the sixth segment had no appreciable effect 
 on the direction of motion. If however the screen was at 
 any time brought forward so that the relatively intense 
 lateral rays fell on the anterior end there was always an 
 immediate response if the specimens were active. If the 
 anterior end chanced to be directed from the source of light 
 in its swinging movements when the exposure was made, it 
 was at once thrown sharply toward the light; if it chanced 
 
20 2 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 to be directed toward the source of light, it was thrown 
 equalK' strongly in the opposite direction. These results, 
 together with some statistical tabulations of direction of 
 head movements with different portions of the body back 
 of the sixth segment exposed to unilateral illumination, indi- 
 cate that, while these portions are no doubt sensitive to 
 light, only the anterior end is immediately functional in 
 regulating orientation. Jennings (i9()6a, p. 442) arrived 
 at practically the same conclusion with reference to other 
 stimuli. 
 
 Parker and Arkin in their experiments used a W'clsbach 
 gas burner as a source of light. Owing to the widtli of the 
 luminous part of the burner it is evident that a shadow of an 
 object in light from such a source will not have sharp edges; 
 and likewise a beam produced by means of a screen con- 
 taining an opening will not have well-defined edges. In 
 such a beam there is a region of uniform highest inten- 
 sity in the middle and a region of graded intensity on 
 either side. By calculations based on the data furnished 
 by Parker and Arkin it was found that in the beam of light 
 at the place where they exposed the earthworms the region 
 of uniform highest intensity was 10 mm. wide and the 
 region of graded intensity on either side was 16 mm. wide. 
 Outside of this on either side there was another region 15 
 mm. wide faintly illuminated by light reflected from the 
 water screen. It is evident that when the middle or pos- 
 terior end of a specimen of Allolobophora foetida, usually 
 only about 4 cm. long, is exposed in the region of greatest 
 intensity in such a field, the anterior end will be exposed to 
 the weaker light in the adjoining region. It may be then 
 that the preponderance of negative head movements found 
 by Parker and Arkin with the posterior portion of Allolo- 
 bophora exposed to relatively strong lateral illumination, 
 was due to the effect of the weak illumination on the an- 
 terior end. Since the light in the beam becomes gradually 
 weaker as one proceeds outward, it is clear that the anterior 
 end of the worm will be in higher light intensity when the 
 
VERMES, FLY LARVAE, AND ECIIIXODERMS 203 
 
 middle is in the region of strongest illumination, than when 
 the posterior end is there. One would therefore expect a 
 greater proportion of negative head mo\'cments under the 
 former conditions than under the latter, which is just what 
 Parker and Arkin found. 
 
 The tabulated conclusions of these authors are formu- 
 lated with the supposition that all positive head movements 
 in AUolobophora exposed to lateral illumination are " due 
 to other stimuli than light " (I.e., p. 153). My observa- 
 tions do not confirm this conclusion. As already stated, it 
 was found that if the anterior end is bent toward either 
 side when the exposure is made, it simply turns toward the 
 opposite side regardless of the direction of the rays. Orien- 
 tation in these forms is by no means entirely due to differen- 
 tial response to localized stimulation. Selection of random 
 movements or trial movements, as Holmes, Harper and 
 Jennings pointed out, undoubtedly plays a very large part 
 in the process of orientation in the earthworm under ordi- 
 nary conditions. 
 
 The swinging movements of the anterior end are in the na- 
 ture of trial movements. They may be induced by external 
 conditions, but their character and direction are determined 
 by the structure of the organism and various physiologi- 
 cal processes. They make it possible for the organism 
 to orient much more accurately than it otherwise could. 
 When the anterior end is directed straight ahead and the 
 organisms are oriented, this end is more or less shaded and 
 not in a position to be readily stimulated by changes in the 
 direction of the greatest illumination. If it were immov- 
 ably fixed to the rest of the body in this position the entire 
 organism might turn toward either side considerably with- 
 out receiving an orienting stimulation. In i^lace of turning 
 the entire body it raises the anterior end so as to magnify 
 the difference of intensity on opposite surfaces, extends it 
 so that it becomes more sensitive, and swings it from side 
 to side so that the different surfaces become alternately 
 shaded and illuminated, thus producing changes of inten- 
 
204 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 sity. If the change of intensity is greater when the oral 
 end is turned toward the right than when it is turned 
 toward the left, it is stimulated and bends farther toward 
 the left. The direction of bending is generally independent 
 of the direction of the rays, but the extent of bending 
 usually is not. 
 
 Holmes and Harper l)oth pointed out that swinging 
 movements toward ihe source of light are checked because 
 the animal becomes more and more sensitive as the anterior 
 end extends toward it. This end is however not only 
 checked under such conditions, it is also stimulated and 
 swings farther in the opposite direction. This is an impor- 
 tant factor in the process of orientation, that can hardly be 
 said to be included in the explanation of orientation by 
 selection of random movements, as described by Holmes. 
 
 It may now be asked: Is the orienting stimulus in this 
 form due to a change of light intensity or to the effect of 
 light acting constantly as a directive stimulus ? 
 
 There is no doubt that a change of intensity causes defi- 
 nite reactions in the earthworm, and that reactions thus 
 produced may result in orientation either by the selection 
 of random or trial movements, or by inducing more defi- 
 nitely prescribed movements, as in the blowfly larvae. But 
 this does not indicate that constant light cannot also pro- 
 duce orienting stimulations. There is however no evidence 
 showing that it does. Even in case a worm lies perfectly 
 quiet and very gradually starts to turn from the light when 
 laterally illuminated, as can be readily demonstrated, it is 
 impossible to say w^hether the stimulus causing the reaction 
 is due to the effect of constant intensity or to change of 
 intensity. And when the worm is in motion, ever extending 
 and contracting the anterior end and changing its position 
 so that the effective intensity is continually changing, it is 
 of course imi)ossible to predict what would take place if 
 the sensitive elements could be exposed to light having a 
 constant intensity. If light is thrown upon a specimen 
 which is perfectly quiet it begins to move, but in this case 
 
VERMES, FLY LARVAE, AND ECIIINODERMS 205 
 
 also it Is impossible to say whether the activity is caused 
 by a change of intensity or by constant intensity. 
 
 The idea of Verworn, Holt and Lee, Loeb, and Torrey 
 that when an organism is oriented both sides are equally 
 stimulated and consequently move at equal rates, and tliat 
 when it is not oriented the two sides are unequally stimu- 
 lated and therefore move at unequal rates, thus causing 
 orientation, has no experimental support in the reactions 
 of the earthworms. 
 
 Summary 
 
 (i) All the earthworms that react to light are ordinarily 
 negative. There is, however, some evidence that some at 
 least are positive in very low light intensity. 
 
 (2) They orient fairly accurately under some conditions 
 and move away from the source of illumination. 
 
 (3) The entire surface of the earthworms is probably 
 sensitive to light, but the anterior end is much more sensi- 
 tive to light than any other part of the body. Our evidence 
 Indicates that in Allolobophora orientation is entirely con- 
 trolled by the sensory elements in the first five or six seg- 
 ments. The anterior end is most sensitive when extended, 
 as shown by Harper. 
 
 (4) These animals have the power of differential response 
 to localized stimulation. Under certain conditions, if one 
 side Is illuminated, they always turn toward the shaded 
 side without preliminary movements and therefore orient 
 directly. 
 
 (5) They frequently swing the anterior end from side 
 to side continuously during the process of locomotion. If 
 light Is thrown on one side under such conditions the>' turn 
 the oral end sharply in the direction opposite that in which 
 it chances to be when it receives the stimulation. The>' 
 may therefore turn toward the source of light first and 
 become oriented only after several more prcliminar\' move- 
 ments. Or they may be in such a state that they are on]\' 
 stimulated when the anterior end is extended toward the 
 
2o6 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 source of light, not when It is turned in the opposite direc- 
 tion. I'nder such conditions they will of course never turn 
 sharply toward the source of light. The swinging of the 
 anterior end may h(3wever be c(jnsidered as a trial move- 
 ment and orientation consequentK indirect. 
 
 (6) The swinging nunements of I lie anterior end increase 
 the possible accuracy of orientation in case of direct as well 
 as indirect orientation. By means of them the animal takes 
 its bearing, if this anthropomorphic term may be permitted. 
 
 (7) The stimulations which lead to orientation are ordi- 
 narily un(iuestionably due to change of intensity on the 
 sensitive surface. 
 
 (8) Light may possibly have some effect on orientation 
 by acting constantly as a directive stimulus, but there is 
 no evidence indicating that it has. There is no evidence 
 showing that these animals are tropic in accord with Loeb's 
 definition. 
 
 4. Planar ia 
 
 It is well known that nearly all the planarians respond 
 to stimulation by light, but it is frequently assumed that 
 they orient only very indefinitely and that the effect of 
 light consists largely in making them more or less active. 
 Loeb (1906, p. 136) says, " If fresh-water Planarians are 
 put into such a circular glass dish, they show very little or 
 no tendency to move in the direction of the rays of light, 
 creeping along in an irregular manner and gathering not at 
 the negative or positive side of the jar, but on both sides 
 . . . where, on account of the refraction of light, the 
 intensity is a relative minimum." 
 
 More recent investigations have however shown that 
 many of these forms orient fairly accurately, and it is pri- 
 marily these investigations that concern us at present. 
 
 In the planarians the power of differential response to 
 localized stimulation seems to be more highly developed 
 than in the earthworms, and orientation in light seems to be 
 brought about largely by such responses. There are how- 
 
VERMES, FLY LARVAE, AND ECIIINODERMS 207 
 
 ever also numerous head movements which hear no definite 
 relation to the location of the stimulus. 
 
 Cole, studying the reactions of Hij)alium kevvense, to 
 sources of light of different size, refers to the i)rocess of 
 orientation as follows (1907, p. 365): "Like most i)lana- 
 rians, it creeps with an even, gliding motion, the head being 
 slightly raised and waved to right and left apparently in 
 searching movements, as the worm crawls forward. . . . 
 Bipalium kewense is exceedingly sensitive to light, (jf e\'en 
 a very low intensity, falling upon it from the side, and 
 responds immediately by turning away from the light." 
 
 Walter (1907) made an extensive study of the light reac- 
 tions of the following species: Planaria maculata; Planaria 
 gonocephala; Phagocata gracilis; Dendrocoelum lacteum; 
 and Bdelloura Candida. He found that all of these species 
 orient more or less accurately under certain conditions and 
 concluded that orientation " is primarily due to asymmetri- 
 cal response resulting from asymmetrical stimulation." He 
 also found that these animals frequently respond by raising 
 the anterior end and throwing it from side to side, and 
 noticed that such movements are caused either by sudden 
 increase or by sudden decrease of intensity. These mo\e- 
 ments, however, he thinks are functional in orientation only 
 " by assisting an organism to secure asymmetrical stimula- 
 tion " (1. c, p. 153). 
 
 I made some observations on the orienting reactions of 
 Leptoplana tremellaris and several other polyclads, all of 
 which were positive in their light reactions, and found that 
 all of these forms orient directly. There is no evidence of 
 preliminary trial movements in the process. Tlie lateral 
 head movements are always slight and frequently appar- 
 ently absent. When exposed to light from two sources all 
 of these forms crawl toward a point between the lights. 
 The location of the point toward which they move, however, 
 depends upon the relative intensity of light from the two 
 sources. It is always nearer the source from which the 
 more intense light comes. This indicates that orientation 
 
2o8 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 is regulated by the relative intensity of light on opposite 
 sides. 
 
 It ma\' now be asked: What is the cause of the orienting 
 stimulus ? Is it a change of intensity or constant intensity ? 
 There is direct evidence showing that the organism responds 
 both to changes of intensity and to constant intensity. If 
 a planarian passes suddenly from a region of a given inten- 
 sit\' into a region of a higher or lower intensity, it responds 
 by suddenly turning the head from side to side; and when 
 subjected .to constant light of different intensities it may 
 become more or less active. In working with fresh-water 
 planarians I frequently observed that if they were exposed 
 to constant illumination after they had been at rest for 
 some time, they did not respond for several minutes; when 
 they did respond they first moved very slowly and very 
 gradually became more active. Walter (1907, p. 63) records 
 similar observations. It may of course be argued that even 
 in this case it is the change of intensity due to turning on 
 the light that arouses the animals. It is however hardly 
 probable that it is, since the response is frequently not 
 apparent until after the animals have been exposed for 
 several minutes. It appears that a difference in constant 
 light intensity not only causes the planaria to become active 
 or to come to rest, but that it also affects the rate of move- 
 ment after they are active. 
 
 In experiments on Planaria gonocephala in constant illu- 
 mination of different intensities from directly above, Walter 
 (1907, p. 57) found an average rateoflocomotionof 0.57mm. 
 per second in darkness, 0.63 mm. per second in 431 ca. m. 
 The highest rate, 0.75 mm. per second, was in 39 ca. m. 
 It will thus be seen that the greatest increase due to a 
 difference in constant intensity is 0.18 mm. per second. 
 
 It is therefore clear that a change of intensity causes a 
 comparatively rapid response, and difference in absolute or 
 constant intensity a comparatively slow response. This leads 
 to a conclusion arrived at previously several times in these 
 chapters, that while an organism may be stimulated both 
 
I 
 
 VERMES, FLY LARVAE, AND ECIIIXODERMS 2 09 
 
 by a sudden change of light intensity and by constant inten- 
 sity, the processes involved are different. A sudden change 
 of intensity acts much like mechanical contact or a change 
 in an electric current. Constant intensity on the other 
 hand acts like constant temperature. 
 
 Our original questions still remain: Is orientation due to 
 light acting through change of intensity? Or is it due to 
 constant intensity, both sides of the organism being stimu- 
 lated constantly, but unequally when one side is more 
 highly illuminated than the other, thus causing difference 
 in rate of movement of the two sides ? The maximum dif- 
 ference in rate due to absolute difference of intensity, as we 
 have seen, is only 0.18 mm. per second. It is evident then 
 that the greatest difference in rate of locomotion on the 
 two sides due to absolute difference of intensity could not 
 be more than 0.18 mm. per second; and if orientation is 
 due to this it is clear that the orienting process would be 
 exceedingly slow, very much more so than it actually is. 
 
 Our evidence then indicates that orientation in these 
 forms is due not to light acting constantly as a directive 
 stimulus similar to the action of a constant electric current 
 in accord with Loeb's theory, but to reactions caused by 
 intermittent action of light through changes of intensity 
 on some part of the sensitive surface. These changes may 
 of course be due to the movements of the organism or to 
 changes in the direction of illumination. 
 
 It is evident that if the anterior end turns from side to 
 side, or if the ray direction Is changed, the Intensity on one 
 side becomes higher while that on the other becomes lower. 
 Is the orientation due to the former or to the latter? In 
 Euglena it was demonstrated that if the specimens are posi- 
 tive the orienting stimulus is due to a decrease of effective 
 intensity; if negative, to an Increase of effective intensity. 
 In Planaria there appear to have been no observations bear- 
 ing directly on this point. I frequently observed, however, 
 that when positive polyclads crawling toward a source of 
 light come into contact with a sharp shadow at either edge 
 
210 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 of the beam in which they are exposed, so as to shade one 
 side, they turn directly tr()in the shaded side, indicating 
 that the orienting stimulus in positive Planaria, as in Eu- 
 glena, is due to a decrease in effective intensity on some part 
 of the sensitive surface and that orientation is brought 
 about directly b>- differential response to localized stimuli. 
 
 (i) Planaria may collect in regions of optimum light 
 intensity either by wandering into such regions and coming 
 to rest or by orienting and crawling directly toward such 
 reg ons and coming to rest. The latter method is of course 
 more efifective than the former. 
 
 (2) In some forms orientation is very indefinite; in others 
 it is fairly accurate. 
 
 (3) In locomotion there are frequent lateral head move- 
 ments. These may be accelerated either by sudden increase 
 or by sudden decrease in light intensity. They appear to 
 be independent of the location of the stimulus. 
 
 (4) Orientation at least in some forms is due largely to 
 differential response to localized stimulation. The lateral 
 head movements no doubt function by increasing localized 
 stimulations, and thus make it possible for the organism to 
 direct its cour-e more efficiently than it otherwise could. 
 
 (5) Light acts on Planaria by virtue of both changes of 
 intensity and constant intensity. Responses to changes 
 of intensity are comparatively rapid; responses to constant 
 intensity comparatively slow. The effect of constant light 
 is similar to the effect of constant temperature. 
 
 (6) The orienting stimulus appears to be due to changes 
 of effective intensity on some part of the sensitive surface. 
 In positive specimens it is, as in Euglena, probably due to a 
 decrease, in negative specimens to an increase of intensity 
 on one side. 
 
 (7) There is no evidence indicating that light owing to 
 constant intensity is functional in the process of orientation. 
 
VERMES, FLY LARVAE, AND ECIIIXODERMS 211 
 
 5. Echinodcrms 
 
 The Echinoderms arc peculiar In that they can move 
 with any side ahead. In reversing the direction of move- 
 ment they ordinarily do not turn around but merely move 
 with the opposite side ahead. Some appear to be perma- 
 nently positive and others permanently negative, while still 
 others may be either positive or negative, depending upon 
 circumstances. 
 
 Washburn (1908, p. 131) says that the starfish and sea 
 urchins depend for their response to light upon pigment or 
 eye-spots on the arms. This conclusion is based largely 
 upon the observations of Romanes, who obtained no reac- 
 tions after the tips of the arms bearing these structures 
 were amputated. In some species however the response 
 appears to be independent of the eye-spots, for Cowles 
 found that in Echinaster crassispina the reactions to light 
 were normal three hours after one centimeter had been cut 
 from the tip of each arm. 
 
 Jennings (1907), working on Asterias forreri, a negative 
 starfish, found that it moves toward the shaded side no 
 matter whether the side is shaded by the substance in the 
 starfish itself or by some other object, showing that the 
 direction of motion is regulated by difference of intensity 
 on the surface regardless of the direction of the rays. If 
 illuminated from one side it therefore moves from the source 
 of light because the side opposite the light is shaded. If 
 the position of the source of light is changed it alters its 
 direction of motion at once, ordinarily by simply proceeding 
 with the side ahead which has become shaded. Bohn (1908), 
 however, working on several different species, found that 
 there is some tendency to turn after the direction of the 
 light is changed so that a given ray will be ahead. 
 
 The lack of orientation in moving from a source of light 
 is much more striking in the holothurlans. which are superfi- 
 cially at least much more definitely bilaterally symmetrical. 
 Pearse (1908, p. 278) describes the process in the holothu- 
 
212 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 rian Thyone briareus as follows: " In a series of twenty-four 
 reactions the locomotion in every case carried the ani- 
 mal away from the light to the end of the di^li, but there 
 was no definite orientation of the bod>- in relation to the 
 light. In ten of these negative responses the anterior end 
 was ahead as the indi\idual moved; in nine instances the 
 posterior end preceded the anterior; and in five the loco- 
 motion was straight toward the right or left. Not one of 
 the eight indi\iduals [used in this experiment] moved in 
 every case with the anterior or posterior end in front." 
 
 Very little is known about the actual mechanism involved 
 in tliese reactions. It seems clear however that it is the 
 shading of part of the body that produces the stimuli which 
 regulate the direction of motion, for if a shadow is thrown 
 on one side of a specimen of Asterias forreri, e.g., in such a 
 way as not to produce any change of intensity on the 
 exposed side, it moves toward the shaded side; and the 
 continued movement toward the shaded side seems to be 
 due to a constant difference in absolute light intensity on 
 opposite sides. If this is true the direction of movement is 
 regulated by light acting constantly as a directive stimulus. 
 It must however be borne in mind that there is no orienta- 
 tion in these organisms. They move with any side ahead 
 much like an amoeba. In Amoeba, it will be remembered 
 that the formation of pseudopods on the illuminated side 
 is probably checked by the action of the light, and that this 
 results in movement toward the shaded side of the organism. 
 It may be that in the echinoderms light has a similar effect 
 on the extension of the tube feet. If it has, direction of 
 movement is of course regulated by changes of intensity on 
 these structures. This idea is supported by the fact dis- 
 covered by von Uexkiill (1897) that a single spine or pedi- 
 cellaria connected with a piece of shell responds to stimuli 
 practically as it does in the entire animal, showing that the 
 parts of this organism are capable of independent action, 
 and the same is probably true of other organs in this form 
 and also in the other echinoderms. 
 
VERMES, FLY LARVAE, AND ECHINODERMS 213 
 
 After I had completed this part of the work, Cowles 
 reported to me personally that he found that positive star- 
 fish, when placed on the dorsal surface, always extend the 
 tube feet toward the shaded side of the body and turn from 
 the source of light, whereas in the normal position they 
 extend the tube feet toward the light. This seems to show 
 clearly that the direction of locomotion is not regulated by 
 the direct effect of light on these structures, as suggested 
 above. 
 
CHAPTER X 
 
 CONCERNING THE QUESTION OF ORIENTATION IN MOL- 
 
 LUSKS, ARTHROPODS AND VERTEBRATES WITH 
 
 SPECIAL REFERENCE TO CIRCUS MOVEMENTS 
 
 AND THEIR BEARING ON THIS QUESTION 
 
 I. General Account of Orientation 
 
 We have found in rnir work on the lower metazoa that 
 as the organisms become more complex and the structures 
 more highly differentiated, the power of differential response 
 to localized stimulation by light becomes more highly 
 developed and plays an increasingly more im])ortant part 
 in orientation; and trial and random movements become 
 of relatively less importance. In the mollusks, arthropods 
 and vertebrates, there is little evidence of preliminary trial 
 movements in the process of orientation in light. If the 
 ray direction is changed these forms turn directly until 
 their direction of motion bears the same relation to the 
 source of light it previoush- had. I found this method of 
 orientation to hold in the following forms: Limnea columella, 
 Cypris sp. (?), Daphnia sp.(?), Scapholeberis armata, zoeae, 
 several species belonging to the Anomura and Brachyura, 
 Caprella sp. (?) and Bufo americanus. The work of Cole, 
 Bohn, Parker, Holmes, Yerkes, Harper, Hadle>% Torelle 
 Radl and others shows the same to be true for numerous 
 other species belonging to these groups. 
 
 The fact that these forms orient in light without prelimi- 
 nary movements does not however indicate the absence of 
 trial movements when subjected to other stimuli and is no 
 argument against the " trial and error " h>^pothesis in gene- 
 ral as Bohn (1908, pp. 77, 82) seems to imply; nor does it 
 show how light acts as a stimulus. It probably means that 
 with reference to stimulation by light the power of differen- 
 
 214 
 
MOLLUSKS, ARTHROPODS AXD VERTEBRATES 21 
 
 
 
 tial response to localized stimulation has become so highly 
 developed that trial movements are largely eliminated. 
 Jennings (1906a, p. 453) makes the following characteris- 
 tically clear statement regarding this cjiiestion: " It is, of 
 course, very true, as Harper ('05) remarks, that definitely 
 localized reaction methj^ds are developed as we rise higher 
 in the scale, yet it appears to be equally true that if we 
 mean by ' trial and error ' the performance of varied move- 
 ments, subjecting the organism to varied conditions, certain 
 of which are selected, then this also becomes more highly 
 developed and more used by organisms as we ascend the 
 scale. We must not forget that this expression ' trial and 
 error ' was originally based on the behavior of such highly 
 developed organisms as the cat, dog and monkey ; and doubt- 
 less there is no organism \vhich uses this method to any such 
 extent as does man. Whenever the external conditions do 
 not furnish a precise determining factor for the movements 
 yet some sort of reaction is required, any organism is forced 
 to have recourse to this style of behavior, performing varied 
 movements till a condition is reached that relieves theorgan- 
 ism of the necessity of continuing these movements. In its 
 highest form we call this experimentation." 
 
 2. Circus Movements 
 
 In Euglena, Stentor and some of the other lower forms 
 it was demonstrated that the orienting stimulus is due to 
 a change of light intensity. Is there any evidence as to how 
 the local stimulus which leads to orientation in the higher 
 forms is produced ? 
 
 It has been found by a number of investigators working 
 on different forms that if one of two symmetrically located 
 sense organs is in any way prevented from functioning, the 
 organism no longer orients but continualh' turns toward 
 one side when stimulated. Loeb and others found this to 
 be the case in several different animals wilii one-half of 
 the brain destroyed. Holmes (1901, p. 220) found that the 
 
2l6 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 positive terrestrial amphipods and several different flies 
 with one eye blackened turn continuously toward the func- 
 tional eye. Parker (1903, p. 463) working on Vanessa 
 antiopa, and Radl (1903, p. 61) on the Hies Dexia carini- 
 frons and Musca d(jniestica, obtained similar results. The 
 fact that these organisms, all of which are positive, thus turn 
 continuously toward the functional eye seems to show that 
 the orienting stimulus is not necessarily and exclusively due 
 to a decrease of intensity in these forms, as it is in positive 
 Euglena and man\- other organisms. It may be due to the 
 continued action of light on the eye. A change of light 
 intensity does however undoubtedly produce a stimulus 
 which may result in orientation. 
 
 The performance of circus movements has frequently 
 been brought forward in support of Loeb's theory of orien- 
 tation stated in the following quotation (1906, p. 139): 
 ''It seems that in animals the region at the oral pole is, as 
 a rule, more sensitive than the rest of the body. Conse- 
 quently the tension of the muscles determining the position 
 of the head or oral pola is more intensely affected by dif- 
 ferences in the intensity of light than that of the muscles 
 of the rest of the body. The head is consequently bent 
 until its symmetrical photosensitive points are again struck 
 at the same angle by the rays of light. The tension of the 
 symmetrical muscles of the head then again becomes ecjual, 
 and the head must remain in this position unless other forces 
 disturb its orientation. The rest of the body follows the 
 orientation of the head." 
 
 The precision with which some organisms with but one 
 functional eye perform circus movements does indeed 
 appear to add support to this explanation of orientation. 
 Recent investigations have however throw^i considerable 
 doubt on the earlier interpretation of these movements. 
 It has been found that they are not so regular and constant 
 as was formerly supposed. Carpenter (1908, p. 486), experi- 
 menting on Drosophila with one eye blackened, observed 
 that they " crept in a fairly direct path toward the light, 
 
MOLLUSKS, ARTHROPODS AND VERTEBRATES 217 
 
 although a tendency to deviate toward the side of the nor- 
 mal eye regularly occurred." Radl (1903, p. 62) says, 
 " Die Calliphora vomitoria bewegt sich fast cbenso gerade 
 mit einem geschwarzten Auge, wie wenn sie auf beiden 
 sieht, und es ist mir nicht leicht, diese Erscheinung zu 
 erklaren." Holmes (1905) discovered that Ranatra with 
 one eye blackened at first deviates strongly toward the 
 functional eye in going toward a source of light, but that 
 this deviation decreases and that the path becomes much 
 more nearly direct after repeated trials, indicating that the 
 animal learns to adjust itself to the new conditions and that 
 its reaction mechanism is not so simple as Loeb's theory 
 demands. This is still more clearly demonstrated l)>- I lie 
 interesting observations of Holmes on the fiddler crab, Uca 
 pugnax. I cannot do better than to quote his conclusions 
 based on these observations (1908, p. 496): " The point of 
 principal interest in the phototaxis of the fiddler crabs is the 
 relation of their lateral orientation to the theories of tro- 
 pisms. Can we regard orientation as a direct response in 
 which the animal is involuntarily forced into line, or is it 
 rather to be considered as coming under the pleasure-pain 
 type of behavior, and as therefore related to the voluntary 
 seeking of a certain end which is exhibited in the beha\'ior 
 of higher forms ? In order to explain the orientation of a 
 highly organized form like an insect or crustacean in which, 
 in most cases, response to light takes place through the 
 eyes, we may assume that light falling more strongly on one 
 eye sets up impulses which are transmitted more or less 
 directly to the leg musculature. We may assume that the 
 extensors of the opposite side are stimulated, or the flexors 
 on the same side, or both, and that in consequence of this 
 distribution of impulses the animal moves until its body 
 is in line with the rays. In such a case the movements 
 involved in orientation are the same as those employed in 
 ordinary locomotion, only the activity of the legs on one or 
 the other side is accentuated according t(^ the position of 
 the body in relation to the direction of the rays. 
 
2l8 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 " In the fiddler crab, however, the case is different, and 
 we cannot explain the phenomenon in this way. The legs 
 of the fiddler move in a plane approximately at right angles 
 to the sagittal plane of the body, but they are capable of a 
 certain amount of forward and backward motion which 
 may be employed to change the direction of locomotion. 
 The movements in^•ol\•ed in orientation are different from 
 those employed in ordinary rimning. They are special 
 movements emi:)loyed to check deviations from a certain 
 course, a circumstance which would greatly complicate any 
 attempt to explain orientation as a comparatively direct 
 response. The results of observations on fiddler crabs tend 
 to conhrm the conclusion reached in studies made on the 
 phototaxis of Ranatra, namely, that light is followed much 
 as an animal pursues any other object of interest, such as 
 prey, or its mate, and until we can give a physiological 
 explanation of these phenomena we are not, I believe, in a 
 position to give a satisfactory explanation of orientation to 
 the direction of the rays of light." 
 
 A similar idea regarding reactions to light was expressed 
 by Graber much earlier. He says (1884, p. 248), " Um ein 
 analoges Beispiel aus einer andern Sinnessphare anzufiihren, 
 so benimmt sich hier [exposed to light differing in color or 
 intensity] die Raupe offenbar ganz ahnlich wie in dem Fall, 
 wenn ihr als Futter einerseits Nesselkraut und andererseits 
 irgend cine andere Pflanze vorgesetzt wird, indem sie con- 
 stant das letztere verschmaht und das erstere ergreift, und 
 in dem Sinne konnen wir also auch ganz gut von einem 
 Farbengeschmacke reden." 
 
 3. Frogs and Toads 
 
 The conclusions of Holmes as far as they refer to orienta- 
 tion in animals with image-forming eyes are strongly sup- 
 ported by the (observations of Miss Torelle on the response 
 of the frog to light and by the orienting reactions of the 
 American toad Bufo americanus described below. Miss 
 
MOLLUSKS, ARTHROPODS AXD VERTEBRATES 219 
 
 Torelle says (1903, p. 471): " Tests were made at midday 
 on a level tract of ground about two acres in extent which 
 contained neither trees nor any object that could cast a 
 shadow. Six frogs were tried. When freed, each moved 
 indifferently toward any point of the compass, but usually 
 kept on moving in the direction in which it began to move. 
 In several trials no movement resulted; the frog crouched 
 low between short bunches of grass, its head held close to 
 the ground. When dark black or dark brown screens were 
 placed in the middle of this area and the frogs placed 
 within five yards of them, the movement was toward and 
 into the shadow of the screen, where they usually remained 
 indefinitely." In various other experiments it was found 
 that frogs exposed in direct sunlight hopped toward shadows 
 in the neighborhood, no matter if this required movement 
 perpendicular to the rays of light. When first exposed the 
 frogs turned toward the light, but after being in this position 
 a few moments they turned and hopped toward the shadows. 
 After they are in the shade they usually turn so as to face 
 the light. These reactions seem to show that the frogs go 
 toward the shadow because they see and perceive it. The 
 following reactions of toads lead to the same conclusion. 
 
 A. Bufo americaniis 
 
 a. Method. — Two horizontal Nernst glowers were so 
 arranged in a large dark room that the rays crossed at right 
 angles above a black table one meter square. The two 
 beams of light, 20 cm. wide at the place of intersection, 
 were parallel with the plane of the table, and the lower edge 
 of the beams was just high enough to clear the table, which 
 was therefore not illuminated. Both beams were absorbed 
 by the dead black walls of the room, which were several 
 meters from the table. The light intensity in the middle of 
 the field from one glower was 12.5 ca. m., and that from 
 the other was 25 ca. m. 
 
 h. Orientation in light from two sources. — On Jul>' 7, 
 at 7 P.M., seven toads, two large ones and five small ones, 
 
2 20 LIGHT AXD THE BEHAVIOR OE ORGAXISMS 
 
 were collected and brought to the laboratory. These speci- 
 mens were exposed one at a lime in li^ht from a single 
 source; they all oriented directly and fairly accurately. If 
 placed on the table in the beam of light so that one side 
 faced the glower they turned slowly but directh' imtil tliey 
 faced the light and then hopped or walked toward its source, 
 stopping frecjuently for a few moments at intervals on the 
 way. The light from both glowers was now turned on and 
 the specimens were exposed, one at a time, in the beam 
 having the lower intensity, in such a position that after 
 they oriented and mo\-ed toward the source of light they 
 soon reached the more intense lateral light from the second 
 glower, which of course illuminated one side. I was some- 
 what surprised to find that ordinarily there was no apparent 
 reaction whatever when the specimens reached the beam 
 of lateral light, although the intensity of this was twice as 
 high as that in which they were oriented. The toads con- 
 tinued on their way just as though there had been no 
 lateral illumination. All the other organisms studied in 
 light from two sources proceeded toward or from a point 
 between the sources of light. The toads always went 
 directly toward one or the other of the two sources. 
 
 Each of the seven specimens under observation was 
 studied while it crossed the field six times, and all but one 
 continued toward the glower which produced the beam in 
 which they were first oriented, without any perceptible 
 deviation on account of the lateral light from the second 
 glower. Only one specimen turned when it reached the 
 lateral illumination. It did not however go toward a point 
 between the two glowers as might have been expected, judg- 
 ing from results of previous experiments on other forms. 
 It proceeded directly toward the glower which produced 
 the lateral illumination. In no instance was there any evi- 
 dence of movement toward a point between the two 
 sources. 
 
 These results do not support Loeb's recent statement 
 regarding orientation in light from two sources. He says 
 
MOLLUSKS, ARTHROPODS AND VERTEBR^iTES 2 21 
 
 (1909, p. 13), " Sind zwei glcich starkc Lichtqucllen in 
 gleichem Abstand vom Tier vorhandcn, so bewegt sich das- 
 selbe senkrecht zur V'erbindungslinic dcr bciden Liclu- 
 quellen weil dann bcidc Augcn in gleicher Weise voni Liclit 
 beeinflusst werden Darin untersche det sich, wie Bohn 
 richtig bemerkt, die maschinenmassige heliotro[)ische Reak- 
 tion der Tiere von der nicht durch Heliotropismus bedingten 
 Bewegung eines Menschen zu einer von zwei Lichtqucllen." 
 Toads, exposed to light from two sources, as slated above, 
 do not proceed toward a point between them. Judged by 
 the criterion of Loeb and Bohn, their reactions under these 
 conditions are therefore like those of a human being under 
 similar conditions and not Hke those of other animals 
 ( Tiere) . 
 
 c. Orientation with one eye destroyed. — Toads with 
 the lenses ^ removed from the eyes so that the power of 
 forming images is destroyed, frequently orient fairly accu- 
 rately and hop or walk toward a source of light. When 
 exposed to light from two sources they move toward a 
 point between them, contrary to what occurs in specimens 
 that can see. 
 
 In specimens with one eye destroyed there is a slight 
 tendency to deflect toward the injured eye. The head in 
 such specimens is inclined toward the blind side as though 
 
 ^ No special precaution was taken to destroy the retina in the eyes from 
 which the lens was removed. This seemed unnecessary since I was not 
 primarily interested in the question of distribution of sensitive elements. 
 Parker (1903 and 1905) demonstrated very clearly that the skin of some 
 frogs and fishes is sensitive to light, and Pearse (1910) showed the same for 
 various amphibia. ISIany of these creatures can orient fairly accurately 
 with the retina of both eyes entirely destroyed. My aim was to test the 
 effect on orientation of unequal stimulation on opposite sides. In the toad 
 with the lens removed, the eye fills with a substance which, while it may not 
 be absolutely opaque, certainly intercepts nearly all the light, so that even 
 if the retina is functional in both eyes, one receives much more light than 
 the other. The important point here is that when the toads with the lens 
 of one eye removed as described in the text are oriented and move toward 
 the light they are not equally stimulated on symmetrically located sensitive 
 points. 
 
222 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 the animals were trying fully to expose the intact eye and 
 still travel toward the li^ht. Hadley (1908, p. 187) ob- 
 served somewhat similar reactions in positive lobster larvae 
 with one eye removed, as did also Miss Torelle (1903) in 
 frogs with one eye covered. Thus it will he seen that the 
 toads and frogs and lobster larvae tend to deflect toward 
 the blind eye, while butterflies, amphipods and flies tend 
 to deflect toward the functional eye. 
 
 On June 27 several toads were exposed to light from one 
 glower, and from these the three which oriented most accu- 
 rateh' were selected. The lens was then removed from one 
 eye in each toad. The following day they were exposed 
 250 cm. from a 50 candle power Nernst glower in a beam of 
 light 45 cm. wide. The beam of course became narrower 
 as the source of light was approached. If set down so that 
 one side was illuminated all of these specimens turned 
 directly so as to face the source of light, but in all the head 
 was turned toward the blind side so that there was a distinct 
 curve in the spine, and after turning thus they hopped or 
 walked toward the glower, deflecting slightly toward the 
 injured side. They almost always reached the edge of the 
 beam of light before getting to its source and one passed 
 out into the shadow a few times. 
 
 On July 14 the lens was removed from one of the eyes 
 of a large active toad which had oriented rather accurately. 
 The toad was then immediately exposed in the beam of 
 light. It hopped toward the glower apparently as accu- 
 rately as it did before the operation, but there was a tend- 
 ency to turn the Intact eye toward the light whenever it 
 came to rest after each leap. The light intensity was much 
 reduced but still the specimen went directly toward the 
 glower. The following day this toad was again exposed; 
 it now went toward the source of light even more nearly 
 directly than on the preceding day. There was no longer 
 any appreciable tendency whatever to turn toward the 
 blind side. 
 
 These results show that in this form and in all the other 
 
MOLLUSKS, ARTHROPODS AND VERTEBRATES 
 
 223 
 
 forms which orient after one eye is destroyed difference of 
 effective intensity on opposite sides does not reguhite ori- 
 entation. " The head is . . . [not necessarily) bent until 
 its symmetrical photosensitive points are . . . struck at 
 the same angle by the rays of light," nor is it necessary 
 that both eyes be influenced alike, as Loeb's theory de- 
 mands. It appears that these organisms, as Holmes says, 
 referring to the fiddler crab, follow light " much as an 
 animal pursues any other object of interest, such as pre>-, 
 or its mate." 
 
 Graber (1884, p. 226) found that Rana esculenta tends to 
 collect in light of relatively low intensity and we have seen 
 that Miss Torelle found that the frogs she studied tended 
 to collect in shaded places, but Parker (1903) and Cole 
 (1907) found that when frogs are exposed to light in a dark 
 room they are positive, apparently regardless of the inten- 
 sity of the light. I found the same true with reference 
 to Bufo. How can these contradictory phenomena be 
 accounted for? In a dark room containing but a single 
 compact source of light such as was used by Parker, Cole 
 and myself in these experiments, it is not likely that an 
 animal can see anything but the source of light. If then 
 the frogs and toads are guided by sight in their movements, 
 as their reactions indicate, it is evident that they must go 
 toward the source of light if they go at all. In these reac- 
 tions light no doubt acts continuously as an orienting stimu- 
 lus, and the direction of the rays must necessarih'. if ihey 
 go toward an object because they see it, guide ihem on 
 their way. This of course does not imply thai the reac- 
 tions are controlled by psychic phenomena. The process 
 of orientation is however, without doubt, far more compli- 
 cated than the theory of Loeb demands. 
 
 4. Caprella 
 
 What can be said with regard to orientation in the 
 higher forms with eyes incapable of forming images ^ Such 
 animals are of course not able to see. They cannot follow 
 
224 
 
 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 an object because of its size or orm. The eyes function 
 merely in distinguishing cHlYerent degrees of intensity or 
 movement perhaps. Caprella seems to possess eyes of this 
 kind. At an>- rate when exposed to Hght from two sources 
 it swims toward any point l^etween them, depending upon 
 
 Fig. 32. Camera outline of Caprella sp. (?). A, dorsal view; B, side view, 
 showing the extent of bending during locomotion. See text. 
 
 their relative intensity, indicating that orientation is not 
 regulated by the same factors as in the toad. 
 
 An idea of the general form and structure of the form on 
 which the following observations were made can be gained 
 by referring to Fig. 32. This creature is found in abun- 
 dance attached to Eudendrium colonies. I did not ascer- 
 tain the species. 
 
 a. Orientation. — While attached this form shows no 
 indication of orientation, but when free it usually swims 
 toward a source of light fairly accurately. When it swims 
 slowly the anterior end, with its appendages spread out, is 
 thrown toward the ventral surface until it is nearly at right 
 
MOLLUSKS, ARTHROPODS AND VERTEBR.ITES 225 
 
 angles with the rest of the body, after which it returns more 
 slowly with the appendages partially folded. When it 
 swims rapidly the posterior end strikes backward at the 
 same time that the anterior end straightens, and moves 
 forward when the anterior end strikes backward. Thus the 
 creature alternately bends and straightens in rapid succes- 
 sion and forces itself through the water. Ordinaril}- it 
 swims on either the right or the left side. When it changes 
 its direction of motion it always turns toward the dorsal 
 surface. If the ray direction is changed it turns directly 
 toward the source of light if the dorsal surface is illumi- 
 nated, but if the ventral surface faces the light it first 
 rotates on its long axis through 180° and then turns. 
 
 Orientation is not brought about by unequal action of 
 symmetrically located sensory and locomotor organs. When 
 the organism turns, either the entire anterior end alone or 
 both ends must strike differently than when it swims 
 straight ahead. The appendages on opposite sides of the 
 body act the same. The organism as a whole must be 
 involved in the process of orientation. This cannot be due 
 to a " compulsory automatic turning of the head " toward 
 the source of light, in accord with Loeb's explanation of 
 orientation. In some way or another these creatures do 
 keep the anterior end directed toward the general source 
 of illumination. Just how this is done I am unable to say. 
 It may be that the organism receives a localized stimulation 
 when it turns so that the anterior end is no longer directed 
 toward the light, and orients by means of a ditTerential 
 response to such a stimulation. There certainl\' is no evi- 
 dence indicating that light acts constantly as a directive 
 stimulation, nor is there any indicating that " the head 
 is . . . bent until its symmetrical photosensitive points are 
 . . . struck at the same angle by the rays of light." The 
 process of orientation is by no means as simple as these 
 explanatory quotations from Loeb demand. 
 
 b. Discussion. — Orientation in Corethra larvae, wliich, 
 according to the work of Harper (1907) bend sharply alter- 
 
2 26 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 nately to the right and left in the process of locomotion, 
 supports the conclusion reached in the study of Caprella, 
 as do also the orienting reactions of zoeae and lobster larvae. 
 Lobster lar\-ac in the earlier stages always orient with the 
 posterior end toward the source of light. When they are 
 positive they swim toward the light with this end ahead. 
 When they are negative they swim in the opposite direction 
 with the anterior end ahead. Hadley (1908, pp. 264-276) 
 showed that if the direction of the rays is changed in any 
 wa>' after tiie larvae are oriented, they immediately and 
 directly turn until the anterior end is shaded again. If the 
 position of the source of light is so changed that the dorsal 
 or the ventral surface is exposed, it is evident that the sides 
 may still be equally illuminated. If the dorsal surface, for 
 example, becomes illuminated when the ray direction is 
 changed they turn toward the ventral surface. This shows 
 very clearly that orientation in these forms is not regulated 
 solely by relative intensity of light on symmetrically located 
 structures, for the illumination on opposite sides throughout 
 this whole reaction may have been equal. I observed simi- 
 lar reactions in the zoeae of several different species of the 
 Brachyura and Caridea. In some of these animals the eyes 
 extend laterally beyond the surface of the body so far that 
 both are illuminated no matter whether the anterior or 
 posterior end is directed toward its source. 
 
 In all these forms it is evident that the turning toward the 
 dorsal or the ventral surface, when the source of light is 
 lowered or raised, must be due to the fact that different 
 portions of the eyes become illuminated, unless it is regu- 
 lated by vision. If it is, the former orientation appears to 
 be a differential response to a localized stimulation. The ob- 
 servations of Holmes and others indicate that orientation in 
 a number of other arthropods also depends upon the surface 
 of the eyes exposed. Holmes (1905) covered different parts 
 of the eyes of Ranatra with an opaque substance and found 
 that it responds just as though the environment in the 
 direction of the blackened portion of the eyes were dark. 
 
MOLLUSKS, ARTHROPODS AND VERTEBRATES 227 
 
 The fact that many animals with image-forming eyes 
 respond to size of the luminous area rather than to intensity 
 difference as shown by Parker (1903) and Cole (1907), is 
 not at all opposed to the idea of Holmes stated above. In 
 this fact we have an answer to the much-discussed and 
 perplexing question as to why moths fly toward a candle at 
 night and not toward the moon. The reason clearly is that 
 in moonlight there arc large illuminated areas all about, 
 while in candle light the objects about are so faintly illumi- 
 nated that the moths do not react to light reflected from 
 them. But this answer is clearly superficial. The question 
 is: Why do the Mourning-cloak butterflies, e.g., fly toward 
 large illuminated patches rather than toward the sun, which 
 is much brighter ? Parker (1903, p. 465) says it is because 
 the larger area makes a larger " spot on the retina." " I 
 therefore believe that Vanessa antiopa stays near the ground 
 on bright sunny days because its flight is directed by large 
 bright retinal spots rather than by small ones, even though 
 the latter are of vastly greater intensity." These answers 
 are no doubt correct, but they are not fundamental. The 
 reactions referred to above are in general adaptive, and an 
 explanation of them must be sought along the same general 
 lines as an explanation for any other question involving 
 adaptation. It is frequently vsaid that organisms in water 
 are limited in their movements toward the sun by the sur- 
 face of the water, but that insects flying in air are not thus 
 limited. Referring to Labidocera Parker says (p. 462): 
 "Their positive phototropism is held in check by their inabil- 
 ity to pass above the surface of the water. No such barrier 
 holds the butterfly to the earth." It is evident that this is 
 true only in a restricted sense. Insects flying up in the air 
 soon find their limit, and of course no one knows how many 
 have tried this very thing during the process of fixing the 
 adaptive habit of reacting positively to large luminous 
 areas rather than to small ones of much higher intensity. 
 Thus it may be that they have learned to go toward the 
 larger luminous areas in preference to smaller ones much 
 
2 28 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 as they have learned to follow their mates or other objects 
 of interest. 
 
 The fact that an insect Hies into a candle flame at night 
 or a bird against a lighthouse tower and leases its life does 
 not indicate that its light reactions are not in general adap- 
 tive, as many assume. One might as well say, if a creature 
 in a room flies toward a window and is injured by striking 
 the glass, that its behavior is not adaptive. The environ- 
 ment in both cases contains artificial factors which animals 
 rarely experience. It remains for future investigators to 
 demonstrate whether or not insects and birds can learn 
 to avoid such pitfalls as candles and lighthouses. 
 
 Whether or not the flight of birds against a lighthouse is 
 tropic, as Cole suggests in the following quotation, depends 
 upon the sense in which the term tropism is used: (1907, 
 p. 410) " The way in which migrating birds often, on stormy 
 nights, gather about lighthouses and dash into the glass only 
 to be killed, recalls strongly the flying of moths into a flame, 
 and it seems possible that this is an expression of photo- 
 tropism in birds which is ordinarily inhibited by other 
 responses." It seems probable that all creatures which fly 
 are guided on their course by sight at least with reference 
 to their immediate environment. It is well known that 
 birds seldom fly against the lighthouse windows unless the 
 night is dark and stormy. Under such conditions the light 
 intensity all about is very low, so that when the birds get 
 near the lighthouse they can see nothing but the light in it, 
 and if they are to fly toward something they can see, they 
 must evidently fly toward this light. If this is true the 
 factors involved in this phenomenon are similar to those 
 involved in regulating their flight toward any other object. 
 
 5. General Summary and Cojiclusions 
 
 The several summaries at the close of the difl'erent sec- 
 tions in this part and the table of contents should be re- 
 ferred to for a general idea of the subjects treated and the 
 
 I 
 
MOLLUSKS, ARTHROPODS AND VERTEBRATES 229 
 
 results and conclusions reached. Here we aim only to 
 bring together the more important factors involved in the 
 process of orientation in different organisms. 
 
 (i) The plumules of Zea mays and i)robahly oi ail the 
 other gramineae bend toward the more highly illuminated 
 side of the sensitive region regardless of the direction of the 
 rays. Our experimental results do not bear on the question 
 as to whether or not orientation is due to a modification of 
 circumnutating movements as maintained by Darwin. Nor 
 do they warrant a conclusion as to whether the stimulation 
 causing bending is due to a change of intensity on some 
 part of the sensitive region in accord w^ith Darwin's sug- 
 gestion, or to constant intensity, all sides being continuously 
 stimulated in proportion to the intensity to which they are 
 exposed, in accord with the theories of De Candolle, Loeb, 
 Verworn and others. The bending may be due to a dif- 
 ferential response to a localized stimulation. 
 
 (2) In the myxomycetes and rhizopods orientation is in 
 all probability due to a local response to a local stimula- 
 tion. Light retards the activity of the protoplasm and 
 thus prevents the formation of pseudopods on the more 
 highly illuminated side. It is impossible to say whether 
 light acts constantly as a directive stimulation, all parts of 
 the organism being continuously stimulated in proportion 
 to the absolute intensity to which they are exposed (Loeb, 
 Verworn, etc.), or whether it acts only through changes of 
 intensity, the prevention of formation of pseudopods on the 
 illuminated side being due to the increase in light intensity 
 on the protoplasm as the pseudopods are thrust out. 
 
 (3) In Euglena, Stentor, Trachelomonas, Chlamydomo- 
 nas, Chlorogonium, Oedogonium swarm spores and prob- 
 ably in all other ciliates and flagellates which orient in 
 light, orientation is due to definite responses to changes of 
 light intensity on the sensitive part of the organism. The 
 changes of intensity are ordinarily due to the movement of 
 shadows of one part of the body over another part. This 
 is caused largely by the rotation on the long axis. In 
 
230 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 Euglena and Trachelomonas the most highly sensitive part 
 is probably restricted to a relatively small mass of proto- 
 plasm in close proximity with the concave surface of the 
 eye-spot. The eye-spot probably functions in shading this 
 structure when its convex surface faces the light. The 
 eye-spot may also function by absorbing light in a manner 
 similar to that in the choroid coat in the eye. In Chlamy- 
 domonas and the volvocineae, judging from its location, the 
 eye-spot cannot function by shading the sensitive part of 
 the organism, as it appears to in Euglena. In these forms 
 it can only function by absorbing light, if it functions in 
 light reactions at all. 
 
 The difference in sensitiveness with different surfaces 
 exposed is surprisingly great in Euglena and Stentor, and 
 probably in all the other ciliates and flagellates. In Stentor, 
 under the conditions of the experiment it requires an in- 
 crease in intensity from 150 ca. m. to 444 ca. m. to induce 
 a reaction when the posterior end faces the light, while a 
 change from a position in which the aboral to one in which 
 the oral surface is exposed causes a reaction without any 
 change of intensity in the field, showing the marked effect 
 of the shadow of the former surface on the latter. There is 
 no evidence that the direction of the rays functions except 
 in so far as it may affect change of intensity. Nor is there 
 any evidence that light acts constantly as a directive stimu- 
 lus similar to the effect of the constant current in accord 
 with Loeb's theory of tropisms. 
 
 In Euglena the avoiding reaction is not a differential re- 
 sponse to a local stimulus, for in the crawling state it bends 
 toward the ventral surface, while in the free-swimming 
 state it bends toward the dorsal surface. The direction 
 of bending is governed by internal factors. The reac- 
 tions caused by changes of intensity result in directing the 
 organisms toward various points of the compass. As soon 
 as they reach a position in which the rotation on the long 
 axis no longer causes a change of intensity on the sensitive 
 region there is no longer any cause for turning; they there- 
 
MOLLUSKS, ARTHROPODS AND VERTEBR^iTES 231 
 
 fore continue in this direction. Orientation in these forms 
 takes place in principle precisely as Jennings' description 
 demands. 
 
 (4) The same factors are involved in the process of 
 orientation in the colonial forms, VoKox, Kudorina and 
 Pandorina, as are involved in the orientation of the ciliates 
 and flagellates. The orienting stimulus is due to a change 
 of intensity on some part of the zooids. This is due (i) to 
 the change in the surface of the zooids exposed as the 
 colonies rotate, and (2) to the transfer of the zooids from 
 the illuminated side of the colony to the shaded side 
 and vice versa. The change of intensity causes a definite 
 response in the zooids, a shock movement or avoiding reac- 
 tion, which consists in an effort to turn toward the side fac- 
 ing the anterior end of the colony. Owing to the wa\- in 
 which the zooids are united there are no errors in the process 
 of orientation in the colonies as a whole. They never turn 
 in the wrong direction as many of the protozoa frequently do. 
 The reaction of Volvox in light is not like the reaction of 
 this form in a constant electric current, as Bancroft assumes. 
 
 (5) Hydra viridis moves fairly accurately toward or from 
 a source of light. When attached it does however not take 
 a definite axial position with reference to the direction of 
 the rays. It may bend toward any point of the compass. 
 There is no definite relation between the direction of bend- 
 ing and the side illuminated. But the anterior end is 
 directed toward the source of light a greater part of the 
 time, and the animal usually travels only in this direction. 
 When the anterior end is shaded the animal tends to turn, 
 but when this end is illuminated it tends to travel. This 
 indicates that the tendency to become oriented and the 
 tendency to travel are not the result of the same factors. 
 The former may be due to the stimulation b\- light owing 
 to a change of intensity, the latter to the action of light 
 owing to its constant intensity. There is no evidence indi- 
 cating that the reaction in light is the same as the reaction 
 in a constant electric current. 
 
232 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 All the other coelenterates studied orient directly. We 
 are however unable to say how light produces the orient- 
 ing stimulus. Orientation probably is due to a differential 
 response to a localized stimulation. 
 
 (6) In Arenicola larvae orientation takes place much as 
 it does in the flagellates and ciliates; but the larxae can turn 
 directly toward the right or the left side. They appear to 
 have the power of differential response to localized stimula- 
 tion; the stimulation is probably due either to a reduction 
 of light intensity on the shaded side, or to an increase of 
 intensity on ihe illuminated side. 
 
 In the blowfly larvae only the very tip of the anterior 
 end is sensitive to light. They turn this tip to the right 
 and left alternately during the process of locomotion. If it 
 becomes more highly illuminated as it turns toward either 
 side it is turned farther in the opposite direction. The 
 larvae apparently continually test their position with refer- 
 ence to the light. The lateral head movements make it 
 possible for these creatures to orient much more accurately 
 than they otherwise could. The orienting stimulus is un- 
 doubtedly due to an increase of intensity. 
 
 The earthworms can turn directly from the light. They 
 probably have the power of differential response to localized 
 stimulation by light. The random movements of the ante- 
 rior end may serve to localize the stimulation. The direc- 
 tion of movement is, however, not entirely determined by 
 localized stimulation. It is largely influenced by internal 
 factors. The worms frequently turn toward the source of 
 light when stimulated. The " selection of random move- 
 ments," essentially as described by Holmes, is an important 
 factor in orientation. 
 
 The anterior end of the earthworm is much more sensitive 
 to light than the rest of the body. Lateral illumination of 
 any portion of Allolobophora back of the sixth segment does 
 not appreciably affect orientation, although this part of the 
 animal is undoubtedly sensitive to light. 
 
 The planarians orient directly. A sudden change of 
 
MOLLUSKS, ARTHROPODS AND VERTEBRATES 233 
 
 intensity causes definite lateral head ni(jvements. In posi- 
 tive specimens, where one side is shaded, they turn toward 
 the side which is not shaded, indicating that it is a decrease 
 of intensity on the shaded side that causes orientation. 
 
 There is no evidence indicating that the direction of the 
 rays or the angle between the rays and the sensitive surface 
 functions in orientation in any of the forms referred to 
 under vermes, except in so far as it may cause change of 
 intensity; nor is there any evidence that light acts con- 
 stantly as an orienting stimulus. 
 
 The echinoderms do not orient; they can move with any 
 part of the body ahead. When the direction of the light 
 is changed they change the direction of motion by moving 
 with another part of the body ahead. Precisely how the 
 direction of motion is regulated is unknown. 
 
 (7) The mollusks, arthropods and vertebrates all orient 
 directly. There is little evidence of preliminary trial move- 
 ments in this process in these forms. Orientation is prob- 
 ably due to differential response to localized stimulation. 
 In some of the positive mollusks the orienting stimulus 
 appears to be due to a decrease of intensity on the shaded 
 side. With reference to the way in which light produces 
 the stimulation in the other forms in these groups our 
 evidence does not warrant a conclusion. 
 
 The animals with image-forming eyes no doubt orient 
 and go toward a source of light much as they go toward 
 any other object of interest to them. 
 
 (8) Orientation may then be due (a) to local response 
 to local stimulation, as in the rhizopods and nnxomycetes; 
 (b) to shock movements, avoiding reactions, as in the cili- 
 ates, flagellates, colonial forms, vermes and larvae of various 
 kinds; (c) to differential response to localized stimulation, 
 as in some of the coelenterates, the vermes and all the 
 higher forms; (d) to sight, as in many animals with image- 
 forming eyes. 
 
 (9) No evidence was found indicating that the direction 
 of light in the field or through the tissue (Sachs) functions 
 
2 34 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 in orientation of any organisms without image-forming eyes 
 except in so far as it may produce difference of intensity on 
 different parts of the organism. Those organisms which 
 orient by sight are of course guided by the direction of the 
 rays reflected from the object toward which they go. 
 
 (10) There is no e\idcnce indicating that symmetrically 
 located points on the surface must be struck by light at the 
 same angle when organisms are oriented, as Loeb maintains 
 (1905). The only organisms which do not travel toward or 
 from a point located anywhere between two sources of light 
 are those with highly developed image-forming eyes. Such 
 forms, as far as experimental evidence indicates, always go 
 toward one or the other of the two sources of light. With 
 but one functional eye these animals still orient fairly 
 accurately, although under such conditions symmetrically 
 located points are unequally stimulated. 
 
 (11) There is no conclusive evidence, except perhaps in 
 animals with image-forming eyes, showing that light acts 
 continuously as a directive stimulus, that symmetrically 
 located sides are continuously stimulated, — equally when 
 the light intensity on them is equal, unequally when it is 
 not, and that this regulates orientation by regulating the 
 rate of motion of the locomotor apparatus on the two sides 
 as is demanded by the theories of De Candolle, Loeb, Ver- 
 worn, Davenport and Radl. 
 
 (12) There is no conclusive evidence showing that orien- 
 tation in light is ever due to tropic reactions in any organ- 
 isms, if the definitions of tropisms given by Loeb, Verworn, 
 or Radl are used as criteria. 
 
 (13) All organisms that respond to light at all respond 
 to changes of intensity. In some the response to such 
 changes results in orientation, in others it does not. They 
 are all tinterschiedseinpfindUch, in accord with Loeb's defi- 
 nition of this term. They all respond to time rate of 
 change in light intensity. The idea of reactions to change 
 of intensity is however not original with Loeb, as is some- 
 times assumed. The explanations of reactions to light 
 
MOLLUSKS, ARTHROPODS AND VERTEBRATES 235 
 
 given by Engelmann, Bert, Graber, Lubbock, Romanes, 
 Darwin and others, all of whom preceded Loeb, were 
 largely founded on this idea. 
 
 (14) Light no doubt acts on organisms without a change 
 of intensity much as constant temperature does, making 
 them more or less active and inducing changes in the sense 
 of orientation; but there is no conclusive evidence showing 
 that light acting thus ever functions in the process of 
 orientation. 
 
PART III 
 
 GENERAL COXSIDERATION OF REACTIONS 
 
 TO LIGHT 
 
 CHAPTER XI 
 
 ADAPTATION, FORMATION OF AGGREGATION IN REGIONS 
 
 OF A GIVEN LIGHT INTENSITY AND DIFFERENT 
 
 METHODS OF RESPONSE IN ATTAINING THIS 
 
 REGION AND REMAINING IN IT 
 
 I. Introduction Showing that Reactions in General 
 
 are Adaptive 
 
 Thus far we have dealt almost exclusively with the mech- 
 anism of orientation. In this and the following chapters 
 we shall deal with reactions to light from a much broader 
 point of view. We shall consider not only the various differ- 
 ent methods of response leading to a general classification, 
 but also the various factors which control the difTerent 
 responses, leading to a discussion of the nature of stimula- 
 tion and the cause of reactions. 
 
 Under natural environmenta conditions organisms are 
 usually found in places well suited for the continuance of 
 their life processes. Sometimes they crowd together and 
 form dense aggregations in such regions. This is especially 
 true in case of unicellular organisms. Euglena, Chlamydo- 
 monas, Vol vox and other similar forms collect in the more 
 highly illuminated regions of their environment during dark, 
 cloudy days, early in the morning and late in the evening, 
 and in shaded places when the sunlight is very intense. A 
 certain amount of light is necessary for the well-being of 
 these organisms since they depend upon photosynthesis in 
 
 236 
 
 ♦ 
 M 
 
ADAPTATION AND AGGREGATION 237 
 
 the process of feeding, but too much is fatal. Stentor 
 coeruleus, Amoeba and myxomycetes thri\'c in total dark- 
 ness. They are always found in regicms (A coniparatix'cly 
 low light intensity. The same is true of fly larvae. Nega- 
 tive response to light tends to keep these creatures buried in 
 cadavers where they find food. It is ordinarily onl>' under 
 artificial conditions that the reactions of organisms to light 
 prove fatal. Positive reactions to candle, lamp and light- 
 house destroy untold numbers of moths and flies and bees 
 and beetles and birds, but who has seen such fatalities under 
 natural conditions ! Under such conditions the responses 
 to light direct these animals to the advantage of their well- 
 being. When an insect or a bird in a room, a bee in a flower 
 or a pomace fly in a wormhole of a decaying apple is excited 
 it flies directly toward the light and ordinarily escapes. It 
 could not be expected to react differently in the presence of 
 a candle surrounded by darkness, since it receives the same 
 general stimulation and has had no experience with the 
 consequences. Many water-inhabiting larvae are strongly 
 positive even in light so intense that it is injurious. They 
 do not become negative and escape danger. Under natural 
 conditions the strong positive response serves to scatter 
 them far and wide, and under such conditions there is no 
 need for a negative response. The surface of the water 
 limits the distance they can proceed toward the light and in 
 their natural environment they experience none of sufficient 
 intensity to be injurious. As they grow older many of them 
 lose their positiveness and become negative, and now their 
 reactions guide them to the bottom into dark places, where 
 they spend most of their adult life. 
 
 Sand fleas are usually found in dark places under masses 
 of seaweed on the beach. If they are disturbed they 
 become strongly positive. This response directs them 
 toward the water, from which the stronger light usually 
 comes. After they are in the water they become negative. 
 This response directs them to the bottom and into dark 
 crevices. Under natural conditions their reactions to light 
 
238 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 are adaptive, but if they are confined in a glass jar with a 
 bunch of seaweeds at one end and an intense light at the 
 other they collect at the illuminated side and remain there 
 and die, whereas if they had become negati\'e they would 
 have been saved by the shelter of the seaweeds. But could 
 they, even from a rational point of view, be expected to 
 react otherwise under conditions which neither they nor 
 their ancestors have experienced? 
 
 If the positive reaction which guides a moth into a candle 
 is a non-adaptive reaction, then the positive reaction which 
 guides the wolf searching for food into a baited pitfall must 
 also be considered non-adaptive. But what would happen 
 to the wolf if he did not react positively to food ; does not 
 the trapper expect him to get into the pitfall ? Is not the 
 flight of the moth into the flame, after all, precisely what 
 one would expect if its reactions to light in general are 
 adaptive? 
 
 Ev^en in case of positive reactions of animals which live 
 in darkness, as e.g. the caterpillar of the willow borer and 
 the mud-inhabiting crustacean Cuma rathkii, referred to by 
 Loeb (1906, p. 159), or the cave-dwelling fishes mentioned 
 by Eigenmann (1899), it is probable that the reactions were 
 inherited from ancestors in which they were adaptive. 
 Reactions to light which are non-adaptive, except under 
 artificial conditions, are certainly rare. And whatever the 
 fundamental cause may be, it is evident that organisms in 
 general in their natural habitats tend to aggregate in regions 
 which have conditions adapted to the needs of their life 
 processes. We shall have occasion to emphasize this in 
 the following paragraphs. These conditions may differ for 
 different species and different individuals and for the same 
 individual from t'me to time. 
 
 Of course the fact that reactions are adaptive does not 
 explain their origin Natural selection may tell us why 
 organisms are as they are. It shows why adaptive charac- 
 teristics continue to exist while the non-adaptive ones do 
 not, but it does not tell us how they originated or why they 
 
ADAPTATION AND AGGREGATION 239 
 
 are at all. It may be, for all that is known to the contrary, 
 that Loeb is correct in his assumption that the reactions of 
 organisms have their origin in fortuitous chemical combina- 
 tions (see Chapter XX), and that those organisms which 
 live in high light intensity do so because they are positive in 
 their reactions to light, while those which live in low inten- 
 sity do so because they are negative. It must however be 
 conceded that we have as yet but very few, if any, acts 
 which bear directly on this question. 
 
 2. Different Reactions Observed in the Process of Collecting 
 in Regions having a given Condition of Illumination 
 
 Let us now proceed to ascertain precisely how the dif- 
 ferent organisms respond so as to get into and remain in 
 favorable light conditions. This has been thoroughly inves- 
 tigated for a number of different forms. It is by no means 
 the same in all. We shall consequently consider the dif- 
 ferent methods under several different headings: — a. Ran- 
 dom movements and avoiding reactions; b. Orientation, 
 change in sense of orientation, and avoiding reactions; c. 
 Orientation and extent of movement limited by environ- 
 mental conditions; d. Orientation and movement directly 
 toward the place where the organism comes to rest; e. Ran- 
 dom movements and coming to rest in a given place It 
 should be emphasized here that our aim in this section is 
 not merely to discuss methods of aggregation but also to 
 set forth all different methods of reactions with the view of 
 classifying them. 
 
 a. Random movements and avoiding reactions. — Quite 
 a number of the unicellular organisms get into the region of 
 optimum intensity by random movements. They swim 
 about aimlessly hither and thither, testing conditions In 
 many different places. Thus they sooner or later get into 
 an optimum intensity, where they remain, not by coming 
 to rest, but because whenever they reach the boun(h\r\- of 
 the optimum region they are stimulated and consequently 
 
240 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 return. Engelmann (1882) was the first to observe and 
 record this method of aggregation in detail. He says (1882, 
 P- 393)' referring to the aggregation in light of Paramecium 
 bursaria, Stentor viridis and other chlorophyll-bearing cili- 
 ates: " Ueberschreiten sie z. B. zufiillig die Griinze von Licht 
 und Dunkel, oder tauchen sie auch nur mit der vorderen 
 Halfte ihres Leibes eine Strecke weit in das Dunkel ein, so 
 kehren sie sofort um nach dem Licht, wie wenn das Dunkel 
 ihnen unangenehm ware." The collection of Euglena in 
 an illuminated area is described as follows (p. 395) : " Dieses 
 wirkt wie eine Falle, denn einmal hineingekommen, gehen 
 die Euglenen in der Regel nicht wieder heraus. Sie kehren 
 an der Grenze des Dunkels immer so gleich wieder um ins 
 Helle. Falls sie, was bei schnellem Vorwartsschwimmen 
 wohl einmal geschieht, ganz ins Dunkel hineingekommen 
 sind, sistiren sie doch so fort die weitere Vorwartsbewegung, 
 drehen um eine ihres kurzen Axcn, probiren — oft unter 
 bedeutenden Gestaltsanderung — in verschiedenen Richt- 
 ungen fortzukommen bis sie endlich wieder ins Licht 
 gerathen." In the last sentence we have an intimation of 
 orientation by trial, a method so thoroughly worked out 
 and clearly stated by Jennings later. The clearest state- 
 ment however which Engelmann made with reference to 
 aggregation by the method under consideration is found in 
 his article on Bacterium photometricum (1883, p. no): 
 " Schwacht mann nun plotzlich das Licht ... so sieht 
 mann alle bis dahin im Gesichtsfeld schwimmenden Bak- 
 terien fast im namlichen Moment eine Strecke weit zuriick- 
 schiessen, einige, meist unter lebhaftester Rotation um ihr 
 Langsaxe stillstehen und danach wieder die gewohnliche 
 Bewegung atifnehmen. Man erhiilt \ollstandig den Ein- 
 druch eines Erschreckens." The reactions to sudden 
 change of intensity in Bacterium photometricum has since 
 been designated " Schreckbewegung." It is in all essentials 
 like the reaction of Paramecium to changes in chemical 
 concentration which Jennings has designated " motor re- 
 flex " or " avoiding reaction," and somewhat similar to the 
 
ADAPTATION AND AGGREGATION 241 
 
 reaction of serpuUds to sudden changes of light intensity. 
 Loeb designated the power in serpuHds to respond tlius 
 " Unterschiedsempfindlichkeit." 
 
 Besides those already mentioned various other organisms 
 have been found to aggregate by this method, notably Sten- 
 tor coeruleus, Trachelomonas, Chlamydomonas, Chlorogo- 
 nium, Phacus and some swarm-spores, but so far as known 
 Bacterium photometricum is the only form which is entirely 
 dependent upon random movements to get into the region 
 of optimum illumination. All the others are only partly 
 dependent upon random movements in this. Under certain 
 conditions they orient and proceed directly toward the 
 region of most favoral)le illumination. Many forms how- 
 ever which make use of orientation at times in reaching the 
 optimum light are entirely dependent upon random move- 
 ments and avoiding reactions in case of other sources of 
 stimulation, notably chemicals. 
 
 Reactions of the sort described above have usually been 
 referred to as photopathic. They are supposed by some 
 to be fundamentally different from orienting reactions, 
 which are often called tropic. The former are supposed 
 to be due to the action of light owing to difference or change 
 of intensity, the latter to the action of light owing to con- 
 stant intensity or direction of rays. This distinction how- 
 ever will not hold, for we have clearly demonstrated that 
 orientation in many organisms is due to changes of light 
 intensity on the sensitive tissue. 
 
 b. Orientation, change in sense of orientation, and avoid- 
 ing reactions. — Many organisms, as stated above, have 
 the power of orienting and moving directly toward the 
 region of favorable illumination. In relati\"cly low light 
 intensities they are positive; in relatively high they are 
 negative; in favorable intensity they do not react to light; 
 consequently they tend to remain, but they do not come 
 to rest.^ The rate of movement is apparenth' not decreased. 
 
 ^ Loeb has recently impugned these statements. \Vc shall refer to this 
 matter in detail later (see footnote, p. 266). 
 
242 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 The organisms simply do not orient but swim about in an 
 apparently aimless wa\ . If in their wandering they get 
 out of the oplimum ihey orient and return. I'nder certain 
 conditions the avoiding reaction serves to prevent them 
 from leaving theof)timum, just as described under (i) above, 
 but this reaction functions mainl> in the process of orienta- 
 tion, as set forth in detail for many different forms in the 
 part of this volume devoted to that subject. 
 
 Practically all the lower motile forms which react to light 
 at all make use of this method of getting into the optimum 
 and remaining there. In this group we may put a few 
 rhizopods, numerous flagellates, some ciliates, several colo- 
 nial forms, a few rotifers, at least one coelenterate, a num- 
 ber of vermes, and some insect larvae. 
 
 The advantage of orientation over random movement in 
 getting to the optimum is evident, for it usually guides the 
 organisms directly there. Conditions can however be so 
 arranged that the greatest amount of light does not come 
 from the portion of the field most highly illuminated. On 
 the floor in front of a window for instance the region of 
 highest illumination is some distance from the window. 
 From this region toward the window the intensity decreases, 
 but the window is still the source of strongest illumination. 
 Under such conditions the organisms may be led astray. 
 If they are positive they proceed in the direction from which 
 the strongest light comes and may thus be carried directly 
 from the optimum. This was the case in a number of 
 Loeb's experiments. It led him and others to the erro- 
 neous conclusion that difference of intensity is of no impor- 
 tance in regulating reactions to light in many forms. They 
 failed to realize that the process of orientation is in itself 
 an attempt on the part of the organism. to attain optimum 
 environmental conditions. Its purpose is essentially the 
 same in all organisms without eyes, plants as well as ani- 
 mals, sessile organisms as well as motile. When an organ- 
 ism is oriented it is in its most favorable light conditions 
 as far as the immediate surrounding is concerned, for in 
 
ADAPTATION AND AGGREGATION 243 
 
 positive individuals the most sensitive part is then fully 
 exposed to the light, whereas in negative indi\i(luals it is 
 shaded. In sessile forms nothing more can be accomj)lishe(l, 
 but in motile forms still more favorable condilicjns are ordi- 
 narily attained by locomotion after orientation, that is, 
 movement toward or from the source of greatest illumina- 
 tion, depending upon whether the organism is positive or 
 negative. 
 
 The fundamental principle involved in orientation, as 
 shown in Part II, is the same in all organisms without 
 image-forming eyes, although the process differs much. Ori- 
 entation is the result directly or indirectly of the effect of 
 illumination on life processes. Movement toward unfa\or- 
 able conditions generally induces orienting reactions. The 
 reaction leading to orientation is however frequently not a 
 response to an immediate unfavorable condition. It may 
 be a response to a sign quite as much as the sudden con- 
 traction which closes the valves of a mussel when a shadow 
 passes over it. The shadow in itself is of no particular con- 
 sequence to the mollusk, but what follows may be. Just so 
 the change of intensity has no particular influence on the 
 life processes of Euglena, e.g., but w^hat follows may have. 
 Thus it is evident that orientation in the lower forms is 
 dependent upon the power of discrimination between dif- 
 ferent degrees of intensity — " Unterschiedsempfindlich- 
 keit " (Loeb), " sensibilite differentielle " (Bohn). In many 
 of these forms orientation is undoubtedly, and in all it is 
 probably, a response to change of light intensity on some 
 part of the organism. At any rate it has in no instance 
 been demonstrated that it is, as Loeb states, " a function 
 of the constant intensity," that orientation in light is like 
 orientation in an electric current. 
 
 c. Orientation and extent of movement limited by 
 environment. — Many organisms orient and proceed toward 
 or from the source of light as far as the physical conditions 
 of the environment will permit and collect there. Areni- 
 cola and Eudendrium larvae, zoeae and many other atjuaiic 
 
244 LIGHT AXD THE BFJIAVIOR OF ORGANISMS 
 
 forms, particularly in the early stages of development, aggre- 
 gate at the surface of the water in this way. They do not 
 have a response similar to the avoiding reaction in many 
 unicellular forms, nor do they become negative even if the 
 light intensity is increased to such an extent that it is 
 undoubtedly injurious. Conditions can readily be so ar- 
 ranged that they will go toward the light into chemical 
 solutions, temperatures and concentrations where they are 
 killed. Still their reactions under natural conditions are 
 highly adaptive. Owing to their strong positive reaction 
 they are brought to the surface of the water and scattered 
 far and wide. Under natural conditions they apparently 
 never or at least seldom experience light of such an intensity 
 that it is injurious, or a combination of other environmental 
 conditions such that their positive reactions to light prove 
 fatal. The power to become negative in excessively high 
 light intensity would consequently be of no special benefit 
 to these organisms. 
 
 d. Orientation and movement directly toward the place 
 where the organism comes to rest. — Most of the organisms 
 with well-developed eyes, together with some few without, 
 may be placed under this head. They go directly to a 
 given place and remain because they come to rest there. 
 Thus we find according to Bohn (1908) that Littorina and 
 some starfishes under certain conditions go directly toward 
 rocks and other objects and come to rest in their shadows. 
 Butterflies and various other forms, according to Parker 
 (1903) and Cole (1907), go directly toward large luminous 
 areas and come to rest there. And frogs in direct sunlight 
 were found by Torelle (1903) to go directly toward shadows, 
 even if in so doing they had to move perpendicular to the 
 direction of the light rays. But after they were in the 
 shadow they turned so as to face the light, an adaptive reac- 
 tion throughout; for under the conditions of the experiment 
 it was probably advantageous for the frogs to be pro- 
 tected by the shade and to face the light when in it, since 
 thus they were most likely to see both food and enemies. 
 
ADAPTATION AND AGGREGATION 245 
 
 e. Random movements and coming to rest in a given 
 place. — Probably all organisms that react lo light at all 
 are affected in their movements by the acti(jn of light owing 
 to the absolute intensity. Such effects are not dependent 
 upon the time rate of change of intensity but upon the 
 actual amount of light energy present and the time of 
 exposure. Light tends to inhibit the movements of Plas- 
 modia, and some bacteria, while total darkness has the 
 same effect on purple bacteria, Volvox, Euglena, Chlamy- 
 domonas. Hydra, some planaria, earthworms and a number 
 of other organisms. In most of these organisms this action 
 of light is either insufficient to have any apparent effect on 
 aggregation or it is masked by other stronger reactions. 
 In planaria however the influence of relatively low light 
 intensity has a marked effect on the process of aggregation. 
 Some of these animals, as Loeb points out (1906, p. 136), 
 do not orient. When exposed in a dish in front of a window 
 they wander about aimlessly until they get into the darker 
 regions, where they come to rest and consequently remain. 
 Thus it is that they collect in the region of lower light 
 intensity. Loeb considers this reaction of planaria " a 
 function of the quotient of the change of intensity over 
 time," i.e. time rate of change. I am however of the 
 opinion that it is a function of the absolute intensity, that 
 the action of light in these reactions is similar to the action 
 of heat, for active planarians respond by raising the head 
 and throwing it from side to side, both when the light is 
 suddenly decreased and w^hen it is suddenly increased. 
 There is no indication of an immediate decrease in locomo- 
 tion under such conditions (Walter, 1907, p. 71), as would 
 be expected if the aggregation were due to a change of 
 intensity. And moreover, if the intensity is suddenly in- 
 creased after the planarians have come to rest, they do not 
 become active at once. Walter says (1907. P- 63) that the 
 interval between sudden increase in illumination and re- 
 sponse under such conditions "was often several minutes." 
 
CHAPTER XII 
 
 REACTIONS TO LIGHT WHICH DO NOT RESULT IN 
 AGGREGATION OR ORIENTATION 
 
 We have demonstrated in the preceding pages that reac- 
 tions to sudden changes of Ught intensity may result in 
 orientation followed by aggregation, or in aggregation with- 
 out orientation. In either case the immediate response 
 to the change of intensity is an abrupt change in direction 
 of motion called Schreckbewegung, or avoiding reaction. 
 Most of the organisms considered respond thus to an 
 increase of intensity under some conditions and to a 
 decrease under others. The immediate response under the 
 two conditions is precisely the same; but in case of orien- 
 tation the former leads to locomotion away from the source 
 of light, the latter to locomotion toward it; and in case of 
 aggregation, the former results in collections in regions of 
 relatively low light intensity, the latter in regions of rela- 
 tively high intensity. 
 
 There are many organisms which respond to sudden 
 changes of light intensity much like those referred to above. 
 They contract suddenly or change their direction of locomo- 
 tion abruptly, but these reactions ordinarily result neither 
 in orientation nor in aggregation. Most of these reac- 
 tions are responses to shadows, i.e. to a sudden decrease 
 in light intensity. These organisms do not all however 
 react in the same way, and may consequently be divided 
 into several groups as follows: — i. Reactions to shad- 
 ows — protective; 2. Reactions to shadows — procuring 
 food; 3. Reactions to sudden increase of light intensity; 
 4. Reactions to light caused by the effect of continued 
 illumination. 
 
 246 
 
REACTIONS TO LIGHT 247 
 
 I. Reactions to Shadows — Protective 
 
 Many animals respond only to shadows, i.e., to a sudden 
 decrease in light intensity, not to a gradual decrease or to 
 an increase, no matter how sudden or how great. Such 
 reactions to shadows are widely distributed. They are 
 highly protective in all instances, and consecjuently vary 
 somewhat in accord with the different habits of the different 
 animals. The holothurian Thyone briareus contracts when 
 the light intensity is suddenly reduced, frequently to such 
 an extent that any portion protruding above the sand and 
 mud in which the animal lives is entirely withdrawn (Pearse, 
 1908, p. 277). Several different sea urchins, according to 
 von Uexkiill (1897), turn the spines toward the shaded part, 
 apparently to ward off an approaching enemy. Various 
 tubicolous annelids have been observed to contract violently 
 and jerk back into their tubes when an object passes be- 
 tween them and the source of light, e.g., Amphitrite bomb>'x 
 (Dalyell, 1853), Branchiomma kollikeri (Claparede, 1868), 
 Serpula (Ryder, 1883), Hydroides dianthus (Andrews, 
 1891, pp. 285, 296), Serpula uncinata (Loeb, 1893, p. 103), 
 Spirographis spallanzani (Nagel, 1896, p. 76), Potamilla 
 oculifera, Sabella microphthalmia and Protula intestimum 
 (Hargitt, 1906, p. 310), and Bispira voluticornis (Hesse, 
 1 899). Shadows cause Pecten (Rawitz, 1888), Avicula, 
 Area, and Cardium (Patten, 1886) to close their valves rap- 
 idly. Nagel (1896, pp. 18-77) observed similar reactions in 
 23 species of lamellibranchs without eyes, 4 species of gastro- 
 pods with the eyes removed, and several blind arthropods. 
 I have also frequently seen the short-neck clam Maya ara- 
 naria close its siphon and contract, and Littorina littorea 
 retract rapidly into its shell. The hermit crab Pagurus 
 also darts back into its stolen home when a shadow is cast 
 on it. Mosquito larvae, ordinarily found at the surface of 
 the water, scurry to the bottom at the approach of a shadow, 
 and the killifish Fundulus responds much in the same way. 
 Barnacles, according to the observation of Pouchet and 
 
248 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 Joubert (1875), stop their respiratory movements and con- 
 tract if they are attached to objects some distance beneath 
 the surface where a shadow might indicate the approach of 
 an enemy, but they do not respond to decrease in liu:ht inten- 
 sity if attached to objects floating on the surface, where 
 shadows can have no such signification. All of these reac- 
 tions are very much Hke the in\'okmtary closing of the 
 eyelids caused by shadows in higher forms. 
 
 All of the organisms in this class are readily acclimated 
 to changes of intensity, and in all, the reduction can be 
 made so gradual that they do not respond, showing that the 
 response is dependent upon the time rate of change. Those 
 organisms which react by contracting expand again very 
 soon, even if the intensity remains constant at the lowest 
 point reached during the application of the stimulus. This 
 shows that the response is due to the process of changing 
 the intensity and not to the absolute difTerence in the 
 amount of light (at different times) before and after the 
 change takes place. Von Uexkiill says that the response 
 in sea urchins usually fails after three or four trials. Nagel 
 found the same with regard to mollusks. He says (1896, 
 p. 29), " Das auffallendste an den Beschattungsreaktionen 
 ist nun aber die Art, wie sich die Tiere an ofter wiederholte 
 Reize gewohnen. 
 
 " Ich lasse beispielsweise den Schatten eines Bleistiftes 
 iiber die ausgestreckten Siphonen eincr Herzmuschel hin- 
 streifen. — Sie schliesst blitzschnell die Siphonen. 
 
 " Ich warte einige Minuten und wiederhole den Versuch. 
 — Jetzt schliesst die Muschel ihre Siphonen nicht mehr, 
 sondern es zucken nur die Rander derselben ein wenig 
 zusammen. 
 
 " Ich warte wieder einige Minuten, und wiederhole den 
 Versuch zum drittenmale, und jetzt bleibt jede Reaktion 
 aus. Nun kann ich aber auch den Grad der Verdunklung 
 um das hundertfache steigern, indem ich als schatten- 
 werfendcn Korper statt des Bleistiftes ein Buch oder 
 ein grosses Stiick Karton verwende: Trotzdem bleibt die 
 
REACTIONS TO LIGHT 
 
 249 
 
 Muschel regungslos, der Schatten geniert sie in keiner 
 Weise. 
 
 " Noch auffallender ist die Erscheinung bei der Auster 
 und unserer Malermuschel. Wenn diese einmal dun h eincn 
 Schatten erschreckt worden sind, dann bleibt jede weitere 
 Beschattung ohne Erfolg. Erst wenn eine Stunde oder 
 mehr seit dem ersten Versuche ohne Storung verflossen ist, 
 sind die Muschehi wieder fiir den Schattenreiz empfiing- 
 lich." I have frequently noticed that the hermit crab 
 (Pagurus), mosquito larvae and tubicolous worms, especially 
 Hydroides, soon fail to respond to ordinary shadows if they 
 are kept in a place where the shadows frequently occur, 
 but under such conditions they still respond to reduction 
 of intensity greater than they ordinarily experience. The 
 experimental results of Mrs. Yerkes (1906) and Hargitt 
 (1906, 1909) on Hydroides dianthus lead to the same con- 
 clusion. Hargitt's observations (1909, p. 158) are of especial 
 interest in showing the relation between the reaction and 
 the habitat. He found that specimens taken from a depth 
 of about twenty fathoms did not respond to shadows which 
 caused very definite reactions in specimens taken in shallow 
 water. Whatever the immediate physiological cause of all 
 these reactions may be, it Is evident that they are admirably 
 adapted to protect the organism against the attack of 
 enemies. There are however animals in which similar re- 
 actions are found which serve quite a different purpose. 
 These are included in the following group. 
 
 2. Reactions to Shadows — Procuring Food 
 
 Whitman observed that even a very faint shadow causes 
 the leech, Clepsine, to become restless and stretch up and 
 sway from side to side, apparently in search of something 
 to seize. Bateson records similar reactions in shrimj:)s and 
 prawns. When a shadow passes near these animals they 
 raise their antennae and swing them about. The primary 
 cause of reaction in these animals is probably the same as 
 
250 LIGHT AND THE BEHAVIOR OF ORGAXISMS 
 
 it is in the preceding group. In both, the reaction has to 
 do with the well-being of the individuals responding. But 
 in the one it has to do with i:)rotection against mechanical 
 injury, while in the other it has to do with the process of 
 procuring focxl. The important point is that the shadow 
 in itself is of no particular importance in either case, but 
 what follows may be. It is a response to a sign just as 
 truly as is the reaction of a dog to which Brooks refers in 
 the following characteristically convincing words (1907, 
 p. 53), " The kick is a sign of something which may follow, 
 and the actions which do follow are not the effect of the 
 kick, for they are directed or adjusted, either consciously 
 or unconsciously, to an event of which it is only the fore- 
 runner." 
 
 3. Reactions to Sudden Increase of Light Intensity 
 
 There are reactions to changes of intensity which appear 
 to be more directly concerned with the effect of light than 
 are those just referred to. They are mostly responses to 
 an increase of intensity and may be due to the effect of the 
 change of intensity or to the effect of the absolute illumina- 
 ti(jn. An earthworm, e.g., jerks back into its burrow when 
 light is flashed upon it, as definitely as Hydroides does when 
 it is suddenly shaded. If however the intensity is gradually 
 increased it may not react at all. This is therefore evi- 
 dently a reaction which is primarily dependent upon the 
 time rate of change of light intensity. A sudden decrease 
 of intensity produces no such reaction. I have frequently 
 observed similar reactions in Stentor coeruleus, as did also 
 Bronn in the actinia, Edwardsia and Cerianthus; Nagel 
 (1896) in the sea squirt, Ciona, and in several different 
 mollusks, and Parker (1908, p. 419) in Amphioxus. Parker 
 says: " In all the tests I carried out, I never observed a 
 reaction to a rapid di?ninutio?i of light, and the reactions 
 to light that did occur were always the result of a rapid 
 increase of intensity. When an animal was resting quietly 
 
REACTIONS TO LIGHT 251 
 
 on its side in a shaded aquarium and a beam of sunlight 
 was suddenly thrown upon it, it would usually respond by 
 one or two vigorous locomotor leaps, after which it might 
 come to rest even in the sunlight. If now the sunlight was 
 suddenly cut off, no response followed. That this failure 
 to respond was not due to exhaustion from over-exposure 
 to light was easily shown by quickly throwing on the sun- 
 light a second time, whereupon a reaction much like the 
 first one usually followed immediately." 
 
 The jellyfish Sarsia contracts, according to Romanes 
 (1885, p. 41), if suddenly illuminated while it is at rest, 
 and Yerkes (1902) observed similar reactions in Gonione- 
 mus. A decrease in illumination produces no response in 
 either of these forms if they are at rest, but in case of 
 Gonionemus in the active -state the movements are imme- 
 diately checked either by an increase or by a decrease of 
 intensity, after which the medusae turn over and sink to 
 the bottom. While a sudden decrease of intensity does not 
 call forth a response in Gonionemus when at rest, prolonged 
 exposure to light of low intensity causes it to become active. 
 Thus its light reactions are such as to keep it in moderately 
 illuminated regions. In Sarsia in the active state, on the 
 other hand, increase in illumination tends to cause increase 
 in activity. It does not inhibit movement as it does in 
 Gonionemus. A number of animals respond to both a 
 decrease and an increase of light intensity. Most of these 
 respond more definitely to the former than to the latter, 
 but there are some which appear to respond to both in the 
 same way and equally definitely. Nagel says (1896, p. 74), 
 e.g., that Helix hortensis draws into its shell when suddenly 
 illuminated, much as it does when suddenly shaded. Such 
 reactions are of considerable theoretic importance: they will 
 be referred to again later. 
 
252 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 4. Reactions to Light Caused by the Effect of Con- 
 tin tied Illumination 
 
 In all of the animals referred to above, the reaction is 
 clearly a response to change of intensity, except in the case 
 where Gonionenius becomes active after ha\'ing been sub- 
 jected to low light intensity for some time. In the actin- 
 ians Aiptasia, Cerianthus and Eloactis the response appears 
 to be due primaril> to the action of light owing to its abso- 
 lute intensity. 
 
 " Aiptasia annulata," Jennings says (1905, p. 459), " is 
 very sensitive to light, expanding in darkness, but con- 
 tracting after a few seconds when exposed to strong light. 
 In ordinary daylight the animal remains contracted for 
 some hours, but after such a period most specimens extend 
 in spite of the light. In comparative darkness the animals 
 direct the disk toward the source of light, through a con- 
 traction on the side of the column exposed to the light. 
 After remaining undisturbed for a long time in an aquarium 
 that is fairly well lighted, the animals give up their orienta- 
 tion with respect to the strongest source of light; with less 
 light they retain it." 
 
 Regarding Eloactis, Hargitt says (1907, p. 277) that they 
 begin to retract almost immediatelyafter exposure to diffuse 
 da>light. " This reaction is not sudden or general at once, 
 as in such creatures as the earthworm, but begins in a some- 
 what indefinite movement of the body, accompanied by 
 similar movements of the tentacles, followed very soon by 
 a slow but definite retraction of the entire body within the 
 tube, often including likewise the tentacles as well." In 
 direct sunlight the reaction is more striking. " In some 
 cases the reaction was so definite and prompt as to leave the 
 impression on the observer that the creature was possessed 
 of something akin to visual sensation." 
 
 While, as stated above, these reactions appear to be pro- 
 duced by the action of light owing to constant intensity, it 
 may even here be due to the effect of the changes of inten- 
 
REACTIONS TO LIGHT 253 
 
 sity. The principal reason for thinking it is due to the 
 effect of constant intensity is the fact that there is no 
 immediate response when the intensity is changed. But 
 this retardation and slowness in reaction may be due to the 
 general character of the animals. As a matter of fact we 
 have as yet presented no conclusive experimental evidence 
 showing that reactions are dependent upon any action of 
 light other than that due to changes of intensity, although 
 we have several times intimated that in all probability the 
 activity of many organisms is affected by constant inten- 
 sity. The strongest evidence we have in support of this 
 is found in connection with observations on the change in 
 sense of reaction. 
 
 However this may be, there is in this group no evidence 
 of a reaction to a sign. The reaction is undoubtedly a 
 direct response to the light itself. 
 
 In organisms with image-forming eyes the reactions are 
 preeminently responses to signs, at least in so far as the eyes 
 function in the responses. These animals are not primarily 
 interested in light as light, but in what may follow a given 
 light condition, e.g., an image on the retina. At first 
 thought it seems as though here were a clear case of stimu- 
 lation due to the action of light through constant intensity, 
 for objects can be seen for some time without changing the 
 light configuration on the retina. No new image is how- 
 ever formed on the retina without change of intensity, and 
 it is consequently evident that here, too, the stimulation 
 may be due to changes of intensity rather than to constant 
 intensity. 
 
 5. Classification of Reactions to Light — Phototropism, 
 
 Photopathy 
 
 Reactions to light have ordinarily been classified as 
 phototropic (phototropism) or phototactic (phototaxis) and 
 photopathic (photopathy). In some instances " helio " has 
 been substituted for " photo." Organisms which orient and 
 
2 54 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 move toward or from a source of light are usually termed 
 phototactic, those which orient but do not move as photo- 
 tropic, and those which do not orient but still react have 
 been termed j)h()i()i)alhic. The adjecli\es positive and 
 negative are ordinarily used in connection with these terms 
 to signif\- whether the organisms go or bend toward the 
 source of light or in the opposite direction; or whether they 
 collect in regions of higher intensity or in those of lower 
 intensiiN'. The terms mentioned above have however been 
 used not only to signify direction of mo\ement, but also 
 to designate the nature of the stimulation and the response 
 as set forth in Part T, under definitions of tropisms. 
 
 Loeb says (1906, p. 135): '* Heliotropism covers only 
 those cases where the turning to the light is compulsory 
 and irresistible, and is brought about automatically or 
 mechanically by the light itself. On the other hand, there 
 are compulsory and mechanical reactions to light which 
 are not cases of heliotropism; namely, the reaction to sud- 
 den changes in the intensity of light. ... In the former 
 case the results are a function of the constant intensity, 
 in the latter a function of the quotient of the change of 
 intensity over time." All cases of orientation are con- 
 sidered by him to be due to heliotropism, i.e., to the effect 
 of light by virtue of its " constant intensity." All other 
 reactions to light are due to changes of intensity. In this 
 class Loeb puts the contraction of the tubicolous annelids 
 Serpula and Spirographis and the collection of Planaria in 
 regions of low light intensity. These reactions, he says, 
 are due to Unterschiedsempfindlichkeit — sensibility to dif- 
 ference of intensity; those resulting in orientation are not. 
 
 Davenport (1897, pp. 210, 211) maintains that "Two 
 kinds of effects are produced by light: one by the direction 
 of its ray — phototactic; the other by the difference in illu- 
 mination of parts of the organism — photopathic. . . . 
 Light acts directly either through difference in intensity on 
 the two sides of the organism, or by the course the rays 
 take through the organism." Here again we have two 
 
REACTIONS TO LIGHT 255 
 
 classes, — phototropism and photopathy. All cases of ori- 
 entation belong to the former, and all cases where organisms 
 aggregate without orientation, to the latter. Davenport 
 mentions Planaria torva, Daphnia and Volvox as examples 
 of organisms which are photopathic. Both he and Loeb 
 affirm that there are some organisms which are both photo- 
 tropic and photopathic. The one mentions Daphnia as an 
 example, the other Spirographis. 
 
 Yerkes (1903, p. 361) refers to this problem as follows: 
 " The motor reactions of organisms to light, so far as known 
 at present, are of two kinds: phototactic and photopathic. 
 In both intensity of the light, not the direction of the rays, 
 is the determining factor. All those reactions in which the 
 direction of movement is determined by an orientation of 
 the organism which is brought about by the light are 
 phototactic; and all those reactions in which the movement, 
 although due to the stimulation of light, is not definitely 
 directed through the orientation of the organism, are photo- 
 pathic. . . . A.n organism which selects a particular in- 
 tensity of light and confines its movements to the region 
 illuminated with that intensity is photopathic." Thus it is 
 seen that Yerkes, like Loeb and Davenport, divides reac- 
 tions into two classes and ascribes both kinds of reactions 
 to a given individual in certain cases. All three authors 
 agree in designating orienting reactions as photopathic or 
 phototropic, but they do not agree in their explanation of 
 the process of orientation. Loeb claims it is due to the 
 effect of constant intensity; Davenport thinks in some cases 
 it is due to the direction in which the rays pass through 
 the organism, and in others to the effect of difference of 
 intensity on opposite sides; and Yerkes maintains that it is 
 due to difference of intensity in all cases. But if the light 
 intensity on the surface of an organism is not uniform 
 almost every movement of the organism produces changes 
 of intensity on some part of it. It is therefore evident that 
 orientation (phototropism), according to Loeb, Davenport 
 and Yerkes, may be due respectively to the effect of con- 
 
256 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 stant intensity, direction through the tissue, or change of in- 
 tensity. We ha\'e however clearly demonstrated in Part II 
 of this volume that there is no experimental e\'idence 
 proving that direction of the rays through the tissue or 
 ditTerence of light intensity on different parts of the body 
 is functional in the (orientation of any organism, except- 
 ing in so far as it may cause changes of intensity on the 
 organism. 
 
 Photopathy, or Unterschiedsempfindlichkeit, as Loeb 
 calls it, is due to change of intensity according to Loeb, to 
 difference of intensity on the organism according to Daven- 
 port, and to difference of intensity in the field according to 
 Yerkes. It is at once evident however that there may be 
 agreement in these apparently different statements. If an 
 organism is so illuminated that the light intensity on dif- 
 ferent parts of the body differs, every moment is almost 
 certain to cause changes of intensity on some part of the 
 organism, and if the intensity is not uniform in the field an 
 organism, of course, cannot move about without causing 
 changes of intensity on its surface. It may be, then, that 
 the fundamental factor involved in photopathy, according 
 to all of these authors, is change of intensity. 
 
 It is thus evident that the reactions grouped under photo- 
 tropism and photopathy by the authors mentioned may all 
 depend upon changes of intensity, and that the two phe- 
 nomena may be fundamentally the same. If this be true, 
 then the classification of reactions to light as photopathic 
 and phototropic is without a foundation. Are there, then, 
 no differences in these reactions which will serve as a basis 
 for a classification? 
 
 6. Reclassification of Reactions to Light 
 
 Reactions to light may conveniently be classified (i) on 
 the basis of the character of the stimulus, and (2) on the 
 basis of the fundamental causes of the response. 
 
 (i) On the basis of the character of the stimulus we 
 
REACTIONS TO LIGHT 257 
 
 obtain the following groups: a. Reaction to change of inten- 
 sity; b. Reactions to constant iUumination; c. Reactions of 
 questionable cause. 
 
 a. Reaction to change of intensity. — Response to change 
 of intensity on the surface of the organism may result in 
 orientation or merely in a change in position or direction 
 of motion, (a) Orientation: Examples — Euglena, Chlam- 
 ydomonas, Trachelomonas, Chlorogonium, swarm-spores, 
 Volvox, Stentor, Planaria, earthworms and fly larvae, (b) 
 Changes in direction of motion: Examples — Shock reac- 
 tions, or avoiding reactions which do not result in orienta- 
 tion, in all the forms mentioned above, (c) Changes in 
 position: Examples — Sudden contraction in the tubicolous 
 worms, Gonionemus, a few actinians, various moUusks and 
 arthropods, and Amphioxus. (d) To these a fourth divi- 
 sion may be added consisting of reactions to shadows in 
 Clepsine, shrimps, prawns, mosquito larvae and Fundulus. 
 
 b. Reactions to constant illumination. — Constant or 
 continuous illumination affects the sense of the reaction of 
 organisms and their general activity, and it may possibly 
 produce orientation in some forms. Whenever a positive 
 organism becomes negative in light or vice versa, it is in all 
 probability due to the action of light owing to its continued 
 intensity, the absolute amount of light energy received, the 
 produ'ct of the intensity and time of exposure. Reversal 
 in the sense of reaction is not common to all organisms which 
 respond to light, but the general activity of all probably 
 depends upon the absolute amount of light energy received, 
 much as the activity depends upon the temperature or heat 
 energy received. The aggregation of Planaria in regions 
 of low light intensity is no doubt in part due to this effect 
 of light, since they come to rest even in a field uniformly 
 illuminated from above in such a way that there is no per- 
 ceptible change of intensity on any part of the organism. 
 The time of exposure is an important element in this 
 response. This is contrary to Loeb's conclusions regarding 
 the cause of aggregation of Planaria. He claims the aggre- 
 
258 LIGHT AXD THE BEHAVIOR OF ORG.'XISMS 
 
 gation of Planaria to be due to Unterschicdsempfindlichkeit, 
 sensibility to changes of intensity, and classifies the reac- 
 tions with those of Serpula and Spirograj^his to shadows. 
 I can howe\er see no siniilarit>' between the responses of 
 these organisms. 
 
 c. Reactions of questionable cause. — There are many 
 responses in which ii is as yet impossible to be certain as 
 to what characierisiir of light caui>es them. The orienta- 
 tion of Amoeba for exami)le is probably due to the direct 
 effect of the increase in light intensity on the protoplasm, 
 but for all that is known to the contrary it may be due to 
 continued illumination rather than to change in illumina- 
 tion. With regard to the orientation of the sessile plants 
 and animals, as well as all animals with well-developed 
 eyes, and many of the lower forms (entomostraca, Hydras 
 sea anemones, and the larvae of Arenicola, Limulus and 
 various crabs), experimental results do not as yet warrant 
 a definite conclusion as to whether the reaction is due to 
 the effect of change of intensity or continued illumination. 
 It should however be emphasized again that in no case has 
 it been demonstrated that orientation is '' a function of the 
 constant intensity " as maintained by Loeb. Nor is there 
 any evidence indicating that the direction of the rays 
 through the tissue has any direct effect on this process. 
 
 (2) On the basis of the fundamental cause of response, 
 the reactions to light may be classified as follows: a. Reac- 
 tions caused by the direct efTect of light on the reacting 
 tissue; b. Reactions caused by an indirect effect of light; 
 c. Reactions due, not to any effect of light in itself, but to 
 what a gi\en light condition or configuration may represent. 
 
 a. Reactions caused by the direct effect of light on the 
 reacting tissue. — Examples: Inhibition of protoplasmic 
 streaming in the rhizopods and plasmodia and in numerous 
 different plant cells, probably reversal in the sense of reac- 
 tion and the change in sensitiveness, and the general activity 
 of some organisms at least. 
 
 b. Reactions caused by an indirect effect of light. — 
 
REACTIONS TO LIGHT 259 
 
 Examples: Shock-movements and orientation in Euglena, 
 Chlamydomonas, Trachelomonas, Chlorogonium, swarm- 
 spores, Volvox and Stentor; orientation in all higher plants, 
 especially those in which the sensory zone is separated from 
 the motory zone, as in the plumules of grasses; and the 
 shock-movements in Edwardsia, Cerianthus, Gonionemus, 
 various mollusks, fly larvae, earthworms, and Amphioxus. 
 Light may have a marked effect on the life processes of all 
 these organisms, but there is no evidence indicating that 
 the slight changes of intensity required to induce reactions 
 affect these processes. The organisms mentioned above 
 are interested, not in the light condition which causes the 
 reaction, but in that which ordinarily follows such a con- 
 dition if the position or the direction of movement is not 
 changed. Euglena for example may respond when the 
 intensity on the colorless anterior end is reduced by a 
 small fraction of a candle-meter. It cannot be of any 
 special importance to the organism to keep this colorless 
 end illuminated, but it is of the greatest importance to 
 keep the green portion back of it illuminated, for light is 
 necessary in the process of photosynthesis. Likewise the 
 slight increase of intensity necessary to cause an earthworm 
 to withdraw into its burrow, or to cause a negative Euglena 
 to give the avoiding reaction, cannot be injurious to either 
 of these organisms, for both thrive in light much stronger 
 than that required to produce these reactions, ^have kept 
 earthw^orms continuously exposed to strong diffuse daylight 
 (150 ca. m.) in excellent condition for weeks, whereas at 
 night a candle-meter of light flashed on them is often suffi- 
 cient to cause violent contraction. The light condition 
 which causes this response in earthworms is not injurious, 
 but the illumination that usually follows if they do not re- 
 spond may be. Then, too, there is another factor Involved 
 here. Exposure ordinarily puts the worms at the mercy 
 of the birds which prey upon them. Thus the light may 
 be a sign of an enemy to the worms and in this regard they 
 belong in the following group, for this phase of the response 
 
26o LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 is not a reaction to light at all, but a reaction to what light 
 represents. 
 
 The distinguishing characteristic which differentiates the 
 responses in the organisms in this and the following group 
 is however superficial. The fundamental principle involved 
 in the reactions of the organisms in both is the same, for 
 the reactions in the former as well as those in the latter are 
 responses to signs. To those in this group the stimulating 
 light condition is a sign of another condition of light either 
 more or less intense than the one to which they respond; 
 to those in the next group it is a sign of an object. And it 
 is this more or less intense light, or the object represented, 
 that is of vital importance to these organisms, not the con- 
 dition of light to which they respond. 
 
 c. Reactions due, not to any effect of light in itself, but 
 to what a given light condition or configuration may repre- 
 sent. — (a) Reactions caused by shadows or a sudden 
 decrease in light intensity, reprCvSenting either enemies or 
 food: Examples — the sudden contraction or movement of 
 tubicolous annelids, numerous echinoderms, mollusks and 
 arthropods, and the response of Clepsine, shrimps, prawns, 
 mosquito larvae and Fundulus. In case of Clepsine the 
 shadows undoubtedly represent food ; in the rest, with the 
 probable exception of shrimps and prawns, it represents 
 enemies, (b) Reactions to sudden exposure to light or 
 increase of intensity probably representing enemies, espe- 
 cially in case of earthworms: Examples — Arenicola and fiy 
 larvae, earthworms and a few mollusks. (c) Reactions 
 caused by the size of the luminous area in connection with 
 intensity: Examples — Butterflies (Vanessa), Water scor- 
 pion (Ranatra) and frogs (Rana and Acris). In some of 
 these organisms the positive reaction to a large area in 
 preference to a small one of the same intensity prevents 
 flight toward the sun, and it probably has something to do 
 with mating, (d) Reactions caused by size, form, varia- 
 tion in shadow and color of luminous area: Example — 
 The higher animals, especially man. In these organisms 
 
REACTIONS TO LIGHT 261 
 
 the reactions are associated with objects which have some 
 vital relation to their existence as food or a source of dan- 
 ger. In the higher animals, man in particular, pleasure 
 and other emotions enter in as factors in the response, and 
 the objects represented by the condition of light which 
 causes the response may consequently have additional signi- 
 fications. I do not however wish to be understood as advo- 
 cating the exclusion of such factors in the reactions of lower 
 organisms, for, while they have not been demonstrated in 
 these organisms, they may exist for all that is known to the 
 contrary. 
 
 In the organisms in Group a all the tissue appears to be 
 equally sensitive to light. In many of those in Group b, 
 Euglena, Stentor and Planaria, for example, some parts of 
 the body are undoubtedly more sensitive than others, and 
 it may be that the sensitive tissue is confined to a small 
 area definitely located, as for instance in Euglena near the 
 eye-spot. This tissue serves to distinguish changes of inten- 
 sity, but owing to its positional relation to non-sensitive 
 tissue (see Fig. 11) which intercepts the light from one side, 
 and the movement of the organism, especially the rotation 
 on the long axis, it serves also to locate the direction from 
 which the strongest light comes. Thus we have in these 
 organisms structures w^hich may be termed direction eyes. 
 In Planaria and Arenlcola larvae the highly sensitive tissue 
 is nearly surrounded by opaque tissue which admits light 
 only from one side (see Fig. 26), and consequently serves to 
 locate more accurately the direction from which the light 
 comes. In these organisms the lateral head movements 
 are also of importance in locating the direction of the light. 
 While the photosensitive tissue may be confined to limited 
 regions in some of these forms, we are certain that it is not 
 in others. Planaria and earthworms e.g. are known to 
 respond after the more highly sensitive anterior end has 
 been removed. Histological investigations in the latter 
 seem to indicate that the photosensitive elements are fairly 
 well distributed over the entire body surface. 
 
262 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 Most of the organisms in Group c ha\e image-forming 
 eyes. The simplest of these appear to serve merely- to distin- 
 guish between clitTerenco in size and location of illuminated 
 areas, while the more complex serve to distinguish form, 
 distance, and color as well. In many of these organisms 
 the tissue which is sensitive to light is not confined to the 
 eyes. Many of the fishes and amphibia respond to light 
 alter thee>es, including the retina, have been removed, and 
 there are also a number of blind species which respond to 
 light. 
 
 Nearly all of the reactions classified above arc probably 
 responses to changes of intensity. They are no doubt asso- 
 ciated with chemical changes caused by changes of light 
 intensity, and affected by other chemical changes dependent 
 upon the effect of continued illumination, the absolute 
 intensity, and the time of exposure. We shall refer to this 
 matter again in the final chapter. 
 
 7. Evolution of Reactions to Light 
 
 It is not my purpose to discuss the problem of the evolu- 
 tion of the reactions to light. Nothing of importance could 
 at present be added, in such a discussion, to the ideas of 
 Jennings on this question expressed in his treatment of the 
 development of behavior (1906, pp. 314-327). I shall there- 
 fore merely suggest, without argument, the order in which 
 the reactions to light seem to have appeared. 
 
 The most primitive responses to light are probably due 
 to the effect of continued illumination on synthetic and 
 growth processes in green plants. Responses of this nature 
 we may assume to have been the basis for the origin of all 
 others, which probably appeared somewhat in the order 
 following: 
 
 (i) Change in the rate of locomotion dependent upon 
 the absolute amount of light energy received. No orienta- 
 tion, but probably aggregation at the optimum intensity — 
 Bacteria. 
 
REACTIONS TO LIGHT 263 
 
 (2) Contraction of naked protoplasm due to sudden 
 changes in the Hght intensity, i.e., changes in the amount 
 of Hght energy received, resulting in orientation in some 
 instances — Amoeba. 
 
 (3) Fixed responses (avoiding reactions) caused by sud- 
 den changes of intensity, the nature of the response depend- 
 ent upon the structure of the organism. No orientation, 
 but aggregation at the optimum — Bacteria. 
 
 (4) Reactions similar to those under (3), but more defi- 
 nitely circumscribed by the structure of the body, especially 
 the localization of tissue sensitive to light. Definite orien- 
 tation and movement directly toward the optimum — Sten- 
 tor, etc. 
 
 (5) Reactions to a sign. The change in illumination 
 which causes the response is of no consequence to the organ- 
 nism, but the illumination which would follow if it did not 
 respond may be — Euglena, Volvox, etc. 
 
 (6) Reactions to a sign. The change of intensity 
 (shadow) which causes the response represents objects 
 which may be beneficial or injurious, food or enemies — 
 Clepsine Hydroides, etc. 
 
 (7) Reactions to a sign. The light condition or con- 
 figuration which causes the response represents objects, not 
 by means of shadows cast by them, but by means of the 
 light reflected from them expressing size, form or color — 
 Animals with image-forming eyes. 
 
CHAPTER XIII 
 
 FACTORS INVOLVED IN REGULATING REACTIONS TO LIGHT 
 — VARIABILITY AND MODIFIABILITY IN BEHAVIOR 
 
 Everyone who has ever attempted observations on the 
 behavior of organisms with precise methods, knows that 
 variabiHty even in the lower forms under constant external 
 conditions is one of the striking characteristics in reactions. 
 There are internal as well as external factors involved in 
 determining what the organism is to do. Just what these 
 are and how they influence reactions is a question of pri- 
 mary importance. 
 
 Many organisms turn or move toward a source of light 
 under certain conditions and away from it under other 
 conditions. They may be either positive or negative; that 
 is, the sense of orientation and response, in general, may be 
 reversed. Nearly all organisms turn through i8o° when 
 the sense of orientation changes so that they always move 
 with the same end ahead. There are however some excep- 
 tions. Radl (1903, p. 91) claims that Daphnia may swim 
 about in various directions with the back constantly facing 
 the source of light. Bohn (1905, p. 8) found that young 
 European lobster larvae always swim with the posterior end 
 directed toward the source of light, so that when they are 
 positive this end is ahead, and when negative it is behind. 
 Hadlcy (1908, p. 260) observed the same in the larvae of 
 the American lobster, as did also Lyon (1906) in several 
 larval stages of Palacmon. I observed similar methods of 
 locomotion in the larvae of several other decapod Crustacea. 
 In many of the lower forms orientation results from re- 
 sponses to changes in light intensity. When these forms are 
 negative they respond only to an Increase of intensity, and 
 when they are positive only to a decrease. It will be our 
 
 264 
 
REGULATION OF REACTIONS 265 
 
 primary aim in this chapter to consider the factors involved 
 in producing these changes. 
 
 I. Change in Sense of Reactions 
 
 a. Effect of intensity of light. — Famintzin (1867, p. 20) 
 appears to have been the first to observe and record, 
 although not very definitely, that the sense of reaction in 
 organisms depends upon the intensity of the light. He 
 placed a shallow dish containing Chlamydomonas and Eu- 
 glena in diffuse daylight and a similar one in direct sunlight, 
 and covered about three-fourths of each at the room side 
 with an opaque screen. In the diffuse light the organisms 
 collected at the window side of the dish; in the direct sun- 
 light they collected in the shadow of the screen at the 
 opposite side. In 1872 Muller recorded the observation 
 that seedlings of Lapidium, which bend toward the source 
 of light if it is moderately strong, turn and bend in the oppo- 
 site direction if it is very intense, e.g. direct sunlight. 
 Strasburger (1878, p. 572) was perhaps the first to actually 
 see motile organisms turn about when the light intensity 
 was changed and swim in the opposite direction. He found 
 that various swarm-spores, which were strongly negative in 
 a given light intensity, became positive when the microscope 
 was moved farther from the window, but that they turned 
 about and swam in the opposite direction when the micro- 
 scope was again brought to the window. " Falls die 
 Schwarmer nicht zu grosse Neigung haben sich niederzu- 
 setzen, kann dies Spiel beliebig wiederholt werden." 
 
 Reactions similar to those mentioned above were seen 
 by Stahl (1880, p. 412) in Vaucheria, by Berthold (1882) 
 in marine algae, by Verworn (1889, p. 50) in diatoms, by 
 Wiesner (1880, p. 38) in tendrils of Ampelopsis and Vitis, 
 by Oltmanns (1897, p. i) in Volvox, Phycomyces and vari- 
 ous seedlings, by Lubbock (1884) and Ostwald (1907) in 
 Daphnia, by Wilson (1891) in Hydra, by Frandsen (1901) 
 in Limax, by Radl (1901, p. 83) in Simocephalus sima, by 
 
266 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 Adams (1903) in Allolobophora foetida, by Parker (1902, 
 p. 119) in Lahidocera, 1)\- Ycrkes (1902) in Gonionemus, by 
 Groom and Locb (1890, p. 169) in nauplii of Balanus, by 
 Loeb (1905, p. 276) in Polygordius larvae,^ by Hadlcy (1908) 
 
 ' In an address published after this part of the manuscript was finished 
 Loeb says (1909, p. 34) that if organisms are positive in a given Hght inten- 
 sity they are positi\e in every intensity to which they respond at all, and 
 that forced suggestions in connection with the theory of natural selection 
 are responsible for the idea that they aggregate in the intensity of light 
 best suited for their general welfare. "Man hat nun auch versucht, zu 
 zeigen, dass die Organismen eine 'Lichtstimmung' besitzen und ihren 
 Ileliotropismus so regulieren, dass sie stets in dicjenige Lichtintensitiit 
 kommen, welche fiir ihr Gedeihen am besten geeignet ist. Ich glaube, dass 
 es sich liier ebenfalls um eine den Forschern durch die extreme Zuchtwahl- 
 theorie aufgezwungene Suggestion handelt. Ich habe an einer grossen Zahl 
 von Organismen \'crsuche angestellt, abcr ich habe bei klarer Anordnung 
 dcr physikalischen X'ersuchsbedingungen auch niemals eine einzige Erschei- 
 nung gefunden, welche fiir eine derartige AnjKissung spricht. Es hat sich 
 stets herausgestellt, dass positiv heliotropische Tiere gegen Licht jeder 
 Intensitiit, sobald nur die Reizschwelle iiberstiegen wird, positiv helio- 
 tropisch sind. . . . Eine 'Auswahl ' einer passenden Beleuchtungsintensitat 
 habe ich nie beobachtet." 
 
 I am unable to understand how anyone can accept the statements quoted 
 above in the face of the numerous records to the contrary; nor can I reconcile 
 these statements with those of Loeb in earher publications. He says (1905, 
 p. 272), "Groom and I described some observations at Naples on the be- 
 havior of the nauplii of Balanus perforatus, and certain other marine animals, 
 which were at times negatively hcliotropic, and at other times positively 
 heliotropic. We found that the intensity of the light determines the sense 
 of heliotropism in these animals. Above a certain intensity light makes 
 these animals negatively heliotropic, and this the more quickly the greater 
 the intensity of the light. By lamplight the animals were always positively 
 heliotropic." (p. 276), "The heliotropism of Polygordius larvae can also 
 be influenced by light. This influence consists chiefly in the fact that direct 
 sunlight makes positively heliotropic animals negative. I did not succeed 
 in making negatively heliotropic larvae positive by exposing them to weak 
 light." The idea that these larvae do not become positive in weak light 
 is however not supported by Loeb's observations as recorded in the same 
 paper a few paragraphs farther on. Referring to Polygordius larvae which 
 had become negative in direct sunlight he says (p. 277), "When later I 
 carried the animals -back into the north room and kept the temperature 
 constant at i5°-i6° C, they again became positively heliotropic in the course 
 
REGULATION OF REACTIONS 267 
 
 in lobster larvae, and by various investigators in a number 
 of other organisms. 
 
 It will thus be seen that reversal in the sense of orienta- 
 tion caused by the effect of light is widely distributed among 
 living organisms. Is this effect of light due to stimulation 
 caused by the process of changing the intensity, as in case 
 of orientation in Euglena, for example, or the contraction 
 of Hydroides ? Or is it due to continuous illumination, as 
 in case of the activity of many organisms ? In other words, 
 is it due to the time rate of change, or to constant intensity ? 
 I have frequently observed that Chlamydomonas, Euglena, 
 Volvox, and other similar forms do not become negative at 
 once if the light intensity is suddenly increased above the 
 optimum. These organisms must be exposed to the higher 
 intensity for some little time before the sense of reaction is 
 
 of twenty minutes "; and on the following page he says, "I again took some 
 animals which had become positively heliotropic in the north room, and 
 convinced myself first of all that at a constant temperature of 20° C. they 
 would become negatively heliotropic in direct sunlight in a few minutes, 
 I then returned them to the north room, and here the animals again be- 
 came positively heliotropic at the same temperature in the course of fifteen 
 minutes." 
 
 It would be difficult to state in more explicit terms that the nauplii of 
 Balanus and the larvae of Polygordius are negative in strong light and 
 positive in weak than Loeb has done in the passages quoted from his pub- 
 lication of 1905, and it would be equally ditlicult to state more explicitly 
 that they are not negative in strong and positive in weak light than he has 
 stated in his address of 1909. 
 
 If these organisms are negative in strong and positive in weak light, as 
 Loeb's experiments indicate, it is evident that their reactions tend to keep 
 them in light of moderate intensity, an idea quite in harmony with those 
 Loeb rejects in his recent address. As a matter of fact it is not at all difficult 
 to find Chlamydomonas, Euglena, Volvox, or any other similar organisms 
 in such a state that they are neutral in a given light intensity, positive in a 
 lower intensity, and negative in a higher. I have repeatedly observed this in 
 all of these forms as well as in several others; and in case of Volvox I have 
 many times observed, as stated elsewhere, that the colonies collect in great 
 numbers in the open spaces between pond-lily leaves and other water plants 
 on dark, cloudy days, but that they collect in shaded places when the sun 
 is bright. 
 
268 
 
 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 reversed. This is clearly shown in the following observa- 
 tions on Volvox, graphicalh- represented in Fig. 33. By 
 referring to path A it will be seen that the colony introduced 
 at n was positive to light from the three glowers as well as 
 
 a 
 
 1 
 
 g 
 
 Fig. Si- The lines A and B represent the courses taken by single Volvox colo- 
 nies as seen in water 2 cm. deep in a plate-glass aquarium c. (The paths are rep- 
 resented in approximately accurate proportions); g, a group of three 222-volt 
 Nernst glowers in a vertical position; a, carbon arc; /, direction of light rays; d, 
 opaque screens; nn', path with glowers exposed and arc shaded; cc', path with arc 
 ejcposed and glower shaded; c'n, path with both glowers and arc exposed. 
 
 to that from the arc, but that it became negative after 
 swimming toward the arc for a short distance from r , turned 
 about and moved across the aquarium to c' . That is, at 
 the end of the experiment the colony was negative to a 
 much lower light intensity than at the beginning. The 
 arc was approximately 250 candle power. It was 15 cm. 
 
REGULATION OF REACTIONS 269 
 
 from the point where the organism became negative. The 
 light intensity at this point was therefore 1 1,1 11 ± ca. m. 
 But the colony was still negative after having crossed the 
 aquarium, a distance of nearly 8 cm., or nearly 23 cm. 
 from the arc, i.e., in an intensity of 4726 ± ca. m., which 
 is 6385 ± ca. m. less than the intensity in which it first be- 
 came negative. Similar results are represented in path B^ 
 but unfortunately the distances between the sources of light 
 and the aquarium, in this exposure, were not recorded. 
 
 The colony which produced path B was positive to the 
 light from the arc when first put into the aquarium at c, 
 but after moving toward the source of light a few centi- 
 meters it became negative, turned about and moved in the 
 opposite direction. When it reached c' the glowers were 
 exposed and the colony promptly changed its direction of 
 motion and proceeded on a course directed from a point 
 between the two sources of light. This point, however, was 
 much nearer the arc than the glowers, the light from the 
 former being much more intense than that from the latter. 
 When the light from the arc was cut off at n, the colony 
 was found to be negative to the comparatively weak light 
 from the glowers. It consequently changed its course and 
 moved from this source; but after continuing about 3 cm. 
 it became positive, turned about and moved toward the 
 glowers to n' , and probably would have continued farther 
 had it not been prevented from doing so by the wall of the 
 aquarium. It will be noticed that the point n' , where the 
 colony was still positive at the end of its course, was about 
 3 cm. nearer the glowers than w, where it proved to be nega- 
 tive, and nearly 7 cm. nearer than the point where it 
 changed its course from negative to positive. That is, the 
 organism was positive at n' in a much higher light intensity 
 than that in which it was negative at n and at the point 
 where it changed from negative to positive. 
 
 This shows that there were very striking changes in the 
 optimum in these colonies. It also shows that the reversal 
 in sense of reaction was not due to an effect produced by 
 
270 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 the processes of changing the intensity; for if it had been 
 the colony on path A would have turned from the source 
 of light at c in place of toward it, and then from it after 
 having been exposed to the higii intensity for some little 
 time. The fact that the colon)- m()\cd toward the arc 
 light some little distance after turning at c, and that it was 
 negative in a much lower light intensity a little later, shows 
 clearl\- that there is some time required to bring about the 
 changes in the organism which determine whether it shall 
 be negative or positive. Re\ersal in the sense of reaction 
 is not mereK' dependent upon the intensity but also upon 
 the time of exposure. It is probably a function of the 
 product of intensity and time. It is therefore evident that 
 the change in sense of orientation in these lower forms is due 
 to continued illumination, while orientation is due to change 
 in the intensityof illumination. If all reactions are regulated 
 b\- chemical changes, there must be at least two different 
 sets of chemicals involved, one which is influenced by 
 changes of intensity, another by constant intensity. I have 
 discussed the possible nature of the chemical changes asso- 
 ciated with reversal in reactions in a former paper (1907, 
 pp. 1 57-161). and shall refer to it in this volume under 
 theoretic considerations. Chapter XX. 
 
 Reversal in the sense of reaction is of the greatest impor- 
 tance to the well-being of organisms, for, as shown in the 
 preceding chapters, it tends to keep them in the optimum 
 illumination. This is true whether the change of intensity 
 causes reversal in orientation or merely a change in the 
 a\'oiding reactions, shock-movements, for both of these 
 methods tend to produce aggregations at the optimum. A 
 change in light intensity does not however induce reversal 
 in all organisms which respond to light. I was unable to 
 obtain positive reactions in Stentor coeruleus, Amoeba, 
 and fly larvae; and the same is true for many of the pla- 
 narians, inclufling the land planarian Bipalitim kewense, 
 and some other worms, especially in certain stages of their 
 development. 
 
REGULATION OF REACTIONS 271 
 
 There are also many organisms which never become nega- 
 tive in their responses. This is true of the great majority 
 of higher plants and various animals. I exposed the ento- 
 mostracan Scapholeberis armata, and Caprella, Leptoplana 
 tremellaris and the early stages of Eudendrium, Arenicola, 
 Limulus, and many other forms in light of 15,000 ± ca. m. 
 intensity, and found that they remained positive although 
 many were soon injured by the intense light. Carpenter 
 (1908) was unable to make Drosophila negative to light, 
 although he exposed specimens in over 480,000 ca. m., an 
 intensity which produced convulsive reflexes and was un- 
 doubtedly injurious. Those organisms mentioned above 
 which do not become positive thrive in darkness. There 
 is, as stated in the preceding chapters, consequently no need 
 for a positive reaction to light. Those which do not become 
 negative thrive in strong light. Under natural environ- 
 mental conditions they rarely meet with intensities so high 
 as to be injurious. In these animals there is, then, no need 
 for negative reactions. 
 
 Holmes says (1901, p. 233), " Talorchestia longicornis is 
 strongly and permanently positive both in weak and strong 
 light." These animals however come to rest in shaded 
 spots and are usually found under drifts of seaweeds. 
 Orchestia agilis, which is found in similar places, is negative 
 when first exposed, but it soon becomes positive, " the more 
 quickly the stronger the light." After it is positive it 
 " remains so even in the strongest light, but it may be 
 rendered temporarily negative to exposure to light of lower 
 intensity." Similarly Holmes (1905) found that Ranatras 
 are negative when first taken from darkness and later posi- 
 tive, after which they remain positive as long as they are 
 in the light, no matter how intense the light may be or how 
 long they may be exposed. In general, exposure to light 
 tends to make them positive, whereas darkness tends to 
 quiet them and make them negative. It should be empha- 
 sized here that not only the intensity but also the time of 
 exposure has to do with these reactions. Holmes says that 
 
272 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 after seventeen Ranatras, negative in a given light inten- 
 sity, had been exposed for an hour and forty minutes, all 
 became positive. 
 
 The positive reactions in all of the animals just referred 
 to pro\e fatal under certain conditions, and L«jeb (1905, pp. 
 42, 74) claims that the caterpillar of the willow borer and 
 the mud-inhabiting crustacean Cuma Rallikii are positive, 
 although in their natural environment they are never ex- 
 posed to light. Here, then, we have a number of reactions 
 which do not lead the organisms to their oi:)limum, reactions 
 which under certain conditions are clearly not adaptive. 
 But, as already shown, these reactions are non-adaptive only 
 under artificial conditions. It was however reactions of this 
 character that led Loeb (1906, p. 159) to conclude "that 
 the tropisms could not have been acquired by the way of 
 natural selection," and to formulate an explanation of their 
 origin which we shall consider later. Chapter XX. 
 
 b. Effect of change in temperature. — It is well known 
 that temperature affects the activity of organisms. If it is 
 increased above normal, organisms ordinarily become more 
 active and more sensitive to other stimuli until the optimum 
 is reached, when their activity and their response to other 
 stimuli decrease, as they usuall>' do when the temperature 
 is decreased below normal. Changes in temperature may 
 however have quite a different effect on some organisms. 
 
 Strasburger (1878, p. 606) found that haematococcus 
 swarm-spores, which were positive in a given light intensity 
 at 16° to 18° C, became negative when the temperature was 
 decreased to 4° C.,and more strongly positive when increased 
 to 35° C. He obtained similar though somewhat less striking 
 results with other swarm-spores. The degree of change of 
 temperature required to cause a reversal in reaction to light 
 was found to vary with the different organisms and with 
 the same organism under different conditions. Strasburger 
 says (p. 610), " Sind die photometrischen Schwarmer, mit 
 denen experimentir.t werden soil, auf sehr hohe Lichtinten- 
 sitat gestimmt, so wird es, um sie auf den negativ-en Rand 
 
REGULATION OF REACTIONS 273 
 
 des Tropfens heriiberzubringen, niederer Temperatur bediir- 
 fen, als wenn sie auf geringere Helligkeitsgrade gestimmt 
 wiiren. Im ersten Falle wirken Licht und Temperatur 
 sich so zu sagen entgegen, im letztercn so zu sagen 
 gleichsinnig." 
 
 The experimental results of Massart (1891, p. 164) on the 
 flagellate Chromulina confirm those of Strasburger in tliat 
 a decrease in temperature causes these organisms to become 
 negative to light. Loeb (1905, p. 274) however observed 
 that an increase in place of a decrease in temperature causes 
 a change in the sense of reaction. He says that Polygordius 
 larvae which were strongly negative in a given light inten- 
 sity at 11° became strongly positive when the temperature 
 was lowered to 6°, and negative again when it was raised ; 
 that others which were positive at 24° became negative at 
 29°; and that still others positive at 17° became negative 
 at 24°. Loeb claims to have found similar reactions in 
 marine copepods. And Miss Torelle discovered a reversal 
 in reactions in the frog Rana clamata, but here it is again a 
 decrease of intensity that causes negative reactions. She 
 says (1903, p. 487), " A rise in the temperature to 30° C. 
 accelerates the rate of the positive response. A lowering 
 of the temperature to 10° C. produces movements away 
 from the light." Holmes (1905, p. 323) observed similar 
 reactions in Ranatra. He says, " Raising the temperature 
 tends to accentuate the positive phototaxis in Ranatra and 
 lowering it tends to produce the negative reaction. In 
 several experiments two dishes containing Ranatras were 
 set before a window so as to receive the same amount of 
 light. As the specimens had been previously kept in the 
 dark, they showed a negative reaction. Into one dish warm 
 water was poured raising the temperature from about 20° C. 
 to nearly 30° C. In a few minutes the specimens in the 
 warmer dish became positive, the ones in the cool water 
 still showing a negative phototaxis. Ranatras transferred 
 to the cooler dish soon became negative, while those which 
 were picked up in the same way and dropped back into the 
 
2 74 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 warm water from whicli they were taken soon resumed their 
 positi\e reaction." 
 
 Change in temperature does not however cause reversal 
 in reactions to Hght in all organisms. Strasburger (p. 608) 
 discoxered that while the swarm-spores of Ulothrix, Ulva 
 lactua, Chaetomorpha aerea, and Chytridium vorax respond 
 much like those of haematococcus, those of Scystosiphon 
 lomentarium, Chilomonas curvata, Botrydium and Bryop- 
 sis could not be induced to reverse by changing the tem- 
 perature; and the same is true for the copepod Labidocera, 
 and for Daphnia pulex and Cypris, according to the work 
 of Parker (1902, p. 117) and Yerkes (1900, p. 417). I have 
 o]^ser\'ed the same in a number of organisms referred to 
 below. It must be admitted that the above statement with 
 reference to the copepods is somewhat too broad, since 
 Parker tested the reactions only in 10°, 30°, and 35°, and 
 Yerkes studied only the effect of increase in temperature 
 above the normal. 
 
 Original observations. — Observations on the effect of 
 changes in temperature on reactions to light in microscopic 
 forms were made by mounting them on a Pfeffer warming- 
 stage under a large cover-glass sealed and supported by 
 means of a ring of vaseline. The Pfeffer warming-stage 
 consists of a glass cell 1X6X8 cm. with three holes in the 
 ends, one for a thermometer, the other two for water inlet 
 and outlet. It is fastened on the stage of a compound 
 microscope and admits of observation under either low or 
 high power. The temperature is regulated by passing hot 
 or cold water through the openings, and recorded by means 
 of the thermometer. It was thus possible to subject the 
 organisms to gradual or sudden changes in temperature 
 ranging from a little above zero to nearly 100°. 
 
 May II, 1908, at 9 a.m. a few drops of solution were 
 taken from some collected the preceding day and mounted 
 on the warming-stage. The solution contained numerous 
 specimens of Euglena viridis, Euglena deses, Phacus trique- 
 ter, and a few specimens of Euglena spiragyra and Phacus 
 
REGULATION OF REACTIONS 275 
 
 longicaudus. A large majority of all of these species were 
 strongly negative at 22°, in light of 250 ca. m., when first 
 mounted, but after they had been exposed from two to 
 three minutes they became strongly positive without any 
 change in temperature or light intensity. The temperature 
 was now gradually lowered, and as this proceeded the organ- 
 isms became less and less active. At about 12° nearly all of 
 them came to rest and the Euglenae contracted and became 
 nearly spherical as if about to encyst. Thus the organisms 
 lay motionless as the temperature decreased to 8° and 
 finally to 5°. But after having been in this low temperature 
 for nearly five minutes, they gradually became active again 
 and swam about, first in an apparently aimless fashion, but 
 later as definitely and rapidly from the source of light as 
 they had been swimming toward it at 22°. They thus 
 became negative in the low temperature without any change 
 in the intensity of the light. Is this reversal in the sense 
 of reaction due to the effect of changing the temperature, or 
 is it due to the absolute difference in temperature? The 
 following has reference only to Euglena viridis, although 
 the reaction of the other species mentioned above is similar 
 to that in this form. After the Euglenae used in the ob- 
 servations referred to above had been subjected to 5° for 
 some minutes, the temperature was gradually raised and it 
 was found that they were still negative at 8°, but positive 
 at 12°. After having been at 12° for six minutes, the tem- 
 perature was againdecreased, and now It was found that the 
 organisms remained active and positive at a temperature 
 even below 5°. They did not come to rest at 8° as they 
 had when first exposed to decrease In temperature. The 
 temperature was now allowed to rise gradually to about 
 22° in 250 ca. m. About half of the Euglenae collected on 
 the side toward the light and the rest on the opposite side. 
 When the slide was turned end for end, the two groups 
 immediately began to swim in opposite directions in two 
 columns which met and passed near the middle of the field, 
 the positive column above, near the cover-slip, the negative 
 
 ►> 
 
276 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 below, near the slide, owing no doubt to the fact that the 
 source (.A light was somewhat above the level of the stage. 
 About half of the Euglenae were now evidently negative and 
 the rest positive, but half an hour later nearly all were nega- 
 tive, alth(jugh there had been no change in light intensity 
 or temperature. When tlie temperature was reduced they 
 became siill more strongly negative. After keeping the 
 tem|)erature between 5° and 8° for three minutes, it was 
 rapidly raised to 22'' \ the Euglenae were now very strongly 
 positive. They fairly streamed toward the source of light. 
 The temperature was now again reduced and held at 5° for 
 a few minutes, during which the organisms were negative. 
 It was then slowly raised, and many became positive at 
 12°, after wliich it was once more reduced and held at 5° 
 for five minutes, during which the organisms were negative, 
 and then again slowly increased. Many of the Euglenae 
 now became positive at 8°. 
 
 We have thus seen the same individuals within the 
 course of a few minutes in constant light intensity reverse 
 in the sense of reaction several times. We have seen them 
 come to rest as the temperature decreased and become active 
 again as it decreased still farther. We have seen them 
 change from a condition in which they were negative at 
 22° and positive at higher temperature to one in which 
 they were positive at 8° and negative at lower tempera- 
 ture. These observations were repeated many times under 
 different conditions with the same general results. Similar 
 changes in reactions to light were also repeatedly produced 
 by changes in temperature in different species of Chlamy- 
 domonas, Trachelomonas, Chlorogonium and Vol vox. 
 
 These results show: (i) That a decrease in heat energy 
 tends to cause a change in the sense of reaction to light 
 from positive to negative and an increase tends to cause a 
 change from negative to positive. A decrease in heat 
 energy, therefore, produces the same changes in the re- 
 actions to light as an increase in light energy. (2) That 
 the reactions to light of a given intensity depend not only 
 
REGULATION OF REACTIONS 277 
 
 upon the absolute temperature at the time of the observa- 
 tion, but also upon the preceding temperature, the time rate 
 of change in temperature, and the time of exposure at a 
 given temperature. (3) That reversal in the sense of reaction 
 may take place without any change in temperature or light 
 intensity. Reversal in the sense of reaction, therefore, 
 seems to be due to the effect of constant temperature and 
 time of exposure rather than to the effect of change in 
 temperature. 
 
 In some of the forms mentioned above the changes are 
 very indefinite, but in Chlamydomonas alboviridis they 
 are even more pronounced and striking than in Euglena. 
 In studying Chlamydomonas it was also found that only 
 under certain conditions will changes in temperature 
 cause reversal in the sense of reaction to light. Thus, for 
 example, it could not be reversed in specimens which had 
 been in total darkness for twenty-four hours. These 
 specimens were negative at 22° in light having an inten- 
 sity as low as 150 ca. m., and they remained negative 
 when the temperature was raised until they died at a little 
 over 40°. 
 
 No change in the sense of reaction to light could be 
 induced by varying the temperature in either direction 
 between zero and the maximum in the following forms: 
 Stentor, various species of zoeae, Scapholeberis armata, 
 Daphnia, Cyclops, Cypris and a small water spider. 
 Change in temperature, however, has certain effects on 
 the reactions in Daphnia, Cyclops, Cypris and the water 
 spider which are similar to those observed in Euglena. 
 On May 16 several specimens of each of these species were 
 exposed in light having an intensity of 160 ca. m. They 
 were neutral at room temperature (22°), and swam about 
 slowly without orienting or aggregating. When the tem- 
 perature was decreased they gradually became more and 
 more quiet, and finally sank to the bottom motionless, but 
 when the temperature reached 8° they became active 
 again, and soon collected at the side of the dish nearest the 
 
278 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 source of light. When the temperature was decreased 
 still more they became strongly positive. This experi- 
 ment was repeated several times with similar results. 
 Increase in temperature above normal ordinarily causes 
 these organisms lo become more strongly positive until a 
 maximum is reached, when the movements become irregu- 
 lar antl reaction to light ceases. In no instance was it 
 found that any of these organisms became negative owing 
 to changes in temperature. The interesting point in these 
 observations is the fact that they become quiet as the 
 temperature decreases and then active again when it 
 decreases still further, just as in case of Euglena, but the 
 former become onK- more strongly positive, whereas the 
 latter change from positive to negative. 
 
 Not all entomostraca can be made positive by decreasing 
 the temperature. On June i, Alona gracilis was found in 
 great abundance in a Paramecium culture jar. A few speci- 
 mens of Cypris were also found in the same jar. They were 
 strongly negative at room temperature (25°) in light of 
 250 ca. m. The temperature was lowered to freezing, but 
 the organisms were continuously negative whenever they 
 responded at all. 
 
 It is thus evident that in some organisms a decrease in 
 temperature causes negative responses to light, whereas 
 in others it causes positive responses. How this is brought 
 about is very dillicult to see from a physico-chemical point 
 of view, although there are chemical compounds in which 
 decrease in temperature facilitates reactions caused by 
 light, as shown in Tart IV of this volume. The fact that 
 the organisms become quiet as the temperature decreases, 
 and then active again as it decreases still more, is particu- 
 larly puzzling. In some organisms the change in the sense 
 of reaction caused by change in temperature is clearly 
 adaptive, and from this point of view we get some light 
 on the causes of the change in reaction, but of course only 
 a superficial explanation for adaptation is itself a problem. 
 In Euglena and frogs, for instance, the negative reaction 
 
REGULATION OF REACTIONS 279 
 
 to light in low temperature takes them from the surface 
 and prevents their freezing, while in Polygordius larvae 
 and those entomostraca which become negative when the 
 temperature increases the reactions take them out of the 
 warm surface water. I am however not positive that 
 the surface water becomes warm enough to injure these 
 creatures. If it does not, then those reactions are appa- 
 rently not adaptive. 
 
 c. Effect of chemicals. — That the reactions to light in 
 some organisms are closely associated with the chemical 
 constituents of the environment was clearly shown by the 
 experiments of Englemann referred to elsewhere. In these 
 experiments, Englemann found that the green ciliates, 
 Paramecium bursaria and Stentor viridis, respond to light 
 only when the oxygen pressure is below normal, but he did 
 not note any actual reversal in reaction due to changes in 
 the chemical condition of the medium. Loeb (1904, p. 2) 
 however states that Gammarus pulex, which is *' naturally 
 negatively heliotropic " can be made positive by adding 
 small quantities of any of the following substances to the 
 water: carbon dioxid, hydrochloric, oxalic or acetic acid, 
 various narcotics, ** such as ether, chloroform, paralde- 
 hyde, alcohol or esters" and "all the ammonium salts, 
 ammonium hydrate included." The alkalis, excepting 
 NH4OH, urea, oxygen and hydrogen, on the other hand, 
 only excite these creatures; they do not cause reversal in 
 the reaction. Similar results were obtained in experiments 
 on Cyclops and Daphnia. The former however can also 
 be made negative, if it is in the positive state, by the addi- 
 tion of NaOH. "Attempts to make sea-water Gdimm^rus 
 positively heliotropic by CO2 have failed." Holmes (1901) 
 observed that the amphipod Jassa becomes positive when 
 placed into foul sea water. 
 
 The fact that chemicals so very different in their general 
 properties as acids, alkalis and narcotics may have the same 
 effect on the sense of the reactions of these organisms seems 
 to show that the effect of the different chemicals is not 
 
2So LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 specific. The chemicals appear to produce changes in the 
 general state of the organism as a whole or a unit. This 
 idea is strongly su|)i)orted by the observations on Arenicola 
 larvae to be presented later, and b>- the work of Holmes 
 (1905, p. 317) on Ranatra. He found that any condition 
 which causes an increase in acti\ity accentuates the posi- 
 tive reactions to light, while any condition which quiets 
 the organisms tends to make them negative. " The causes 
 that prcxluce the negative reaction are, as a rule, those 
 which lead to diminished activity and excitement. Cold, 
 exposure to darkness, and the quieting effect of contact 
 stimuli lead to a condition of lessened excitability and, 
 perhaps as a result of this, to a negative reaction to light." 
 The same is probably true of many other insects. When 
 a moth becomes quiet it is likely to crawl into dark crevices, 
 but when it is disturbed it Hies tow^ard the light, and the 
 more it is stimulated the more energetically positive it 
 becomes. The pomace fly Drosophila is often found in 
 dark cavities in decaying fruit. If it is disturbed it im- 
 mediately flies out and escapes. Carpenter showed that 
 the stronger it is stimulated the more strongly positive it 
 becomes. Many similar instances could be cited. The 
 strong positive reactions to light in these forms may lead 
 them into fatal surroundings, but ordinarily they are of 
 the greatest importance, for they guide them to places of 
 safety. 
 
 Original observations. — On May 29, 1908, a solution 
 containing numerous specimens of Daphnia, Cypris, Cy- 
 clops, a small water spider about 0.5 mm. in diameter, and 
 various insect larvae, all taken from a shallow pond the 
 preceding day, were exposed in light of 200 ± ca. m. Some 
 of the individuals of the different species were negative 
 but most of them were neutral. In these there was no 
 apparent response to light. They remained equally scat- 
 tered throughout the acjuarium and swam slowly about. 
 Pure CO2 was now allowed to bubble through the water 
 very slowly. Nearly all of the organisms except the water 
 
REGULATION OF REACTIONS 281 
 
 spiders soon became more active and began to swim 
 toward the light side of the aquarium, where in the course 
 of a very few moments they formed a dense aggregation. 
 Those which had been negative as well as those which were 
 neutral had become positive. They remained at the more 
 highly illuminated side of the aquarium only four to five 
 minutes, then gradually scattered about again. A little 
 more CO2 was then added to the water and the organisms 
 became positive again. This process was repeated several 
 times. When air was forced through the water they 
 scattered almost immediately, and became indifferent to 
 light, or sometimes slightly negative. It is consequently 
 not the agitation which makes them positive when CO2 is 
 allowed to bubble through the water. These results seem 
 to indicate that the change in reaction to light is dependent 
 upon the change in amount of CO2 as well as upon the 
 absolute amount. 
 
 In these experiments the water spiders became quiet 
 when the CO2 was added and sank to the bottom, but in 
 some later experiments they also became strongly positive. 
 In Stentor, Chlamydomonas, Volvox and Scapholeberis no 
 change in sense of reaction could be induced by means of 
 adding CO2. They become quiet and sink to the bottom 
 after the CO2 reaches a certain concentration. I was 
 unable to change the sense of reaction to light in several 
 different zoeae and in the larvae of Hydroides dianthus 
 by means of hydrochloric-acid solutions. The hydroides 
 larvae were exposed in sea-water solutions of HCl varying 
 in strength from w/250, in which they were immediately 
 killed, to w/7250, in which their response was normal in 
 every respect, both in light intensities so low that they were 
 positive and so high that they were negative. 
 
 In Arenicola larvae however the sense of reaction to 
 light can be reversed by means of various chemical solu- 
 tions. These larvae are strongly positive during the first 
 few days, even in very intense light. They swim freely 
 through th^ water near the surface. Later they settle to 
 
282 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 the bottom and become slightly negative to light. On 
 August 6, 1909, a considerable number of larvae, a few 
 hours after the>' had emerged from the egg-strings, were 
 put into each of several small glass aquaria containing sea 
 water. The lar\ae were strongly positive in the light 
 intensity in which they were exposed. To one of the 
 aiiuaria distilled water was added drop by drop, until the 
 larv^ae no longer responded to light; to another concentrated 
 sea water was added, and to each of the others a weak 
 solution of one of the following chemical compounds: 
 chloroform, adrenalin, atropin, carbonic acid, hydro- 
 chloric acid, acetic acid, magnesium sulfate, magnesium 
 chlorid, ammonia and sodium hydrate. 
 
 After the larvae became neutral in each solution, they 
 were left undisturl)ed. If they became positive in the 
 course of a few minutes, as frequently happened, the solu- 
 tion was made stronger until they became neutral again. 
 In the solutions containing carbon dioxid, caffeine, dis- 
 tilled water or concentrated sea water, the larvae became 
 negative in the course of a few minutes and collected at the 
 side of the aquaria farthest from the source of light, but in 
 no instance was the negative reaction as marked and 
 precise as the positive had been. There was no very 
 definite orientation in the negative specimens, no such 
 streaming from the source of light as there is toward it 
 under normal conditions. Many of the larvae in each 
 solution settled to the bottom of the aquaria and remained 
 there, having apparently lost all power to respond to light. 
 The larvae in the vSolution containing magnesium sulfate, 
 magnesium chlorid, hydrochloric acid, acetic acid, ammonia, 
 sodium hydrate or atropin, also became negative, but it 
 required a much longer time. In some of these solutions 
 the larvae did not become negative until several hours 
 after the compounds had been added. 
 
 After becoming negative the larvae usuall}' remain so 
 permanently, or at least for several hours. For example, 
 those which became negative in the afternoon of August 6 
 
REGULATION OF REACTIONS 283 
 
 were still negative the following morning. Many of those 
 in diluted sea water and in sea water containing CO2 were, 
 however, positive after being in over night. In these 
 aquaria there were two collections, one at the end toward 
 the light and one at the opposite end. In those solutions 
 containing ammonia, sodium hydrate or magnesium sul- 
 fate the aggregation at the negative side of the aquarium 
 was much more pronounced the following morning than it 
 had been the preceding evening. Larvae taken from any 
 of these solutions and put into normal sea water became 
 positive in the same light intensity almost immediately in 
 every instance. I did not succeed in producing reversal 
 in reactions with chloroform or adrenalin, nor did I suc- 
 ceed by changing the temperature. The experiments under 
 these conditions were, however, not very extensive. 
 
 It is at once evident that there is a striking difference 
 between the reversal in reaction in such forms as Chlam>'do- 
 monas and Arenicola larvae. In the former the change is 
 comparatively sudden, sharp and definite, and the nega- 
 tive orientation is as accurate and precise as the positive 
 orientation. In the latter the change is comparatively 
 slow and indefinite, and negative orientation is much less 
 precise than positive orientation. In Arenicola larvae it 
 appears that any condition which acts as a depressant 
 tends to cause the young positive larvae to become nega- 
 tive. These larvae become negative under normal condi- 
 tions as they grow older. Depressants apparently hasten 
 the appearance of this state, and under their influence 
 larvae become negative earlier than they otherwise would. 
 
 d. Effect of concentration of the medium and mechani- 
 cal stimuli. — We have already stated the fact that Areni- 
 cola larvae become negative both in concentrated and in 
 diluted sea water. Loeb (1893, pp. 94, 96) was able to 
 make negative Polygordius larvae positive by adding i to 
 1.3 per cent. NaCl to the sea water and positive individuals 
 negative by diluting the sea water with 40 to 60 per cent, 
 fresh water. Similar results were obtained with cope- 
 
2 84 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 pods. Minkiewicz (1907, p. 50) says that Linens ruber, 
 which is ordinarily nej^ative in the regions of the spectrum 
 toward the \ iolei end and positive in those toward the red 
 end, becomes positive in the former and negative in the 
 latter if subjected to a solution consisting of 25 to 80 c.c. 
 of (Hstilled water and 100 c.c. of sea water, but that the 
 reactions to white hght remain negati\e. There may then 
 be, according to Minkiewicz, a reversal in the sense of reac- 
 tion to light of given wa\e lengths without a reversal in the 
 sense of reaction to white light. 
 
 In working on the light reactions of Temora longicor- 
 nis, a copepod, Loeb (1905, p. 282) noticed that the animals, 
 ordinarily negative, were frequently positive immediately 
 after being caught. This change in the sense of reaction 
 was due probably to mechanical agitation. Miss Towle 
 (1906, p. 345) obtained similar results. She found that 
 the light reaction of Cypridopsis could be temporarily 
 changed from negative to positive by taking the animals 
 up in a pipette or by making them pass through a maze 
 constructed with needles, but that they could not be 
 changed in the opposite direction. In certain organisms 
 however precisely the opposite change takes place. Holmes 
 (1905, p. 319) observed that Ranatra becomes negative if 
 it is handled under water or taken from the water and 
 dropped in again. He also (1901) thinks that the fact 
 that Orchestia gracilis Is positive in air and negative in 
 water may be due to the contact stimulus of the water. 
 It is of interest to note that while these animals are per- 
 manently negative in sea water they become positive in 
 fresh water shortly before they die. The copepod Labi- 
 docera, which is ordinarily positive to light, can, according 
 to Parker (1902, p. 117), be made temporarily negative by 
 vigorously ejecting it from a pipette into sea water several 
 times. 
 
 e. Effect of internal changes. — There are many organ- 
 isms which respond to light in one way during part of their 
 existence and in a different way, or perhaps not at all, 
 
REGULATION OF REACTIONS 285 
 
 during another. Thus we find the plumules of many of the 
 grasses (gramineae) very sensitive to light during the early 
 stages of development and not at all later. Fly larvae are 
 strongly negative, but the imagos are positive. Loeb 
 (1906, p. 134) found that the nauplii of Balanus are positive 
 when they leave the Qgg, but that they become negative 
 soon afterward. I have observed similar changes in reac- 
 tions in the larvae of Arenicola, Limulus, and Hydroides 
 dianthus, in various larvae of crabs and in the medusae of 
 Bougainvillea. There is a striking peculiarity connected 
 with the change in the sense or reaction of Limulus larvae. 
 When these animals proceed toward the source of light 
 they always swim, but when they proceed from it they 
 always crawl. They usually swim most of the time 
 when they are young and are positive, but when they get 
 older they nearly always crawl on the bottom and are 
 negative. If specimens which are crawling from the 
 source of light are agitated until they swim they proceed 
 toward the light, but as soon as they touch the bottom 
 and begin to crawl they go away from the light. I have 
 repeatedly seen specimens in a glass dish swim toward the 
 source of light against the side of the dish, sink to the bot- 
 tom and crawl from the light several centimeters, then 
 start up again and swim toward the light, and so on, 
 repeating the process many times. Contact seems to 
 have something to do with the sense of reaction here, but 
 the fundamental causes of the changes are no doubt rooted 
 in the developmental changes in the organism associated 
 with its habits. In nearly all of the species mentioned 
 above the changes in the sense of the reactions are un- 
 doubtedly adaptive. When the larvae first leave the egg 
 they are strongly positive, and swim out in various direc- 
 tions from the site of their birth so as to become widely 
 scattered. Later, when the developmental processes pre- 
 pare them for sedentary life, they become negative and 
 consequently go to the bottom, where they become at- 
 tached or burrow in the mud. It is not probable that the 
 
286 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 different conditions of light are of vital importance to these 
 creatures. The reactions are in reality responses not to 
 light, hut to what light represents. They appear to have 
 learned to use light as a guide in directing their course in 
 accord with the demands of their state of development 
 and general habits. 
 
 Hadley (190S) made a very thorough study of the 
 changes in the photic reactions of lobster larvae. He 
 found that tliey are positi\'e for about two days after 
 hatching, after which they become negative and remain 
 so until shortly before molting, when they again become 
 positive. Both the early second-stage, and the third- 
 stage larvae are negative but as in the first stage they 
 become positive before molting. The fifth and later stages 
 are persistently negative. 
 
 It has long been known that changes in light cause 
 daily periodic movements in plants, the so-called sleep 
 movements of leaves and flowers, and that these move- 
 ments continue for some time if the plant is kept in con- 
 tinuous illumination. Pfeffer (1906, p. 108) says, "The 
 periodic movements are at first pronounced, both in con- 
 stant light and in darkness, in the case of the leaves 
 of Acacia lophantha, Mimosa pudica, Impatiens noli-me- 
 tangere, and Sigesbeckia orientalis, and they continue 
 to be perceptible until after the lapse of four to eight 
 days." 
 
 Similar after effects have been noted in certain animals. 
 Mitsukuri (1901) observed that the mollusk Littorina is 
 negative when under water during high tide and positive 
 when it is exposed to the air at low tide. Bohn (1905 and 
 1907) made similar observations on Littorina, Hedista 
 diversicolor, and Actinia equina, and claims for them 
 that these periodic changes in the sense of reactions to 
 light continue in harmony with the tide for some days in 
 specimens confined in aquaria where they are not directly 
 affected by the tides. I was unable to confirm the results 
 recorded by Bohn in observations on Littorina littorea at 
 
REGULATION OF REACTIONS 287 
 
 Woods Hole. Nor was I able to confirm them In obser- 
 vations on several related species at the Tortugas. 
 
 We have thus presented various instances in which an 
 organism is positive under given external conditions at 
 one time and negative under precisely the same condi- 
 tions at another time. In some cases this change in 
 reaction requires a long time, in others only a few moments, 
 as e.g., in the reaction of Vol vox represented in Fig. 33. It 
 is evident that such changes must be regulated by inter- 
 nal factors, that they must be due to alterations within 
 the organism itself. As a matter of fact, all reactions are 
 directly controlled by internal factors which are in turn 
 influenced by external factors. The interesting point here 
 is however the fact that we may have movements and 
 change in movements without any immediate changes in 
 the environment. Many instances of this have been cited 
 by Jennings (1906), especially in Chapter XVI. 
 
 The facts (i) that the reactions may be affected in the 
 same way in a given organism by so many contrasting 
 conditions, including concentration and dilution of medium, 
 high and low temperature, acids, alkalis, narcotics and 
 salts; (2) that the same change in external conditions may 
 cause opposite reactions in different organisms, e.g., a rise 
 in temperature causes some to become negative and others 
 positive; and (3) that the sense of reaction may change 
 without any immediate external change, — indicate that 
 these responses are due not to a direct and specific effect 
 of the environment on some definite chemical compound 
 within the organism, but rather to the effect on the organ- 
 ism as a whole. 
 
CHAPTER XIV 
 
 FACTORS INVOLVED IN REGULATING REACTIONS TO 
 
 LIGHT — VARIABILITY AND MODIFIABILITY IN 
 
 BEHAVIOR (continued) 
 
 I. CJia?iges in Sensitiveness, in the Optimum, and iii Vari- 
 ous Other Features Regarding Reactions 
 
 The sensitiveness and the optimum vary greatly in 
 different organisms and in the same organism under differ- 
 ent conditions. In some the optimum is nearly total 
 darkness, in others it is direct sunlight, 5000 ± ca. m. 
 Some are negative in extremely low intensities, others are 
 positive in equally low intensities. The flatworm Bipa- 
 lium kewense, e.g., avoids light so weak that it barely 
 affects the human eye, and responds to the slightest 
 changes in illumination; and the plants Lepidium sativum, 
 Amaranthus melancholicus ruber, Papaver paeoniflorum, 
 and Lunularia biennis bend toward the source of light 
 in an intensity as low as 0.00033 ca. m. (Figdor, 1893). 
 Some organisms are usually negative in direct sunlight, 
 and the intensity may be changed thousands of candle 
 meters without a response. Other organisms are positive 
 in equally high light intensity. What interests us here 
 chiefly is not the difference in response in different species, 
 but variability in response, and the changes in sensitive- 
 ness and in the optimum in given individuals and the 
 regulation of such changes. 
 
 Strasburger (1878) found that if swarm-spores are kept 
 in light of relatively high intensity their optimum is much 
 higher than if they are kept in weak illumination. These 
 organisms, then, adapt themselves in some way to the 
 
 288 
 
REGULATION OF REACTIONS 289 
 
 environmental conditions. They tend to become attuned, 
 as Strasburger puts it, to the Hght intensity of their en- 
 vironment; they become accHmated. The sensitiveness and 
 the optimum, as well as the reactions in general, at any 
 given time, depend upon the preceding exposure of the or- 
 ganism. This is well illustrated by the behavior of Volvox 
 as observed by the writer. 
 
 On July 30, 1904, at 5 p.m., it was found that Volvox, 
 which had been collected at 6 a.m. and kept in the dark all 
 day, responded definitely to light of 0.16 ca. m. intensity, 
 and quite definitely to light of 0.14 ca. m. This is the 
 lowest intensity to which any response was obtained at 
 any time. Specimens collected shortly after 12 M., July 14 
 and 15 respectively, and tested as soon as brought into 
 the laboratory, responded to light of 0.50 to 0.83 ca. m. 
 The sky was clear on both of these days, but the organ- 
 isms were found among the water plants in more or less 
 shaded places. 
 
 It was found at different times that after being exposed 
 to direct sunlight a few moments the colonies did not 
 respond even to an intensity as high as 500 ca. m. We 
 have thus observed the threshold to vary from 0.14 to 
 500 ca. m., and this variation seems to have been due 
 largely to preceding exposure to light. The threshold is 
 higher in colonies pre\dously exposed to strong light than 
 in those exposed to weak light. 
 
 The optimum light intensity for practically all Volvox 
 colonies is somewhat lower than that of direct sunlight, 
 5000 ± ca. m., but sometimes it is very much lower; it 
 varies greatly. This variation is clearly shown in the fol- 
 lowing observation : 
 
 After a few very cloudy days the sun came out at il A.M., 
 July 24, 1904, and the sky became exceptionally clear and 
 remained so the remainder of the day. At 2 p.m. Volvox 
 colonies were found in abundance freely exposed to the 
 sunlight. Some of the colonies were collected and taken 
 to the laboratory, where it was accidentally discovered that 
 
290 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 they were ncgati\'e in light intensities in which this organ- 
 ism had formerly always been luund Lo be strongly posi- 
 tive. I then tested the colonies for the optimum and was 
 greatly surprised to find that they were ncgati\'c to all 
 light intensities above 0.57 ca. m. In li^ht from 0.57 to 
 0.29 ca. m., the lowest intensity to wliich they were ex- 
 posed, iheir reactions were indefinite. There was no indi- 
 cation of any positive reaction whatever. 
 
 Ai different times a number of colonies were taken from 
 a given jar and half of them put into each of two similar 
 vessels containing equal amounts of water. One of the 
 vessels was then exposed to direct sunlight and the other 
 covered so as to exclude all light. After having been in 
 this condition a short time the reactions of the colonies in 
 the two \ essels were compared by exposing both to the 
 same light intensity. In such cases it was always found 
 that the specimens which had been in direct sunlight were 
 negative to light of lower intensity than those which had 
 been in darkness. These results indicate that the colo- 
 nies had not become acclimated to the high light intensity. 
 But they do become acclimated under certain conditions," 
 judging from the observations of Oltmanns, who says 
 (1892, p. 190), that he covered two lots of Volvox with 
 the same kind of prisms, July 31, in the evening. One of 
 these lots with its prism was kept in darkness until 9 a.m., 
 August I, the other was exposed to light. During the 
 hjllowing three days it was found that those which were 
 in darkness until 9 A.M. collected in regions of lower light 
 intensity than the others. Strasburger found the same to 
 be true with reference to the reactions of swarm spores. 
 It seems strange that the effect upon the optimum in 
 colonies exposed for so short a time could, as Oltmanns 
 states, be still observed after three days. 
 
 There are some indications that when Volvox is negative 
 to light of low intensity- it becomes positive when exposed 
 to a much higher intensity. This is shown by the follow- 
 ing observations: 
 
REGULATION OF REACTIONS 29 1 
 
 August 23, 1904, was a bright, clear day. At 4 p.m. 
 specimens were collected in a place which had been well 
 exposed to the sun much of the afternoon. Soon after 
 reaching the laboratory, these specimens were found to be 
 positive in light intensities varying from 230 to 1400 ca. m. 
 The colonies not used in these tests were put into a liter 
 jar and placed in strong diffuse sunlight in a west window. 
 Here many of the colonies soon aggregated on the side of 
 the jar farthest from the source of light. At 5.45 p.m., 
 after having been in the window about an hour, they were 
 found to be negative to an intensity of 230 ca. m. and at 
 6.45 P.M. to an intensity as low as 3 ca. m. They seemed 
 to become more strongly negative the longer they were left 
 in the window, although the light from 6.30 p.m. on was 
 quite dim. At the close of the experiment, 7 p.m., certain 
 colonies which had been strongly negative to an intensity 
 of 230 ca. m. were found to be positive to an intensity 
 of 400 ca. m. The following day these organisms were 
 exposed again to light of 1400 ca. m. and to various 
 lower intensities, but there were no indications of negative 
 reactions. 
 
 In certain cultures of attached specimens of Stentor 
 coeruleus kept in low light intensity I have seen some 
 specimens respond definitely by violent contraction to a 
 sudden increase of illumination of even less than 120 ca. m., 
 while other specimens in the same culture did not re- 
 spond at all, even to a much greater increase. In other 
 cultures under the same environmental conditions none 
 of the specimens could be made to respond even by flash- 
 ing the most intense direct sunlight (5000 ± ca. m.) upon 
 them. 
 
 Free-swimming individuals at times avoid even the 
 faintest illumination, while at other times they are found 
 in strong, diffuse daylight. These creatures apparently 
 become accustomed to light very readily. They were 
 often observed to give very definite responses in diffuse 
 light when first taken from a culture jar, and none at all 
 
292 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 after they had been exi:)osed five minutes. Many similar 
 instances ha\ c been cited in the preceding pages, notably 
 those with reference to reactions to shadows. 
 
 It will thus ])e seen that there is a tendency in organisms 
 toward adajnation to enxlronmental conditions. Exposure 
 to low intensity tends to lower the optimum and increase 
 the sensiti\eness, while exj^osure to high intensity tends 
 to produce the opposite effect. But momentary exposure 
 to high intensity, as we have seen in X'oKox, may actually 
 lower the optimum. The reaction of an organism depends 
 not only upon the rate of change in illumination and the 
 intensity, but also upon the time it is exposed. 
 
 Among the most interesting and conclusive observations 
 on variation and modification in reactions to light are 
 those of Mrs. Yerkes (1906) and Professor Hargitt (1906) 
 on the annelid, Hydroides dianthus. Hydroides, as pre- 
 viously stated, ordinarily jerks rapidly back into its tube 
 when the light intensity is suddenly decreased, but it does 
 not respond when the intensity is increased. This is 
 clearly a reaction to a sign. The decrease of intensity, 
 the shadow, is of no direct consequence to these creatures, 
 but what ordinarily follows the shadow, an attack of an 
 enem\', may be. 
 
 Mrs. Yerkes was primarily interested in modification of 
 behavior. She selected two specimens, one of which did 
 not respond at all to a given reduction of light intensity 
 and the other responded only once. Both however re- 
 acted definitely when lightly touched. For ten days 
 these two specimens were subjected to a series of stimu- 
 lations consisting of shadows followed by light tactile 
 stimuli. The first day one responded to the shadow, 
 alone, three times in forty trials, and the other only once. 
 In the former there was a great increase in the number of 
 responses to shadow from the fourth to the eighth day, 
 then a slight falling off. In the latter the increase was not 
 so great, but still it was definite, especially from the second 
 to the fifth day. It thus appears that these creatures 
 
REGULATION OF REACTIONS 293 
 
 learned to react to the shadow, the sign of the tactile 
 stimulus that regularly followed it. 
 
 . Hydroides becomes acclimated to a given stimulus with 
 surprising rapidity. Mrs. Yerkes (1906, p. 442) found that 
 in sixteen tests out of twenty-seven with different spec- 
 imens the animals responded to shadows passed over 
 them at regular intervals only from one to three times, 
 after which the decrease of intensity did not appear to 
 affect them at all. 
 
 Hargitt records similar results in a paper published a 
 few months earlier than that of Mrs. Yerkes, and again in 
 a later paper. He observed (1909, p. 179) that specimens 
 taken at a depth of from eight to fifteen fathoms react to 
 shadows only in an indefinite way, and that many do not 
 respond at all, indicating clearly that the response depends 
 upon past experience as well as upon present conditions. 
 Jennings (1906) and others have observed similar effects 
 of other stimuli on numerous different species. 
 
 One of the most interesting features in the behavior of 
 Hydroides, and one that has been most accurately recorded, 
 is the variation in the time that these animals remain in 
 the tubes after responding to a given reduction in light 
 intensity. In a series of ten trials Mrs. Yerkes (1906) 
 found the time to vary from 15 to 240 seconds, and in 
 another series of sixty trials from 10 to 710 seconds. 
 There is no apparent regularity in this variation. The 
 author says, referring to the last series mentioned above 
 (p. 447): " The period of retraction is short the first three 
 times — 19 to 34" — but the fourth time it is nearly four 
 minutes. For the next thirteen times it ranges from 
 eighteen to ninety-three seconds; then conies another 
 period of nearly four minutes followed by nineteen con- 
 tractions which last from twelve to eighty-five seconds 
 each and then a contraction of nearly twelve minutes' 
 duration. Thus after the fourth, eighteenth, thirty- 
 eighth and sixtieth trials the animal remained contracted 
 for a relatively long period, varying from four to twelve 
 
294 
 
 LIGHT AXD THE BEHAVIOR OF ORG AX IS MS 
 
 minutes, whereas the intervening contractions seldom 
 lasted more than one and a half minutes and are usually 
 less than thirty seconds." 
 
 Hargitt (1909) extended these observations on the vari- 
 ability in reactions of the tubicolous annelids. Of especial 
 interest are his results with experiments on specimens 
 taken in deep water where shadows are very faint as com- 
 pared with specimens taken from shallow water where 
 changes of light intensity are striking. The following 
 tables illustrate the character of these reactions (pp. 170, 
 
 TABLE \III 
 Showing reactions of specimens from deep waters^ 
 
 August Q, II A.M. August 9, 2 P.M. 
 
 Tc 
 
 mperature, 2 
 
 2*»C. 
 
 
 
 Temj 
 
 )erature, 22.5 
 
 "C. 
 
 
 
 A 
 
 B 
 
 c 
 
 ; D 
 
 E 
 
 A 
 
 B 
 
 c 
 
 D 
 
 E 
 
 I 
 
 _^ 
 
 __ 
 
 ^_ 
 
 ^_ 
 
 
 
 
 
 
 
 
 
 
 
 ^_ 
 
 2 
 
 — 
 
 — 
 
 — 
 
 — 
 
 — 
 
 — 
 
 — 
 
 — 
 
 — 
 
 — 
 
 3 
 
 4 
 
 5 
 
 — 
 
 + 10 
 
 
 
 — 
 
 — 
 
 — 
 
 — 
 
 + iS 
 
 + 
 
 
 +30 
 
 + 20 
 
 — 
 
 + 
 
 + 
 
 — 
 
 — 
 
 — 
 
 — 
 
 — 
 
 6 
 
 — 
 
 — 
 
 
 
 
 
 — 
 
 — 
 
 — 
 
 
 
 — 
 
 — 
 
 7 
 
 8 
 
 + 12 
 
 + 10 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 "— " 
 
 ^^ 
 
 9 
 
 10 
 
 _ 
 
 + 
 
 
 
 
 I 
 
 
 
 I 
 
 I 
 
 
 
 
 
 
 I 
 
 II 
 
 12 
 
 13 
 
 14 
 
 15 
 
 — 
 
 
 
 
 
 
 
 
 — 
 
 
 
 — 
 
 +35 
 
 
 
 
 
 
 
 — 
 
 — 
 
 16 
 
 17 
 
 18 
 
 19 
 
 20 
 
 
 — 
 
 — 
 
 — 
 
 — 
 
 \ 
 
 \ 
 
 
 
 
 
 
 
 — 
 
 = 
 
 * The numbers preceded by -f represent time in seconds animals re- 
 mained in tubes after stimulation. Minus sign indicates failure to respond; 
 zero indicates that animal was in tube when stimulus was applied. The 
 stimulus consisted in turning off a i6-candle-power electric lamp. The 
 intensity is not recorded. Interval between successive stimuli, usually 
 5 minutes. (Hargitt, 1909, pp. 159 and 170.) 
 
REGULATION OF REACTIONS 
 
 295 
 
 TABLE IX 
 
 Showing reactions of two specimens F and G, from shallow water. 
 Legend same as for Table VIIL See footnote. 
 
 I 
 
 o 
 
 4 
 
 5 
 6 
 
 7 
 
 8 
 
 9 
 
 ID 
 II 
 
 12 
 
 13 
 14 
 
 15 
 i6 
 
 17 
 
 i8 
 
 19 
 
 20, 
 
 ID A.M. 
 
 22° 
 
 40 
 
 30 
 
 30 
 
 32> 
 60 
 
 45 
 100 
 
 300 
 40 
 
 30 
 180 
 
 43 
 180 
 
 75 
 
 75 
 470 
 
 50 
 60 
 
 150 
 
 360 
 60 
 
 180 
 60 
 
 50 
 
 50 
 90 
 
 60 
 
 35 
 120 
 
 300 
 
 60 
 
 90 
 
 105 
 90 
 
 130 
 
 150 
 
 150 
 225 
 
 90 
 
 2 P.M. 
 23° 
 
 15 
 
 20 
 
 10 
 
 30 
 
 12 
 
 45 
 
 10 
 18 
 
 30 
 
 45 
 
 15 
 
 50 
 
 12 
 13 
 
 35 
 80 
 
 — 
 
 100 
 
 IS 
 
 40 
 
 12 
 
 no 
 
 45 
 
 90 
 
 75 
 
 IS 
 
 20 
 
 90 
 
 20 
 
 90 
 
 12 
 
 105 
 
 — 
 
 80 
 
 15 
 
 85 
 
 II 
 18 
 
 45 
 
 The fact that there Is no " definite law In relational 
 sequence " in the reactions of Hydroldes, especially the 
 fact that the time that a given specimen remains In the 
 tube varies so much without any observable regularity or 
 relation with environmental changes, has led Hargitt to 
 conclude that behavior of organisms cannot be explained 
 by the application of purely physical principles and to 
 sympathize "with a tendency to postulate the presence of 
 certain psychic factors." 
 
 However one may regard Hargitt's conclusion, his results 
 seem to show clearly that the Immediate environment 
 at any given time will not account for the reactions of 
 Hydroldes at that time, that they are dependent upon 
 
296 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 internal as well as external factors, and that if the inter- 
 nal processes, physiological changes, do account for the 
 varial)ility in the reactions these processes cannot be run- 
 ning their course with any degree of regularity. 
 
 Among the crustaceans and the higher forms varia- 
 bilit\- and moditiability in reactions to ligiil are com- 
 mon. Holmes (1905) found that Ranatra with the left 
 eye blackenctl lends to turn to the right in going toward 
 a source of light, but after several trials it goes nearly 
 directly toward it. "In the first trial the insect veered 
 over constantly to the left, passed by the lamp and went 
 off fn^m the table before it turned around. In the fol- 
 lowing trials a marked tendency to turn to the left is also 
 shown; frequently the insect makes one or more complete 
 circus movements to the left before reaching the light. 
 At the eleventh trial its course is corrected for the first 
 time by a turn to the right side, but, instead of going 
 straight up to the light, it performed a complete circus 
 movement to the left before reaching it. The next time 
 the course was corrected by a sharp turn to the right and 
 the circus movement was dispensed with. At the next 
 trial the course was corrected in the same way, and at the 
 fourteenth attempt the insect deviated only slightly to 
 the left side and then turned to the right to reach the 
 lamp. In the following ten trials it reached the light by 
 a nearly straight path. Whenever it began to turn away 
 from the light to the left it corrected its course by a direct 
 turn in the opposite direction instead of going around in 
 a complete circle as at first." 
 
 Spaulding (1904) observed that hermit crabs are ordi- 
 narily positive. They usually collect in the more highly 
 illuminated regions of an aquarium. But he found that 
 after shading the part of the aquarium in which the crabs 
 were fed every time that food was introduced, they soon 
 came to the part shaded even before food was put in, 
 quite contrary to their ordinary reaction to light. 
 
 In even casually studying the behavior of bees, wasps, 
 
REGULATION OF REACTIONS 297 
 
 ants and various insects in their natural environment, one 
 can hardly fail to see that their reactions to light are any- 
 thing but fixed. Ants, for example, ordinarily avoid the 
 light. They are said to be negative. But they are not 
 always found in the dark recesses of their nests. Does 
 this mean that the sense of reaction changes? Light 
 undoubtedly guides them at times, and the sense of 
 reaction changes frequently, but sight no doubt plays a 
 part in the reactions. If a nest containing pupae or 
 larvae is opened, a given ant may often be seen, in caring 
 for the young, to travel back and forth repeatedly from 
 the brightest sunlight to the dark cavities of the nest. 
 Here it is evident that the ordinary negative response to 
 light has been modified. Again, the flight of bees from the 
 extreme darkness of the hive out into the brightest sun- 
 light, through shadow and sunshine, into and out of the 
 cavities of flowers and back into the darkness of the hive 
 again, offers another striking example of variability in 
 response to light, for it is no doubt light that guides these 
 organisms in many of their movements, although that in 
 which they are primarily interested is not light, but the 
 objects represented by different conditions or configurations 
 of light. 
 
 We have thus seen that the reactions to light depend 
 upon various agents, and that they are modifiable and 
 variable to a certain degree in all organisms from the 
 lowest to the highest, i.e., that they are in general adap- 
 tive and regulatory. But in none of the lower forms 
 have such striking adaptive changes in reaction to light 
 been observed as Jennings records with reference to other 
 stimuli in his interesting description of the behavior of 
 Stentor and some other organisms (1906, pp. 170-179). 
 There is at present no greater need in the study of the 
 behavior of lower organisms than a comprehensive and 
 thorough quantitative study of the relative activity of 
 the difi"erent factors involved in regulation. 
 
298 LICnT AND THE BEHAVIOR OF ORGANISMS 
 
 General Summary of Part III 
 
 (i) Reactions to light arc in general adaptive. There 
 arc, however, certain reactions which are clearly injurious 
 and often fatal; as, for exaini)le, the thing of insects into a 
 llanie and the positive reactions of organisms which live in 
 darkness. I^ut the positive reactions of insects are ordi- 
 naril\- adwantageous. It is only under artificial condi- 
 tions that the\' prove fatal, and the ancestors of many 
 animals which now live in darkness lived in the light. 
 Positive reactions were probably advantageous to them, 
 and the power to respond thus was probably inherited i)y 
 the offspring, in which it is useless. 
 
 (2) Organisms ordinarily collect in light conditions 
 which facilitate life processes. They get into an optimum 
 light condition either by orienting and moving directly 
 toward it or by random movements; and they remain 
 eitlier because they come to rest there or because they 
 respond with the avoiding reaction when they reach the 
 limit of the region of optimum light. All of the reactions 
 involved in aggregation are responses to changes of inten- 
 sit\', with the probable exception of those in which the 
 organisms come to rest at the optimum. In these, aggre- 
 gation is no doubt due to the effect of continued illumina- 
 tion or constant intensity. 
 
 (3) Many organisms react to light without orienting 
 or aggregating. In nearly all cases these reactions are 
 sudden contractions or changes in direction of movement 
 caused by sudden changes in light intensity, as, for ex- 
 ample, the jerking into its tube of Hydroides when a 
 shadow passes over it. Most of these reactions are highly 
 protective against the attack of enemies, l)ut some serve to 
 indicate the presence of food, as in th.e case of Clepsine. 
 These are clearly reactions to signs. It is not the shadow 
 in which these organisms are interested, but what ordi- 
 nariU' follows. There are, however, organisms which re- 
 spond to the effect of light more directly, as, for example, 
 
REGULATION OF REACTIONS 299 
 
 the contraction of the sea anemone Edwardsia when light 
 is flashed on it. It is the change of intensity that causes the 
 response, but there is no evidence of a reaction to a sign 
 here. These creatures are directly interested in the effect 
 of the Hght which produces the response. Continued illu- 
 mination probably affects the activity of all organisms that 
 respond to light, and change in the sense of reaction 
 when due to the action of light is in all probability due to 
 the effect of continued intensity rather than to change of 
 intensity. 
 
 (4) The actions of organisms may change without any 
 change whatever in external conditions. They may, for 
 example, be positive to light of a given intensity under 
 given conditions at one time and negative to the same 
 intensity under precisely the same external conditions at 
 another. This change in reaction must be due to internal 
 factors. It may take place in course of a few moments, as 
 in Volvox, or it may require weeks, as in the case of the de- 
 velopment of the fly, the larva of which is negative while 
 the imago is positive. Then, again, it may be periodic, 
 as in the sleep movements of many of the plants. 
 
 (5) High light intensity ordinarily causes organisms in 
 the positive state to become negative. There are, how- 
 ever, organisms which do not become negative no matter 
 how high the intensity is, e.g., many plant structures and 
 numerous aquatic larvae. Under natural conditions they 
 do not experience illumination so strong that it is injuri- 
 ous; there is, therefore, no need for a negative response. 
 The positive reactions of insects and other forms which 
 frequently prove fatal under artificial conditions are adap- 
 tive under natural conditions. 
 
 Reversal in the sense of reaction is not a response to a 
 change of intensity. It does not take place until some 
 time after the change is made. There is a time element 
 involved here. It is due to changes occurring within the 
 organisms, caused by continued light conditions, not by 
 changes in such conditions. 
 
300 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 (6) Decrease in temperature causes swarm- spores, Eu- 
 glena, Chlamydomonas, Volvox and other similar organ- 
 isms in the positive state to become negative. Increase 
 of intensity causes the opposite change. Decrease in heat 
 energ>-, therefore, causes the same change in reaction in 
 these forms as increase in Hght energy. In other forms 
 however this is not true. Polygordius larvae, for example, 
 become negatixe when the temperature is increased. In 
 many organisms changes in temperature do not cause re- 
 versal in the sense of reaction. 
 
 (7) In Gammarus pulex, Cyclops, Daphnia, C>pris, a 
 small water spider, and various insect larvae, addition of 
 CO2 causes the reactions to light to become strongly 
 positive. In Gammarus various acids and narcotics and 
 all the ammonium salts also cause strong positive reactions. 
 In Cyclops sodium hydrate causes positive specimens to 
 become negative. In Stentor, Chlamydomonas, Vol vox, 
 and Scapholeberis carbon dioxid does not cause a change 
 in the reaction. In Arenicola larvae various narcotics, 
 acids, alkalis and neutral salts — in general, apparently 
 any substance which acts as a depressant — cause a change 
 from positive to negative reactions. In Ranatra any con- 
 dition which tends to make the animal quiet produces 
 negative reactions, while any condition which excites it 
 tends to produce positive reactions. 
 
 (8) The sense of reaction in most organisms is only 
 temporarily affected by concentration of the medium or 
 mechanical stimulation. Arenicola larvae become nega- 
 tive in both concentrated and diluted sea water, while Poly- 
 gordius larvae become positive in the former and negative in 
 the latter. 
 
 Mechanical stimulation appears to cause Temora and 
 Cypris to become positive, while it causes Ranatra, Or- 
 chestia and Labidocera to become negative. 
 
 (9) The fact that change in sense of reaction can be 
 produced by such a variety of different means seems to 
 show very clearly that this change is not due to a specific 
 
REGULATION OF REACTIONS 301 
 
 interaction between external and internal chemical con- 
 stituents, as, for example, a relation between acids and 
 alkalis, but to an effect on the general state of the organ- 
 ism as a whole. 
 
 (10) Variability and modifiability in response to a given 
 external condition are striking characteristics in the be- 
 havior of all living beings. The optimum and sensitive- 
 ness in swarm-spores, diatoms, Euglena, Stentor, Volvox 
 and other similar organisms have been found to change in 
 accordance with the environment. If they are exposed to 
 strong illumination for some time the optimum intensity 
 increases and the sensitiveness decreases. If exposed to 
 low intensity the opposite change takes place. Not only 
 the light intensity, but also the time of exposure, is active 
 in these changes. Momentary exposure may produce 
 results just the opposite from those due to continued 
 exposure. Changes in response frequently occur with- 
 out any immediate changes in the environment. These 
 changes are regulated by internal factors, physiological 
 processes. Many of the responses to a given light con- 
 dition are extremely variable. This is due to the fact that 
 numerous factors, both internal and external, are involved 
 in these responses. Concerning the internal factors little 
 is as vet known. 
 
PART IV 
 
 REACTIONS IX LIGHT OF DIFFERENT WAVE- 
 LENGTHS OR COLORS 
 
 CHAPTER XV 
 
 ENERGY, PHOTOCHEMICAL REACTIONS AND BRIGHTNESS 
 
 1 1 is assuniL'd by some investigators that the reactions 
 to hght in lower forms, plants as well as animals, are all 
 induced by waves of approximately the same length, and 
 that these waves are the more refrangible in the spectrum, 
 the so-called actinic rays, the rays which are generally sup- 
 posed to have the greatest effect on chemical reactions. 
 
 Davenport (1897, p. 202) closes a brief review of the 
 literature on this subject with the following words: " Thus, 
 without multiplying cases, the results of experiments may 
 be summed up as follows: positively phototactic or posi- 
 tively photopathic organisms are such only in the presence 
 of the blue rays." Referring to experiments with numer- 
 ous different animals, Loeb says (1905, p. 294),^ " [I] found 
 a universal confirmation of the fact . . . that the more 
 strongly refrangible rays of the visible spectrum are the 
 most active heliotropically, as in the case of plants." 
 
 Other investigators have, however, arrived at different 
 conclusions. They claim that all the rays in the visible 
 spectrum and some in the ultra-violet may be active in 
 stimulating organisms and that not all organisms are 
 equally stimulated by the different rays. After disagree- 
 ing with Loeb's statement that the shorter waves are the 
 more efficient in all plants and animals, Nagel adds (1901, 
 p. 294), " Es ist sehr wahrscheinlich dass ftir sehr viele 
 
 ^ Original in Pflugcr's Arch., Vol. 54, 1893. 
 
 302 
 
ENERGY, PHOTOCIIEMICALS AND BRIGHTNESS 303 
 
 lichtempfindliche Thiere das Maximum der Reizwirkung 
 im Gelbgriin liegt, dass sie mit anderen Worten lichtemp- 
 findliche Substanzen besitzcn die dem Sehpurpur der 
 Wirbelthieraugen ahnlich sind." Pfeffer (1906, p. 175) 
 maintains that " the relative efficiency of the different rays 
 is not the same in all plants," and Verworn says (1889, 
 p. 60), '' Es hat sich herausgestellt, dass die mcisten Protis- 
 ten nur auf bestimmte Farben, d. h. Strahlen von bestimm- 
 ten Wellenlangen reagiren, welche durchaus nicht fiir alle 
 die gleichen sind." It is therefore evident that with ref- 
 erence to the reactions of the lower forms there are con- 
 tradictory opinions as to the efficiency of the different rays 
 of light. The same may be said in regard to animals with 
 image-forming eyes. 
 
 Let us review the more important of the experiments 
 which have led to these contradictory opinions and try to 
 formulate the conclusions to which they lead. 
 
 In this review we shall first attempt to ascertain the 
 efficiency of different parts of the spectrum in producing 
 reactions in various plants and animals, then we shall 
 compare this with the distribution of energy in the spec- 
 trum, with the distribution of brightness as judged by the 
 human eye, and with the distribution of actinic or photo- 
 chemical effect. 
 
 Sunlight, as is well known, consists of ethereal vibrations 
 composed of waves varying in length from approximately 
 390'''' to 760"''. Aside from varying in length, light waves 
 may also vary in amplitude, and then there may be innu- 
 merable combinations of waves of different lengths. These 
 three different physical characteristics of light are said to 
 produce different specific subjective sensations in man, 
 known respectively as color-tone or hue, brightness or 
 shade, tint or saturation. The subjective sensations are 
 in all probability in some way associated with the effects 
 of light on chemical changes in the retina. 
 _ Monochromatic light, or light having a fixed color-tone, 
 consists of waves which are equal in length. In accord 
 
304 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 with the above supposition there are, therefore, theoreti- 
 cally as many ditterent color-tones in the visible solar 
 spectrum as there are different wave lengths. Practically, 
 however, these numerous theoretical hues are divided, usu- 
 ally into six classes, — violet, blue, green, yellow, orange and 
 red. Authorities differ but little as to the point of division 
 in the spectrimi between the different colors. Selecting 
 the classification most commonly found we shall refer to 
 wave lengths 390 to 430''" as violet, 430 to 490"" as blue, 
 490 to 560"" as green, 560 to 590"'' as yellow, 590 to 630'''' 
 as orange, and 630 to 760'''' as red. 
 
 I. Energy Distribution in the Spectrum 
 
 The distribution of energy in the solar spectrum as well 
 as that in various artificial spectra has been thoroughly 
 investigated, as has also that of the effect of different 
 rays on a number of different chemical reactions and the 
 distribution of brightness as judged by the human eye. 
 In a recent paper Nichols (1905) has summarized much of 
 the work on the energy in the visible spectrum. He gives 
 the curves of distribution for the normal or grating spec- 
 trum of the following sources of light: Hefner lamp, 
 acetylene lamp, petroleum lamp, illuminating gas with 
 different burners, Welsbach mantle, various electric incan- 
 descent carbon filaments, magnesium flame, direct and 
 diffuse sunlight and a few others. While the distribution 
 of energy in the spectrum differs considerably with the 
 different sources of light, it corresponds in that the energy 
 toward the red end is much greater than that toward the 
 violet (Fig. 34). There is only one exception to this: in 
 case of magnesium oxide the energy is " greater in the 
 violet than in the yellozu and green " {I.e., p. 159). 
 
 In the normal gas spectrum the energy increases very 
 gradually from the beginning of the violet to the begin- 
 ning of the green at about 500"", then it increases very 
 rapidly and reaches its maximum at the end of the red. 
 
ENERGY, PHOTOCIIEMICALS AND BRIGHTNESS 305 
 
 At 450''^' (with a bat's wing burner) the energy is repre- 
 sented by o.ii; at 500^''' by 0.52; at 700''^' by 12.70. In 
 the red, then, the energy is 100 times as great as in the 
 violet and about 20 times as great as in the green. In 
 case of direct sunUght the maximum energy in the normal 
 spectrum is between the yellow and the orange. From 
 this point there is a gradual decrease toward the red end 
 and a somewhat more rapid decrease toward the violet. The 
 difference in energy in different parts of the normal sun- 
 light spectrum is not nearly so great as in the normal gas 
 spectrum. In the prismatic spectrum of both gas and 
 sunlight it is, however, much greater than in the normal 
 spectrum, owing to the relatively greater condensation of 
 the rays toward the red end. Whereas the maximum in 
 the normal sunlight spectrum is between the yellow and the 
 orange at about 600^", in the prismatic spectrum it is, 
 according to Langley (1884), at the end of the red or per- 
 haps even in the infra-red. (Fig. 34.) 
 
 2. Brightness Distribution in the Spectrum 
 
 The determination of relative brightness of different 
 parts of the spectrum is a matter of considerable com- 
 plexity, owing largely to the difficulty of comparing differ- 
 ent colors with reference to brightness and to the fact that 
 the relative brightness of the different colors varies with 
 the absolute intensity, a characteristic known as Pur- 
 kinje's phenomenon. In spite of these difficulties the 
 results obtained by eight or more different methods, all 
 refined in every detail, are in close agreement, indicating 
 that the conclusions stated below are in all probability 
 reliable. 
 
 The method most widely employed is the direct com- 
 parison of the different colors with white light of known 
 intensity. By means of this method Fraunhofer in 18 17 
 located the maximum brightness in the prismatic solar spec- 
 trum in the neighborhood of the line D. In 1871 Vierordt 
 
3o6 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 obtained results which agree fairly well with those of 
 Fraunhofer, b\- means of finding the amount of white light 
 required to make a given color imperceptible when added 
 to it. The results of Vierordt are graphically given in 
 Fig- 34- 
 
 •100 400 520 580 G40 700 TOO 
 
 Fig. 34- Curves representing the relative distribution of brightness and energy 
 in the spectrum, i, brightness curve for color-blind persons showing that the 
 red end is considerably shortened (after Konig, iSgi); 2, brightness curve for 
 normal eye in prismatic solar spectrum (from Davenport, 1897, p. 159, after Vier- 
 ordt, 1873, p. 17); 3, 4, and 5, brightness curves for normal eye in normal gas- 
 light spectrum of low, medium, and high intensities respectively (after Haycraft, 
 1897, p. 141); 6, energy curve for solar prismatic spectrum (after Langley, 1884, 
 P- 233); 7, energy curve for normal gas-light spectrum constructed from data 
 given by Nichols, 1905, p. 151. The value of the ordinates in most of these curves 
 is arbitrary. A-G, approximate positions of Fraunhofer lines in spectrum. 
 
 These curves show clearly that the distribution of brightness in the spectrum is 
 not proportional to the energy of the different parts and that consequently bright- 
 ness must depend upon the length of the waves as well as upon their amplitude. 
 
 Haycraft (1897, p. 140) ascertained the brightness dis- 
 tribution in a normal gas spectrum produced with Hilgar's 
 large spectroscope and diffraction grating, by means of 
 the so-called flicker method. The results obtained by this 
 method, graphically recorded in Fig. 34, agree very well 
 
ENERGY, PIIOTOCIIEMICALS AND BRIGHTNESS 307 
 
 with those Haycraft obtained by measuring the size of 
 the pupil in different parts of the spectrum and by deter- 
 mining the distance at which small areas differing in color 
 become invisible. It will be seen by referring to Fig. 34 
 that the maximum brightness obtained in these experiments 
 in light of high intensity is very near the Fraunhofer line D, 
 that is, between the yellow and the orange, which agrees 
 with the maximum in the solar spectrum obtained by 
 Fraunhofer and Vierordt. It is interesting to note that 
 in low light intensity the maximum is in the green, and 
 that this corresponds fairly well with the maximum for 
 color-blind individuals. 
 
 In case of color-blind individuals the difficulty of com- 
 paring intensity of different colors is of course obviated. 
 The brightness distribution in the spectrum has been 
 ascertained in several cases, all of which are in approxi- 
 mate agreement, the maximum being in the green near 
 the Fraunhofer line E (Fig 34), in fairly close agreement 
 with the maximum for the normal eye in the spectrum of 
 low light intensity. * 
 
 By comparing the curves in Fig. 34 it is at once evident 
 that the brightness distribution in the spectrum is not 
 proportional to the energy. The distribution of brightness 
 in the normal solar spectrum, the prismatic solar spectrum 
 and the normal gas spectrum agrees fairly well in certain 
 respects, while the distribution of energy in these spectra 
 is very different. Brightness sensation is therefore asso- 
 ciated with some specific effect of the length of light waves, 
 as well as with the amplitude of the wave. Color sensa- 
 tion, on the other hand, is associated with the specific effect 
 of the length of the waves and with the effect of combina- 
 tion of waves of different lengths. The specific effect of 
 waves of a given length and amplitude is no doubt due to 
 chemical changes in the retina. Visual purple is most 
 rapidly bleached by the rays in the spectrum between the 
 lines D and E, the region containing the rays which are 
 absorbed most readily, and the region which contains the 
 
3o8 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 maximum l)rightness. There is probably some interrela- 
 tion between these phenomena. It is however not our 
 jHirpose to discuss theories of vision. We wish merely to 
 emi)iiasize thai experimental results appear to show that 
 briglitness is not proportional to the energy in light, that 
 it is a function of wave length as well as of amplitude. 
 
 3, Distribution of Actinic Effect in the Spectrum 
 
 That light causes profound changes in chemical com- 
 pounds is a matter of common information to all familiar 
 with the process of photography. The fact that the 
 shorter waves of the spectrum, the ultra-violet, violet and 
 blue are chieily active in causing changes in the halogen 
 salts of silver and various other metals used in this pro- 
 cess, is at least in part responsible for the idea that photo- 
 chemical changes in general are largely if not entirely 
 brought about by the action of the shorter waves, w^hich 
 are usually referred to as the actinic rays. 
 
 Photochemical reactions are far more numerous in both 
 the inorganic and the organic realms than is generally 
 supposed. Davenport (1897, pp. 161-165) brought to- 
 gether many instances under the following heads: syn- 
 thetic, analytic, substitutional, isomeric, polymerismic, 
 fermentative effects of light. Recent investigations have 
 made known others which are of especial interest to us. 
 Most important among these are numerous reversible 
 reactions, reactions which take place in one direction in 
 daylight or in light of a given wave length, and in the 
 opposite direction in darkness or in light of a different 
 wav^e length. 
 
 The following reversible equations are referred to in a 
 recent paper by Stobbe (1908) on photochemical reactions. 
 The first five are quoted by Stobbe, the rest were dis- 
 covered by him. In these equations the arrow^s indicate 
 the direction in which the reaction takes place in the 
 different conditions of light with which they are labeled. 
 
ENERGY, PIIOTOCIIEMICALS AND BRIGHTNESS 309 
 
 In the first equation, for example, the reaction proceeds 
 toward the right in the Hght and toward the left in 
 darkness. 
 
 light 
 
 1. 2 AgCl ^=f AgaCl +C1. 
 
 dark 
 
 Hght 
 
 2. 2 CuHio < ' C28H20. 
 
 dark 
 
 light 
 
 3. 5AgI^==^ Agl3 + 2Ag2l. 
 
 dark 
 
 4. Tetraphenyldihydrotriazin. 
 
 light 
 White i. rose red. 
 dark 
 
 5. Dimethyloxalessigesterphenylhydrazon. 
 
 light 
 
 White < citron yellow. 
 
 dark 
 
 CeHs— C — CeHs CeHoCH 
 
 6. Triphenylfulgid, OC-C C-CO 
 
 I o i 
 
 light 
 Orange yellow ?=^ light brown. 
 
 dark 
 
 blue or violet (440 to 550""') 
 Orange yellow < ^ =^=^ ^^^^ brown. 
 
 red or yellow (550 to 700"") 
 
 H CI2 
 
 C C 
 
 ^ \ / \ 
 
 HC C CCl 
 
 7. i3-Tetrachlor-a:-ketonaphtalion, I II II 
 
 HC C CCl 
 
 "^ / \ / 
 
 c c 
 
 H O 
 
 ultra-violet 
 
 White < I violet. 
 
 yellow green 
 
310 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 In the last two equations it is clearly shown that the 
 longer waves as well as the shorter are actinic. Stobbe 
 investigated the reactions of numerous so-called fulgides in 
 the different rays of the spectrum ("Steinheilschen Spectro- 
 graphen") and found seventeen which behave much like 
 triphen\lfulgid. There is howexer considerable varia- 
 tion as to the specific effect of the rays in the different 
 forms. In general the shorter waves cause the fulgides to 
 become darker in color, while the longer ones cause them 
 to become lighter. But in some it is the violet whic h pro- 
 duces the dark shades, while in others it is the ultra-violet 
 or the blue. " Je mehr sich die Farbe eines Fulgides ver- 
 tieft, je weiter sich die Absorption eines Fulgides nach 
 dem rothcn Ende des Spectrums erstreckt, um so weiter 
 riickt audi die Erregungszone nach derselben Richtung 
 vor" (1908, p. 31). 
 
 In white light the fulgides turn dark, just as in mono- 
 chromatic light, but strange as it may appear the reac- 
 tion is much less pronounced, even if the white light has 
 more of the effective rays than the monochromatic light of 
 any given region in the spectum. The relatively feeble 
 effect of white light must be due to the presence of the 
 longer waves, which, as represented in the equation above, 
 tend to produce the lighter shades and consequently retard 
 the production of the darker. It may be well to call 
 attention to the fact in passing that the investigations of 
 Lubbock on Daphnia, of Wilson on Hydra and of Wiesner 
 on some of the higher plants show that as in the ful- 
 gides, monochromatic light consisting of certain rays is 
 more effective in causing reactions than the same light in 
 combination with other rays. 
 
 It is evident from the last two equations that the longer 
 rays as well as the shorter may have a specific photo- 
 chemical effect. Triphenylfulgid, e.g., is changed from dark 
 brown to orange yellow by the longer waves and not by 
 the shorter. There are many other reactions which are 
 induced only by the longer waves. Among the most 
 
ENERGY, PIIOTOCIIEMICALS AND BRIGHTNESS 31 1 
 
 important of these the process of photosynthesis in plants 
 furnishes an excellent example. The maximum for this 
 process lies in the red very near the Fraunhofer line C. 
 This is not solely due to the fact that the rays in this 
 region are more readily absorbed than those in the adjoin- 
 ing regions, for the violet rays are also absorbed, and here 
 there is no appreciable effect on photosynthesis. In 
 solutions which contain ferrocyanide or certain other 
 coloring matter the longer waves are also more effective 
 than the shorter, and pure ozone, which is changed to 
 oxygen only in the ultra-violet, is similarly acted upon by 
 the visible rays if chlorine be added. These various 
 examples inevitably lead to the conclusions that while the 
 shorter rays may induce chemical changes in more sub- 
 stances than the longer, they cannot be considered as the 
 only actinic rays. The relative efficiency of the different 
 rays depends first of all upon one or more of the com- 
 pounds between which the photochemical reaction is tak- 
 ing place, but it also, at least in certain cases, depends upon 
 the presence of substance in which no apparent change is 
 taking place. 
 
 Many of the photochemical reactions are exothermal. 
 For example, the light conditions which induce the ful- 
 gides to become dark are much more effective in lower 
 temperature than in higher. According to Stobbe it 
 requires nearly ten times as much light energy to produce 
 a given change at 100° as it does to produce the same 
 change at 87°. A decrease in heat energy, therefore, pro- 
 duces the same effect as an increase in light energy, a 
 statement which at first thought appears self-contradictory. 
 As a matter of fact, however, it merely demonstrates the 
 independence of these two forms of energy in producing 
 chemical reactions. 
 
 It is evident from what has thus far been presented 
 that the actinic distribution in the spectrum is not 
 proportional to distribution of energy. There are many 
 well-known photochemical reactions which occur only in 
 
312 LIGHT AXD THE BEHAVIOR OF ORGANISAfS 
 
 ultra-violet, the region of the spectrum which contains 
 least energy. 
 
 Precisely how light produces chemical changes is un- 
 kn(3wn, but it is clear that only those rays which are 
 absorbed can be elTective. The efficiency is however not 
 proportional to the absorption. According to the excel- 
 lent researches of Luther and Forbes (1909), the reaction 
 between quinine and chromic acid is only affected b>' the 
 rays absorbed by the quinine, and not at all by those 
 absorbed by the chromic acid. Only about 4 per cent of 
 the light absorbed by the quinine is changed to chemical 
 energy. The ultra-violet and violet are most readily 
 absorbed, but the green is most efficient, i.e., a greater 
 amount of chemical action is caused by a given amount 
 of light energy absorbed in the green than by the same 
 amount absorbed in the violet and ultra-violet, showing 
 clearly that the efficiency is not proportional to the ab- 
 sorption. The same is true in case of photosynthesis, 
 which is supposed to be due to the action of light ab- 
 sorbed by the chlorophyll. Chlorophyll dissolved in alcohol 
 has, according to Reinke (1884), a prominent absorption 
 band in the red, a weak band in the orange, the yellow 
 and the green, while from 500^^ on, i.e., in the blue and 
 violet, practically all light is absorbed. The maximum rate 
 of photosynthesis however takes place in the red, from 
 which it decreases rapidly in either direction, so that be- 
 yond the green in the region of maximum absorption there 
 is scarcely any photosynthesis. 
 
 The specific effect of the different rays on chemical 
 reactions as well as on brightness sensation is evidently a 
 function of the length of the waves and the rate of vibra- 
 tion. The effect of the different rays is not proportional 
 to the energy in these rays, but the effect of light of a given 
 wave length is of course dependent upon the amplitude of 
 the waves, their intensity, as well as upon their length. 
 
CHAPTER XVI 
 
 EFFECT OF DIFFERENT RAYS ON THE REACTIONS OF 
 
 SESSILE PLANTS 
 
 It is evident that as in inorganic and organic compounds 
 and in man, so in the lower organisms, the reactions to 
 light may be due to or at least associated with a specific 
 action of the length of the light waves, or with the ampli- 
 tude of the waves, or with a combination of waves of 
 different lengths. In experiments on the effect of colored 
 light on organisms it is therefore essential to know what 
 sort of light is being used as a stimulating agent; many 
 results are unreliable because this was not known, or at 
 least is not recorded. The colors used were frequently 
 produced by means of solutions or colored glass which 
 transmit waves varying much in length. In case of red 
 glass, e.g., there is usually some orange and yellow and fre- 
 quently a little blue, violet or ultra-violet transmitted as 
 well as the red. A reaction in such light, apparently due 
 to the longer waves may actually be due to the shorter, or 
 to the specific effect of the combination. Then, too, the rela- 
 tive intensity of the different colors was often not consid- 
 ered. There was then the possibility that the reactions 
 were due to intensity rather than to color. 
 
 The bearing of this discussion becomes evident when 
 we consider the fact that some organisms are sensitive to 
 light of extraordinarily low intensity; e.g., Figdor (1893) 
 found the plants Lepidlum sativum, Amaranthus melan- 
 chollcus ruber, Papaver paeonlflorum, and Lunularia biennis 
 to respond to light as weak as 0.00033 ca. m. In organ- 
 isms so extremely sensitive It is evidently impossible to 
 be certain as to what causes a reaction If they are not 
 subjected to monochromatic light of known intensity. In 
 
 313 
 
314 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 organisms which arc not very sensitive it is, however, un- 
 likely that a few stray foreign rays mixed with a given 
 color will alter the reactions. In reviewing the work on 
 reactions in colored light and formulating conclusions it 
 will conse(iuently be necessary to consider carefully the 
 methcKis used in such work. 
 
 Poggioli (1817) was the first to study the relative effect 
 of light waves of different lengths on the reactions of 
 plants. He exposed seedlings of Brassica and Raphanus in 
 different parts of the spectrum and found that they turn 
 toward the source of light in the red as well as in the 
 violet, but that the reaction in the latter occurs much 
 more rajiidly than in the former. He does not mention 
 the reaction in other parts of the spectrum. These 
 results seem to have remained unchallenged for twenty- 
 five years, when Payer (1842), after studying the reac- 
 tions of different seedlings in a solar prismatic spectrum 
 and behind different color media, came to the conclusion 
 that red, orange, yellow and green act like darkness, and 
 that blue is more active than violet. This conclusion, 
 however, although supported by Sachs, is not in harmony 
 with the experimental results of Gardner, Dutrochet and 
 Pouillet, Guillemin, Wiesner and others. Gardner (1844) 
 found that all the seedlings in a trough which extended 
 beyond the solar prismatic spectrum into the ultra-violet 
 and the infra-red bent toward the sources of light, but that 
 they deflected slightly toward the indigo. The deflection 
 toward the indigo was no doubt due to light reflected by 
 the seedlings in this region of the spectrum. Dutrochet 
 and Pouillet (1844) obtained similar results in their experi- 
 ments with the roots of white and black mustard in a 
 strong solar prismatic spectrum. They concluded that 
 all the rays, including ultra-violet and infra-red, cause the 
 roots to bend from the source of the light, but that the 
 blue is most active. 
 
 Guillemin (1858) made a more detailed study of this 
 subject than had previously been made. His methods 
 
REACTION OF PLANTS IN COLORS 315 
 
 were excellent. He studied the reactions of white mus- 
 tard and cress seedlings in three different solar prismatic 
 spectra produced respectively by means of flint glass, 
 rock salt, and quartz. All diffuse light was eliminated by 
 means of suitable screens, and twenty-five different tests 
 were made. In all three spectra there were two regions 
 of maximum effect. The primary maximum lay in the 
 violet or ultra-violet in all cases, and the secondary maxi- 
 mum between the infra-red and the green. The minimum 
 effect in all cases was in the blue near the Fraunhofer 
 line F. 
 
 These conclusions were in the main confirmed by the 
 thorough work of Wiesner (1879). Wiesner studied the 
 reactions of stems and roots of several different seedlings 
 behind thoroughly tested absorbing media and in differ- 
 ent parts of a direct sunlight spectrum produced by means 
 of the " Soleil'sche apparat mit Flintglasprisma." The 
 spectrum " showed the Fraunhofer lines clearly " and was 
 consequently relatively pure. I shall mention but one of 
 the numerous experiments the results of all of which agree 
 in general. In this experiment numerous pots of Vicia 
 sativa were distributed in the spectrum from infra-red to 
 ultra-violet. The seedlings between the violet and the ultra- 
 violet began to bend first, then those to the right and left, 
 and later those in the red. In the yellow and orange there 
 was no reaction. Wiesner states the results in the fol- 
 lowing words (1879, p. 190): " Schon nach ij Stunde 
 waren die an der Grenze zwischen Violett und Ultra- 
 violett (H-J) befindlichen Pflanzchen nach vorn geneigt. 
 Nach Ablauf von etwa \ Stunde folgten die im mittlercn 
 Violett und Ultraviolett aufgestellten; eine Viertelstunde 
 spater neigten sich die im Indigo stehenden, 10 Minutcn 
 hierauf die im Blau, nach weiteren 20 Minuten die im 
 Griin und Ultraroth stehenden, sodann, nach einer Viertel- 
 stunde die im aussersten Roth, und nach einer weiteren 
 Viertelstunde die im Roth von B-C. Die Keimlinge in 
 Gelb und Orange standen jetzt, d. i. nach vollen 3 Stunden, 
 
3i6 
 
 UGITT AND THE BEHAVIOR OF ORGANISMS 
 
 noch volllg aufrecht. Eine Stunde spater hatten die vom 
 lndii;(j l)is ins Ultraviolett reichenden Keimlinge sich 
 stark hankcnformig gegen die Lichtciuelle hingewendet, 
 gleichzeitig ncigte sirh das iin Orange stehende Ptianzchen 
 schwach vor. Der ini CvW) betindliche Keiniling blieb 
 aber bis ans Ende des Versuches \ullkonimen aufrecht." 
 The roots of Sinapis alba (white mustard) were found to 
 respond in the spectrum in all essentials like the stems of 
 Vicia sativa, except that they turned from the light in place 
 of toward it. 
 
 I 
 
 Fig. 35. Graphic representation of the reaction of several plants in light differ- 
 ing in wave-length. A-II represent Fraunhofer lines in the spectrum. The 
 curves i, 2, ajid 3 were constructed by tabulating as ordinates the reciprocals of 
 the time required to induce a response under the different light conditions: i. stem 
 of Vicia seedling; 2, stem of cress seedling; 3, etiolated willow shoot. After Wiesner 
 (1879, p. 191). 
 
 The results of numerous observations on the reactions 
 of different seedlings behind different absorbing media all 
 of which were spectroscopically tested are graphically rep- 
 resented in Fig. 35. It is interesting to note that these 
 results agree fairly well with those obtained in the spec- 
 trum, although the different colors were in no instance 
 monochromatic. Wiesner claims that yellow is not merely 
 
REACTION OF PLANTS IN COLORS 317 
 
 neutral, but that it actually causes a retardation in reac- 
 tion when mixed with other active rays. 
 
 Miiller (1872) obtained varying results in his study of 
 the reactions of seedlings in the solar prismatic spectrum. 
 He found the region of maximum effect for cress to be in 
 the blue at line F, and that for Sinapis alba in the yellow 
 and upper part of the green between lines D and E. He 
 claims that this difference is due to the difference of absorp- 
 tion of light under different conditions and in different 
 plants. The farther a plant is from the prism the farther 
 the maximum extends toward the red end of the spectrum. 
 It may even extend into the infra-red. In case of cress 
 seedlings, e.g., set in a row extending from the prism, it 
 was found that those farthest away were neutral in violet 
 and blue but still reacted in green and yellow, whereas 
 those nearer the prism responded most strongly In the 
 violet and blue. Miiller thinks this is due to difference in 
 absorption of different rays under different conditions. 
 
 The work of Sachs in 1864 and later, as already stated, 
 led to conclusions similar to those of Payer. Sachs 
 studied the reactions of various seedlings under double- 
 walled bell jars, some of which were filled with ammoniacal 
 solution of copper hydrate and others with potassium 
 bichromate. The former transmitted violet to green in- 
 clusive, the latter yellow to red. The seedlings under 
 the copper solution curved strongly, while those under 
 the chromate remained straight. Similar results were ob- 
 tained behind cobalt and ruby glass. These results are 
 not in accord with those of Guillemin, Wiesner and others, 
 who found that seedlings responded in the longer wave 
 lengths. The fact that Sachs could get no reaction in red 
 produced by sunlight passed through a solution of potas- 
 sium bichromate was probably, as Pfeffer points out 
 (p. 176), " the result of feeble intensity of the light used, 
 or of the special properties of the experimental material." 
 The experiments of Kraus (1876) with colored screens 
 show that the stalks of the perithecial heads of the fungus 
 
3l8 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 Claviceps microcephala turn toward the source of light 
 nearly as rapidh* in the red as in tlie blue, and Brefeld 
 obtained similar results for Pilobolus microsporus and 
 Pilobolus crystallinus. 
 
 Some of ilie contradictory results mentioned above are 
 evidently due to the fact that the authors did not take into 
 account the effect of the time of exposure and the inten- 
 sity of I he W'^ht. If the blue, for instance, is more active 
 than the red, the maximum curvature will take place in 
 the blue in weak illumination, while a minimum curvature 
 will take place in this region in very strong illumination; 
 for plants either become negative in high light intensity or 
 fail to respond. Recently Blaauw (1909) made a thorough 
 investigation of the reactions of plants in different regions 
 of the spectrum with these facts in mind. He calculated 
 the relative efficiency of different rays in terms of energy 
 contents and time of exposure of the reacting organ, and 
 found for oats seedlings, Avena sativa, that in medium light 
 intensity and time of exposure there is a slight reaction from 
 the red end of the spectrum to the green, 500"'", then a 
 rapid increase to a maximum in the indigo 465''", and a 
 decrease to zero well in the ultra-violet. For equal energy 
 and time of exposure the reaction is 2600 times greater in 
 the region of maximum efficiency than in the red, yellow 
 and green, twice as great as between the violet and the 
 ultra-violet, 390"'', and only four times as great as in the 
 ultra-violet at 365''''. 
 
 Tn case of the fungus Phycomyces, Blaauw found the 
 effect of red and yellow relatively much greater than in 
 Avena. In other respects the distribution of efficiency in 
 the spectrum was found to be similar in the two forms. 
 
 In spectra of very high intensity produced by means of 
 a grating it was found that the minimum rate of curvature 
 for Phycomyces is in the indigo, and that there are two 
 regions of maximum rate of curvature, one in the red, the 
 other in the violet. This, the author maintains, is due to 
 the fact that Phycomyces is positive in weak light (100- 
 
REACTION OF PLANTS IN COLORS 3^9 
 
 150 ca m. seconds), neutral in strong light (100,000- 
 200,000 ca.m. sec.)/ and negative in very strong light 
 (2 000,000-12,000,000 ca. m. sec). However, the red and 
 yellow rays are, under all conditions, relatively more eth- 
 cient in the molds than in green plants. The difference 
 in the distribution in the spectrum of the stimulating 
 efficiency in different intensities of light may possibly 
 account for the discrepancies in the results of former 
 investigation in this field. 
 
 Summary 
 
 (i) According to Gardner all visible rays are active in 
 producing curvature in plants; according to Dutrochet 
 and PouiUet, GuiUemin and Muller, plants respond to all 
 visible rays and some ultra-violet and infra-red as well; 
 according to Wiesner they respond to all rays in the visi- 
 ble spectrum except some yellow and orange, and possibly 
 some beyond at either end ; according to Kraus and Bre- 
 feld the red is nearly as active as the blue in causing 
 reactions in the molds; according to Blaauw all the visible 
 rays of the spectrum and some ultra-violet produce cur- 
 vature in oats seedlings and molds, but the longer waves 
 are relatively more active in the latter than in the former; 
 according to Sachs and Payer only the shorter waves are 
 active. Thus it is seen that all but Sachs and Payer 
 obtained reactions to the longer as well as to the shorter 
 waves of the spectrum, and since nearly all of these inves- 
 tigators used relatively pure prismatic colors and efficient 
 methods in other respects, it is evident that the great bulk 
 of evidence goes against Sachs' conclusion that only the 
 shorter waves are active in light reactions in plants. 
 
 (2) The experimental results of all the investigators 
 
 1 '' Candle-meter seconds " indicates the product of the time of exposure 
 and the intensity of the light. This, Blaauw maintains, is a constant for 
 the threshold of a given plant, that is, the higher the intensity, the shorter 
 the time of exposure required to produce a reaction. 
 
320 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 mentioned above agree in that they indicate that the 
 region of maximum effect for all the plants tested is lo- 
 cated somewhere toward the \ iolet end of the spectrum. 
 It may therefore be definitely concluded that the distri- 
 bution in the spectrum of stimulating efficienc\- for j^lants 
 is not i)rimarily dependent ui)on energy or brightness as 
 judged by the human eye, since the maximum for these is 
 located in the yellow. 
 
 (3) The reactions to light in plants are in all proba- 
 bility associated with photochemical changes induced by 
 the light. We liave seen that photochemical changes are 
 specific. If, e.g., one chemical reaction takes place only in 
 blue and another only in green, other conditions being the 
 same, it may be concluded that the chemical constituents 
 taking part in the two reactions are not the same. The 
 experimental results presented above show that the effect 
 of the different rays on reactions is not the same for all 
 plants. It is therefore probable that the photochemical 
 changes associated with the reactions to light are not the 
 same in all plants. I do not consider this point definitely 
 established, owing to the possible effect of difference in 
 selective absorption of light in the different plants. It is 
 however more strongly supported by the observed reac- 
 tions in unicellular forms than by those in sessile plants. 
 
I 
 
 CHAPTER XVII 
 
 THE RELATIVE EFFECT OF DIFFERENT RAYS ON THE 
 REACTIONS OF UNICELLULAR FORMS 
 
 The first observations on the effect of different colors 
 on the movement of unicellular forms were made by 
 Cohn in 1865, nearly fifty years after similar observations 
 had been made on sessile plants by Poggioli (18 17). 
 Cohn's account of his observations is very brief. He 
 studied the movements of swarm spores in colors differ- 
 entiated by means of colored glass and concluded that the 
 blue rays are the most effective and that the red act like 
 total darkness. He says (1865, p. 222), " Die Organismen 
 werden von den blauen Strahlen am stiirksten angezogen, 
 wahrend sich die rothen wie totale Finsterniss verhalten." 
 
 I. Strashtirger' s Experiments 
 
 Much more extensive and conclusive results were 
 obtained by Strasburger (1878), who also studied swarm- 
 spores of various kinds, but principally Botrydium. 
 Strasburger exercised the utmost precaution in his experi- 
 ments. Many of the observations were made in a dark 
 room in light of different colors produced by a quartz 
 prism in a horizontal beam of direct sunlight. The slit 
 in the opaque screen over the prism was only 0.4 mm. 
 wide and the spectrum at the point of exposure 55 mm, 
 long. The Fraunhofer lines could be clearly seen. It is 
 therefore evident that there was but little intermingling of 
 rays of different lengths in the spectrum. In addition to 
 the spectrum, colored glass and various solutions were 
 used. The results led Strasburger to conclude that the 
 blue, indigo and violet light alone cause orienting reactions, 
 but that yellow, red and green cause a quivering movement 
 
 321 
 
322 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 (" zitternde Bewegung ") in some swarm-spores. " Die 
 blauen indigofarbigen und \iulcUeii Strahleii siiid allein 
 auf die pholotaktischen Schwarmer von Einfiuss und liegt 
 das Maximum der W'irkung im Indigo " (p. 623). These 
 conclusions in general support lh(jse of Cohn, but it appears 
 quite probable that in liighcr liu,lit intensity the swarm- 
 spores would have been found i3ositi\e in the hunger waves 
 as well as the shorter, since, as Strasburger states, the red 
 and \ellow produced indefmite reactions in the low inten- 
 sity in which the\- were exposed. It is worthy of note 
 that Strasburger and Wiesner both obtained practically 
 the same effect on reactions with slightly impure colored 
 light produced by means of filter screens as they did with 
 mon(^chromatic light produced by means of prisms. And 
 the same appears to be true in all other experiments on 
 organisms without eyes, in which such a comparison has 
 been made, as, e.g., in the observations of Harrington and 
 Leaming on Amoeba and in my own work on the same 
 organism referred to in detail later. This indicates that 
 the reactions of these organisms in waves of a given length 
 are not appreciably affected b>' the presence of waves of a 
 different length and that extreme precautions to eliminate 
 all foreign rays are not necessary in studying their reac- 
 tions to different colors. In more sensitive forms, however, 
 especially in those with well -developed eyes, monochro- 
 matic light is indispensable. 
 
 2. Rjigelmanns Experiments 
 
 Engelmann's experiments on unicellular forms in spec- 
 tral colors are of the highest character and the greatest 
 interest. He exposed the organisms in monochromatic 
 light of solar, gas, and electric microspectra thrown on the 
 slide by means of a prism attached to a microscope, and 
 noted not only the regions in which they aggregated, but 
 also the reactions during the process of aggregating. He 
 divides these creatures into three classes based upon their 
 different reactions, as follows: 
 
UNICELLULAR FORMS AND COLOR 323 
 
 a. Diatoms and oscillaria with different species of Na- 
 vicula and Pinnularia as types (1882, p. 390). — These 
 organisms are active in dayhght or in the yellow, orange 
 and red of the spectrum, but become quiet if the light 
 intensity is decreased or if they are transferred to the green, 
 blue or violet regions. In these colors they may however 
 become active again if the intensity is sufficiently in- 
 creased. It is evident that owing to these reactions they 
 would tend to collect in the spectrum toward the violet 
 end, or at least leave the opposite end, since they become 
 quiet whenever they chance to get out of the yellow, 
 orange or red. This region however has the greatest 
 stimulating efficiency. Engelmann thinks that the reac- 
 tions of these organisms are dependent upon the liberation 
 of oxygen, and that they react primarily to the longer 
 light waves because of the liberation of oxygen when they 
 are exposed to them. This conclusion is based upon the 
 fact that a reduction of oxygen pressure without a change 
 of light intensity or color causes cessation of movement. 
 It appears then that these organisms do not respond to 
 sudden changes in light intensity and that the reaction to 
 light is dependent upon the absolute intensity rather than 
 upon change of intensity. 
 
 b. Ciliates which have chlorophyll with Paramecium 
 bursaria as a type (1882, p. 393). — Engelmann found that 
 these organisms do not react to light at all, unless the 
 oxygen pressure is reduced below the normal. If the air 
 surrounding the preparation is replaced with hydrogen 
 they become very active (" sehr unruhig ") and aggregate 
 in the region of highest light intensity. Under such con- 
 ditions they also collect in the red of the gas or solar micro- 
 spectrum between the lines B and C at 650-700''''. 
 Engelmann describes the process of aggregation as fol- 
 lows: '' Ueberschrciten sie z. B. zufallig die Granze von 
 Licht und Dunkel, oder tauchen sie auch nur mit der vor- 
 deren Halfte ihres Leibes eine Strecke weit in das Dunkel 
 ein, so kehren sie sofort um nach dem Licht, wie wenn das 
 
324 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 Dunkel ihnen unangenehm ware." The process of col- 
 lecting in regions of given light conditions in these forms 
 is therefore precisely the same as that which Jennings 
 found some fifteen years later with reference to the col- 
 lection of Paramecium caudatum and aurelia in regions 
 containing carbon dioxide. 
 
 It is e\'ident that in Paramecium bursaria a reduction of 
 light intensity or a change from regions illuminated by 
 the longer waves to one illuminated by the shorter causes 
 a definite response. Whether or not this response would 
 under certain conditions result in orientation in these 
 organisms as it does in Euglena was unfortunately not 
 ascertained. Nor was it ascertained whether or not they 
 ever become negative, i.e., respond to an increase of inten- 
 sity or to a change from the shorter waves to the longer. 
 
 Engelmann thinks that the reactions of these organisms 
 to light is regulated by the oxygen liberated, that it is a 
 change in the oxygen pressure that produces a stimula- 
 tion. The response which results in aggregation in these 
 organisms is however, undoubtedly, at least indirectly 
 dependent upon the time rate of change of light intensity 
 rather than upon the absolute intensity, and their reaction 
 system and method of collection in a given region are 
 evidently quite different from those in diatoms. 
 
 c. Flagellates with Euglena viridis as a type (1882, 
 P- 395)-' ~~ The light reactions of the organisms in this 
 class are not primarily dependent upon oxygen pressure. 
 They respond to light in the same way whether the oxygen 
 pressure is normal or above or below normal, unless it is 
 carried to such extremes that all activity ceases. 
 
 Tiiese organisms were found to form dense aggregations 
 in the more highly illuminated regions, just as Paramecium 
 bursaria does when the oxygen pressure is below normal, 
 but in the microspectrum they collect in the blue near 
 the Fraunhofer line F, 470-490^'', not in the red where 
 
 ^ Bacterium photomctricum (1883, pp. 95-124), a form on which Engel- 
 mann worked later, might also be included here. 
 
UNICELLULAR FORMS AND COLOR 325 
 
 P. bursaria collects. The method of collection of Euglena 
 is described as being the same as that of P. bursaria. The 
 region in which they aggregate " wirkt wie eine Falle, 
 denn einmal hineingekommen, gehen die Euglenen in der 
 Regel nicht wieder aus. Sie kehren an der Grenze des 
 Dunkels immer sogleich wieder nur ins Helle." Engelmann 
 does not mention the fact that these forms orient and that 
 they may become negative, and of course did not realize 
 that owing to these reactions they can move directly 
 toward the optimum, so that the aggregation is not 
 entirely due to random movements. According to the 
 work of Engelmann, then, a reduction of light intensity or 
 a change from blue in the region of the Fraunhofer line F 
 to any other color of the spectrum stimulates Euglena and 
 causes it to turn and proceed in a different direction. 
 
 Like Euglena, so specimens of Bacterium photometricum 
 collect in the more highly illuminated regions of their 
 environment, but in the microspectrum, unlike Euglena, 
 most of them collect in the infra-red between 800 and 
 900"', some in the orange between 580 and 610"" and a 
 few elsewhere. In general they collect in those regions 
 where the absorption bands for the coloring matter they 
 contain are found. Their reactions to light are independ- 
 ent of oxygen pressure. The method of collection is 
 described as being similar to that of Euglena. A reduc- 
 tion of intensity or a change in color from that in which 
 they collect to any other causes the bacteria to swim sud- 
 denly backward (" zuriick schiessen ") 10 to 20 times 
 their length, after which they proceed in the ordinary 
 way again. This reaction has been designated " Schreck- 
 bewegung." An increase of intensity or movement into 
 the regions of the spectrum where they collect does not 
 produce the " Schreckbewegung," neither does a gradual 
 decrease of intensity. Engelmann found but little evi- 
 dence indicating that these organisms orient, and did not 
 ascertain whether or not they become negative in very 
 high intensity. 
 
326 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 3. Verworns Experiments 
 
 The experiments of Verworn (1889) yielded results 
 which led him to conclude that the diatom Navicula bre- 
 vis reacts only to the shorter waves, while Oscillaria reacts 
 to all the waves in the visible spectrum. *' Als die allein 
 wirksamen Lichtstrahlen erwiesen sich auch bei den Dia- 
 tomeen die kurzwelligen " (p. 49); " Die Versuche, welche 
 sich auf die Ermittelung der wirksamen Strahlen bezogen, 
 hatten das ganz unvermuthete Ergebniss, dass Strahlen 
 von alien Wellenlangen ungefahr von der Linie a bis 
 uber G hinaus die Bewegungen der Oscillarien beeinflussen. 
 . . . Die Ansammlungen der Oscillarien war nach Ein- 
 schaltung von Rubinglas oder Kalibichromatlosung, auch 
 im Halbdunkel, ebenso vollkommen, wie bei Anwendung 
 von griinem Glas, Kobaltglas, Kupferoxydammoniaklosung 
 oder reinem Sonnenlicht " (p. 51). In these experiments 
 Verworn mounted the organisms on a slide and studied the 
 movements under a microscope, surrounded by a tight case 
 which was black inside. The case contained an opening 
 15 X 20 mm. in one side to admit light. Colored glass 
 plates or flat flasks containing different solutions could be 
 so adjusted as to admit only light which passed through 
 them. The colored media used were thoroughly tested 
 spectroscopically ("genau spektroskopisch untersucht "). 
 Five different media were used. The red glass transmitted 
 red and orange (600-740"") ; the cobalt glass, blue and violet 
 (410-510"") and a little infra-red; the green glass, yellow 
 and green, and a little orange and blue (480-600"''); the 
 potassium bichromate solution, red, orange, yellow, and a 
 little green (550-740"") ; and the ammoniacal solution of 
 copper hydrate, blue and considerable green (430-520"") • 
 
 The Oscillaria were positive; they collected at the side 
 nearest the light, even if it was of very low intensity 
 (" Halbdunkel "), no matter which one of the media was 
 in the opening. The diatoms, on the other hand, moved 
 toward the light only in the shorter waves; they did not 
 
UNICELLULAR FORMS AND COLOR 
 
 327 
 
 respond behind the ruby glass or the potassium bichro- 
 mate solution, and only indefinitely behind the green 
 glass even in direct sunlight. It Is therefore evident that 
 the distribution in the spectrum of stimulating efficiency 
 is not the same In these two forms, for the diatoms react 
 only to the shorter waves, while Oscillaria reacts to the 
 longer as well as to the shorter. Whether or not the 
 reaction of Oscillaria to the different rays is proportional 
 to the energy can however not be definitely ascertained 
 from the data at hand. 
 
 4. Experiments of Harrington and Learning on Amoeba 
 
 One of the most Interesting of the Investigations on the 
 effect of different colors on the reactions of protozoa is 
 that of Harrington and Leaming (1900). These authors 
 projected amoebae on a screen with a Zeiss photomicro- 
 graphic apparatus, and studied the effect of sudden changes 
 in color and Intensity on the movements. The light rays 
 were differentiated by means of Bierstadt's colored celloi- 
 din plates. The results of numerous observations, all 
 recorded In detail, are summarized In the following table, 
 which has been slightly modified (p. 16). The table shows 
 clearly that the violet is more effective than any other 
 color tested, except white, in causing retardation In move- 
 ment, and since orientation Is the result of such retarda- 
 tions in movement the violet must also be more effective 
 in regulating this phenomenon. 
 
 TABLE X 
 
 
 White 
 
 * — 
 
 Violet 
 
 Green 
 
 Yellow 
 
 Red 
 
 ^^'hite following 
 
 
 
 -5? 
 
 — I 
 
 — I 
 
 — I sec. 
 
 Violet 
 
 + 5 
 
 
 
 — 20 
 
 -24 
 
 -9 " 
 
 Green " 
 
 + 5 
 
 + 12 
 
 
 
 ?+i2 
 
 " 
 
 Yellow " 
 
 + 1 
 
 + 3 
 
 
 
 
 
 " 
 
 Red 
 
 + 1 
 
 + 2 
 
 
 
 
 
 " 
 
 Zero indicates that there was no change in movement when the light 
 conditions were changed as indicated; the numbers preceded by — indicate 
 the average time in seconds before streaming stopped or decreased; and the 
 numbers preceded by -\- indicate the average time before streaming started 
 or increased. 
 
328 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 It is claimed that a sudden increase of intensity causes 
 an immediate cessation of movement either in white light 
 or in light containing only the shorter waves, but that 
 " after a few minutes [exposure] streaming will commence 
 untler any liglit if the amoeba be a fairly active individual" 
 (p. 1 6). The authors appear to think that some colors 
 actually cause an increase in the rate of movement in the 
 amoebae: " That red is the most powerful excitant to flow 
 is indicated by the shorter latent period after quiescence 
 in white light." There is however little evidence sup- 
 porting this conclusion. 
 
 Harrington and Leaming did not ascertain the intensity 
 of light transmitted by the different filters, and the light 
 was not spectroscopically examined, so that the purity of 
 the colors used remains unknown. Similar results were 
 however obtained in spectral colors, but the authors un- 
 fortunately have not described how the spectrum was 
 produced or what kind of light was used as a source. 
 Their results have occasionally been seriously questioned. 
 
 5. Original Observations on Amoeba 
 
 Owing to the questionable character of some of the 
 results of Harrington and Leaming and to the fact that 
 the region in the spectrum of maximum effect on the 
 movement could not be definitely located from their data, 
 it seemed desirable to have the experiments repeated. 
 
 a. Experiments with color filters. — Early in June, 
 1909, Ur. H. S. Jennings put at my disposal an excellent 
 culture of Amoeba proteus, which had come up in hay 
 infusion used in rearing Paramecia. Specimens of this 
 culture were studied both in a solar prismatic spectrum 
 and in different colors produced by means of filters which 
 were kindly furnished by Dr. R. P. Cowles. The filters 
 were prepared and spectroscopically tested in the physical 
 laboratory of Johns Hopkins University. The red was 
 transparent from 620'''' out, opaque from 450 to 590'"' and 
 
UNICELLULAR FORMS AND COLOR 329 
 
 faintly transparent from 380 to 450''''. The blue was trans- 
 parent from 430 to 490"" and from 690"'' out, and opaque 
 from 590 to 670'"'. The green was transparent from 380 to 
 400^'', from 450 to 550'''' and from 680'"' out. It was 
 opaque between 580 and 660'"' and faintly transparent 
 between 400 and 450"^. 
 
 Several amoebae were mounted under a large cover- 
 glass surrounded by a thin ring of vaseline so as to pre- 
 vent evaporation, and give ample space for free movement. 
 In this inclosure they were found to remain active and 
 in excellent condition for several days. The observations 
 were made on the stage of a compound microscope under 
 a magnification of about 150 diameters with very faint 
 illumination from the mirror. A beam of direct sunlight 
 which passed through 8 cm. of water was thrown on the 
 slide at an angle of about 45° with the stage. 
 
 The organisms were exposed to light of different colors 
 by intercepting the beam with the colored filters. It was 
 found that amoebae which moved actively in weak diffuse 
 light ceased moving shortly after being suddenly exposed 
 to strong red light, but soon began again. If they were 
 now exposed to green the movement again ceased; the 
 same was true for blue after green and for direct sunlight 
 after blue. A change from direct sunlight to blue, blue 
 to green, or green to red, produced no apparent effect. 
 After being exposed to any color or any combination of 
 colors for a short time the movement was resumed. In 
 direct sunlight or in blue light it required longer than in 
 green or red. As a matter of fact, in these two colors, in 
 the red in particular, there was no cessation of movement 
 in some specimens, and only a slight decrease in others, 
 while in still others the movement stopped entirely. In 
 case of direct sunlight or blue, on the other hand, the 
 movement stopped abruptly in nearly every specimen 
 almost as soon as exposed. Similar but somewhat more 
 detailed results were obtained in the spectrum. 
 
 b. Experiments with solar spectrum. — In these expcri- 
 
330 LIGHT AXD THE BEHAVIOR OE ORGAXISMS 
 
 mcnts a horizontal beam of direct sunlight was passed 
 through a vertical prism and thrown on the mirror below 
 the stage of the microscope, from which it was reflected 
 to the slide. By manipulating the mirror the amoebae on 
 the slide could be suddenly subjected to light in any part 
 of the spectrum, and the color to which they were exposed 
 could be instantaneously changed. 
 
 The vertical slit in the opacjue screen over the face of 
 the lens was 2 mm. wide and the spectrum on the slide 
 ncarh' 3 cm. long. There was consequently some over- 
 lapping of rays in adjacent parts of the spectrum, but there 
 was no intermingling of rays in distant parts. For example, 
 in the red there was some orange, but no rays of shorter 
 wave lengths. 
 
 The amoebae were examined in daylight so faint that 
 they could scarcely be seen. After a specimen active in 
 this light had been selected, it was suddenly exposed to 
 any desired part of the spectrum and the reaction noted. 
 
 Many observations were made on numerous indixiduals 
 between 10 a.m. and i p.m., June 16 and 18. The sky 
 was clear, and the intensity of light consequently at a 
 maximum, approximately 5000 ca. m. Without going into 
 details with reference to reactions of individual specimens 
 it may be stated that the effect of sudden exposure to 
 red,' yellow or violet after very faint diffuse sunlight was 
 essentially the same. There was in many specimens a 
 slight decrease in rate of movement, in some a momentary 
 cessation, and in others no apparent reaction whatever. 
 In the green the effect was similar to that in red, yellow 
 and \iolet, only somewhat more marked. To obtain the 
 
 * The wave k-ngths are designated in terms of color. 
 
 Red = 630-760'^'' 
 
 Orange = 590-630'''* 
 
 Yellow = 560-5Q0'''* 
 
 Green = 490-560'*'' 
 
 Blue = 430-490'''' 
 
 Violet = 395-430''" 
 
 Ultra-violet = 340-395'''* 
 
 # 
 
UNICELLULAR FORMS AND COLOR 331 
 
 effect described above it is necessary (i) to have amoebae 
 in a certain condition, (2) to keep them in as low Hght 
 intensity as possible before exposing, and (3) to use very 
 intense light. When exposed in blue after having become 
 active in any other color or in diffuse sunlight, all move- 
 ment stopped instantly in nearly all specimens observed. 
 But there was no apparent contraction; the animals re- 
 tained almost the exact form they had before the exposure. 
 After remaining quiet a few seconds, the streaming of the 
 protoplasm in the anterior pseudopods slowly began again, 
 but now it nearly always proceeded in the same direction. 
 Gradually new pseudopods were formed, usually at the 
 posterior end, and as these developed the old ones were 
 slowly withdrawn. The rate of movement ordinarily in- 
 creased at such a rate that after 30 to 60 seconds it was 
 again normal. If any other part of the spectrum was 
 flashed on an amoeba which had become active in the blue 
 there was no apparent reaction, but when such a specimen 
 was exposed to direct sunlight it was clearly seen, in some 
 instances, that the streaming ceased again. 
 
 The results obtained in these experiments lend support 
 to the general conclusions of Harrington and Leaming. 
 They differ from their results only in a few details. I 
 found red and yellow to have a slight effect on the move- 
 ment of Amoeba. Harrington and Leaming did not, prob- 
 ably owing to deficiency in light intensity or to exposure 
 in too great an intensity preceding the exposure to red or 
 yellow. I found only a very slight stimulation in the 
 violet, whereas they recorded no difference between the 
 effect of blue and violet. 
 
 It may then be definitely concluded that the blue rays, 
 430 to 490"", have a very marked effect on the rate of 
 movement of Amoeba, while the violet, green, yellow, 
 orange and red rays have only a slight effect; and since 
 the direction of movement is in all probability regulated 
 by changes in the rate, it is evident that the blue rays are 
 also of primary importance in this process. 
 
332 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 It will be seen at once that the effect of different parts 
 of the prismatic solar spectrum on the movement of 
 Amoeba is not proportional to the energy contents, for 
 the energy gradually increases as one proceeds from the 
 violet toward the red end, whereas the region of maximum 
 stimulation for this animal is in the blue, from which it de- 
 creases toward both ends. Nor is it i:)r()portional to the 
 brightness as judged by the human e>e, for the >ellow is 
 much brighter than any other part of the spectrum. In 
 fact, under the conditions of the experiment, one could 
 hardh' bear to look through the microscope when the 
 yellow was reflected, while in the case of blue, the region of 
 maximum stimulation for Amoeba, there was no unpleasant 
 stimulation whatever to the eye. 
 
 One of the most interesting characteristics in the re- 
 actions of unicellular forms is the variation in the loca- 
 tion of the region of maximum stimulating efficienc>' in 
 the spectrum. The experimental results presented above 
 indicate that for the swarm-spores it is in the indigo, for 
 Amoeba it is in the blue, while Oscillaria appears to be 
 stimulated equally by all the visible rays. This indicates 
 that, as in plants so in unicellular forms, the chemical 
 changes associated with the reactions to light are not the 
 same in all of the different species (see p. 320). 
 
CHAPTER XVIII 
 
 REACTIONS OF MULTICELLULAR ANIMALS IN LIGHT 
 CONSISTING OF WAVES DIFFERING IN LENGTH 
 
 I. Experiments of Wilson on Hydra 
 
 Among the most thorough and rehable experiments on 
 the reactions to Ught of different wave lengths are those 
 of Wilson (1891) on Hydra. Wilson had "a fraternity of 
 Hydras five hundred to a thousand strong all of which 
 had arisen in [a large] aquarium [in a north room] from a 
 group of three or four progenitors" (Footnote, p. 415). 
 The window side of the aquarium was divided into equal 
 areas which were covered with sJLrips of red, yellow, green, 
 blue or colorless glass. In some instances there were two 
 areas of each color, one with a single thickness of glass, 
 the other with two, producing two fields of the same color 
 but of different intensity. 
 
 The activities of the animals and the changes in posi- 
 tion were studied and recorded for a week, during which 
 the glass strips were frequently rearranged. It was 
 found that the Hydras tend to collect in the colorless 
 region and in the more intensely illuminated regions of 
 each color, i.e., back of the areas covered with only one 
 thickness of glass, but that with reference to the different 
 colors they tend to aggregate in the blue even if the inten- 
 sity in it is much lower than that in any other region. 
 The tabulated results of two series of observations will 
 serve to emphasize this. These results were obtained 
 after rearranging the plates so as to change the color of 
 the different regions. 
 
334 LIGUT AND THE BEHAVIOR OF ORGANISMS 
 
 TABLE XI. (After Wilson, iSgi, p. 424-) 
 
 Yellow decrease [in numl)er of Hvdras] . . . . 56 per cent. 
 
 Red " " " " '" 55 " " 
 
 Green " " " " " 70 " " 
 
 Blue increase " " " " 9-' " " 
 
 TABLE XI L (After Wilson, p. 427.) 
 
 Total increase [in number of individuals during period of 
 observation] 421 to O74, i.e., Oo per cent. 
 
 Blue, increase 327 per cent. 
 
 Yellow, decrease 30 
 
 Dark screen, decrease 37 
 
 Daylight, increase 30 
 
 u 
 
 Wilson says that hydras arc positive in blue, that they 
 go fairly directly toward the source of light, and that the 
 other colors are inactive, but he does not show definitely 
 how the aggregations in the blue are formed. We shall 
 discuss the movement in different colors more in detail 
 later. 
 
 The colors produced by the plates of glass used were 
 not monochromatic. Thorough spectroscopic examina- 
 tion showed that the red transmitted a little orange, the 
 yellow some green, orange and red, the green some yellow 
 and a trace of red, and the blue some indigo and violet 
 and a trace of green and red. It is not at all likely that 
 such slight transmission of foreign colors as represented 
 above modifies the reactions of organisms that have not 
 well-developed eyes and are no more sensitive to light 
 than Hydra, although much has been said regarding this 
 and many results have been branded worthless owing to 
 the use of slightly impure colors. Wilson fortunately con- 
 firmed the results obtained with colored glasses by critical 
 tests in a spectrum produced by focusing light from an 
 Argand gas burner on a narrow slit in an opaque screen 
 in front of a large carbon bisulphide prism. " The appa- 
 ratus was placed in a perfectly dark underground room and 
 every pains was taken, by the use of suitable screens, etc., 
 
MULTICELLULAR ANIMALS AND COLOR 335 
 
 to exclude from the aquarium all light excepting that 
 proceeding from the prism" (Footnote, p. 430). In the 
 spectrum, which was about three inches long, the hydras 
 showed a very marked tendency to collect in the lower 
 blue, from line G to line F, and for a slight distance in the 
 green. They were wholly indifferent to the lower rays, 
 the violet and ultra-violet, as well as to all those above the 
 lower green, including the infra-rCd. It should be empha- 
 sized that they are not negative in these colors. This was 
 shown both by their reactions in the spectrum and by 
 those under colored glass. 
 
 The observations of Wilson seem to prove conclusively 
 that the blue is most active in stimulating both Hydra 
 viridis and Hydra fusca. This stimulating activity of the 
 blue is specific; it bears no definite relation to the distri- 
 bution of energy or of brightness, both of which are fairly 
 definitely located for the gas-light spectrum, the region of 
 maximum energy and brightness being well toward the 
 red end. 
 
 One of the striking peculiarities in the results obtained 
 by Wilson is the fact that whereas hydras collected most 
 abundantly in the regions of highest intensity under given 
 color conditions, more were regularly found in the blue than 
 in daylight, when they were given a choice between these 
 two conditions of light (see Table XII), although the latter 
 contained at least as much blue as the former and was of 
 course more intense owing to the presence of other rays. 
 This result is similar to that obtained by Lubbock (1888) 
 on daphnias, which were found to collect more freely in 
 yellow and green light than in daylight. 
 
 Loeb appears to doubt the accuracy of these results. 
 Referring to those of Lubbock he says (1905, p. 10): 
 "One half of a dish was covered by a yellow screen; the 
 other half was left uncovered. In the uncovered half, 1,904 
 animals collected, while 3,096 gathered under the yellow 
 screen. From this Lubbock concludes that Daphnia has 
 a ' preference ' for ' yellow.' But one would suppose that 
 
336 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 in the uncovered part of the dish there was at least as 
 much yellow light as under the >ellow screen; or did the 
 majority 'hate' the blue light?" Referring to observa- 
 tions on Porthesia chrysorrhoea he says (1905, p. 29), 
 " This experiment shows that the ynorc refrangible rays have 
 the same effect as mixed rays.'' 
 
 The methods of both Lubbock and Wilson were how- 
 ever such as to leave littfe room for doubt regarding their 
 results. It is interesting to note that the recent work 
 of Stobbe (1908) on the photochemical changes in the 
 organic compounds known as fulgides demonstrates reac- 
 tions which proceed more rapidly in monochromatic light 
 of a given intensity than in the same light in combination 
 with other rays. These experiments have been referred to 
 under the section on photochemical reactions, p. 310. It is 
 likely that the chemical changes in Hydra and Daphnia 
 associated with their light reactions are of the nature of 
 certain fulgides. 
 
 2. Bert's Experiments on Daphnia 
 
 The first experiments dealing with the effect of differ- 
 ent colors on the reactions of multicellular animals were 
 made by Paul Bert in 1868. Bert was interested in 
 color vision. He attempted to answer the question as to 
 whether the specific effect on the different rays in the 
 spectrum is the same in the inferior animals as it is in 
 man. He attacked the problem from a purely psychologi- 
 cal point of view, as the title of his paper published in 
 1869 indicates: " Sur la question de savoir si tous les ani- 
 maux voient les memes rayons que nous." 
 
 An electric-light spectrum was thrown on the flat side 
 of an aquarium covered with an opaque screen containing 
 a narrow vertical slit. Daphnias were exposed to the 
 different colors of the spectrum, and it was found that 
 they collected at the side of the aquarium nearest the 
 light, no matter which part of the spectrum was allowed 
 
MULTICELLULAR ANIMALS AND COLOR 2>2>1 
 
 to enter the slit. The organisms were therefore positive 
 in all colors; the reactions however were more rapid in 
 the yellow and green than in other parts of the spectrum. 
 Bert says (1869), " II fut facile de rcmarquer, qu'elles 
 accouraient beaucoup plus rapidement au jaune ou au 
 vert qu'a toute autre couleur."^ 
 
 When the opaque screen was removed so as to expose 
 the daphnias to the entire spectrum at once, most of them 
 collected in the yellow and green. Bert was of the opinion 
 that the distribution of effect in the spectrum is the same 
 in Daphnia as it is in man, and he concluded that the col- 
 lection of Daphnia in the yellow and green part of the 
 spectrum and the great activity in these colors is not 
 due to color vision, but to the fact that the light intensity 
 in this part of the spectrum is higher than elsewhere. 
 He was of the opinion that light affects these animals 
 much as it does the human being, with reference to 
 brightness, that the yellow for them as for man is the 
 brightest and consequently the most effective part of the 
 spectrum. 
 
 Results similar to those recorded by Bert were obtained 
 by Merejkowsky (1881) on Dias longiremis and larvae of 
 Balanus, by Lubbock (1881) on Daphnia, and by Yerkes 
 (1900) on Simocephalus. 
 
 Merejkowsky states that he exposed Dias longiremis and 
 Balanus larvae in light of different colors but of equal 
 brightness and found no evidence of preference. The valid- 
 ity of these results is, however, questionable, since it is by 
 no means certain that the brightness of the different colors 
 used in these experiments was actually equal. 
 
 3. Lubbock's Experiments on Daphnia 
 
 Lubbock's experimental methods and results are far 
 more convincing than those of Merejkowsky. He pub- 
 lished his interesting observations on Daphnia in the 
 
 ^ Taken from Loeb (1905, p. 9). Page in original not given by Loeb. 
 
33 S LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 Journal of the Linnean Society in 1881. The following 
 account is however taken from a later publication (1882, 
 pp. 212-231). 
 
 In these experiments Lubbock projected a prismatic 
 solar spectrum arranged by Professor Dewar at the Royal 
 Institute, verticalK' downward on a wooden trough 14 
 inches long and 4 inches wide. In this trough he put 
 50 specimens of Daphnia pulex, scattered them equally 
 through the water, and after ten minutes inserted glass 
 partitions so as to divide the trough into compartments 
 corresponding in size with the five principal colors of the 
 spectrum. He then recorded the numl)er of individuals in 
 each, after which he repeated the process. The total for 
 ten trials follows: 5 in the violet, 32 in the blue, 298 in 
 the green, 74 in the yellow, 90 in the red and one in the 
 dark part of the spectrum. 
 
 In comparing these results it is necessary to consider 
 the fact that in a prismatic spectrum the red and green 
 are each much more than twice as wide as the yellow; and 
 the blue and violet are each wider than the green. Lub- 
 bock allowed three-fourths of an inch for the yellow and 
 two inches for the green. Correcting for this difference 
 in width the calculated number in the yellow would have 
 been 196. Lubbock concludes (p. 214), " It will be 
 observed . . . [that] there were more Daphnias in pro- 
 portion, as well as absolutely, in the green, although the 
 yellow is the brightest portion of the spectrum." 
 
 It was also found that when daphnias were exposed in 
 the green, yellow and red of a normal spectrum, they col- 
 lected in the green rather than in the red. In these experi- 
 ments the region of highest intensity in the middle of the 
 field was shaded. After ten minutes' exposure 410 speci- 
 mens were found in the green end, 14 in the shaded area 
 and 76 in the red. These results indicate clearly that if 
 brightness alone controls the reactions of Daphnia, it 
 must be different for them than it is for the human eye. 
 Numerous convincing experimental results showing that 
 
MULTICELLULAR ANIMALS AND COLOR 339 
 
 this animal is positive in ultra-violet indicate the same 
 thing. 
 
 Lubbock's primary object in this work, however, was to 
 test the color vision of Daphnia. I can do no better than 
 to quote at length the experiments which bear on this 
 question : 
 
 '' I placed (March 26) fifty Daphnias in a trough (i), 
 covering over one half of it with a pale green, and another 
 fifty in a trough (2) half of which was covered with yellow 
 (aurine). On one side was a similar trough (3), one end 
 of which was shaded by a porcelain plate; and on the 
 other side a fourth trough (4), one end of which had a 
 little, though but little, extra light thrown on it by means 
 of a mirror. As before, I counted the Daphnias from 
 time to time, and turned the troughs round. All four 
 were in a light room, but not actually in direct sunshine. 
 Thus, then, in one trough I had half the water in some- 
 what green light; in the second trough, half the water in 
 yellow light; in the third, one half was exposed and the 
 other somewhat darkened; while the fourth, on the con- 
 trary, gave me a contrast with somewhat more vivid 
 light. If, then, the Daphnias went under the green and 
 yellow glass, not on account of the color, but for the sake 
 of shade, then in trough 3 a majority of them would have 
 gone under the porcelain plate. On the other hand, if the 
 porcelain plate darkened the water too much, and yet the 
 open water was rather too light for the Daphnias, then in 
 the fourth trough they would, of course, have avoided the 
 illuminated half. The results show that the third trough 
 was unnecessary, still I may as well give the figures; the 
 fourth proves that the Daphnias preferred a light some- 
 what brighter than the ordinary diffused light of the room. 
 Of course it does not follow that the effect of color is the 
 same as with us " (p. 226). 
 
 The results of twenty trials are recorded, but since all 
 are essentially the same I shall quote only the following 
 five (p. 227) : 
 
340 UGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 • 
 
 TABLE 
 
 XIII. 
 
 (After Lubbock, p. 2: 
 
 ^7.) 
 
 
 
 Trough I 
 
 Trough 2 
 
 Trough 3 
 
 Tro 
 
 Ligh 4 
 
 
 
 
 
 
 Ex- 
 
 Durk- 
 
 Illumi- 
 
 Unillumi- 
 
 
 Green 
 
 White 
 
 Yellow 
 
 White 
 
 posed 
 
 encil 
 
 nated 
 
 nated 
 
 
 light 
 
 light 
 
 light 
 
 light 
 
 half 
 
 half 
 
 half 
 
 - 
 
 half 
 
 March 28 
 
 
 
 
 
 
 
 
 
 7-30 
 
 33 
 
 17 
 
 34 
 
 16 
 
 35 
 
 15 
 
 30 
 
 20 
 
 7-50 
 
 32 
 
 18 
 
 37 
 
 13 
 
 27 
 
 23 
 
 32 
 
 18 
 
 S.io 
 
 34 
 
 16 
 
 33 
 
 17 
 
 29 
 
 21 
 
 30 
 
 20 
 
 ^•35 
 
 36 
 
 14 
 
 35 
 
 15 
 
 26 
 
 24 
 
 33 
 
 17 
 
 9-05 
 
 26 
 
 24 
 
 27 
 
 23 
 
 35 
 
 17 
 
 35 
 
 15 
 
 
 161 
 
 89 
 
 166 
 
 84 
 
 150 
 
 100 
 
 160 
 
 90 
 
 In another series of experiments in which one half of 
 one dish was covered with a rul)y glass and that of another 
 dish with blue glass, the majority of the animals collected 
 in the uncovered portion of both dishes. It is evident 
 from the results in troughs 3 and 4 above that the daph- 
 nias were positive to the highest light intensity used in 
 these experiments. It may then be assumed that they 
 w^ere positive to the light conditions in which the majority 
 collected in troughs i and 2, and in the experiments with 
 red and blue. In the former however they collected in 
 the part of the trough having the lower intensity, i.e., in 
 the green and yellow respectively, in preference to white, 
 whereas under all the other conditions they collected in 
 that portion of the trough having the higher light intensity. 
 
 While these results do not demonstrate subjective color 
 sensation, I am unable to see how they can be explained 
 without assuming a specific effect depending upon the 
 length of the waves or the color independent of intensity 
 or brightness. 
 
 Lubbock's conclusions are very cautiously summed up 
 in the f(jll()wing paragraphs (p. 231): 
 
 " My experiments, I think, show that while the Daph- 
 nias prefer light to darkness, there is a certain maximum 
 of brilliancy beyond which the light becomes inconven- 
 
MULTICELLULAR ANIMALS AND COLOR 
 
 341 
 
 iently bright to them, and that they can distinguish be- 
 tween hght of different wave-lengths. 1 sui)pose it would 
 be impossible to prove that. they actually perceive colors; 
 but to suggest that the rays of various wave-lengths pro- 
 duce on their eyes a different impression from that of 
 color, is to propose an entirely novel hypothesis. 
 
 " At any rate, I think I have shown that they do dis- 
 tinguish between rays of different wave-lengths, and prefer 
 those which to our eyes appear green and yellow." 
 
 The striking positive reaction to yellow and green In 
 preference to white light of a higher intensity seems to 
 indicate that Daphnia is negative to the other rays of the 
 spectrum. This question has been discussed elsewhere. 
 
 4. Experiments of Yerkes on Simocephaliis 
 
 Yerkes (1899) made a very thorough study of the re- 
 actions of Simocephalus vetulus, a form similar to Daphnia, 
 both in gas and sunlight spectra. Every reasonable pre- 
 caution was taken in the manipulation of the apparatus. 
 The method employed was like that used by Lubbock on 
 Daphnia. The following table shows the relative numbers 
 which collected in the different regions of the two spectra: 
 
 TABLE XIV. (After Yerkes, 1899.) 
 
 
 Gas spectrum 
 
 Sunlight spectrum 
 
 Red 
 
 24. 7 per cent 
 
 6.2 " " 
 
 5-3 " " 
 0.8 " " 
 
 9. 7 per cent 
 
 35-2 " " 
 14.6 " " 
 
 25-5 " " 
 8 '* " 
 
 Yellow 
 
 Green 
 
 Blue 
 
 Violet 
 
 
 
 Since Simocephalus is positive even in direct sunlight, it 
 is safe to say that it is positive to the light conditions of 
 that part of the spectrum in which it aggregates. The 
 table above shows clearly that in the gas spectrum most of 
 
342 LIGHT AyD THE BEHAVIOR OF ORGANISMS 
 
 the specimens collected in the red and yellow, whereas in 
 the sunligiit si)ectrum most of them collected in the yel- 
 low, green and blue. In the former, then, ihey collected 
 nearer the red end than in the latter, and since the bright- 
 est part of the gas spectrum is somewhat nearer the red 
 end than it is in the sunlight spectrum, Yerkes concluded 
 that the reactions of Simocei)halus are dependent upon 
 intensity rather than upon color. 
 
 The results ai)pear to me to demonstrate that intensity 
 is undoubtedly a factor in the reactions of Simocephalus, 
 but they do not appear to demonstrate that the reactions 
 are independent of the length of the waves of light. The 
 distribution in the spectrum of the power to stimulate 
 Simocephalus agrees roughly with that of brightness for 
 the human eye. If corrections are made for the difference 
 in width between yellow and blue in the sun spectrum, 
 then the 25 per cent for the blue, which is fully twice as 
 wide as the yellow, will be reduced to about twelve per 
 cent, and the percentage in the green will also be reduced 
 somewhat, showing that the yellow is the most active by 
 far. This however is not in opposition to the conclusions 
 reached from Lubbock's results, that the reactions depend 
 upon the length of the waves as well as upon the ampli- 
 tude. On the contrary, the fact that the distribution in 
 the spectrum of stimulating efficiency in these forms 
 does not correspond with the distribution of energy, indi- 
 cates that the reactions are dependent upon the wave 
 length, possibly in some such way as brightness is depend- 
 ent upon the wave length. This does not mean that the 
 chemical changes and the mechanism in general is the 
 same in these forms as that associated with brightness 
 sensations in man; and of course it does not demonstrate 
 the presence of brightness sensations in these Crustacea, 
 as the conclusions of Bert and Lubbock might lead one 
 to suspect. 
 
 Considering the results of all the experiments on the 
 daphnias referred to above, it may be concluded: (i) that 
 
MULTICELLULAR ANIMALS AND COLOR 343 
 
 these organisms, contrary to the hypothesis of Loeb and 
 Davenport, are most strongly affected by the green and 
 yellow rays; (2) that this effect is dependent primarily 
 upon the length of the waves and secondarily upon the 
 amplitude or energy; (3) that these organisms can be 
 stimulated by the ultra-violet and by all the rays in the 
 visible spectrum, except perhaps those near the infra-red; 
 (4) that the stimulating efficiency is not proportional to 
 the energy contents; and (5) that the distribution in the 
 spectrum of stimulating efficiency in these organisms dif- 
 fers from that in a majority of the lower forms, in which 
 it has been clearly demonstrated that the blue or violet 
 rays are the most active. This indicates that the chemical 
 changes associated with the reactions are not the same in 
 all organisms. 
 
 5. Experiments of Graher on Various Animals 
 
 The experiments of Graber on the reactions of animals 
 to colored light are the most extensive of any yet made in 
 this line. He tested 54 different species, — 5 mammals, 
 7 birds, 2 reptiles, 3 amphibia, 2 fishes, 3 mollusks, 2^ 
 insects, 2 spiders, and 3 worms. Nearly all of these 
 species have well-developed eyes. Besides these in the 
 normal state a blinded amphibia and a blinded insect were 
 tested. 
 
 Following the work of Bert and Lubbock, Graber at- 
 tacked the problem from the psychological point of view. 
 The foremost question with him appears to have been: 
 Do the animals perceive color? In nearly all the experi- 
 ments he studied the change of distribution of the animals 
 in suitable boxes or troughs, the two halves of which were 
 illuminated with light of different colors or different inten-r- 
 sities, and recorded the number which collected luiow. 
 half of the inclosure. He does not state how tJMn the quo- 
 reacted so as to aggregate in one or the ot>actually perceive 
 conditions. The aggregations may 1^ 
 
344 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 positive orientation to the light conditions in which the 
 animals collected, or to negative orientation to the other 
 condition; or the organisms may have wandered into the 
 light in which they collected by random movements and 
 have remained because of a definite reaction when the 
 border of this area was reached, somewhat similar to the 
 reactions of I'araniccium on reaching the limit of an area 
 containing carbon dioxide; or they may have remained 
 because th(^\' came to rest somewhat as planarians come 
 to rest in a given light condition under some circum- 
 stances. It is therefore impossible to be certain as to the 
 interpretation of many of the results. Moreover only 
 a limited number of different colors was used, so that 
 the maximum effect in the spectrum cannot be located. 
 We shall consequently consider only a few of Graber's 
 observations. 
 
 Colored glass and solutions were used almost exclusively 
 to differentiate the rays. These were all, however, thor- 
 oughly examined spectroscopically, and the relative inten- 
 sity of the light transmitted was also fairly accurately 
 ascertained. In this work Graber had the assistance of 
 the physicist Professor Mach. The lower forms only are of 
 interest to us here. 
 
 Lumbricus is well known to be negative in its light reac- 
 tions. Graber's results are in harmony with this. In a 
 trough one half of which was shaded he found over five 
 times as many in the shaded region as in the illuminated 
 region. When one half of the trough was red and the other 
 blue there were nearly five times as many in the former as 
 in the latter, and in the case of red and green, and green 
 and blue, the majority collected in the light having the 
 longer wave lengths. In all these experiments the worms 
 M'cre undoubtedly negative to the light conditions they 
 avoi(!i.^ 
 
 The reu in these experiments contained rays between 
 6io and yio'"-, the blue rays between 550 and 570"'', 
 those between 700 and 720"'^, and those below 540'''^; the 
 
MULTICELLULAR ANIMALS AND COLOR 345 
 
 green rays between 450 and 600'^'^. In the red-blue and 
 green-blue tests the intensity of the red and green was 
 twice as high as that of the blue. In the red-green test it 
 was a little more than twice as high in the red as in the 
 green. In these tests, however, the worms consistently 
 collected in the light having the higher intensity, whereas 
 in white light the opposite was true. However, when 
 white light containing ultra-violet was contrasted with 
 white light without, but of decidedly higher intensity, the 
 great majority collected in the light having the higher 
 intensity. 
 
 Considering the conditions of these experiments I do 
 not hesitate to conclude that blue, violet and ultra-violet 
 are more efficient in causing reactions in Lumbricus than 
 green or red. Similar conclusions are strongly supported 
 by the reacfions of various other species, notably the 
 snail Limnaeus stagnalis and several insect larvae as well 
 as imagos. In numerous other instances, however, as 
 already intimated, the results are not conclusive; in these 
 it is questionable whether the quality of light has any 
 specific functions and it is impossible to say to what the 
 change in distribution is due. 
 
 The following conclusion of Graber is undoubtedly not 
 warranted (1884, p. 245): " Als eines der allerwichtigsten 
 und interessantesten Ergebnisse meiner vergleichenden 
 Lichtgefiihl-Studien betrachte ich die Tatsache, dass die 
 leukophilen oder weissholden Tiere mit geringen Ausnahmen 
 alle blauliebend, die leukophoben oder dunkelholden hingegen 
 rotliebend sind.'' 
 
 The results of these experiments appear to indicate that 
 in general animals which are positive in white light are 
 also positive in blue, whereas those which are negati\e in 
 white are negative in blue, not positive to red, as Graber's 
 conclusions would suggest. This is however not univer- 
 sally true, as the experiments on Daphnia clearly show. 
 
 The ideas in Graber's conclusions expressed in the quo- 
 tations above and elsewhere, that animals actually perceive 
 
346 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 color and that the reactions are controlled by subjective 
 sensation, are evidently without experimental foundation. 
 They are reached purely through human analogies. 
 
 6. Loeb's Observations 
 
 The accounts of Loeb's experiments on the effect of 
 different colors on reactions are found in two papers, 
 published in 1890 and 1893 respectively. Of the two 
 colors, red and blue, used in these experiments, the former 
 was produced by means of a solution of potassium bichro- 
 mate or ruby glass, and the latter by means of " cobalt 
 glass or an ammoniacal solution of copper." In each of 
 these two colors the reactions of the following animals 
 were studied: Musca larvae, plant lice, caterpillars of 
 Porthesia chrysorrhoea, moths of Sphinx euphorbia and 
 Geometra piniaria, various copepods, the meal worm Tene- 
 brio molitor, and larvae of the June bug Melolontha vul- 
 garis, Limulus polyphemus, and Polygordius. 
 
 Loeb maintains that these forms react in blue light 
 essentially as they do in white; that the negative forms are 
 negative in the blue light and the positive forms positive; 
 and that the red rays have a slight effect on the move- 
 ment of some of the animals but none on that of others. 
 He concludes that in all of these animals " the more 
 strongly refrangible rays of the visible spectrum are the 
 most active heliotropically, as in the case of plants " 
 (1905, p. 294). He also says (1905, p. 73), " I have con- 
 firmed the identity of animal with plant heliotropism on 
 crabs (Gammarus locusta, Cuma Rathkii), naked snails 
 and worms (leeches, planarians, earth-worms and others)," 
 but he does not state definitely that he studied the reac- 
 tions of these forms in colored light. 
 
 In harmony with the results of Graber's experiments, as 
 well as with those of various other investigators, Loeb's 
 results show fairly clearly that blue is more efficient than 
 red in stimulating the organisms he tested, but they do 
 
MULTICELLULAR ANIMALS AND COLOR 347 
 
 not show that it is more efficient than yellow or green or 
 any other rays in the spectrum. It is difficult to under- 
 stam. how he could conclude that in animals '' the more 
 strongly refrangible rays of the visible spectrum are the 
 mo't active heliotropically, as in the case of plants," and 
 " that the more refrangible rays have the same effect as 
 m.xed rays " (1905, p. 29), after studying reactions in but 
 two different colors. 
 
 ^posited 
 
CHAPTER XIX 
 
 BRIEF CONSIDERATION OF THE REACTIONS OF MULM- 
 
 CELLULAR ANIMALS WITH WELL-DEVELOPED EYES 
 
 IN LIGHT DIFFERING IN COLOR -WITH SPECIAL 
 
 REFERENCE TO COLOR VISION 
 
 In the lower animals with image-forming eyes the reac- 
 tions to colors are very much more complicated than in 
 those wiihout them. In these there is but little evidence 
 that an>' one color is much more efficient than another. 
 They may be positive to or may select a given color at 
 one time and a very different one at another. Investiga- 
 tions in this line are still few and methods inadequate. We 
 shall present only a few of the more conclusive. Foremost 
 among these may be mentioned those of Lubbock on ants, 
 bees and wasps. Of these we shall devote special atten- 
 tion to the work on ants and bees. 
 
 I. Ants 
 
 In his earlier experiments with ants Lubbock (1895, 
 p. 186) placed strips of glass differing in color, or glass jars 
 containing colored solutions, side by side over an artificial 
 nest.^ After leaving them for a few minutes he recorded 
 the number in each of the different colors, then rearranged 
 the color media and repeated the process. In twelve 
 different observations there was a total of 890 ants under 
 the red, 544 under the green, 495 under the yellow and 
 only 5 under the violet. The results of numerous other 
 observations under similar conditions were in all essentials 
 Uke them. 
 
 "" color media were spectroscopically tested by Professor Dewar. 
 
 348 
 
COLOR VISION 349 
 
 In later experiments Lubbock used more refined methods. 
 He had a prismatic electric-light spectrum thrown upon a 
 nest especially prepared for the purpose. Ten different 
 experiments were made with this, the results of which were 
 similar. I shall quote one in full: 
 
 " Professor Dewar kindly prepared for me a condensed 
 pure spectrum (showing the metallic lines) with a Siemens' 
 machine, using glass lenses and a mirror to give a perpen- 
 dicular incidence when thrown on the nest. ... I arranged 
 the light and the ants as before, placing the pupae in the 
 ultra-violet, some being distinctly beyond the bright 
 thalline band. The ants began at once to remove them. 
 At first many were deposited in the violet, some, however, 
 being at once carried into the dark beyond the red. When 
 all had been removed from the ultra-violet, they directed 
 their attention to those in the violet, some being carried, 
 as before, into the dark, some into the red and yellow. 
 Again, when those in the violet had all been removed, 
 they began on the pupae in the red and yellow, and car- 
 ried them also into the dark. This took nearly half an 
 hour. As I had arranged the pupae so that it might be 
 said that they were awkwardly placed, we then turned the 
 nest round, leaving the pupae otherw^ise as they had been 
 arranged by the ants; but the result of moving the nest 
 was to bring some of them into the violet, though most 
 were in the ultra-violet. They were, as before, all carried 
 into the dark space beyond the red in about half an hour. 
 
 " We then turned the glass round again, this time 
 arranging the end about the length of the spectrum be- 
 yond the end of the violet visible to our eyes. They began 
 clearing the thalline band, carrying some into the violet, but 
 the majority away further from the spectrum. In a quarter 
 of an hour the thalline band had been quite cleared; and 
 in half an hour a band beyond, and equal to the thalline 
 band, those in the violet being left untouched. After 
 the pupae in the ultra-violet portion had all been moved, 
 those in the violet were also carried away and deposited 
 
350 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 about twice as far from the edge of the violet as the further 
 edge of the bright thalhne band " (1895. pp. 203-205). 
 Considering the results of all these different experi- 
 ments, Lubbock concluded (p. 199) " that: (i) ants have the 
 power of distinguishing colours; (2) that they are very 
 sensitive to violet; and it would also seem (3) that their 
 sensations of colour must be very different from those 
 produced upon us." We shall discuss these conclusions 
 
 later. 
 
 Many experiments with various color and intensity con- 
 ditions were performed, in which the light in one part of 
 the nest was passed through carbon bisulphide so as to 
 eliminate the ultra-violet. It was found in general that, 
 other conditions being equal, the ants avoid the light con- 
 taining ultra-violet rays. These rays, although invisible 
 to man, appear therefore to stimulate the ants somewhat 
 like the rays which are visible. These results agree with 
 those of Lubbock on Daphnia and those of Graber on the 
 earthworm, and ten different species of insects as well 
 as a few other forms. It is also well known that para- 
 mecia and various bacteria can be stimulated by ultra- 
 violet. Stimulation by these rays therefore appears to be 
 fairly common among animals. 
 
 Lubbock assumes that ultra-violet as well as the visible 
 rays in the spectrum produces color sensation in ants. 
 He says (1895, p. 220): " These experiments seem to me 
 very interesting. They appear to prove that ants per- 
 ceive the ultra-violet rays. Now, as every ray of homo- 
 geneous light which we can perceive at all appears to us 
 as a distinct colour, it becomes probable that these 
 ultra-violet rays must make themselves apparent to the 
 ants as a distinct and separate colour (of which we can 
 form no idea), but as unlike the rest as red is from yellow, 
 or green from violet." 
 
 Very few will agree with Lubbock in assuming that he 
 has demonstrated color vision — subjective sensation in 
 ants. His results however are reliable. There can be no 
 
COLOR VISION 
 
 351 
 
 question but that these creatures while in their nests 
 avoid white Hght, and particularly rays of the shorter 
 wave lengths, and that red is much less efficient in stimu- 
 lating them tlian any other color. The effect of the dif- 
 ferent rays is at least to some degree specific. The dis- 
 tribution of efficiency in stimulating ants in the prismatic 
 solar spectrum is certainly not proportional to the dis- 
 tribution of energy or brightness as judged by the human 
 eye. But why this is true and what mechanism is involved 
 in the process of avoiding the shorter waves are questions 
 upon which Lubbock's results have no definite bearing. 
 
 Can the reactions of ants to colors be explained by 
 assuming that they are negative to rays of the shorter 
 wave lengths, and that they are oriented by the light in 
 the sense of Loeb's definition of heliotropism, or in any 
 other definite way? In the absence of larvae or pupae there 
 is some evidence indicating that ants orient and move 
 from light containing the shorter waves, and that their 
 movements are fairly definitely controlled by external con- 
 ditions, but in the presence of these organisms there is no 
 evidence showing that their reactions are thus definitely 
 controlled. Under such conditions internal factors must 
 have much to do with the reactions. In transferring 
 larvae and pupae, as in the experiment of Lubbock quoted 
 above, a given individual may pass back and forth many 
 times from the red end of the spectrum to the violet and 
 ultra-violet before all the young are deposited in the red 
 or beyond. It cannot be maintained that they become nega- 
 tive to violet light when they are carrying their young and 
 positive when they are not; for this opposes the fact that 
 in the absence of larvae and pupae they avoid the violet. 
 During the process of transferring their young the ants 
 cannot therefore be considered either negative or positive 
 to the violet or to any other color or condition of illumi- 
 nation. These reactions must be regulated primarily by 
 internal factors. What these factors are is a question con- 
 cerning which there is yet very little knowledge. That the 
 
352 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 reactions are adaptive, that it is to the advantage of the 
 larvae and pupae, as well as to the adults, to be in darkness 
 or in rays of longer wave lengths while in their nests, 
 rather than in those of shorter, can scarcely be questioned. 
 And Lubbock's suggestion that " ants do not like light in 
 their nests, probal)l> i)ecause they do not deem it safe," 
 if liberally interpreted, may not be so far from the truth 
 as some investigators assume (see Loeb, 1905, p. 13). 
 
 2. Bees 
 
 In the study of the effect of different colors on the 
 behavior of bees Lubbock showed the same characteristic 
 thoroughness and ingenuity manifested in his work on 
 ants. Several pieces of paper which differed in color were 
 pasted to glass slips, upon each of which a drop of honey 
 was placed. A bee was then taken from a hive, marked 
 and placed near the honey on one of the glass slips. After 
 the bee had taken honey to the hive and returned several 
 times the glass slip was removed to a distance of from one 
 to three feet, and one of a different color was put in its 
 place. When the bee now returned it seldom went to the 
 honey over the new color in the old position; it usually 
 returned to that over which it had been accustomed to 
 collect honey, although it was now in a new position. 
 The reactions to various different colors were tested in this 
 way. In case of blue being the original color visited by 
 the bee, it returned 31 times to the blue, two times to the 
 green and not at all to the yellow, orange, red and white, 
 one of which was substituted for the blue between each 
 \isit. In case of green being the original color the bee 
 returned to the green 20 times, to the blue twice, to the 
 yellow once and not at all to the other colors. In case of 
 orange it returned 20 times to the orange, and but twice to 
 other colors which were not recorded. These experiments 
 extended ov^er several days and a number of different bees 
 were used. 
 
COLOR VISION 353 
 
 In a second set of experiments Lubbock trained a bee 
 to come to a lawn for honey placed on a piece of colorless 
 glass. He then procured several similar pieces of glass of 
 different colors, arranged them on the lawn so that they 
 were all about one foot apart, and put a drop of honey on 
 each. After the bee returned it was frequently disturbed, 
 so that it was compelled to sip honey several times, either 
 over the same color or over different ones, before it left for 
 the hive. The order in which it visited the different colors 
 was recorded, and every time the bee left for the hive the 
 relative position of all the glass plates was changed. 
 
 Many different series of observations were made in this 
 way under various conditions; the results in all are however 
 essentially the same. I shall therefore present only a por- 
 tion of one series. In this series (1895, p. 307) the bee came 
 first to the blue 31 times, to the green 10, to the orange 11, 
 colorless 5, red 14, white 19, and to the yellow 9 times. 
 This shows that the blue is visited much oftener than any 
 other color, although the bee was trained to get honey 
 from the colorless piece of glass. 
 
 Graber (1884, pp. 167-174) obtained similar results in 
 comparing the effect of red with that of blue by means of 
 a different method. He inclosed the bees in a box one 
 half of which was illuminated with red, the other half with 
 blue light. In some experiments the blue was much 
 brighter than the red, in others the red was brighter than 
 the blue. In every test a majority of the individuals 
 inclosed collected in the blue. 
 
 The first set of experiments led Lubbock to conclude 
 " that bees possess the power to distinguish colours " 
 (p. 302) ; and the second, that they prefer blue. 
 
 Perhaps the most interesting of Lubbock's results is the 
 demonstration that honey bees can be trained to select 
 any given color. This shows that they can in some way 
 distinguish color and that the different rays and combina- 
 tions of rays must have a specific effect on them; but it 
 does not prove that they have color vision, for color-blind 
 
354 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 persons can also distinguish different colors if they differ 
 in brightness as did those in Lubbock's experiments. 
 It does, however, demonstrate that internal factors are of 
 primary importance in these reactions. The selection of 
 a given color now and a different one some other time, 
 flight into the pitch darkness of their home at one moment 
 and out into the brightest sunlight the next, is surely not 
 the result of orientation unequivocally controlled by the 
 immediate environment. These reactions can be explained 
 only upon the assumption that some internal condition 
 regulates the change in reaction. 
 
 Concerning the second conclusion of Lubbock, that bees 
 " prefer blue," it must be said that if this is anthropo- 
 morphically interpreted there is no solid foundation for 
 the conclusion, but if it is merely intended to indicate that 
 shorter waves having a given amount of energy stimulate 
 bees more strongly than longer waves having the same 
 amount of energy, there can be no doubt as to its validity, 
 unless the bees used in the experiments of Lubbock and in 
 those of Graber had been accustomed to collect honey from 
 blue flowers before the tests were made. 
 
 Aside from those already mentioned there are numerous 
 other references to the reactions to color in ants, bees 
 and other arthropods in the literature on these subjects. 
 Among these may be mentioned those of Minkiewicz (1907), 
 Keeble and Gamble (1900), and Bell (1906) on decapod 
 Crustacea, those of the Peckhams and McCook on spiders, 
 and those of Graber, Forel, Plateau, Buttel-Reepen, Bethe, 
 Bulman, Miss Fielde, Darwin, Miiller and Bonnier on ants, 
 bees, wasps and other insects. Much of the work of the in- 
 vestigators in the last group was directed toward the ques- 
 tion as to the influence of the color of flowers on the visits 
 of insects with its bearing on their evolution. Most of 
 the results of this work favor the negative of this question, 
 but nearly all of these investigators agree that insects have 
 color vision, although their evidence is far from conclusive. 
 With reference to reaction to color, none of the work of any 
 
COLOR VISION 355 
 
 of the authors mentioned above is as thorough as that 
 of Lubbock. And since it leads to no essentially new or 
 contradictory conclusions, with the exception of that of 
 Bethe, it would be of but little value to review it here. 
 That of Minkiewicz is nevertheless somewhat out of the 
 ordinary, and it may consequently not be out of place to 
 devote a few paragraphs to it. 
 
 3. Higher Crustacea — Experiments of Minkiewicz 
 
 Before presenting the work of Minkiewicz on the Crus- 
 tacea, it will be necessary to refer briefly to his earlier obser- 
 vations on the nemertean Lineus ruber, since these form 
 the basis of his later work. 
 
 Minkiewicz exposed these worms in horizontal beams 
 of light of different colors produced by means of a prism, 
 colored glass or tissue paper. Under normal conditions 
 they were found to be negative in blue or green and positive 
 in red or yellow. But if left for several hours in 100 c.c. 
 of sea water diluted with 25 to 80 c.c. of distilled water, 
 they became positive to the more refrangible rays of the 
 spectrum. The striking peculiarity of these reactions is 
 tliat in colorless light the organisms were negative under 
 all conditions. He says (1907, p. 48): " I have not as yet 
 found, in spite of long continued researches, a single means 
 of transforming the negative phototropism of Lineus into 
 positive phototropism by agents either chemical, osmotic 
 or thermic. Thus, for example, the animal remains nega- 
 tive until its death in the presence of white light whatever 
 the concentration of the sea water." 
 
 The author concludes that all the chromatic rays have a 
 specific action independent of each other and of white light. 
 
 Among the Crustacea, Minkiewicz experimented with 
 spider crabs and hermit crabs. His studies on the former 
 were devoted primarily to Maja verrucosa and Maja squi- 
 nado, but he claims to have made analogous observations 
 on different species of Pisa, Inachus and Stenorynchus. 
 
356 LIGHT AND THE BEHAVIOR OF ORGAXISMS 
 
 It is well known that many of the spider crabs fasten 
 seaweeds and other objects to the carapace. Minkiewicz 
 placed the crabs into aquaria with the sides and bottom 
 uniformly colored and added bits of colored paj^er, some 
 corresponding to the walls of the aquarium and others not. 
 He claims that the animals selected those pieces which 
 harmonized with their environment in color, and fastened 
 them to the surface of the body and legs so that they became 
 inconspicuous. " Les resultats sont . . . vraiment frap- 
 pants, la couleur du costume correspondant toujours 
 precisement a celle du milieu " (1907, p. 41). In a black 
 aquarium however there was no evidence of selection, and 
 it appears that the animals were unable to distinguish 
 between green and yellow. 
 
 The author also says that if the crabs are left on a 
 given color for some time and then transferred to an 
 aquarium which is variegated in color, they come to rest 
 in that part which corresponds in color with that from 
 which they were taken. 
 
 In the experiments on the hermit crabs Minkiewicz 
 illuminated the two halves of an aquarium with light 
 of different colors, placed the animals so that the two 
 eyes were exposed to light of different colors, and found 
 that the creatures turned toward the color indicated by the 
 arrows below; i.e., in case of black and red in the aquarium 
 they went toward the red, in case of red and yellow, toward 
 the yellow, etc., as indicated: 
 
 black -^ red -^ yellow — > blue -^ violet— > green — > white. 
 
 If the crabs are kept in a jar and exposed to their own 
 excreta for some time their reactions to colors gradually 
 change as follows: 
 
 Normal : red -^ blue — * green, 
 red — * green — > blue, 
 green — > red — > blue, 
 green -^ blue -^ red. 
 
COLOR VISION 357 
 
 Minkiewicz maintains that these reactions cannot be due 
 to intensity difference, since the Hght in the yellow and green 
 under the conditions of the experiments was more intense 
 than that in any other color, and the organisms were posi- 
 tive to blue in the presence of yellow, but positive to green 
 in the presence of violet. 
 
 The most interesting of the results obtained by Minkie- 
 wicz refer to the change in reaction to different colors. 
 The spider crabs apparently become positive to the color 
 wiiich is dominant in the environment. Lincus and 
 Pagurus, positive to a given color under certain conditions, 
 become negative to that color under different conditions, 
 or positive to some other color, while they remain con- 
 tinuously positive to white light. These reactions have 
 much in common with those ot the honey bee to different 
 colors. They show that the creatures, especially the spider 
 crabs, can distinguish colors. This of course does not 
 demonstrate the power of subjective color sensations. 
 It does however indicate that the different rays cause 
 different changes in the organisms; in other words, that 
 they have specific effects which are in some way related 
 to the wave lengths. There is, however, no evidence bear- 
 ing on the question as to whether or not these effects are 
 analogous to those associated with brightness sensation or 
 with color sensation in man or with neither. 
 
 The results of Minkiewicz must unfortunately be accepted 
 with reserve, since he does not describe his methods in 
 sufficient detail to warrant definite conclusions as to their 
 validity, and they have as yet not been confirmed, although 
 similar experiments have been made on other forms. 
 
 Pearse has recently (1909) repeated the experiments 
 of Minkiewicz at Woods Hole, Mass., using Libinia emar- 
 ginata in place of Maja. He obtained no evidence what- 
 ever of decoration in harmony with the environment. I 
 have observed many of Pearse's experiments and repeated 
 some myself, and feel justified in saying without going 
 into details, that there was no evidence of color selec- 
 
358 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 tion. Bateson (1887) records similar results in work on 
 Stenorynchus. Recently I have again tested crabs for 
 selection of color. At the Tortugas Islands numerous 
 specimens of each of three species, not yet definitely 
 identified, were used in these tests. A large proportion 
 of all of the animals observed decorated profusely in the 
 colored aquaria used; but I found no e\idence whatever 
 of harmony between the color of the substance selected 
 and that {predominating in the environment, although the 
 methods used by Minkiewicz were closely imitated. 
 
 4. Fishes 
 
 I shall refer to but two other experiments on the subject 
 of reactions to colors, one by Washburn and Bentley on 
 the creek chub Semotilus atromaculatus, the other by 
 Reighard on the marine gray snapper, Lutianus griseus. 
 
 Washburn (1908, p. 140) gives the following description 
 of their experiment: * Two dissecting forceps were used, 
 alike except that to the legs of one were fastened, with 
 rubber bands, small sticks painted red, while to those of 
 the other similar green sticks were attached. The forceps 
 were fastened to a wooden bar projecting from a wooden 
 screen, which divided the circular tank into two compart- 
 ments, and hung down into the water. Food was always 
 placed in the red pair of forceps, which were made fre- 
 quently to change places with the green ones; and the fish 
 was caused to enter the compartment half of the time on 
 one side and half of the time on the other. This was to 
 prevent identification of the food fork by its position or 
 the direction in which the fish had to turn. The animal 
 quickly learned to sinojle out the red fork as the one impor- 
 tant to its welfare, and in forty experiments, mingled with 
 others so that the association might not be weakened, 
 where there was no food in either fork, and where the for- 
 ceps and rul)bcr bands were changed so that no odor of 
 food could linger, it never failed to bite first at the red. 
 
COLOR VISION 
 
 361 
 
 Moreover, the probability that its discrimination 
 based upon brightness was greatly lessened by using, \vi. 
 we experimented without food, a dilTerent red much lighter 
 than that in the food tests. The fish successfully discrimi- 
 nated red from blue j)aints in the same way, and it was 
 afterwards trained, by putting food in the green fork, to 
 break the earlier association and bite first at the green." 
 
 Reighard made his experiments on a school of gray 
 snappers, a form which usually inhabits the water under 
 a dock at one of the Dry Tortugas Islands. The gray 
 snappers feed on atherina, a small fish found in abundance 
 near the shore. They take these fish, even if they have 
 been killed in formalin and stained any color, but if Cassi- 
 opea tentacles are fastened to the atherinas they soon learn 
 to avoid them. After the gray snappers had learned to 
 reject red atherinas with tentacles it was found that they 
 also rejected red ones without, but that they still took 
 those stained any other color. For example, when blue 
 and red atherinas were thrown in together they took only 
 the blue, and this was true even if some of the red ones 
 were of a much brighter shade and others of a much darker 
 shade than the blue ones. 
 
 This seems to prove that the selection could not have 
 been due to difference in brightness, such as a color-blind 
 person can perceive in the different colors, and it led the 
 author to conclude that ^ray snappers have color vision. 
 
 It will be seen that the conclusion that fishes have color 
 vision, both in the work of Washburn and Bentley and in 
 that of Reighard, rests primarily upon the fact that the 
 animals discriminated between red of different shades and 
 blue or green, and upon the assumption that the brightness 
 of the different parts of the spectrum is practically the same 
 for fishes as it is for man — that their eyes are stimulated by 
 all the rays from the infra-red to the ultra-violet somewhat 
 as ours are. While this may be true, it has not been posi- 
 tively demonstrated. As a matter of fact, there are reasons 
 for believing that the red end of the spectrum for fishes 
 
3-8 LIGHT '^^^^^ THE BEHAVIOR OF ORGANISMS 
 
 tion. ^"^^ other vertebrates, e.g., the dancing mouse and 
 5l^pjr-blind {persons, is much darker than it is normally 
 for man, and that the visible s{:)ectrum for these forms is 
 somewhat shortened at this end. It is evident that the 
 red, which appeared brighter than the blue to the human 
 eye, may ha\e actually apj)eared darker to the fishes, and 
 if this be true the discrimination may have been made 
 on the basis of brightness. There consequently remains 
 some doubt as to the \alidity of the conclusion stated 
 above. 
 
 P^ven in the birds and mammals the question of color 
 vision is not settled, although these animals can undoubt- 
 edly distinguish different regions in the spectrum. But 
 since it is not our object to discuss this subject we shall 
 refer the reader to the excellent researches of Porter (1904, 
 1906) on the birds, Yerkes (1907) on the dancing mouse, 
 Kinnaman (1902) and Watson (1909) on the monkey, and 
 Cole (1907) on the raccoon. 
 
 5. General Summary and Conclusions of Part IV 
 
 (i) The energy curve in both normal and prismatic 
 spectra is much the same for nearly all sources of light. 
 Beginning with the violet end it rises more or less gradu- 
 ally to a maximum at the red end, 760'"'. In the normal 
 or grating spectrum for sunlight, however, the maximum 
 is in the orange at 610'"'. From this point the energy 
 decreases slightly toward the red end. 
 
 (2) The location in the spectrum of the maximum efTect 
 on photochemical reactions depends primarily upon the wave 
 length and the chemical substances wdiich take part in the 
 reaction; and secondarily upon the absorption of light, the 
 distribution of energy and the presence of substances which 
 apparently do not take part directly in the reaction. The 
 reaction between quinine and chromic acid, e.g., takes place 
 most rapidl>' in the ultra-violet, whereas Triphenylfulgid is 
 changed from the black form to the yellow most rapidly in 
 
COLOR VISION 361 
 
 the presence of red, orange and yellow rays, and photosyn- 
 thesis in plants proceeds most rapidly in the red and orange. 
 The photochemical reaction in a given substance is therefore 
 primarily dependent upon the length of the light waves. 
 The reaction between quinine and chromic acid is affected 
 only by the light absorbed by the (|uinine, but the effect 
 is not proportional to the absorption. The maximum ab- 
 sorption takes place in the ultra-violet, but the maximum 
 efficiency is in the green. This shows that the location 
 of the maximum effect is dependent upon the power of 
 absorption and upon the distribution of energy as well as 
 upon wave length. Ozone in the presence of chlorine is 
 changed to oxygen by the visible light rays, while pure 
 ozone is not, showing that photochemical reaction in a 
 given substance may depend upon the presence of other 
 substances. 
 
 (3) The distribution of brightness as judged by the human 
 eye is approximately the same for both normal and pris- 
 matic spectra of sunlight and gaslight. But the distri- 
 bution of energy in these spectra differs considerably. It 
 is therefore evident that brightness must be primarily a 
 function of the length of the waves and secondarily a func- 
 tion of the amplitude or energy contents. In lower in- 
 tensity the maximum is near the green, in higher it is in 
 the orange. In color-blind persons it is usually in the 
 green. 
 
 (4) In the higher plants the maximum rate of curvature 
 in the spectrum takes place at the lower limit of the violet, 
 but there is a secondary maximum in the red. In some 
 fungi, however, the rate of curvature takes place under 
 potassium bichromate as rapidly as it does under an 
 ammoniacal solution of copper hydrate. That is, it takes 
 place as rapidly in the longer waves of the spectrum as it 
 does in the shorter. In the swarm-spore the region of 
 maximum stimulation in the solar prismatic spectrum is in 
 the indigo near the Fraunhofer line G. In Amoeba, Euglena 
 and Hydra it is in the blue, in Paramecium in the ultra- 
 
362 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 violet, in Daphnia and Simocephalus in the yellow or green, 
 in Bacterium photometricum probably in the infra-red 
 with a secondary maximum in the orange, while in Oscillaria 
 all rays appear to be ecjually efhcient. In plasmodia, 
 protoplasmic streaming in cells, earthworms, some mollusks, 
 and a number of insects and spiders, the region of maximum 
 stimulation is probably somewhere toward the violet end 
 of the spectrum, although it has not been definitely lo- 
 cated. In nearly all organisms without image-forming 
 eyes^ the relative stimulating efficiency of the different rays 
 is apparently constant under different conditions, but in 
 the forms with eyes there is evidence that it varies. Some 
 of the spider and hermit crabs, a number of insects and 
 spiders and many higher forms may be positive to certain 
 rays under certain conditions and to others under other 
 conditions. Bees and fishes can undoubtedly distinguish dif- 
 ferent regions in the spectrum. They can be trained to select 
 any of the primary colors of the spectrum by associating 
 these colors with food. That is, they are positive to (or 
 select) one color at one time and another at a different time. 
 Just what mechanism is involved in this power of selection 
 is unknown. Whether it is on the basis of brightness or on 
 the basis of color vision or neither is a matter concerning 
 which experimental evidence does not warrant a definite 
 conclusion. Many organisms react to ultra-violet much 
 as they do to visible rays. This is in harmony with the 
 following quotation from Schafer referring to man (1898, 
 p. 1055): " The invisibility of the infra-red rays is prob- 
 ably due to insensitiveness of the retina, while the ultra- 
 violet rays fail to be seen, partly, at any rate, owing to 
 absorption by the ocular media." 
 
 (5) The presence of certain rays retards the reaction to 
 others in a number of organisms. According to Wiesner, 
 some jilants react to red more strongly than to red mixed 
 with yellow. And according to Lubbock and Wilson 
 
 ^ Euglena appears to be an exception to this. According to the researches 
 of Engelmann it becomes positiv^e to red in low oxygen pressure. 
 
COLOR VISION 363 
 
 Daphnia reacts more strongly to green or yellow and Hydra 
 to blue than to white light. These reactions have much 
 in common with the reversible photochemical reactions of 
 certain chemical compounds, i)articularly the fulgides, in 
 which the reaction in one direction proceeds most rapidly 
 in the shorter wave lengths and in the opposite direction 
 in the longer, while in a mixture of rays the reaction pro- 
 ceeds more slowly in one or the other direction, depending 
 upon the relative amount of the different rays. 
 
 (6) Considering the results set forth above it is evident 
 (a) that, contrary to the hypothesis of Sachs, Loeb and 
 Davenport, the shorter waves are not the more active in 
 all plants and animals; the different rays do not have 
 the same relative stimulating efficiency in all organisms; 
 and {h) that the stimulating efficiency of the different 
 rays, probably in all organisms, is not proportional to the 
 energy they contain, but that for a given ray or color there 
 is a definite relation between the energy and the stimula- 
 tion which is probably in accord with Weber's law. 
 
 (7) Light, as we have seen, causes reactions between 
 many different chemical compounds. In these reactions 
 the different rays have a specific effect. That is, certain 
 reactions are produced only by waves of a given length and 
 others only by waves of a different length. If reactions 
 in a given chemical solution take place in light waves of 
 a given length, and those in another solution in waves of a 
 different length, we may be fairly certain that the reacting 
 compounds in the two solutions differ. The reactions of 
 organisms are caused by, or at least associated with, chem- 
 ical changes in the organisms. The organisms probably do 
 not react to the external agents directly, but to the chem- 
 ical changes within produced by these agents. Since the 
 reaction to the different rays is not the same in different 
 organisms, it is clear that the chemical changes associated 
 with the reactions in the different organisms are not the 
 same. For example, in Amoeba the maximum power of 
 stimulation is in the blue; this must be associated with 
 
364 LIGHT AXD THE BEHAVIOR OF ORG AX IS MS 
 
 certain chemical changes which are produced by the blue. 
 In Daphnia the maximum is in the \ello\v and green, and 
 in many of the plants it is in the violet. These reactions, 
 too, are associated with chemical changes, but since these 
 chemical changes are caused by rays differing in wave 
 length from those which are most efficient in Amoeba, the 
 chemical compounds must also be different unless the dif- 
 ference in the effect of the different rays can be accounted 
 for by assuming the presence of certain inactive substances 
 which intluence the chemical reaction, or by assuming 
 selective al)sorption on the part of the organism. It is 
 however not likely that the difference in reaction to differ- 
 ent rays can be explained thus. We may then conclude 
 that the chemical changes associated with reactions are 
 not the same in all organisms. While we do not at present 
 know what these changes are, there are prospects that 
 future investigations along this line may demonstrate the 
 nature of some of them at least, especially after the photo- 
 chemical reactions in organic and inorganic substances have 
 been more thoroughly investigated. 
 
 (8) In plants and the lower organisms on which the 
 relative stimulating efficiency of the different rays is fairly 
 constant the chemical changes accompanying the reactions 
 may be relatively simple, but in the higher forms in which 
 the relative stimulating efficiency of the different rays 
 varies it seems evident that the chemical changes must be 
 very complicated. If this is true, it contradicts Loeb's 
 general conclusion that the reaction mechanism associated 
 with photic responses in plants is the same as that in 
 animals, that " the dependence of animal movements on 
 light is in every point the same as the dependence of plant 
 movements on the same source of stimulation " (1905, 
 p. 81). 
 
 (9) Honey bees, some fishes, birds and mammals, and 
 probabl}' some of the decapod Crustacea and spiders, can 
 unquestionably, and many of the lower forms with well- 
 developed eyes can probably, distinguish the different 
 
COLOR VISION 365 
 
 regions of a spectrum. Whether the mechanical processes 
 associated with this discrimination are analogous to those 
 associated with brightness or color vision in the human 
 being or neither is not known, but the processes in these 
 forms are undoubtedly very different from those associ- 
 ated with the reactions in simpler forms, e.g., in Amoeba 
 or plants. 
 
CHAPTER XX 
 
 THEORETIC CONSmERATIONS 
 
 The following points have been established in the pre- 
 ceding pages: 
 
 (i) Movement and change in movement, both in rate 
 and direction, may take place without any immediate exter- 
 nal change. 
 
 (2) Sudden changes in light intensity on any sensitive 
 structure in an organism may cause reactions; ^ for exam- 
 ple, the orientation of Euglena and the retraction of the 
 tubicolous annelids. 
 
 (3) Continued illumination without any variation of 
 intensity probably affects the rate of locomotion in all 
 organisms which respond to light, and it may cause changes 
 in direction of movement by inducing a reversal in the 
 sense of reaction. The time of exposure, as well as the 
 absolute intensity, is functional in this. In fact the prod- 
 uct of the time of exposure and the intensity is probably, 
 within certain limits, constant in producing a given stimu- 
 lus, no matter what the relative value of the two factors is. 
 
 (4) A sudden increase and a sudden decrease of light 
 intensity may under certain conditions produce the same 
 reaction, e.g., the contraction of Helix hortensis, the avoid- 
 ing reaction in Euglena and the raising and throwing of 
 the anterior end from side to side in planarians. Such 
 responses are more striking in some cases of stimulation 
 by temperature than in case of stimulation by light. Para- 
 mecium, for instance, gives the avoiding reaction to de- 
 
 ^ In this discussion we shall consider anything which causes a change 
 of movement a stimulus, and any response to a stimulus a reaction. A 
 reaction, then, is cither a change in rate of movement or in direction of 
 movement. 
 
 366 
 
THEORETIC CONSIDERATIONS 367 
 
 crease as well as to increase of temperature, even if the 
 change is only slight. 
 
 (5) A given condition of illumination may inhibit one 
 kind of movement in an organism and cause movement 
 of another kind. When the oral end of Hydra viridis 
 is fully illuminated the swinging about the point of at- 
 tachment is inhibited and locomotion is produced. (See 
 Chapter VIII). 
 
 (6) An increase in the general illumination of an organ- 
 ism may cause an increase in activity, while a sudden de- 
 crease of intensity causes a still greater increase in activity 
 in the same organism at the same time. If the light inten- 
 sity on a Volvox colony under certain conditions is increased, 
 all of the zooids in the colony become more active, but those 
 on the shaded side of the colony become most active. The 
 rotation of the colony on the longitudinal axis causes a sud- 
 den decrease of intensity on the sensitive part of the zooids 
 as they are carried to the shaded side of the colony (see 
 Chapter VII), and the greater increase in activity of the 
 zooids on the shaded side is no doubt due to this sudden 
 decrease of intensity, while the activity of all the zooids 
 is probably augmented by the effect of the continued 
 illumination. 
 
 (7) An increase in light energy may produce the same 
 effect on reactions as a decrease in heat energy. Chla- 
 mydomonas, for example, becomes negative in constant 
 temperature if the light intensity is increased or in con- 
 stant illumination if the temperature is decreased. (See 
 Chapter XIII). 
 
 (8) Acids, certain narcotics and salts, and at least one 
 alkali, may cause a change in the sense of reaction from 
 negative to positive in Gammarus pulex. Any condition 
 which acts as a depressant may cause Ranatra or Arenicola 
 larvae to become negative. 
 
 (9) The stimulating effect of the different rays in the 
 spectrum is specific. But it is not the same in all organisms. 
 With a given amount of energy some are most strongh 
 
368 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 stimulated by blue, others by violet or ultra-violet, others 
 by green and yellow, and still others by red and infra-red 
 (see Summary to Part I\', Chapter XIX). 
 
 (10) Reactions to light are variable, modifiable, and in 
 general adajnix'e. (a) An attached specimen of Stentor 
 coeruleus, for example, may contract suddenly when light 
 of a given intensity is Hashed upon ii, or it may merely 
 swing about its point of attachment or it may not respond 
 at all. Hydroides may remain in its tube after stimulation 
 by a gi\'en decrease of intensity only a few seconds, or it 
 may remain for several minutes. This difference in re- 
 sponse to the same external conditions must be due to 
 internal factors, (b) Wherever there is a reaction to a 
 sign, it is probable that the response to a given external 
 condition has been modified. For example, Euglena under 
 certain conditions responds to a very slight decrease in 
 light intensity on the colorless anterior end, which is in 
 itself of no consequence to the organism, but this slight 
 decrease in illumination is usually followed by a greater 
 decrease on the entire body if there is no change in the 
 direction of locomotion, and it is of course for the welfare 
 of the organism to prevent this. It is probable that 
 originally no response was given until the injurious condi- 
 tion was realized. Many similar illustrations are found 
 in organisms that respond to shadows which announce 
 the approach of an enemy, (c) Adaptation and regulation 
 are striking characteristics in nearly all reactions to light. 
 The reactions are adaptive not only under constant con- 
 ditions, but also under varying conditions, for if the environ- 
 ment is changed the reactions change to meet the demands 
 of the new circumstances. Jennings has well said (1906, 
 P- 338): "Regulation constitutes perhaps the greatest 
 problem of life. How can the organism thus provide for 
 its own needs? To put the question in the popular form, 
 How does it know what to do when a difficulty arises? 
 It seems to work toward a definite purpose. In other 
 words, the hnal result of its action seems to be present in 
 
THEORETIC CONSIDERATIONS 369 
 
 some way at the beginning, determining what the action 
 shall be. In this the action of Hving things appears to 
 contrast with that of things inorganic." 
 
 Let us now see in how far the various theories concern- 
 ing behavior account for tlie plienomena set forth above. 
 The more prominent of these are those of Loeb, Jennings 
 and Driesch. We have already referred to some theories 
 elaborated to account for the reactions of plants to light 
 (see Chapter IV). These we shall not consider again here. 
 
 Loeb's theories refer to two features in behavior: (i) the 
 direct cause and regulation of any given reaction and (2) 
 the origin of adaptive reactions, (i) He says (1906, p. 130) 
 that the reactions " are caused by a chemical effect of 
 light " and then continues as follows, showing how the 
 reactions are regulated: "We assume . . . that if light 
 strikes the two sides of a symmetrical organism with 
 unequal intensity, the velocity or the character of the chem- 
 ical reactions in the photosensitive elements of both sides 
 of the body is different; that in consequence of this differ- 
 ence the muscles, or contractile elements, on one side of the 
 organism are in a higher state of tension than their antago- 
 nists." He claims (p. 131) that " it [is] possible by the use 
 of chemicals to control the precision and sense of the 
 heliotropic reactions " and that this and other facts prove 
 that reactions to light are caused by the chemical changes 
 produced by the light. Very few will agree that Loeb 
 has proved his point here. But practically every one 
 assumes that light does cause chemical changes in organisms 
 and that these changes affect the reactions. Many, how- 
 ever, do not agree with Loeb in the idea that they are the 
 direct and immediate cause of the reactions to light, as 
 his elucidation indicates. The fact that acids, narcotics, 
 salts, alkalis or any condition which acts as a depressant 
 may produce the same effect on the reactions of certain 
 organisms to light seems to indicate that the reactions 
 are, at least in some instances, due to a general effect on the 
 organism as a whole. 
 
370 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 There are however photochemical reactions which are 
 suggestively similar to some of the photic reactions in 
 organisms. We have shown in Part IV that in some com- 
 pounds the reactions proceed in one direction in one light 
 condition and in the opposite in another; that the action 
 of the different rays of light is specific, and that in some 
 of these chemical reactions heat and light tend to produce 
 opposite effects. The first two of these reactions are 
 somewhat similar to the change in the sense of reactions 
 produced by changes in light intensity in many organisms 
 and to the specific reactions to different rays. The last 
 is similar to the effect of heat and light on the sense of 
 reaction in Chlamydomonas and various other organisms 
 referred to under (7) above. If then these organisms 
 contain chemical compounds which are affected by light 
 like those referred to above, we can account for the reac- 
 tions mentioned by assuming that they are due to the 
 effect of the light on the chemical changes in the organism. 
 In case of Volvox I was also able to account for a number 
 of peculiarities in the process of reversal in the sense of 
 response by the assumption of reversible photochemical 
 reactions within the organism (Mast, 1907, pp. 1 57-1 61). 
 And we might account for the fact that an increase in 
 illumination produces the same effect as a sudden decrease, 
 as in the case of the zooids of Volvox (see (6) above), 
 by assuming two photochemical reactions, one dependent 
 upon the time rate of change of intensity, the other upon 
 the absolute intensity and the time of exposure. In this 
 same way the inhibition of one sort of movement and the 
 augmentation of another in the same organism (Hydra) 
 might be accounted for. So we might continue and account 
 for modifiability, variability, adaptation, etc., by various 
 other assumptions. But all of these assumptions regard- 
 ing chemical changes are so extremely hypothetical that 
 speculation based on them has at present but little 
 value. And it is important to realize that the common 
 belief that light in some way influences the activity 
 
THEORETIC CONSIDERATIONS 371 
 
 of organisms by chemical changes which it causes within 
 them, is as yet founded almost entirely on such hypo- 
 thetical assumptions — assumptions which are problems, 
 not solutions. 
 
 (2) Loeb's explanations of the origin o{ adaj^iive reac- 
 tions to light is found in the following cj notation (1906, 
 p. 160): '' The fact that cases of tropism occur even where 
 they are of no use, shows how the play of the blind forces 
 of nature can result in purposeful mechanisms. There is 
 only one way by which such purposeful mechanisms can 
 originate in nature; namely, by the existence in excess of 
 the elements that must meet in order to bring them about. 
 In green plants and in some animals the positive heliotrop- 
 ism is useful; yet there exists probably an endless number 
 of heliotropic animals for which their heliotropism is about 
 as useless as is galvanotropism. The prerequisites for 
 heliotropism are a symmetrical body form, which seems 
 to be present in almost all organisms — although some 
 asymmetries exist — and the presence of photosensitive 
 substances, which is not quite so common, but certainly 
 not infrequent. Some of the regular substances found 
 in protoplasm seem to turn readily into a photosensitive 
 form. As the two conditions mentioned above are quite 
 common, the laws of probability make it necessary that 
 in a certain number of cases both conditions will be fulfilled, 
 and then we may expect heliotropic actions. If it now 
 occurs that in an organism the turning to the light helps 
 it to find its food, as is the case with certain caterpillars, 
 e.g., Porthesia chrysorrhoea, or the stems of green plants 
 whose starch is manufactured by light, we have a ' purpose- 
 ful mechanism.' Again, according to the laws of probabil- 
 ity, the number of animals in which the three groups of 
 conditions meet is much smaller than where only two meet. 
 The tropisms thus furnish an insight into the origin of 
 purposeful reactions by the blind forces of nature." The 
 difficulty with this hypothesis is that it does not fit the 
 facts. It rests primarily upon the assumption that there 
 
372 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 exists an endless number of organisms whose reactions to 
 light are useless. Indeed, according to this theory, there 
 must be more organisms in which the reactions to light 
 are or have been useless than there are in which they 
 are purposeful, for the author states, as quoted above, 
 that " the number of animals in which the three groups 
 of conditions [purposeful mechanism] meet is much smaller 
 than where only two meet [useless reactions]." We have 
 demonstrated that, while there are isolated instances, 
 mostly under artificial conditions, in which orientation and 
 subsequent locomotion (heliotropism) lead to fatal results, 
 the orienting reactions are in general useful to the organism 
 in its life processes, and the same may be said regarding 
 all other reactions to light. Thus, it is evident that Loeb's 
 theor>- of the origin of purposeful reactions is not in har- 
 mony with the observed facts. 
 
 Jennings' theory of behavior is founded upon the idea 
 that the reactions are fundamentally " purposeful." He 
 admits that light and other external agents cause chemical 
 changes in the organism, and that all reactions are deter- 
 mined by chemical changes or states; but that the chemical 
 change or state which causes a given reaction is not directly 
 and entirely the result of the external condition which 
 precedes the reaction; that what an organism does under 
 a given condition depends upon what it and its ancestors 
 have done and experienced in the past as well as upon the 
 present external conditions. The reactions are above all 
 things regulatory. External conditions are not the direct 
 cause of reactions. 
 
 Reactions are defined as changes in the activity of or- 
 ganisms. Such changes may occur under constant exter- 
 nal conditions. They are therefore due primarily to 
 internal changes. External factors cause reactions not 
 directly, but indirectly, by altering internal processes 
 (physiological states). Variability in reaction to given 
 external conditions is due to changes in physiological 
 states. If an organism responds to light of a given inten- 
 
THEORETIC CONSIDERATIONS 
 
 373 
 
 sity in a given way now, and to the same intensity in another 
 way later, it is because the physiological state of the organ- 
 ism has changed. Wlien external changes persistently 
 follow each other, as, for example, shadow and contact in 
 case of the attack of an enemy on Hydroides, the shadow 
 produces a certain physiological state. This state is 
 resolved into another by contact, and this results in a 
 reaction. Repetition tends to cause the resolution of the 
 first physiological state into the second, without contact, 
 and consequently a reaction to the shadow which was 
 formerly given only to a contact stimulus. Thus we have 
 the origin of a reaction to a sign, response to a representative 
 stimulus, as Jennings terms it. The shadow in the case 
 mentioned above represents the contact; it.is a sign of the 
 approach of danger. All of this the author has elaborated 
 in a most masterful way in his book " Behavior of the 
 Lower Organisms" (1906). Every step in the develop- 
 ment of the theory is supported by numerous experimental 
 facts and all seems to fit what is known concerning the 
 reactions of organisms. Reactions, according to this 
 theory, are, as stated above, primarily due to physiologi- 
 cal states. External agents ordinarily produce reactions 
 through the effect they have on these states. By the 
 application of this idea all the different phenomena con- 
 nected with reactions to light as summarized at the begin- 
 ning of this chapter can be accounted for. 
 
 But what are these physiological states and of what do 
 they consist? That there are such states in organisms 
 cannot reasonably be doubted, and that the reactions are 
 dependent upon them much as Jennings assumes, seems to 
 me to have been well established in his work. But what 
 regulates the physiological states is a question concern- 
 ing which we have as yet but little knowledge. Jennings 
 assumes that they are regulated entirely objectively, i.e., 
 by the interaction of external and internal physico- 
 chemical processes. This is of course a legitimate assump- 
 tion, an assumption which indeed has some experimental 
 
374 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 support, especially in the fact that changes in metabolism 
 cause changes in reaction. '* Hungry animals react 
 positively to possible food while satiated ones react nega- 
 tively lo the same stimuli." Paramecium bursaria is 
 positive to light in solutions deficient in oxygen, whereas 
 it does not react under normal conditions. After Volvox 
 has been resting in darkness for some time it responds 
 to light in a manner very different from the response given 
 when it is active. Jennings (1906, pp. 251-253 and else- 
 where) cites several other similar instances indicating that 
 reactions depend upon physiological states, but he frankly 
 admits that " it is rarely possible to observe them [physio- 
 logical states] directly," especially in the lower organisms, 
 in which "the real data of observation are the actions; 
 if we considered these alone, we could only state that a 
 given organism reacts under the same external conditions 
 sometimes in one way, sometimes in another. This would 
 give us nothing definite on which to base a formulation 
 and analysis of behavior, so that we are compelled to assume 
 the existence of changing internal states. This assump- 
 tion, besides being logically necessary, is, of course, sup- 
 ported by much positive evidence drawn from diverse 
 fields, and there is reason to believe that in time we shall 
 be able to study these states directly. Before we can 
 come to a full understanding of behavior, we shall have to 
 subject the physiological states of organisms to a detailed 
 study and analysis, as to their objective nature, causes, 
 and effects " (p. 251). 
 
 And again, after giving a most excellent description of 
 the reactions of Stentor, in which he shows that these 
 creatures sometimes respond in at least five different ways 
 to the same stimulus, Jennings says (p. 177): "Since in 
 each of these cases the external conditions remain through- 
 out the same, the change in reaction must be due to a change 
 in the organism. The organism which reacts to the carmine 
 grains by contracting or by leaving its tube must be differ- 
 ent in some way from the organism which reacted to the 
 
THEORETIC CONSIDERATIONS 375 
 
 same stimulus by bending to one side. No structural 
 change is evident, so that all we can say is that the physio- 
 logical state of the organism has changed. The same (organ- 
 ism in different i:)hysiological states reacts (iiffcrently 
 to the same stimuli. It is evident that the anatomical 
 structure of the organism and the different ph>'sical or 
 chemical action of the stimulating agents are not sufficient 
 to account for the reactions. The varying jjhysiological 
 states of the animal arc equally important factors. In 
 Stentor we are compelled to assume at least five differ- 
 ent physiological states to account for the five different 
 reactions given under the same conditions." It is thus 
 evident, without further argument, that while there is 
 some evidence bearing on physiological states, we know 
 but little about their nature and regulation. Even in 
 those cases where it appears evident that they are depend- 
 ent upon metabolic processes, it must be said that we 
 know practically nothing about their regulation, since we 
 know almost nothing concerning the fundamentals in 
 metabolism. It is evident, then, that for all that is known 
 to the contrary, subjective factors, entelechies, or psy- 
 choids, factors foreign to inorganics, may have a hand in 
 controlling physiological changes and consequently the 
 reactions. Such factors have been postulated by the vital- 
 ists and neovitalists, notably by Hans Driesch. 
 
 Driesch postulated a non-energetic factor to account for 
 form regulation and regulation in behavior. He claims 
 that if certain organisms, starfish eggs, for instance, are 
 divided into halves in any direction, each half will produce 
 a new individual. Such organisms, he says, form harmo- 
 nious equipotential systems. Every part has the same 
 potency (future possibilities) as every other part, no 
 matter how the whole is divided. No machine^ (using 
 the term in its broadest sense), he holds, could account for 
 
 ^ "A machine is a typical confip;uration of physical and of chemical 
 constituents, by the acting of which a typical effect is attained." (Driesch, 
 Vol. I, pp. 138, 139.) 
 
376 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 this. Referring to genesis, Driesch asserts that an egg 
 must be considered as a whole, a unit, an entity, an individ- 
 ual, but during every step in the process of development it 
 is still a wholr, an in(li\idual, although it may have been 
 dixided man>' thousands of times. Now he asks (Vol. I, 
 p. 225), ** Can you imagine a very complicated machine, 
 differing in the three dimensions of space, to be divided 
 hundreds and hundreds of times and in si)ite of that to 
 remain always the same whole ? " and adds (p. 226), " We 
 say it is a mere absurdity to assume that a complicated 
 machine, t\pically different in the three dimensions of 
 space, could be divided many many times, and in spite 
 of that always be the whole: therefore there cannot exist 
 any sort of machine as the starting-point and basis of 
 development." Acting, too, he affirms, cannot be ex- 
 plained by the application of physico-chemical principles 
 alone; and it is this part of his analysis which concerns us 
 in particular. " In acting," he says (Vol. II, p. 69), " there 
 may be no change in the specificity of the reaction when the 
 stimulus is altered fundamentally, and again, there may 
 be the most fundamental difference in the reaction when 
 there is almost no change in the stimulus." In other 
 words, each constituent of the effect does not depend upon 
 each constituent of the stimulus, " but one whole depends 
 on the other whole, both ' wholes ' being conceivable in 
 a logical sense exclusively " (p. 81). The author supports 
 his contention still further by referring to the historical 
 basis of acting. He says (p. 81), " Firstly, the effects that 
 are given off in acting occur in a field of natural events very 
 different from that of the stimuli received historically: 
 sensations belong to one, movements to another field. 
 Secondly, the historical basis serves only as a general 
 reservoir of faculties, the specific combinations of the 
 stimuli received historically being preserved by no means 
 in their specificity, but being resolvable into elements; these 
 elements then — transferred, however, to another sphere 
 of happening — are rearranged into other specificities 
 
THEORETIC CONSIDERATIONS 377 
 
 according to the individuality of the actual stimulus in 
 question." 
 
 Thus it is maintained that acting or behavior cannot 
 be accounted for on the basis of physics and chemistry. 
 There must be a factor involved here which is not acti\e in 
 the inorganic realm. This factor is postulated as a non- 
 energetic regulatory factor. It is supposed to prevent 
 reactions — activity or becoming — by compensating po- 
 tentials, i.e. J by transferring kinetic into potential energy, 
 and to regulate reactions and becoming by setting " free 
 Into actuality what it has itself prevented from actuality, 
 what it has suspended hitherto " (p. 180). Thus it is 
 that this non-energetic factor, entelechy, psychoid, is sup- 
 posed to regulate development, becoming and action in 
 organisms. It requires the same amount of energy under 
 certain conditions to fire a gun to-morrow at 10 a.m. as it 
 does at II a.m., and just as much to fire it east as it does 
 to fire It west. In some such way, I understand the author 
 to assume that psychoid can regulate behavior without 
 energy. It does not create reactions, but it regulates 
 them with regard to time and direction. 
 
 Admitting the operation of a factor of this sort it Is a 
 simple matter to explain the regulation of physiological 
 states and all of the puzzling phenomena in behavior previ- 
 ously referred to, but in such an explanation we still ha\e an 
 unknown factor to account for, the psychoid ; and concern- 
 ing this some maintain nothing can be learned, for it is evi- 
 dent that if there is such a factor at work in behavior dif- 
 ferent things can happen under precisely the same ph>sIco- 
 chemical conditions. And If this be true, how can we hope 
 to progress experimentally? According to Driesch's theory 
 every act is definitely and absolutely determined, although 
 not mechanically. With a given physico-chemical con- 
 stellation and a psychoid in a given state, precisely the same 
 things will always occur. If there is a psychoid of this 
 sort, a factor which has different states but which always 
 acts the same in any given state, it seems to me that by 
 
378 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 experimental investigation it will be possible to learn some- 
 thing concerning its nature, just as we have learned, at 
 least in part, the character and manifestation of electricity, 
 gra\ it\' and other similar concepts. 
 
 It is however highly essential in all invCvStigation and 
 discussion bearing on such concepts to differentiate clearly 
 the two points of view from which the\ may be considered, 
 the scientific and the metaphysical. From a scientific 
 point of \iew entelech>' and psychoid, like gravity, electri- 
 city and chemical affinit>-, can be used only to indicate the 
 facts observed, not the cause of the phenomena. From this 
 point of \icw gra\it\' indicates merely the fact that bodies 
 tend to approach each other, not the cause of this tendency. 
 It is only in the realm of metaphysics that all of these con- 
 cepts, psychoid and entelech\-, as well as gravity, electricity 
 and chemical affinity-, are looked upon as causal agents. 
 
 Entelechy, then, from a scientific point of view, merely 
 indicates certain facts concerning regulation which ap- 
 parentl}- do not fit into an}- of our physical or chemical 
 concepts. It has no more to do with the cause of these 
 acts than chemical affinity has with the cause of chemical 
 reactions. It is a name for certain phenomena just as is 
 electricity. W hether or not there are any such phenomena 
 is the question at issue, and our only hope of agreement 
 in an answer lies in further investigations. But until this 
 question is settled it must be said that those w ho maintain 
 that there are no factors functional, no phenomena, in liv- 
 ing matter that are not also found in irorganic matter, that 
 there are no entelechies, are certainly no more scientific 
 than those who maintain the opposite, for the fundamental 
 phenomena, the distinguishing characteristics of li\ing 
 matter, have not as yet been accounted for mechanically. 
 To say that they can be is prejudging the future quite as 
 much as to say that they cannot be. Convictions are 
 valuable, but dogmatic statements as to what can or cannot 
 be done in the future have no place in science, as has 
 been repeatedly demonstrated. 
 
 PnonRTY LIBRARY 
 
BIBLIOGRAPHY 
 
 The following bibliography contains all of the essential references con- 
 sulted in the preparation of this book. It is hoped that all of the im- 
 portant works which have a bearing on reactions to light in both animals 
 and plants will be found in it. Those referring to vision are, however, 
 not included. 
 
 Adams, G. P., 1903. On the Negative and Positive Phototropism of the 
 
 Earthworm AUolobophora foetida (Sav.) as Determined by Light of 
 
 Different Intensities. Amer. Jour. Physiol., Vol. 9, pp. 26-34. 
 Albrecht, G., 1908. Uber die Perzeption der Lichtrichtung in den Laub- 
 
 blattern. Ber. d. deutsch. bot. Ges., Vol. 26, pp. 182-191. 
 Andrews, E. A., 1891. Compound Eyes of Annelids. Jour. ^lorph., 
 
 Vol. 5, pp. 271-299. 
 AxENFELD, D., 1899. Quelques observations sur la vue dcs arthropodes. 
 
 Arch. ital. biol., t. 31, pp. 370-376. 
 Bancroft, F. W., 1907. The Mechanism of the Galv'anotropic Orientation 
 
 in Volvox. Jour. Exp. Zool., Vol. 4, pp. 157-163. 
 Baranetzski, J., 1876. Influence de la lumiere sur les plasmodia des 
 
 Myxomycetes. Mem. Soc. Sc. nat. Cherbourg, Vol. 19, pp. 321-360. 
 Barrows, W. M., 1907. The Reactions of the Pomace Fly, Drosophila 
 
 ampelophila Loew, to Odorous Substances. Jour. Exp. Zool., Vol. 4, 
 
 PP- 515-537. 
 Bateson, W., 1887. Notes on the Senses and Habits of Some Crustacea. 
 
 Jour. Mar. Biol. Assoc. United Kingdom, Vol. i, p. 211. 
 , 1887a. On the Sense-organs and Perceptions of Fishes. Ibid., 
 
 Vol. I, p. 225. 
 Beer, Th., 1893. Studien iiber die Accommodation des Vogelauges. 
 
 Arch. f. d. ges. Physiol., Bd. 53, pp. 175-237. 
 , 1894. Die Accommodation des Fischauges. Ibid., Bd. 58, pp. 
 
 523-650. 
 , 1897. Die Accommodation des Kephalopodenauges. Ibid., Bd. 67, 
 
 PP- 541-586. 
 , 1898. Die Accommodation des Auges bei den Reptilien. Ibid., Bd. 
 
 69, PP- 507-568. 
 ■ , 1898a. Die Accommodation des Auges bei den Amphibien. Ibid., 
 
 Bd. 73, PP-.. 501-534- _ 
 
 1901. Uber primitive Sehorgane. Wiener klin. \\ ochenschr., 
 
 Jahrg., i90i,Nr. 11-13. 
 
 Beer, Th., Bethe, A., u. Ue.xkiill., J. v., 1899. Vorschlage z. einer objectivi- 
 
 render Nomenclatur in der Physiologic des Nervensystems. Biol. Cent., 
 
 Bd. 19, pp. 5 1 7-5-" I- 
 Bell, J. C, 1906. The Reactions of the Crayfish. Harvard Pysch. 
 Studies, Vol. 2, pp. 615-644. 
 
 379 
 
380 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 Bert, P., 1869. Sur la question de savior si tous les animaux voient les 
 memes rayons que nous. Arch, de physiol., t. 2, p. 547. 
 
 , 1870. Influence de la lumiere vertc sur la sensitive. Compt. Rend., 
 
 Vol. 70, pp. 338-340. 
 -, 1878. Intluence de la lumiere sur Ics ctres vivants. Revue Scient., 
 
 \'ol. 21, pp. 981-990. 
 Bkrthold, G., 1882. Beitriifije zur Morphologie und Physiologie der ]Meeres- 
 
 algen. Jahrb. f. wiss. Bot., Bd. 13, pp. 569-717. 
 Bethk, a.. 1898. Diirfen wir den Ameiscn und Hicnen psychiscbe Qualitii- 
 
 ten zuschrciben? Arch f. d. gcs. Ph\siol., Hd. 70, \)p. 15-100. 
 , 1902. Die Heimkehrliihij^keit der Ameiscn und Bienen. Biol. Cent., 
 
 Bd. 22, pp. 193-238. 
 BiNET, A., 1S94. The Psychic Life of Micro-organisms. Authorized 
 
 Translation. Chicago. 120 pp. 
 Bl.\..\uw, a. H., 1908. The Intensity of Light and the Length of Illumina- 
 tion in the Phototropic Curvature in Sccdh'ngs of Avena satixa (Oats). 
 
 Ron. Ak. Wet. Amsterdam. Proc. Mccth., Sci)t. 26, 1908. l\c\ic\v in 
 
 Bot. Cent.. 1909, \'ol. no, p. 655. 
 , 1909. Die Perzeption des Lichtes. Rec. d. Trav. bot. Neerl. W 
 
 Review in Bot. Cent.. 1910, \'ol. 113, pp. 353-356. 
 BoHX, 0., 1902. Contributions a la psychologic des annelidcs. Bull. Mus. 
 
 d'hist. nat.. t. 9. p. 62. 
 , 1903. Sur les mouvements oscillatoires des Convoluta roscoffensis. 
 
 C. r. Acad. Sci., Paris, t. 137, pp. 576-578. 
 , 1903a. Actions tropiques de la lumiere. C. r. Soc. Biol., Paris, 
 
 t- 55. PP- 1440-1442. 
 , 1903b. Sur le phototropism des artiozoaires superieurs. C. r. 
 
 Acad. Sci., Paris, t. 137, pp. 1 292-1 294. 
 , 1904. Periodicite vitale des animaux soumis aux oscillations du 
 
 niveau des hautes mcr. Ibid., t. 139. pp. 610-61 1. 
 , 1904a. Oscillations des animaux littoraux synchromes des mouve- 
 ments de la maree. Ibid., t. 139. pp. 646-648. 
 , 1904b. Mouvements de manege en rapport avec les mouvements 
 
 de la maree. C. r. Soc. Biol., Paris, t. 57, pp. 297-298. 
 , 1904c. Theorie nouvelle du phototropisme. C. r. Acad. Sci., 
 
 Paris, t. 139, pp. 890-891. 
 -, 1905. Attractions et oscillations des animaux marins sous I'in- 
 
 fluence de la lumiere. Mcmoires Inst. g6n. psych., Paris, Vol. i, 
 no pp. 
 
 — , 1906. Sur les mouvements de roulement influences par la lumiere. 
 C. r. Soc. Biol., Paris, t. 61, p. 468. 
 
 — , 1907. I>e rhythme nycthcmcral chez les actinies. Ibid., Paris, 
 t. 62, p. 473. 
 
 — , 1908. Introduction a la psychologic des Animaux a symctrie 
 rayonnec, II. Les J'^ssais et Erreurs chez les Ivtoiles de mcr ct les Oph- 
 iures. Paris, 86 pp. 
 -, 1909. Les Troj)ismes. Rapport au VImc Congres International de 
 
 Psychologic. Geneve. 15 pp. 
 BoNXiER, G., 1879. I-cs Nectaircs. Ann. des. sc. nat. Bot., 6 ser.. Vol. 8, 
 
 pp. 46-48. .. 
 Bosch, F., 1907. Uber die Perzeption bcim tropistischcn Reizprozess der 
 
 Pflanzen. Doctorate Dissertation. Bonn, Cicrmany. 
 Brooks, W. K., 1907. The Foundations of Zoology. New York. 339 pp. 
 BuLMAX. G. \V., 1890. On the Supposed Selective Action of Bees on 
 
 Flowers. The Zoologist, Vol. 14, 3 ser., p. 422. 
 
BIBLIOGRAPHY 38 1 
 
 , 1899. Bees and the Origin of Flowers. Nat. Sci., Vol. 14, pp. 128- 
 
 130. 
 
 BuNSEN, R., and Roscoe, H. E., 1859. I'hotochemischer Untersuchungen, 
 V. Die Sonne. PoggendorfT's Annalcn, \'ol. 108, pp. 193-273. 
 
 Buttel-Reepen, H. v., 1907. Are Bees Rellex Machines? Kxperimenlal 
 Contribution to the Natural History of the Iloney-bee. Translated 
 by Mary H. Geislcr. Published \)y A. I. Root Co., Medina, O. 48 pp. 
 Original in Biol. Cent., igoo, \'<)1. :?o. 
 
 Byk, a. v., 1908. Die Fortschritte der Photochemie im Jahre 1908. Zeit- 
 schrift f. Klektrochemie, Bd. 15, pp. 331-338. 
 
 Candolle, a. P. de, 1832. Physiologic vegetal. A German trans, by 
 Roper, pub. in 1835. 
 
 Carpenter, F. \V., 1905. Reactions of the Pomace Fly Drosophila am- 
 
 pelophila Loew to Light, Gravity, and Mechanical Stimulation. .\mer. 
 
 Nat., Vol. 39, pp. 157-17 1. 
 , 1908. Some Reactions of Drosophila, with Special Reference to 
 
 Convulsive Reflexes. Jour. Comp. Neur. and Psych., \'ol. 18, pp. 483- 
 
 491. 
 
 CiESiELSKi, T., 1875. Untersuchungen iiber die Abwartskrummung der 
 
 Wurzel. Cohn's Beitriige z. Biol., Vol. i, pp. 1-30. 
 Claparl:de, E., 1868. Annelides Chetopodes du Golfes de Naples. 
 CoHN, F., 1865. Uber die Gesetze der Bewegung mikroscopischer Thiere 
 
 und Pflanzen unter Einfluss des Lichtes. Jahresber. d. Schles. Ges. f. 
 
 Vaterl. Cult., Vol. 42, pp. 35-36. 
 
 Cole, L. J., 1901. Notes on the Habits of Pycnogonids. Biol. Bull., 
 
 Vol. 2, pp. 195-207. 
 , 1907. Influence of Direction vs. Intensity of Light in Determining 
 
 the Phototropic Responses of Organisms. Abstract in Jour. Comp. 
 
 Neur. and Psych., Vol. 17, p. 193. 
 
 1907a. .\n Experimental Study of the Image-forming Power of 
 
 Various T3-pes of Eyes. Proc. Amer. Acad. Arts and Sci., \'ol. 42, pp. 
 
 335-417- 
 Cole, L. W., 1907. Concerning the Intelligence of Raccoons. Jour. Comp. 
 
 Neur. and Psych., Vol. 17, pp. 211-261. 
 CoNGDON, E. D., 1908. Recent Studies upon the Locomotor Responses of 
 
 Animals to White Light. Jour. Comp. Neur. and Psych., Vol. 18, 
 
 PP- 309-328. 
 CowLES, R. P. Reaction to Light and Other Points in the Behavior of the 
 
 Starfish. To appear in Pubhcation of the Carnegie Institution of 
 
 Washington. 
 
 . Stimuli Produced by Light and by Contact with Solid Walls as 
 
 Factors in the Bcha\ior of Ophiuroids. To appear in Jour. Exp. Zool., 
 
 Brooks ^Memorial \'olume. 
 Czapek, F., 1895. Uber den Nachweis der geotropischen Sensibilitiit der 
 
 Wurzelspitze. Jahrb. f. wiss. Bot., Bd. 27, pp. 311-366. 
 , 1898. Weitcre Beitriige zur Kenntniss der geotropischen Rciz- 
 
 bewegungen. Ibid., Bd. ^2, pp. 175-308. 
 , 1900. Untersuchungen iiber Geotropismus. Ibid., Bd. 35, pp. 
 
 243-339- 
 Dalyell, Sir J. G., 1853. The Powers of the Creator Revealed. London. 
 Darwin, C, 1876. The Effect of Cross- and Self-fertilization in the \'ege- 
 
 table Kingdom. London. 482 j^p. 
 , 1881. The Formation of \'c^etal)le Mold through the .Action of 
 
 Worms, with Observations on their Habits. New York. 326 pp. 
 
382 LIGHT AND THE BEHAVIOR OF ORGANISMS 
 
 Dar\mn, Charles and Francis, 18S0. Power of Movement in Plants. 
 
 London. 59:? pp. 
 Darwin, Fr.\ncis, 1907. Lectures on the Physiology of ^Movement in 
 
 Plants. New Philologist, \'ol. 6, pp. 10, 35, 69, 120. 
 Davenport, C. H., 1S97. Experimental Morphology. Vol. i. New York. 
 
 280 pp. 
 
 , 1899. The Same. \'ol. 2. pp. 281-509. 
 
 Davenport, C. B., and Cannon. W. P., 1S97. On the Determination of the 
 
 Direction and Kate of Mowment of Organisms by Light. Jour. 
 
 Physiol., \'oI. 21, pp. 22-32. 
 Dan'ENPort, C. H. and Lewis, F. T., i8qq. Phototaxis of Daphnia. Science, 
 
 X. S., \'ol. 9, p. 368. 
 Driesch, IL, 1890. Heliotropismus bei Hydroidpol> [)cn. Zool. Jahrb. 
 
 .\bth. f. Syst., Vol. 5, pp. 147-156. 
 , 1907. The Science and Philosophy of the Organism. London. 
 
 Vol. I, 329 pp. 
 
 1908. The Same. Vol. 2, 381 pp. 
 
 Dubois. R.. 1890. Sur la perception dcs radiations lumineuses par la peau, 
 chez Ics protees aveugles des grottes de la Carniole. C. r. Acad. Sci., 
 Paris, t. no. pp. 358-361. 
 
 DuTROCHET, AL De I'inlle.xion des tiges vegetales vers la lumiere coloree. 
 .\nn. des. sc. nat.. 2 ser., t. 20, pp. 329-339. 
 
 DuTROCHET, ]\L, and Pouillet, 1844. Ann. des. sc. nat., 3 ser., t. 2, pp. 96- 
 
 "3- 
 EiGENMANN, C. H.. 1 899. The Blind Fishes. Biol. Lectures, ^larinc BioL 
 
 Lab.. Woods Hole, 1899. pp.. 11 3-1 26. 
 Engelmann, T. W., 1879. Uber Reizung contraktilcn Protoplasmus 
 
 durch plotzliche Beleuchtung. Arch. f. d. ges. Physiol., Bd. 19, 
 
 pp. 1-7. 
 
 , 1881. Zur Biologic dcr Schizomyceten. Ibid., Bd. 26, pp. 537-545. 
 
 , 1882. Uber SauerstolTausscheidung von Pflanzenzellen in Micro- 
 spectrum. Ibid., Bd. 27, pp. 485-489. 
 , 1882a. ijber Licht- und Farbenperception niederster Organismcn. 
 
 Ibid., Bd. 29, pp. 387-400. 
 
 , 1883. Farbe und Assimilation. Bot. Ztg., Vol. 41, pp. 1-13, 17-29. 
 
 , 1883a. Bacterium Photometricum. Arch. f. d. ges. Physiol., Bd. 
 
 30, pp. 95-124. 
 . 1884. Untersuchungen iiber die quantitativen Beziehungen 
 
 zwischen .Absorption des Lichtes und Assimilation in Pflanzenzellen. 
 
 Bot. Ztg., \o\. 42, pp. 81-106. 
 , 1885. Uber Bewegungen der Zapfen und Pigmentzellen der Netz- 
 
 haut unter dem ICinlluss des Lichtes und des Nervensystems. Arch. 
 
 f. d. ges. Physiol., Bd. 35. pp. 498-508. 
 
 1888. Die Purpurbacterien und ihre Beziehungen zum Lichte. 
 
 Bot. Ztg., Vol. 46, pp. 661-669; 677-689; 693-701; 709-720. 
 EsTERLY. C. O.. 1907. Reactions of Cyclops to Light and to Gravity. 
 
 Amcr. Jour. Physiol., Vol. iS. pp. 47-57. 
 EwALi). \V. F., 1910. Uber Oricntierung. Lokomotion und Lichtreak- 
 
 tionen ciniger Cladoceren und deren Bedeutung fiir die Theorie der 
 
 Tropismen. Krlangen. 34 pp. 
 Ew'ART, .\. J., 1903. On the Physics and Physiology of Protoplasmic 
 
 Streaming in Plants. Oxford Press. London. 131pp. 
 Famintzin, A., 1867. Die Wirkung des Lichtes auf .\lgen und einige 
 
 andere nahe verwandte Organismen. Jahrb. f. wiss. Bot., Bd. 6, pp. 
 
 1-44. 
 
BIBLIOGRAPHY 383 
 
 FrELDE, Adele M., 1902. Notes on Ants. Proc. Acad. Nat. Sci., Phila- 
 delphia, Vol. 54, pp. 599-625. .„.,.• , 
 
 FiGDOR, W., 1893. Versuche iiber die heliotropische Emi)fmdhchk.cit dcr 
 Pflanzen. Sb. d. Akad. Wiss., Wien, Bd. 102, pp. 45-59- 
 
 , 1908. P>x]:)erimcntellc Studien iiber die heliotropische Empfind- 
 
 lichkeit der Pflanzen. Wicsner Festschr., pp. 287-307. 
 
 Fitting, H., 1907. Die Leilung tropistischer Reizc in jjarallelotropen 
 Pflanzenteilen. Jahrb. f. wiss. Bot., Vol. 44, pp. 177-253. 
 
 , 1908. Lichtperzejition und phototropischc IOmi)findlichkeit, zu- 
 
 gleich ein Beitrag zur Lehre vom Etiolement. Ibid., Bd. 45, pp. 83-136. 
 
 Fleure, H. J., and Walton, C. L., 1907. Notes on the Habits of Some 
 Sea-anemones. Zool. Anz., Bd. 31, pp. 212-220, 
 
 FOREL, A., 1886-1888. Experiences et remarqucs critiques sur les sensa- 
 tions des insectes. 2 parties avec appendices. Rec. Zool. Suisse, 
 Vols. 2 and 4, pp. 1-50; 145-240; 515-523- 
 
 , 1 900-1 90 1. Sensations des insectes. Rivista di biologia generate. 
 
 Vol. 2, pp. 561, 641; Vol. 3, pp. 7, 241, 401. 
 -, 1904. Ants and Some Other Insects. Trans, by \\ . M. \\ heeler. 
 
 Chicago. 
 
 FoREL, A., and Dufour, H., 1902. Uber die Empfmdlichkcit der Ameisen 
 fiir ultraviolett und Rontgensche Strahlen. Zool. Jahrb. Abth. f. Syst., 
 Bd. 17, pp. 335-338. 
 
 Frandsen, P., 1 90 1. Studies on the Reactions of Limax maximus to Direc- 
 tive StimuH. Proc. Amer. Acad. Arts and Sci., Vol. 37, pp. 185-227. 
 
 Frank, A. B., 1870. Die natiirliche wagerechte Richtung von Pflanzen- 
 teilen, u. s. w. Leipzig. 
 
 Franze, R., 1893. Zur IMorphologie und Physiologic der Stigmata der 
 Mastigophoren. Zeitschr. f. wiss. Zool., Bd. 56, pp. 138-164. 
 
 Gardner, D. P., 1844. Sur Taction de la lumiere jaune dans la production 
 de la couleur vertc des plantes et sur celle de la lumiere indigo dans la 
 production de leurs mouvements. BibHotheque universelle de Geneve. 
 Feor. 1844. 
 
 Gaulhofer, K., 1908. Die Perzeption der Lichtrichtung im Laubblatte 
 mit Hilfe der Randtiipfel, Randspalten und der windschiefen Radial- 
 wande. Sb. d. Akad. Wiss., Wien, Bd. 117, pp. 153-190. 
 
 Gius, L., 1907. tJber den Einfluss submerser Kultur auf Heliotropismus 
 und fixe Lichtlage. Sb. d. Akad. Wiss., Wien, Bd. 116. pp. 1593-165 1. 
 
 Graber, v., 1883. Fundamentalversuche uber die Helligkeits- und Ear- 
 benempfindlichkeit augenloser und geblendeter Thiere. Sb. d. Akad. 
 Wiss., Wien, Bd. 87, pp. 201-236. 
 
 , 1884. Grundlinien zur Erforschung d. Helligkeits- und Farbcn- 
 
 sinnes der Thiere. Prag u. Leipzig. 322 pp. 
 
 1885. tJber die Helligkeits- und Earbenempfindlichkeit ciniger 
 
 Meerthiere. Sb. d. Akad. Wiss., Wien, Vol. 91, pp. 129-150. 
 Grant, R. E., 1829. On the Influence of Light on the Motions of Infusoria. 
 
 Edinb. Jour, of Sci., \'ol. 10, pp. 346-349. 
 Groom, T. T., and Loeb, J., 1890. Der Heliotropismus der Nauplicn von 
 
 Balanus perforatus und die periodischen Tiefenwanderungen pelagischer 
 
 Tiere. Biol. Cent., Vol. 10, pp. 160-177. 
 Guillemin, 1858. Production de la chlorophyll et direction des tiges sous 
 
 I'influence des rayons ultraviolette, caloriiiques et lumincux du spectra 
 
 solaire. Ann. des sc. nat., 4 ser., t. 7, pp. 154-172. 
 Haberlandt, G., 1904. Die Perzeption des Lichtreizes durch das Laub- 
 
 blatt. Bcr. der deutsch. bot. Ges., Bd. 22, pp. 105-119. 
 
384 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 , 1905. Die Lichtsinnesorgane der Laubblatter. Leipzig. 138 pp. 
 
 1906. Ein experimentaler Beweis fiir die Bedeutung der papillosen 
 
 Laubblattepidermis als Lichtsinnesorgan. Bcr. d. deutsch. bot. Ges. 
 
 Vol. 24, pp. 361-366. 
 — , 1907. Die Bedeutung der papillosen Laubblattepidermis fur die 
 
 Lichtperzeption. Biol. Centr., Bd. 27, pp. 289-301. 
 — , 1908. Ubcr dieX'crbrt'ilung der Lichtsinnesorgane der Laubblatter. 
 
 Sb. d. Akad.. \\'iss., Wien. math.- naturw. Klasse, Bd. 117. 
 — , i9oSa. UixT Reizbarkcit und Sinnesleben der Pflanzen. (Vortrag.) 
 
 Wien, .\. Holder. 27 pp. 
 
 1009. Zur Physiologie der Lichtsinnesorgane der Laubblatter. 
 
 Jahrb. f. wiss. Bot.. \'ol. 46, pp. 377-417. 
 II.VDLEY, P. B., 1906. The Relation of Optical Stimuli to Rheota.xis in the 
 
 American Lobster (Homarus Americanus). Amcr. Jour. Physiol., 
 
 Vol. 17. pp. 3^6-342. 
 , 1908. The Reaction of Blinded Lobsters to Light. Ibid., Vol. 21, 
 
 pp. 180-199. 
 
 1908a. The Behavior of the Larval and Adolescent Stages of the 
 
 American Lobster (Homarus Americanus). Jour. Comp. Neur. and 
 
 Psych., \'ol. 18, pp. 199-301. 
 Handl, A., 1887. tjber d. Farbensinns der Thicre u. die Vertheilung der 
 
 Energie im Spcktrum. Sb. d. Akad. Wiss., Wien, math.-naturw. 
 
 Klasse, Bd. 94. p. 935. 
 H.\nssex, O., 1908. Recherches experimentales sur la sensibilisation 
 
 optique du protoplasma. Bull. Ac. roy. Sc. et Lettr. Danemark, 
 
 Vol. 3, pp. 1 13-132, 4 pis. 
 H.VRGiTT, G. W., 1904. The Early Development of Eudendrium. Zool. 
 
 Jahrb., Bd. 20, Heft 2, pp. 257-276. 
 , 1906. Experiments on the Behavior of Tubicolous Annelids. 
 
 Jour. Exp. Zool., Vol. 3, pp. 295-320. 
 , 1907. Notes on the Behavior of Sea-anemones. Biol. Bull., 
 
 Vol. 12, pp. 274-284. 
 
 1909. Further Observations on the Behavior of Tubicolous Anne- 
 
 lids. Jour. Exp. Zool., Vol. 7, pp. 157-187. 
 Harper, /E. H., 1905. Reactions to Light and Mechanical Stimuli in the 
 
 Earthworm, Perichaeta bermudensis (Beddard). Biol. Bull., Vol. 10, 
 
 pp. 17-34. 
 , 1907. The Behavior of the Phantom Larvae of Gorethra plumi- 
 
 cornis P'abricius. Jour. Gomp. Neur. and Psych., \'ol. 18, pp. 435-456. 
 Harrington, N. R., and Leaming, E., 1900. The Reactions of Amoeba to 
 
 Light of Different Golors. Amer. Jour. Physiol., Vol. 3, pp. 9-16. 
 Haycraft, J. B., 1897. Luminosity and Photometry. Jour. Phys., Vol. 21, 
 
 pp. 126-146. 
 Herrick, F. H., 1896. The American Lobster: A Study of its Habits and 
 
 Development. Bull. U. S. Fish Gommission, for 1895, PP- ^~^5~- 
 Hertel, E., 1904. Ubcr Becinflussung dcs Organismus durch Licht, 
 
 speziell durch die chemisch wirksamen Strahlen. Zeitschr. f. allg. 
 
 Physiol., Vol. 4, pp. 1-43. 
 Hesse, R., 1896. Untersuchungen iibcr die Organe der Lichtempfind- 
 
 ungen bci niederen Thieren, L Die Organe der Lichtempfindungen bei 
 
 den Lumbriciden. Zeit. f. wiss. Zoo!., Bd. 61. pp. 393-419. 
 , 1897. Untersuchungen u. s. w., H. Die Augen der Platyhelminthen, 
 
 insonderheit der tricladen Turbellarien. Ibid., Bd. 62, pp. 527-582, 
 1897a. Untersuchungen u. s. w., HI. Die Sehorgane der Hirudineen. 
 
 Ibid., Bd. 62, pp. 671-707. 
 
BIBLIOGRAPHY 385 
 
 — , 1898. Die Lichtempfmdung des Amphioxus. Anat. Anz., Bd. 14, 
 
 PP- 556-557- 
 — , 1898a. Untersuchungen u. s. \v., IV. Die Sehorgane des Amphioxus. 
 
 Zeit. f. wiss. ZooL, Bd. 63, pp. 456-464. 
 — , 1899. Untersuchungen u. s. w., V. Die Augen der polychaeten 
 
 Annclidcn. Ibid., Bd. 65, pp. 446-516. 
 — , 1900. Untersuchungen u. s. w., V'l. Die Augen einiger Mollusken. 
 
 Ibid., Bd. 68, pp. 379-477- 
 — , 1901. njntersuchungcn u. s. w., VII. Von den Arthropodenaugen. 
 
 Ibid., Bd. 70, pp. 347-473- 
 
 1902. Untersuchungen U.S. \v., VIII. WeitereThatsachen. Allge- 
 
 meines. Ibid., Bd. 72, pp. 565-656. 
 
 He"\vitt, C. G., 1908. The Structure, Development and Bionomics of the 
 
 Housefly, Musca domestica (Linn.). Quar. Jour. Micr. Sci., Vol. 52, 
 
 PP- 495-545- 
 
 HoFMEiSTER, W., 1 863. Uber das Eindringung der Wurzeln in den Boden. 
 Jahrb. f. wiss. Bot., Bd. 3, pp. 77-114. 
 
 , 1867. Die Lchre v. d. Pflanzcnzellc. Leipzig. 664 pp. 
 
 Holmes, S. J., 1901. Phototaxis in the Amphipoda. Amer. Jour. Physiol., 
 Vol. 5, pp. 211-234. 
 
 , 1902. Observations on the Habits of Hyallella dcntata. Science, 
 
 N. S., Vol. 15, p. 529. 
 
 , 1903. Phototaxis in Volvox. Biol. Bull., Vol. 4, pp. 319-326. 
 
 , 1905. The Selection of Random IVIovements as a Factor in Photo- 
 taxis. Jour. Comp. Neur. and Psych., Vol. 15, pp. 98-112. 
 
 • , 1905a. The Reactions of Ranatra to Light. Ibid., \'ol. 15, 
 
 PP- 305-349- 
 
 1908. Phototaxis in Fiddler Crabs and its Relation to Theories 
 
 of Orientation. Ibid., Vol. 18, pp. 493-497. 
 
 Holt, E. B., and Lee, F. S., 1901. The Theory of Phototactic Response. 
 Amer. Jour. Physiol., Vol. 4, pp. 460-481. 
 
 Jennings, H. S., 1904. Contributions to the Study of the Behavior of 
 Lower Organisms. Carnegie Inst, of Washington, Pub. Xo. 16, 
 256 pp., 81 figures. 
 
 , 1905. ModifiabiHty in Behavior, I. Behavior of Sea-anemones. 
 
 Jour. Exp. Zool., Vol. 2, pp. 447-472. 
 
 — , 1905a. The Basis for Taxis and Certain Other Terms in the Be- 
 havior of Infusoria. Jour. Comp. Neur. and Psych., Vol. 15, pp. 138- 
 
 143. 
 , 1906. Behavior of the Lower Organisms. New York. 366 pp. 
 
 , 1906a. ModifiabiHty in Behavior, II. Factors Determining Direc- 
 tion and Character of ^Movement in the Earthworm. Jour. Exp. 
 Zool., Vol. 3, pp. 435-455- 
 
 , 1907. Behavior of the Starfish Asterias Forreri de Loriol, L^niv. of 
 
 Cal. Pub. in Zool., Vol. 4, pp. 53-185. 
 
 1909. Tropisms. Rapport au VIme Congres International de 
 
 Psychologie. Geneve. 20 pp. 
 Keeble, F., and G.\mble, F. \V., 1904. The Colour Physiology of the 
 
 Higher Crustacea. Phil. Trans. Roy. Soc, London, Vol. 196 B, 
 
 PP- 295-388. 
 KiNNAMAN, A. J,, 1902. Mental Life of two Macacus rhesus Monkeys in 
 
 Captivity. Arner. Jour. Psych., Vol. 13, pp. 9S-14S; 173-21S. 
 Kniep, H., 1907. Uber die Lichtperzeption der Laubbliitter. Biol. Centr., 
 
 Vol. 27, pp. 97-106; 129-142. 
 KoNTG, G., 1891. Beitr. z. Psych, u. Phys. d. Sinn., p. 311. 
 
2,S>6 LIGHT AXD THE BEHAVIOR OF ORGAXISMS 
 
 Kr.\bbe, G., 1889. Zur Kcnntniss der fken Lichtlage der Laubblatter. 
 
 Jahrb. f. wiss. Bot., Bd. 20, pp. 211-260. 
 Kraus, G., 1S76. V'ersuchc mil niaiizcn im farbigen Licht. Ber. Sit- 
 
 zungs d. Naturf. Ges. Halle. Jahre 1876, pp. 4-8. 
 Kr.\use,\V., 1897. Die Farbencmpfindungendcr Amphioxus. Z ool. Anz., 
 
 Bd. 20. pp. 513-515- 
 Langley, S. p., 1S84. Researches on Solar Heat and its .Absorption by 
 
 the Earth's .Atmosphere. Profess. Papers Sig. Serv. X\'I, Wash- 
 ington, Gov't Print. Ollice, 242 pp. 
 LiLUE, R. S.. 1003. Structure and Development of the Xephridia. Mit. 
 
 a. d. zool. Sta. z. Neapel, Bd. 17, pp. 341-405. 
 LoEB. J., 1888. Die Orientirung der Thiere gegen das Licht. (Thierischcr 
 
 HeliotrofMsmus). Sb. d. phys. med. Ges., Wiirzburg, pp. 1-5. 
 , i8S8a. Die Orientirung der Thiere gegen die Schwerkraft der 
 
 Erde. Ibid., Wurzburg, 1888, p. 5. 
 , 1889. Der Heliotropismus der Thiere und seine Ubereinstimmung 
 
 mit dcm Heliotropismus der Pflanzen. Wurzburg. n8 pp. 
 , 1893. Uber kiinstliche UmwandJung positiv heliotropischer Thiere 
 
 in negativ hcliotropische und umgekehrt. .Arch. f. d. ges. Physiol., 
 
 Bd. 54, pp. 81-107. 
 -, 1897. Zur Theorie der physiologischen Licht- und Schwerkraft- 
 
 wirkungen. Ibid., Bd. 66, pp. 439-466. 
 
 — , 1900. Comparative Physiology of the Brain and Comparative 
 Psychology. Xew York. 309 pp. 
 
 — . 1904. The Control of Heliotropic Reactions in Fresh-water Crus- 
 taceans by Chemicals. Univ. of Cal. Pub, in Physiol., Vol. 2, 
 
 PP- 1-3- 
 
 — , 1905. Studies in General Physiology. Chicago. Vol. i, 423 pp. 
 
 — , 1906. The DNTiamics of Living ^Matter. New York. 233 pp. 
 
 — , 1906a. Uber die Erregung von positiven Heliotropismus durch 
 Saure, insbesondere Kohlensaure, und von negativen Heliotropis- 
 mus durch ultra violette Strahlen. .Arch. f. d. ges. Physiol., Bd. 115, 
 pp. 564-581. 
 
 — , 1907. Concerning the Theorj^ of Tropisms. Jour. Exp. Zool., 
 Vol. 4, pp. 151-156. 
 -, 1909. Die Bedeutung der Tropismen fur die Psychologic. Leipzig, 
 
 51 PP- 
 LovELL, J. K., 1909. The Color Sense in the Honey-bee; is Conspicuous- 
 
 ness an Advantage to Flowers? Amer. Nat., Vol. 43, pp. 338-349. 
 Lubbock, Sir J., 1881. On the Sense of Color among Some of the Lower 
 
 .Animals, Part I. Jour. Linn. Soc. (Zocil.), Vol. t6, pp. 1 21-127. 
 , 1882. On the Sense of Color, etc., Part II. Ibid, Vol. 17, pp. 205- 
 
 214. 
 , 1888. On the Senses, Instincts, and Intelligence of Animals, with 
 
 Special Reference to Insects. New York. 292 pp. 
 , 1895. Ants, Bees and Wasps. New York. 448 pp. Preface to 
 
 original edition dated 1881. 
 Luther, R., and Forbes, G. S., 1909. A Quantitative Study of the Photo- 
 chemical Reaction between Quinine and Chromic Acid. Jour Amer. 
 
 Chem. Soc, Vol. 31, No. 7, pp. 770-783. 
 Lyon, E. P., 1906. Note on the Heliotropism of Palaemonetes larvae. 
 
 Biol. Bull., Vol. 12, pp. 23-25. 
 Massart, J., 1888. Recherches sur les organismcs inferieurs, I. La loi de 
 
 Weber verifiee pour I'heliotropisme du champignon. Bull. Belg. Acad., 
 
 ser. 3, t. 16, pp. 590-597- 
 
BIBLIOGRAPnV 387 
 
 , 1891. Rechercnes, etc. La sensibilite a la concentration chez les 
 
 etres unicellulaires marins. Ibid., ser. 3, t. 22, pp. 148-167. 
 Mast, S. O., 1903. Reactions to Temperature Changes in S{)irillum, Hydra, 
 
 and P>esh- water Planarians. Amer. Jour. Pliysiol., Vol. 10, pp. 165- 
 
 190. 
 , 1906. Light Reactions in Lower Organisms. I. Stentor coeruleus. 
 
 Jour. Exp. Zool., Vol. 3, pp. 359-399- 
 , 1907. Light Reactions in Lower Organisms. IL Volvox. Jour. 
 
 Comp. Neur. and Psych., Vol. 17, pj). 99-180. 
 -, 1909. The Reactions of Didinium nasutum (Stein), with Special 
 
 Reference to the Feeding Habits and the Function of Trichocysts. Biol. 
 
 Bull., Vol. 16, pp. 91-118. 
 McCooK, H. C, 1889-1893. American Spiders and their Spinning Work. 
 
 3 vols. 
 Merejkowsky, M. C, 1881. Les Crustaces inferieurs distinguent-ils les 
 
 couleurs? C. r. Acad. Sci., Paris, t. 93, pp. 1160-1161. 
 MiNKiEwicz, R., 1907. Analyse experimentale de I'instinct de deguise- 
 
 ment chez les Brachyures oxyrhynques. Arch. d. Zool. Exper. et 
 
 Gen., t. 7, pp. 37-67. 
 ,1907a. Chromotropism and Phototropism. Trans, in Jour. Comp. 
 
 Neur. and Psych., Vol. 17, p. 89. 
 -, 1908. Sur le chlorotropisme normal des Pagnres. Compt. rend., 
 
 Nov. 1908. 3 pp. 
 MiTSUKURi, K., 1901. Negative Phototaxis and Other Properties of Litto- 
 
 rina as Factors in Determining its Habitat. Annot. Zool. japonenses, 
 
 Vol. 4, pp. 1-19. 
 Moore, S. LeM., 1887. Studies in Vegetable Biology. IH. The Influence 
 
 of Light upon Protoplasmic Movement, Part I. Jour. Linn. Soc. 
 
 (Bot.), Vol. 24, pp. 200-251, PI. V. 
 Morgan, C. L., 1900. Animal Behavior. London. 344 pp. 
 Morse, M., 1907. The Beha\aor of Gonionemus. Amer. Nat., Vol. 41, 
 
 pp. 683-688. 
 MtJLLER, N. J. C, 1872. Uber die Kriimmung der Pflanzen gegen das 
 
 Sonnenlicht. Bot. Ztg., Vol. 30, p. 446. 
 MtJLLER, H., 1873. I^ie Befruchtung der Blumen durch Insekten und die 
 
 gegenseitigen Anpassungen beider. Leipzig. 
 
 , 1876. Uber Heliotropismus. Flora, Vol. 59, pp. 65-70; 88-95. 
 
 • , 1882. Versuche uber d. Farbenhebhaberei der Honigbiene. Kos- 
 
 mos, Bd. 12, pp. 273-299. 
 MuRBACH, L., 1909. Some Light Reactions of the Medusa Gonionemus. 
 
 Biol. BuU., Vol. 17, pp. 354-368. 
 Nageli, C, i860. Ortsbewegungen der Pflanzenzellen und ihrcn Theile 
 
 (Stromungen). Beitr. z. wiss. Bot., Heft 2, pp. 59-108. ■ 
 Nagel, W. a., 1894. Beobachtungen iiber den Lichtsinn augenloser 
 
 Muscheln. Biol. Cent., Bd. 14, pp. 385-390. 
 , 1894a. Experimentclle sinncsphysiologische Untcrsuchungen an 
 
 Coelenteraten. Arch. f. d. ges. Physiol., Bd. 57, pp. 495-552. 
 
 , 1896. Der Lichtsinn augenloser Thiere. Jena. Fischer. 120 pp. 
 
 , 1 90 1. Phototaxis, Photokinesis, und Unterschiedsempfmdlichkeit. 
 
 Kritische Betrachtungen. Bot. Ztg., Vol. 59, pp. 289-299. 
 
 1901a. Der Farbensinn der Thiere. Ein Vortrag. Wiesbaden. 
 
 Nathansohn, a., and Pringshefm, E., 1908. Uber Summation inter- 
 mittierender Lichtreize. Jahrb. f. wiss. Bot., Bd. 45, pp. 137-190. 
 
 Newcombe, F. C, 1902. The Rheotropism of Roots. Bot. Gaz., Vol. 2,2* 
 pp. 177-198; 263-283; 341-362. 
 
7,S>8 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 Nichols, E. L., 1905. On the Distribution of Energy in the Visible Spec- 
 trum. Phys. Rev., \'ol. 21, pp. 147-165. 
 
 N1C0L.AI, (1. F. (and Dk. Uaudolix), igo8. Das Lerncn dcr Ticrc (auf 
 (irund \on \'ersuchen mit Pawlowscher Spcichellistelj. Ccntr. f. 
 Physiol., Hd. 22, pp. 36.2-364. 
 
 NoRDHAiSKN, M., iQc;. Ubcr (lie BcdeutunR der papilloscn Epidcrmis als 
 Organ fur die Lichlpcrzcption dcs Laubblattcs. Ber. d. deutsch. bot. 
 Ges., \'ol. 25, pp. 398-410. 
 
 NuEL, J. P., 1904. La vision. Paris, 376 pp. 
 
 Oeij^ki.t-Xkuin, .\.. 1906. Rcobachtungen ul)cr das Leben der Protozocn. 
 Zeit. f. Psych, und, Physiol, dcr Sinntsorganc. Hd. 41, [)p. 349-382. 
 
 Oltmanns, F., 1892. Ubcr die photomclrischcn licwcgungen dcr Pflanzcn. 
 Flora, Vol. ..75, pp. 183-266. 
 
 , 1897. Ubcr positiven und negativen Heliotropismus. Ibid., 
 
 Vol. 83, pp. 1-32. 
 
 Orbeli, L. a., 1908. Conditioned Reflexes resulting from Optical Stimu- 
 lation of the Dog. Dissertation. St. Petersburg, 1908. (Russian.) 
 
 OsTWAi.i), W.. 1907. Zur Thcorie der. Richtungsbewcgungen nicdcrer 
 schwimmender Organismen, III. Ubcr die .Abhangigkeit gcwisser 
 hcliotropischer Reaktionen von der inncren Rcibung des Mediums, 
 sowie u. d. Wirkung "mechanischer Sensibilatoren." Arch. f. d. ges. 
 Physiol., Hd. 117, pp. 384-408. 
 
 Parker, G. H., 1901. Reactions of Copepods to Various Stimuli and the 
 Bearing of this on Daily Depth jMigrations. Bull. U. S. Fish Com. 
 for 1 901, pp. 103-123. 
 
 , 1903. The Phototropism of the Mourning-cloak Butterfly. IMark 
 
 Anniversary \'olume, pp. 453-469. 
 
 , 1903a. The Skin and the Eyes as Receptive Organs in the Re- 
 actions of I'rogs to Light. Amer. Jour. Physiol., Vol. 10, pp. 
 28-36. 
 
 , 1905. On the Stimulation of the Integumentary Nerves of Fishes 
 
 by Light. Ibid., Vol. 14, pp. 413-420. 
 
 1908. The Sensory Reactions of Amphioxus. Proc. Amer. Acad. 
 
 Arts and Sci., Vol. 43, PP- 415-455- 
 Parker, G. H., and Arkin, L., 1901. The Directive Influence of Light on 
 
 the Earthworm AUolobophora foetida (Sav.). Amer. Jour. Physiol., 
 
 Vol. 4, pp. 151-157- 
 Parker, G. H., and Burnett, F. L., 1901. The Reactions of Planarians 
 
 with and without Eyes to Light. Ibid., Vol. 4, pp. 373-385. 
 Patten, W., 1886. Eyes of Molluscs and Arthropods. IMit. a. d. Zool. 
 
 Sta. z. Neapel, Bd. 6, pp. 542-756. 
 Payer, J., 1842. Memoire sur la tendance des tiges vers la lumicre. 
 
 Compt. rend., t. 15, pp. 1194-1196. 
 Pearl, R. J., 1901. Studies on the Effects of Electricity on Organisms. 
 
 II. The Reactions of Hydra to the Constant Current. Amer. Jour. 
 
 Physiol., Vol. 5, pp. 301-320. 
 , 1903. The ]\Iovemcnts and Reactions of Fresh-water Planarians. 
 
 Quar. Jour. Micr. Sci., Vol. 46, pp. 509-714. 
 -, T904. On the Behavior and Reactions of Limulus in Early Stages 
 
 of its Development. Jour. Comp. Neur. and Pysch., Vol. 14, pp. 138- 
 
 164. 
 Pearl. R. J., and Cole, L. J., 1901. The Effect of Very Intense Light on 
 
 Organisms. Report Mich. Acad. Sci., 1901, pp. 77-78. 
 Pearse, \. S., 1906. Reactions of Tubularia crocea. Amer. Nat., Vol. 40, 
 
 pp. 401-407. 
 
BIBLIOGRAPHY 3^9 
 
 igo8. Observations on the Behavior of the Holothurian, Thyone 
 
 bri'areus (Leseur). Biol. Bull., Vol. 15, pp. 259-288. 
 -, 19 10. The Reactions of Amphiljians to Light. Proc. Amer. Acad. 
 
 Arts and Sci., Vol. 45, PP- 161-208 ,u at , 1 
 
 Peckham, G. \V., and K. G., 1887. Some Observations on the jMental 
 
 Powers of Spiders. Jour. Morph., \'ol. i, pp. 383-419. 
 ^ 1887a. Some Observations on the Special Senses of Wasps. 1 roc. 
 
 Nat. Hist. Soc. Wisconsin, 1887, p. 105. , ^ ^, 
 1894 The Sense of Sij^ht in Spiders, with Some Observations on 
 
 the Color Sense. Trans. Wis. Acad. Sci., Arts and Letters, Vol. 10, 
 
 ?! ^1005. Wasps, Social and Solitary. Boston. 306 pp. 
 
 Pfeffer W., 1884. Locomotorische Richtungsbewegungen durch chem- 
 
 ische Reize. Untcrs. a.d. bot. Inst. Tubingen, Vol. i, pp. 364-482. 
 . ^ 1894. Geotropic Sensitiveness of the Root-tip. Ann. of Bot., 
 
 ^-,^19^6^^ 'The Physiology of Plants. Trans, by A. J. Ewart. Oxford. 
 
 Vol. ill. 451 PP- , 1 
 
 Plateau F 1885. Rechcrches experimentelles sur la vision chez les arthro- 
 
 pode's. 'Les insectes distinguent-ils la forme des objects ? Bull. Acad. 
 
 roy. Bclgique, 3 ser. t. 10, pp. 231-350. _ . 
 . 1 886. Rechcrches sur la perception de la lumiere par les mynapodes 
 
 av'eugles. Jour, de I'anat. et de la physiol., t. 22, PP- 431-457- 
 ^ 1897. Comment les fleurs attirent les insectes. Troisicmc partie. 
 
 Bull. Acad. roy. Eelgique, 3 ser., t. t>2>. PP- i7-4i- ^^, . ^ , 
 ^ 1899. La choix des couleurs par les insectes. Memoires Soc. zoo). 
 
 France, t. 12, pp. 336-370- . . ^ c < 
 
 -, 1899a. La vision chez I'Anthidium manicatum. Ann. Soc. ent. 
 
 Belgique, t. 43, pp. 452-456- , , . 
 
 — . 1902. Observations sur les erreurs commises par les hymenoptcres 
 
 \isitant les fleurs. Ibid., t. 46, PP- 113-129 
 PoGGiOLi, S., 1817- Opuscoli scientif^ci. Bologna. -n , r:. 
 
 Pollock, J. B., 1900. The ]\Iechanism of Root Curvature. Bot. Crtiz., 
 
 PortIr" j'^P.f?904. ^A Preliminary Study of the Psychology of the English 
 
 Sparrow. Amer. Jour. Psych., Vol. 15, P- 313-346- 
 . 1^ igo6. Further Study of the English Sparrow and Other Birds. 
 
 Ibid., Vol. 17, pp. 248-357. , , .^ , - A A- ^- ^. 
 
 PoucHET G 1872. Sur de Tin fluencede la lumiere sur les larves des dipt eres 
 
 priv6es d'organes exterieurs de la vision. Rev. et Mag. de Zool 
 
 2 ser., t. 23, pp. 110-117; 129-138; 183-186; 225-231; 261-264; 312-316. 
 PoucHET and JouBERT, 1875. La vision chez les Cirrhipedes. C. r. et 
 
 Memoires Soc. Biol., 6 ser., t. 2, pp. 245-247. 
 PouLTON, E. B., 1887. Notes in 1886 upon Lepidopterous Larvae, etc.. 
 
 Trans. Ent. Soc.Lond. for 1887, pp. 281-321. 
 Preyer, W., 1886. Uber die Bewegungen der Seesteme. Mit. a. d. zool. 
 
 Sta'. z. Neapel, Bd. 7,..PP. 27-127, 191-233- 
 Pringsheim N., 1879. Uber Lichtwirkung und Chlorophyllfunction in 
 
 der Pflanze. Jahrb. f. wiss. Bot., Bd. 12, pp. 288-437- .„. , ^ , 
 Radl, E., 1901. XJl^er d. Phototropismus einiger Arthropoden. Biol. Lent., 
 
 Bd. 21, pp. 75-86. . , . .1 1 
 
 . , 1901a. Untersuchungen iiber die Lichtreactionen der .Vrthropoden. 
 
 Arch. f. d. ges Physiol., Bd. 87, pp. 418-466. _ 
 
 -, 1903. Untersuchungen uber den Phototropismus der J lere. 
 
 Leipzig. 188 pp. 
 
3 go LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 -, 1906. Einige Bemerkungen und Beobachtungen iiber den Photo- 
 
 tropismus dcr Tiere. Biol. Cent., Bd. 26, pp. 677-690. 
 Rauitz, B., 1888. Dcr Mantelrand der Acephalen. Jena. Zeitsch., Bd. 22, 
 
 pp. 415-556. 
 Reinke, J., 18S3. Untersuchungen iiber die Kinwirkung des Lichtes aul 
 
 die SauerstolTausschcidung der Pllanzen, 1. Mitt. Bot. Ztg., \'oI. 16 
 
 pp. 697-707; 713-7-M; 73-'-738. 
 
 , 1884. The Same, II. Ibid., Vol. 42, pp. i-io; 17-29; 33-46; 46-59. 
 
 Ro-MANES, G. J., 1883. .\nimal Intelligence. Xew York. 520 j)p. 
 
 , 1S85. Jellyfish,. Starfish and Sea-urchins. New York. 323 pp. 
 
 RoTHKRT, \V., 1894. Uber Ilcliotropismus. Cohn's Beitriige z. Biol., 
 
 \ ol. 7, pp. 1-212. 
 
 , 1901. Beobachtungen und Betrachtungen uber taktische Reizer- 
 
 .scheinungcn. Mora, Bd. 88. pp. 371-421. 
 RvDLK, J. A., 1883. Primitive \isual Organs. Science, X. S., Vol. 2, p. 739. 
 Sachs, J. v., 1864. Wirkungen farbigen Lichtes auf Pllanzen. Bot. Ztg., 
 
 Bd. 22, pp. 353-358; 361-367; 369-372. (Also in his Gesammelte 
 
 .\bh. iiber Pflanzenphysiologie, pp. 261-292.) 
 , 1876. Uber Kmulsionsfiguren und Gruppirung der Schwiirmsporen 
 
 im Wasser. Flora, \'ol. 59, pp. 241-248; 257-264; 273-281. 
 -, 1887. The Physiology of Plants. Transl. by H. M. Ward. 0.\- 
 
 ford. 836 pp. Original edition, 1882. 
 , 1S90. History of Botany. Transl. revised by I. B. Balfour. 
 
 O.xford. 568 pp. Original Edition, 1875. 
 ScHAFER, E. A., 1898. Te.xtbook of Physiology. PMited by E. A. Schiifer. 
 
 New York. \'ol. .2, 1365 pp. 
 Seefried, F., 1907. Uber die Lichtsinnesorgane der Laubbliitter einheim- 
 
 ischer Schattenpflanzen. Sb. d. Akad. Wiss., ^^■ien, Bd. 116, pp. 131 1- 
 
 1357. 4 Taf. 
 Senn. G., 1908. Die Gestalts- und Lagenveranderung der Pflanzen- 
 
 Chromatophoren. Leipzig. 
 Sharp, B., 1884. On the \'isual Organs in Lamellibranchiata. Mit. a. d. 
 
 zool. Sta. z. Neapel, Bd. 5, pp. 447-469. 
 Smith, .\melia C, 1902. The Influence of Temperature, Odors. Light, and 
 
 Contact on the Movements of the Earthworm. Amer. Jour. Physiol., 
 
 \'ol. 6, pp. 459-486. 
 Spauldixg, E. G., 1904. An Establishment of Association in Hermit 
 
 Crabs, Eupagurus longicarpus. Jour. Comp. Neur. and Psych., 
 
 Vol. 14, pp. 49761. 
 Stahl, E., 1878. Uber d. Einfluss des Lichtes auf die Bewegungserschein- 
 
 ungen der Schwiirmsporen. Bot. Ztg., Vol. 36, p. 715. 
 , 1879. Uber den Einfluss des Lichtes auf die Bewegung der Des- 
 
 midicn nebst cinigen Bemerkungen iiber den richtenden Einfluss des 
 
 Lichtes auf Schwiirmsporen. Verh. phys.-med. Ges. Wiirzburg, Vol. 14. 
 
 pp. 24-34. .. 
 , 1880. Uber den Einfluss von Richtung und Stiirke der Beleuchtung 
 
 auf einige Bewegungserscheinungen im Pflanzenreiche. Bot. Ztg., 
 
 Vol. 38, pp. 298-413. 
 , 1884. Zur Biologic der Myxomyceten. Ibid, Vol. 40, pp. 146- 
 
 155; 162-175; 187-191. 
 Stobbe, H., 1908. Phototropieerscheinungen bei Fulgiden und anderen 
 
 StolTen. Liebig's Annaien der Chemie, Bd. 359, pp. 1-48. 
 Stockard. C. R., 1908. II. Habits, Reactions, and Mating Instincts of 
 
 the "Walking Stick," Aplopus Mayeri. Publication 103, Carnegie 
 
 Institution of Washington, pp. 43-59. 
 
BIBLIOGRAPHY 391 
 
 Strasburger, E., 1878. Wirkunj:^ des Lichlcs und dcr Wiirme auf Schwiirm- 
 
 sporen. Jena. Zeitschr., N. F., Hd. 12, pp. 551-625. 
 ToRELLE, Ellen, 1903. The Response of the I'rog lo Light. Amer. 
 
 Jour. Physiol., Vol. 9, pp. 466-488. 
 ToRREY, H. B., 1907. The Method of Trial and the Tropism Hypothesis. 
 
 Science, N. S., Vol. 26, pp. ^i^'i^i- 
 TowLE, Elizabeth W., 1900. A Study in the Heliotropism of Cypridopsis. 
 
 Amer. Jour. Physiol., Vol. 3, pp. 345-365- 
 Trembley, a., 1744. Mcmoires pour servir a Thistoire d'un genre de 
 
 polypes d'eau douce a bras en forme de cornes. Leyden. 324 pp. 
 Uexkull, J. v., 1897. Uber Reflexe bei den Seeigeln. Zeitschr. f. Diol., 
 
 Bd. 34, pp. 298-318. 
 , 1897a. Der Schatten als Reiz fiir Centrostephanus longispinus. 
 
 Ibid., Bd. 34, pip. 319-339- 
 , 1900. Die Wirkung von Licht und Schatten auf die Seeigeln. 
 
 Ibid., Bd. 40, pp. 447-476. 
 Verworn, M., 1089. Psycho-physiologische Protisten studien. Jena: 
 
 Fischer. 218 pp. 6 pis. 
 , 1899. General Physiology, Trans, by F. S. Lee. New York. 
 
 615 pp. Original Edition, 1894. 
 ViERORDT, K., 187 1. Die .\nwendung des Spektralapparats zur Messung 
 
 d. Starke des farbigen Lichts. Tubingen. 
 , 1873, Die Anwendung des Spektralapparats zur Photometric 
 
 der Absorptionspectren und zur quantitativen chemischen Analyse. 
 
 Tubingen. Laupp. 169 pp. 6 Taf. 
 Villa, Guido, 1903. Contemporary Psychology. Trans, by H. IMana- 
 
 corda. London. 396^ pp. 
 VdcHTiNG, H., 1888. Uber Lichtstellung der Laubbliitter. Bot. Ztg., 
 
 Vol. 46, pp. 501-559- 
 Wager, H., igoo. On the Eye-spot and Flagellum of Euglena viridis. 
 
 Jour. Linn. Soc. (Zool.), London, V^ol. 2j, pp. 463-481. 
 Walter, H. E., 1907. The Reactions of Planarians to Light. Jour. Exp. 
 
 Zoal., Vol. 5, pp. 35-162. 
 Washburnt, Margaret F., 1908. The Animal Mind. New York. 333 pp. 
 Washburn, M. F., and Bentley, I. M., 1906. The Establishment of an 
 
 Association invohing Color Discrimination in the Creek Chub, Semot- 
 
 ilusatromaculatus. Jour. Comp. Neur. and Psych., Vol. 16, pp. 1 13-1 25. 
 Watson, J. B., 1909. Some Experiments Bearing upon Color-vision in 
 
 Monkeys. Jour. Comp. Neur. and Psych., Vol. 19, pp. 1-28. 
 Wheeler, W. M., 1910. Ants. New York. 663 pp. 
 Whitman, C. O., 1898. Animal Behavior. Biol. Lectures, Marine Biol. 
 
 Lab., Wood's Hole, i8g8, pp. 285-338. 
 WiESNER, J., 1879. Die heliotropische Erscheinungen im Pflanzenreiche. 
 
 Eine physiologische Monographic. Theil I. ]:)enkschr. Wien Akad., 
 
 Bd. 39, pp. 143-209. 
 , 1881. Die heliotropischen Erscheinungen u. s. w., Theil II. Ibid., 
 
 Bd. 43, pp. 1-92. 
 —, 1893. Photometrische LTntcrsuchungen auf pflanzcnphysiolog- 
 
 ischen Gebiete. Erste Abhandlung. Orientircndc Vcrsuche iibcr den 
 
 Einfluss der sogenannten chemischen Lichtintcnsiliit auf den Geslal- 
 
 tungsprocess der Pflanzenorgane. Sb. d. Akad. Wiss., Wien., Bd. 102, 
 
 pp. 291-350. 
 Willem, v., 1892. De la vision chez les mollusques gasteropodes pul- 
 
 mones. Arch, de biol., t. 12, pp. 57-147. 
 
392 LIGHT AXD THE BEHAVIOR OF ORGANISMS 
 
 Wilson, E. B., 1S91. The Heliotropism of Hydra. Amer. XaL., \'ol. 25, 
 
 pp. 413-433- 
 W1N0GR.A.DSKY, S., 1887. Uber Schwefelbacterien. Bot. Ztg., Bd. 45. 
 
 pp. 489-610. 
 Yerkhs, Ada \\'., 1906. Modifiabilily of Behavior in Hydroides Dianthus. 
 
 V. Jour. Comp. Xeur. and Psych.. \'oI. 16, pp. 441-449. 
 Vf.RKKS, R. M., 1899. Reactions of Entomostraca to Stimulation by Light. 
 
 Amer. Jour. Physiol.. \'ol. 3, pp. 157-182. 
 , 1900. Reactions of Entomostraca, etc.. II. Reactions of Daphnia 
 
 and Cy|)ris. Ibid.. \'oI. 4. pp. 405-422. 
 , 1902. .\ Contribution to the Physiology of the Nervous System 
 
 in the Medusa (ionionemus murbachii, I. The Sensory Reactions of 
 
 Cionionemus. Ibid., \'ol. 6, pp. 434-449. 
 , 1903. Reactions of Daphnia puie.x to Light and Heat. Mark 
 
 Anniversary \'olume. pp. 361-377- 
 
 1004. The Reaction Time of Gonionemus murbachii to Electric 
 
 and Photic Stimuli. Biol. Bull., Vol. 6, pp. 84-95. 
 Vi-:rkks. R. M.. and .\ver, J. B., Jr., 1903. A Study of the Reactions and 
 
 Reaction Time of the Medusa Gonionema murbachii to Photic Stimuli. 
 
 Amer. Jour. Physiol.. Vol. 9. pp. 279-307. 
 YuxG, E., 1878. Contributions a I'histoire de I'influence des milieux 
 
 l>hysiques sur les etres vivants. Arch, de Zool., Bd. 7, pp. 251-282. 
 , 1892. La fonction dermatoptique chez le ver de terre. C. r. des 
 
 Trav. Sec. Helv. Sci. nat., 1892, pp. 127-128. 
 , 1893. La psychologic de I'escargot. Ibid., 1S93, pp. 128-131. 
 
INDEX 
 
 Acacia, 286. 
 
 Acclimatization, Euglena, 103; Vol- 
 vox, 141; Hydra, 152; Stentor, 
 119; Alusca larvae, 189, 190, 197; 
 to change of intensity, 248, 249; 
 288-297. 
 
 Acids, effect of, on reversal of reac- 
 tions, 279-283, 300. 
 
 Acris, 260. 
 
 Actinia, reaction to sudden increase 
 of intensity, 250; reaction to con- 
 tinued illumination, 252; 257. 
 
 Actinia equina, 286. 
 
 Adams, orientation in earthworms, 
 198; 201; 266. 
 
 Adaptation, Verworn on, 36; in re- 
 actions in plants, 72; in Arenicola 
 larvae, 167; in reactions of but- 
 terflies, 227; in Euglena, Chlamy- 
 domonas, Volvox, Stentor, 
 Amoeba, etc., 236-239; natural 
 selection, 238, 239; 285; 292; 
 297; 298; in ants, 351; 368; 
 chemical regulation of, 370. 
 
 Aggregation, method of in: Euglena 
 (Engelmann), 16, Paramecium 
 (Jennings), 45, Volvox, 144, coe- 
 lenterates, 149, Planaria, 206, 
 frogs, 219, general, 239-245; cause 
 of, 242, 243. 
 
 Aiptasia annulata, reactions of 
 (Jennings), 252. 
 
 Algae, 265. 
 
 Alkalis, effect of, on reversal of re- 
 actions, 279-283, 300. 
 
 Allolobo])hora foetida, 199; 266. See 
 Earthworm. 
 
 Alona, 278. 
 
 Amaranth us, 288; 313. 
 
 Amoeba, 48; reactions to light, 74- 
 80; process of orientation, 76-79; 
 effect of change of light intensity 
 on movement of, 76-79; adapta- 
 tion, 237; 257; 263; 270; 322; 
 reactions of, in spectrum, 327-332, 
 361; effect of change of intensity 
 and color on movement of, 328, 
 
 330, 2>i^\ 2>^Z\ 365. 
 
 Ampelopsis, 265. 
 
 Amphibia, function of skin in re- 
 sponse, 262; 343. See Bufo. 
 
 Amphioxus, reaction to sudden in- 
 crease of intensity, 250; 257; 259. 
 
 Amphipods, 216. 
 
 Amphitrite bombyx, 247. 
 
 Andrews, 247. 
 
 Animal behavior, effect of theory of 
 evolution on, 9, 10; relation to 
 psychic phenomena, see Psychic 
 phenomena; summary of Loeb's 
 ideas on, 34, 35; analysis of (Jen- 
 nings), 49, 50; Drieschon,374-378. 
 
 Annelids, tubicolous, reactions to 
 shadows, 247. Sec Ilydroidcs. 
 
 Anomura, 214. 
 
 Ants, modification in reactions of, 
 296, 297; reactions to colors, 
 348-352; effect of ultra-violet 
 rays on, 349; sensation in, 350; 
 change in reactions of, 351. 
 
 Area, 247. 
 
 Arenicola larvae, orientation in light 
 from two sources, 87; description 
 of, 166, 167; locomotion, 1O7; 
 
 393 
 
394 
 
 INDEX 
 
 accuracy of orientation, 167; me- 
 chanics of orientation, 1O8-171, 
 174, 232; orientation compared 
 with that in Kuglena and X'olvox, 
 169, 171; distribution of sensitive 
 tissue. 172; aggregation of, 243; 
 258; 260; reversal in reactions, 
 271, 280-283, 285; 367. 
 Aristotle, regulation of behavior, 
 
 51 7- 
 
 Arkin, orientation in earthworms, 
 igS; 201; 202. 
 
 Arthropods. 2^^; 257; 260. 
 
 Asterias forreri. 211; 212. 
 
 Atherina, 359. 
 
 A vena, reactions in spectrum, 318. 
 
 Avicula, 247; 323; 326. 
 
 Avoiding reaction. 17; defined, 45; 
 compared with "Schreckbe- 
 wegung, " 82, no; in Kuglena, 
 82-86, 92-106; in Stentor, 113- 
 121; in swarm-spores, 125, 126; 
 137; in Volvox, 142; in Pandorina 
 and Eudorina, 147; in Euden- 
 drium, 161; 233; efTect of, on 
 aggregation, 239-243; 246; 257. 
 
 Bacteria, 36; 37; 350; 362. 
 Bacterium photometricum, 16; 17; 
 
 45; aggregation of, 240, 241; 263; 
 
 324; reactions of, in spectrum, 
 
 325- 
 Balanus, nauplii of, 266; change in 
 
 reactions, 285; 337. 
 Bancroft, reaction to electricity in 
 
 Volvox, 145, 146; 231. 
 Baranetzsky, movements of myxo- 
 
 mycetes, 74. 
 Barnacles, reactions to shadows, 
 
 247. 
 Barrows, definition of tropism, 55. 
 Bateson, 249; decoration in crabs, 
 
 357- 
 Bdelloura Candida, 207. 
 
 Bees, adaptation, 237; modification 
 in reactions of , 296, 297; reactions 
 to colors, 352-355; change in 
 reactions, 352-354; 3<^4- 
 
 Bell, 354. 
 
 Hentley, 358. 
 
 Bert, 10; 14; 24; 27; 34; 235; reac- 
 tions of Daphnia in spectrum, 
 
 33<^, 337; 342; 343- 
 
 Berthold, 265. 
 
 Bethe, 354. 
 
 Bierstadt, 327. 
 
 Bipalium kewense, 182; reactions 
 to light, 207; 270; sensitiveness of, 
 288. 
 
 Birds, flight of, into lighthouse, 228; 
 adaptation, 237; color vision in, 
 343, 360, 364. 
 
 Bispira voluticornis, 247. 
 
 Blauuw, reactions of plants in spec- 
 trum, 318, 319. 
 
 Blowfly larvae. See Musca larvae. 
 
 Bohn, 17; definition of tropism, 56; 
 211; 214; 221; 243; 244; 264; 
 periodic movements, 286. See 
 Criticism. 
 
 Bonnier. 354. 
 
 Borelli, founder of iatromechanical 
 school, 6; 51. 
 
 Botrydium. 274; 321. 
 
 Bougainvillea superciliaris, medusae 
 of, orientation in light from two 
 sources, 87, 165; orientation, 164, 
 165; description of, 165; 285. 
 
 Brachyura, 214; orientation in zoeae 
 of, 226. 
 
 Branchiomma koUikeri, 247. 
 
 Brassica, 314. 
 
 Brefeld, 318; 319. 
 
 Brightness, distribution in spectrum : 
 normal, 305-308, 361, in color- 
 blind individuals, 307; cause of, 
 307; distribution of, in spectrum 
 compared with that of energy, 361. 
 
INDEX 
 
 395 
 
 Bronn, 250. 
 
 Brooks, response to a sign, 250. 
 
 Bryopsis, 274. 
 
 Bufo americanus, orientation in 
 light from two sources, 89, 219- 
 221 ; 214; orientation with one eye 
 destroyed, 221, 222. 
 
 Bulman, 354. 
 
 Buttel-Reepen, 354. 
 
 Butterfly, Mourning-clock. See Va- 
 nessa anliopa. 
 
 Calliphora vomitoria, 217. 
 
 Caprella, 214; structure and loco- 
 motion, 224; 271. 
 
 Carbon dioxid, effect of, on rever- 
 sal of reactions, 279-283, 300. 
 
 Cardium, 247. 
 
 Caridea, orientation in zoeae of, 226. 
 
 Carpenter, on circus movements, 
 216; reactions of Drosophila, 271, 
 280. 
 
 Cartesian doctrine, 9. 
 
 Cassiopea, 359. 
 
 Cerianthus, 250; 252; 254. 
 
 Change of intensity, 17; orientation 
 by, in Euglena, 99. See Light. 
 
 Chaetomorpha aerea, 274. 
 
 Chemicals, changes in, related to 
 reactions, 270, 278, 308-312, 320, 
 3(^3^ 364, 370; effect of, on re- 
 versal in reactions, 279-283, 300, 
 367; extent of effect of, on reversal 
 in reactions, 280; reactions of, 
 reversible in light (Stobbe), 308- 
 312; effect of different rays on re- 
 actions of, and cause of, 312, 360, 
 363; effect of mixed rays of light 
 on reactions of, 310, 363; same 
 compared with effect of mixed 
 rays on organisms, 335, 336; effect 
 of, on reactions of organisms in 
 spectrum, 323-325- 332, 343. 
 Chilomonas curvata, 274. 
 
 Chlamydomonas, orientation in 
 light from two sources, 87; func- 
 tion of eye-spot, 109, 133; struc- 
 ture of, 131; mechanics of orien- 
 tation, 132, 133; 134; 136; 14O; 
 229; 230; adaptation, 236; aggre- 
 gation of, 241; 257; 259; reversal 
 in reactions and effect of tempera- 
 ture on, 265, 267, 277, 280, 300; 
 reversal in reactions compared 
 with same in Arenicola, 283; 367; 
 
 370- 
 Chlorogonium, structure of, 134; 
 
 function of eye-spot in, 134; ag- 
 gregation of, 241; 257; 259. 
 
 ChromuHna, 273. 
 
 Chytridium vorax, 274. 
 
 Ciesielski, 20. 
 
 Ciliates, 229; 230; 232; 233; 240; 
 aggregation of, 242; reactions in 
 spectrum, 323, 324. 
 
 Ciona, 250. 
 
 Circus movements, in various species, 
 215-218. 
 
 Claparede, 247. 
 
 Classification, of reactions to light, 
 253-262. 
 
 Claviceps, 318. 
 
 Clepsine, reactions to shadows, 249; 
 257; 260; 263; 298. 
 
 Cohn, ray-direction and movement 
 of organisms, 15; effect of different 
 rays (unicellular forms), 321, 322. 
 
 Cole, L. J., 182; reactions of Bi- 
 palium to light, 207; 214; 223; 
 effect of size of light area on re- 
 actions, 227; 2 28; 244. 
 Cole, L. W., 360. 
 
 Color, wave-length of, 304; energy 
 in, 304, 305; brightness of, 305- 
 308; effect of, on reactions: 
 chemical, 308-312, Daphnia, 310, 
 336-341, Hydra, 310, 333-33^, 
 plants, 310, 313-320, unicellular 
 
396 
 
 INDEX 
 
 forms, 321-332, Simoccphalus, 
 341-343, higher animals, 343-346, 
 ants, 348-352, bees, 352-355, 
 higher Crustacea, 355-358, fishes, 
 358-360, birds and mammals, 360; 
 effect of impurity of, on reactions, 
 310, 313, 322, 335, 340, 362; 
 selection of, in crabs, 356-358- 
 See Spectrum. 
 
 Concentration of medium (mechani- 
 cal stimuli). See Reactions. 
 
 Copepods, 273; 274. 
 
 Corethra larvae, orientation in (Har- 
 per), 225. 
 
 Cowles, 211; direction of righting 
 reactions in starfishes, 213; 328. 
 
 Crab, fiddler: reactions to light, 217, 
 218; Hermit, see Pagiirus; 258; 
 effect of color on reaction (decora- 
 tion), 355-358. 
 
 Criticism, of Darwin by Sachs, 21; 
 of Loeb's theories of orientation, 
 26, 27, 31, 70, 80, 83, 87-89, 104, 
 
 . no, III, 119, 122, 137, 144, 164, 
 168, 171, 173, 177-180, 182, 183, 
 188, 192-195, 198, 205, 206, 209, 
 220, 221, 223, 225, 229, 230, 233- 
 23s, 242, 258, 351, 363, 364; of 
 Loeb and Sachs by Vervvorn, 38; 
 of Sachs' ray-direction theory, 70, 
 80, 87, III, 137, 144, 158, 182, 
 192, 198, 233; of Pollock's theory 
 of curvature in roots, 71; of Jen- 
 nings by Torrey, 84, 85; of Torrey 
 on orientation in Euglena, 85, 86, 
 loi, 104, III, 205; of Radl's the- 
 ory of orientation, 43, 234; of 
 Bancroft on orientation, 145; of 
 Verworn's theory of orientation, 
 104, 122, 168,, 171, 173, 205, 229, 
 234; of Parker and Arkin on ori- 
 entation in earthworms, 202, 203; 
 of Holt and Lee on orientation, 
 205; of Bohn on orientation, 220, 
 
 221; of Davenport on orientation, 
 234; of Loeb on cause of aggre- 
 gation in Planaria, 245; of classi- 
 fication, 255, 256; of Loeb on 
 regulation and adaptation, 266, 
 267, 272, 369, 371, 372; of Loeb 
 on cause of change in reactions, 
 287, 301; by Loeb on Lubbock's 
 experiments on Daphnia, 335, 336; 
 of Loeb on reactions in spectrum, 
 336, 346, 347, 363, 364; of Sachs' 
 hypothesis on effect of different 
 rays, 363; of Davenport on reac- 
 tion in spectrum, 363; of Jennings 
 on regulation, 377; of Driesch on 
 regulations, 378. 
 Crustacea, 42; 238; reactions to 
 shadows, 249; decapod, 264; 272; 
 354; modifiabihty in reactions, 
 
 355-358; 364. 
 Cryptomonas, 132. 
 Cuma rathkii, adaptation, 238; 272; 
 
 346. 
 
 Cyclops, 277; 279; 280-283; 300. 
 
 Cypridopsis, 284. 
 
 Cypris, 214; 274; 277; 278; 280-283; 
 300. 
 
 Czapek, curvature of roots con- 
 trolled by root-tip, 21; 59; 72. 
 
 Dalyell, 247. 
 
 Daphnia, 214; 255; orientation of, 
 264; 265; 274; 277; 279; 280-283; 
 300; effect of different wave- 
 lengths on reactions of, 310; reac- 
 tions in spectrum, 335-341, 362; 
 
 345; 350; 364- 
 Darkness, effect of, on movement: 
 in Hydra, 152; in Hydra, etc., 
 
 245- 
 Darwin, 9; 10; 12; theory of orienta- 
 tion in plants, 18-21; transmis- 
 sion of stimuli in plants, 19; 23; 
 31; 47; 52; definition of tropism, 
 
INDEX 
 
 397 
 
 54; 57; 59; 60; 63; 70; location of 
 sensitive structure in leaves, 71; 
 229; 235; 354. 
 
 Davenport, theory of orientation, 
 40-42; definition of tropism, 55; 
 58; reactions of Amoeba to light, 
 74; movement in Amoeba affected 
 by change in light intensity, 78; 
 reactions of Stentor, 113; 234; 
 phototaxis and photopathy com- 
 pared, 254-256; effect of different 
 rays on reactions: 302, criticism 
 of, 343, 363; photochemical reac- 
 tions, 308. 
 
 De Candolle, vitalism, 8; sleep 
 movements of leaves, 11; theory 
 of orientation in plants (heliotrop- 
 ism), 12; 13; 14; 27; 52; defini- 
 tion of tropism, 53; 54; 229; 234. 
 
 Decoration, in crabs, 356-358. 
 
 Dendrocoelum lacteum, 207. 
 
 Descartes, philosophy of movement, 
 
 6; 51- 
 Dewar, 338; 349. 
 Dexia carinifrons, 216. 
 Dias longiremis, 337. 
 Diatoms, 265; reactions of, in spec- 
 trum, 323, 326. 
 Didinium nasutum, 126. 
 Dodart, curvature of roots, 7. 
 Driesch, definition of tropism, 56; 
 
 369; theory of vitalism, 374-378. 
 Drosophila ampelophila, 216; effect 
 
 of intense light on, 271; change in 
 
 reaction of, 280. 
 Du Bois-Reymond, 9. 
 Du Hamel, cause of curvature in 
 
 plants, 7. 
 Dutrochet, osmosis and movement 
 
 of plants, 12. 
 Dutrochet and Pouillet, 314; 319. 
 
 Earthworms, orientation of (Daven- 
 port), 40, 41; 50; locomotion, 199; 
 
 trial in orientation, 200-206, 232; 
 distribution of sensitive tissue in, 
 201-205; accuracy of orientation, 
 205; reaction to sudden increase 
 of light intensity, 250; 257; 259; 
 260; 343; 346. 
 
 Echinaster crassispina, 211. 
 
 Echinoderms, method of locomo- 
 tion, 211; orientation of, 211-213; 
 233; aggregation of starfishes, 244; 
 reactions of sea urchins to shad- 
 ows, 247; 260. 
 
 Edwardsia, 250; 259; 299. 
 
 Eigenmann, 238. 
 
 Eloactis, reactions of (Hargitt), 
 252. 
 
 Energy, distribution in spectrum, 
 304-308, 360; distribution in spec- 
 trum compared with stimulating 
 efficiency, 332, 343. 
 
 Engelmann, reactions of unicellular 
 forms to light, 16, 17; 42; 44; 57; 
 stimulation of pseudopods, 74; 
 method of aggregation in Euglena, • 
 etc., 82; 94; function of eye-spot, 
 106-109; 235; methods of aggre- 
 gation compared with ideas of 
 Jennings, 240, 323, 324; 279; reac- 
 tions of unicellular forms in spec- 
 trum, 322-325. 
 
 Entelechy, as a factor in regulation, 
 
 377- 
 Entomostraca, 258; 271; 278. 
 
 Eudendrium, hydranths, mechanics 
 of orientation, 163, 164. 
 
 Eudendrium, planulae of, orienta- 
 tion in light from two sources, 87; 
 description and locomotion, 159; 
 accuracy of orientation, 160; me- 
 chanics of orientation, 161-163; 
 aggregation of, 243; 271. 
 
 Eudorina, orientation in light from 
 two sources, 87, 147; function of 
 eye-spot, 109, 147; structure of, 
 
398 
 
 INDEX 
 
 146; locomotion of, 147; orienta- 
 tion and change in sense of , 147, 
 231; 171; aggregation of, 242. 
 
 Euglena, viridis, 16; method of ag- 
 gregation (Engelmann), 17; 36; 
 45; description of, 80-82; aggrega- 
 tion of (Engelmann), 82; orienta- 
 tion of (Jennings), 83, 84, (Torrey), 
 84, 85; orientation in light from 
 two sources, 86, 87, no; dilTerent 
 species and collection of, 89, 90; 
 locomotion of, 90, no; accuracy 
 of orientation, 92; mechanics of 
 orientation, 92-99, 102-104, no; 
 discussion of orientation, 99-102; 
 distribution of sensitive tissue, 
 104-106, in; function of eye- 
 spot, 98, 99, 102, 106-109, in; 
 sensation in, 112; 115; 118; 122- 
 137; 142-148; 156; 161; 174; orien- 
 tation compared with that in Are- 
 nicola larv^ae and Musca larvae, 
 169, 171, 195; 209; 210; 215; 216; 
 229; 230; adaptation, 236; 243; 
 257; 259; 261; 263; reversal in 
 reaction and effect of tempera- 
 ture, 265, 267, 274-279; deses, 
 viridis, spiragyra, 274; 301; reac- 
 tions in spectrum, 324, 325, 361; 
 366; 368. 
 
 Evolution, effect of theory of, on 
 behavior, 9, 10; 52; natural selec- 
 tion, 238, 239; of reactions to 
 light, 262, 263. 
 
 Ewart, protoplasmic streaming, 74. 
 
 Eyes, function of, in reactions: flies, 
 216, 217, fiddler crab, 217, Rana- 
 tra, 218, toads and frogs, 219- 
 223, arthropods, 226, 227, birds, 
 228, 233; function of, in aggrega- 
 tion, 244. 
 
 Eye-spot, in Euglena: structure of, 
 81, 82, function of, 98, 99, 102, 
 106-109, in; in Trachelomonas: 
 
 structure of, 128, 129, function 
 of, 109, 130; in Chlamydomonas: 
 function of, 109, 133; in Chloro- 
 gonium, function of, 134; in Eu- 
 dorina and Pandorina, structure 
 and function of, 147, 148; lumi- 
 nosity of, in direct sunlight, 147; 
 in Arenicola larv^ae: 166, function 
 of, 171, 172, 174; function of, in 
 starfish, 211; summary of function 
 of, 230. 
 
 Famintzin, change in sense of re- 
 action, 265. 
 Fechner, 9. 
 Fielde, 354. 
 Figdor, sensitiveness of plants, 288, 
 
 313- 
 Fishes, adaptation in blind, 238; 
 
 function of skin in response, 262; 
 
 343; color vision in, 358-360, 
 
 364- 
 Flagellata, 36; 37; 229; 230; 232; 233; 
 
 aggregation of, 242; reactions in 
 
 spectrum, 324, 325. 
 Flies, circus movements in, 216, 
 
 217; function of eyes in reactions, 
 
 216, 217; adaptation, 237. See 
 
 Musca. 
 Forbes, 312. 
 Forel, 354. 
 Frandsen, 265. 
 Frank, geotropism, 12. 
 Fraunhofer, brightness in spectrum, 
 
 305-307- 
 
 Frogs, aggregation of, 244; 260; re- 
 versal in reactions, 273; 278; See 
 Biifo. 
 
 Fulgides, reversible in light, 308- 
 310. 
 
 Fundulus, reactions to shadows, 247; 
 257; 260. 
 
 Fungi, reactions of, in spectrum, 
 
 317-319- 
 
INDEX 
 
 401 
 
 Light, cb'-' 
 (Darwi 
 prism, 
 
 suppc INDEX 
 
 grad' 
 effc 
 int^en experiments on animals, 
 
 E.ra'notropism, ^orav^fjt 
 "heliotropism, 28, 29-, 56-,i^9, 
 
 S Volvox, 145, 146- 
 ^ 'Gamble, 354- . 
 
 Gammarus, 279; 3-;346, 3 ^ 
 Gardner, reactions of seedling 
 
 spectrum, 3^4, S^Q- 
 Geometra, 346- 
 
 Geotropism, ^3- orientation 
 
 Gonionemus murbacnu, 
 
 in,x54;^5x-,^5-,^57;^59,^66 
 
 Graber, preference -^thod^^ -.J^' 
 ,.■ 27- u; theory oS reactions, 
 
 reactions ot liigher animals, 343 
 
 '^'■''"if/r. 59-. change in 
 
 Grammeae, 68, 229, ^:)y) 
 
 reactions, 285. 
 
 Smitreactions oi seedlings in 
 spectrum, 3i4,-3i7, 3i9- 
 
 399 
 
 \ 
 
 \ 
 
 Haberlandt. SO', function of epider- 
 nial cells in reactions to light, 72. 
 
 Hadley, definition d "opism 5 ■ 
 ,ii- 222; orientation in lobster 
 arte, 226; reversal in reactions 
 of lobster larvae, 264, 266, 286. 
 
 Haematococcus, 272; m- 
 
 Hargitt. r6o; 247; '-^""f "^f/, 
 droides, 249; reactions of Eloacus 
 
 ,„. variation and modification 
 
 of tactions of Hydroides, 292- 
 
 Ha%^er, orientation in e-—. 
 
 199; 200-205; 214, 215, " 
 
 tion in Corethra larvae, 225^ 
 Harrington and ^eammg, mo^e 
 ^ent of Amoeba, 74; 322, reac 
 tions of Amoeba in spectrum, 327, 
 
 Harty^'xperiments of, on the cir- 
 
 Haycraft, brightness in spectrum, 
 
 306, 307. 
 Hedista, 286. 
 Heliotropism, origin of term, 12. 
 
 Sachs- theory of, r3-r6; compared 
 
 with galvanotropism, 2 5-3 1 i t-oeo s 
 TheorVof, 1-8-33; compared with 
 Unterschiedsempfindlichkeit, 3^, 
 „ 254-256; Verworn's theory of, 
 'i.X defined, 53-56, 253-^56; 
 'of Euglena, 85, r°4;. compared 
 ^th galvanotropism mWvox, 
 
 145 i46-,i64;"7;inh'«is(Cole, 
 2* 26;-, in plants and amma's 
 compared, 345. S« Or««.a(, 
 Helix hortensis, 251; 300- ' 
 
 Helmboltz, 9- , . ,^ 
 
 Hertel, response to ultra-viole 
 
 Paramecium, i34- 
 
 Hesse, 247. 
 
 Hewitt, 184. . ) 
 
 Hofmeister, heliotropism, 12, cuv^ 
 
 Ce in single-celled structu, 
 71; Plasmodia, 74- , 
 
 Holmel orientation-. select»n of 
 
 - ^^'^S-.'^ect of light inten- 
 sity on rate of movement loo, 
 Z; orientation in Musca larva . 
 n6 X83; r89; trial movement .n 
 
 eirthworms, 198; -3; »4- -t4. 
 on circus movements, 2:5, ^7. 
 on orientation, 2,8; 223, 226, »7. 
 
 ,„■ reversal in reactions, 271, 
 11' 2,0 280; effect of contact on 
 
 "' ,«!• modification m re- 
 
 reactions, 284, mou. 
 
 actions ot Ranatra. 296. 
 
 Holothurian, orientation <> - ^ ^ 
 
 Holt and Lee, 4^, reactions of Sten 
 
 '"I' "'■' IteRect of light inten- 
 Hydra, 34; 48, '^^ct 
 
 sity on activity, 1507152-° _ 
 
\ 
 
 400 
 
 IXDEX 
 
 231; acclimatization, 152; reaction 
 of negative specimens, 154; dis- 
 tribution of sensitive tissue in, 
 156; reaction to electric current 
 compared with reaction to light, 
 158; 258; 265; effect of different 
 wave-lengths on reactions of, 310; 
 reactions in spectrum, 333-335; 
 361, 363; 367; 370. 
 Hydroides dianthus, orientation in 
 light from two sources, 87; reac- 
 tions to shadows and acclimatiza- 
 tion, 247, 249; 250; 263; 280; 285; 
 variation and modification in reac- 
 tionsof, 292-295,368; 298;366;373. 
 
 function 
 lomonas: 
 
 stimulation of Amoeba, 75 ; mo\ • p 
 
 ment of Amoeba, 80; 82; orient . 
 
 'ucture 
 tion in Euglena, 83, 84; 89; 91; 9c ^^^^_ 
 
 104; 112; orientation of Stentor 
 
 T ■ 
 
 113; 114; reactions in Chlamydo- 
 monas, 132; orientation in Hydra, \ 
 156; 202; 203; orientation in 
 Asterias, 211; "trial and error" 
 defined, 215; 231; theory of orien- 
 tation compared with that of 
 Engelmann, 240, 323, 324; reac- 
 tions of Aiptasia, 252; 262; 287; 
 293; 297; 328; 369. 
 
 Johnston, 11. 
 
 Joubert, 248. 
 
 Impatiens, 286. 
 Inachus, 355. 
 
 Indian corn. See Zea mays. 
 Infra-red, effect of : on seedlings, 314- 
 319, on bacteria, 325, on Daphnia, 
 343; cause of invisibihty, 362; 368. 
 Infusoria; 36. 
 Insects, 42, ^43. 
 
 Intensity-difference, 3, 4; effect of, 
 on moven.ent of organisms, 15. 
 See Light. 
 
 Jassa, 279. 
 
 Jellyfishes. See Sarsia, Gonionemus 
 and Bougainvillea. 
 
 Jennings, 17; work compared with 
 that of predecessors, 44; method 
 of aggregation of lovver organisms, 
 45, 46; motor reaction, motor 
 reflex, avoiding reaction defined, 
 45; theory of orientation (trial and 
 error), 46-49; external stimulus 
 not necessary for activity in or- 
 ganisms, 47; direct orientation, 
 48; analysis of behavior, 49, 50, 
 372-375; on regulation in be- 
 havior, 50, 368, 372; put; c defi- 
 
 Keeble and Gamble, 354. 
 
 Kinnaman, 360. 
 
 Kneip, function of epidermal cells in 
 orientation of leaves, 72. 
 
 Knight, eft'ect of gravitation on 
 curvature of roots, 11, 12; 13. 
 
 Krabbe, 71. 
 
 Kraus, reactions of fungi in spec- 
 trum, 317, 319. 
 
 Labidocera, 227; 266; reversal in 
 reactions, 274, 284, 300. 
 
 Lacrj'maria olar, 51. 
 
 Langley, 305. 
 
 Lapidium, 265, 288, 313. 
 
 Larvae, Limulus, 87; Musca, 87; 
 Arenicola, 87; lobster, 226; adap- 
 tation, 237; aggregation of, 242; 
 mosquito, 247; Palaemonetes, 264; 
 Polygordius, 266. 
 
 Leaves, orientation of, 71-73. 
 
 Leeches, 346. See Clepsine. 
 
 Leeuwenhoek, 136. 
 
 Leptoplana tremellaris, orientation, 
 in light from two sources, 87; 207; 
 orientation of, 207; 271. 
 Liebig, 9. 
 
 primitive ideas of. i^. -T. 
 
INDEX 
 
 401 
 
 Light, change of intensity on plants 
 (Darwin), 19; graded by means of 
 prism, 39, 60; pressure of, and 
 supposed effect on orientation, 43; 
 graded by means of lens, 60, 61; 
 effect of constant, and change of 
 intensity of, on reactions of: 
 Euglena, 16, 83, 85, 98, 99, 100- 
 112, Stentor, 113-115, 118-123, 
 Swarm-spores, 127, Trachelomo- 
 nas, 130, Chlamydomonas, 132, 
 133, Volvox, 139, 143-145, Eudo- 
 rina and Pandorina, 147, Hydra, 
 157, Eudendrium, 161, 163, Me- 
 dusae, 165, Arenicola larvae, 172- 
 175, Fly larvae, 191-197, earth- 
 worms, 204, Planaria, 208, 210, 
 general, 229-235, 241, 245, 253- 
 258, 299, 366, 367; effect of, on 
 movement, 245; reactions to sud- 
 den decrease of, 247-250; reac- 
 tions to sudden increase of, 250, 
 251; reactions to continued illumi- 
 nation, 252, 253; effect of, on 
 reversal in reactions, 265-272, 
 299; characteristics of, 303; effect 
 of, on chemical reactions, 308-312. 
 
 Light grader, 60-62. 
 
 Lillie, 166. 
 
 Limax, 265. 
 
 Limnaeus stagnalis, 345. 
 
 Limnea columella, 214. 
 
 Limulus polyphemus, larvae of, orien- 
 tation in light from two sources, 
 87; 88; 258; 271; reversal in reac- 
 tions, 285; 346. 
 
 Lineus ruber, 284; effect of color on 
 reactions, 355. 
 
 Littorina, aggregation of, 244; 247; 
 periodic movements caused by 
 tides, 286. 
 
 Lobster larvae, orientation and 
 change in sense of orientation, 
 226, 264; 267. 
 
 Loeb, 11; 17; 21; object of observa- 
 tions on reactions of animals, 23, 
 24, 34; control of movement in 
 animals and plants identical, 25, 
 
 26, 164, 346; on relation between 
 sensations and animal behavior, 
 
 27, 28; first theory of orientation 
 (ray direction), 24, 25, 34; second 
 theory of orientation (angle of 
 rays), 28, 29, 35; third theory of 
 orientation (intensity difference), 
 29-31, 35, 221; effect of constant 
 intensity compared with change 
 of intensity, 32, 33; extent of 
 application of theories, 33, 34; 
 ideas on animal behavior sum- 
 marized, 34-36; theory of orien- 
 tation (tropism) compared with 
 Verworn's, 39; 40; 42; 52; 53; 
 definition of tropism, 54-58; 70; 
 80; 83; 86; theories criticized. See 
 Criticism; theories of orientation 
 applied to; Volvox, 137, 144, 146, 
 Hydra, 149, 150, 159, Arenicola 
 larvae, 173, earthworms, 2c , Pla- 
 naria, 209, Caprella, 225, ants, 
 351; 363; 364; on orientation in 
 Eudendrium, 164; 168; 171; orien- 
 tation in Musca larvae, ^75, 176, 
 182, 183; 177; 178; 180; 192; 194; 
 198; reactions of planarians to 
 light, 206; on circus movements, 
 215; 216; 220; 223; 229; 230; 
 234; 238; on origin of reactions, 
 239-243; cause of aggregation in 
 Planaria, 245; 247; heliotropism 
 compared with Unterschiedsemp- 
 findhchkeit, 254-257; 258; on 
 adaptation and change in sense 
 of reaction, 266, 267, 272, 273, 
 285; effect of chemicals on reac- 
 tions, 279; effect of concentration 
 of medium on reactions, 283; 284; 
 effect of different colors on reac- 
 
402 
 
 INDEX 
 
 tions: 302, 346, 347, criticism of, 
 
 335, 336, 343, 346, 347; on cause 
 and regulation of reactions, 369; on 
 origin of adaptiv'e reactions, 371. 
 
 Lotze, 9. 
 
 Lubbock, 10; 24; 235; 265; on effect 
 of different colors on reactions of: 
 Daphnia, 310, 335-343, 362, ants, 
 348-352, bees, 352-355. 
 
 Lumbricus, 199; reactions in colors, 
 344. Sec Earthworms. 
 
 Lunularia, 288; 313. 
 
 Lupinus albus, 21. 
 
 Luther and Forbes, photochemical 
 reactions, 312. 
 
 Lutianus griseus, 358. 
 
 Lyon, 264. 
 
 Mach, 344. 
 
 Machine, defined (Driesch), 375. 
 
 Maja, verrucosa and squinado, 355; 
 
 357. 
 
 Mammals, 343; 360; color vision in, 
 ^64. 
 
 Mass rt, 12; reversal in reactions 273. 
 
 Maya aranaria, 247. 
 
 McCook, 354. 
 
 Medusae reactions to light, 164, 165. 
 
 Melolontha, 346, 
 
 Merejkowsky, 337. 
 
 Metazoa, 47. 
 
 Mimosa, 7; 286. 
 
 Minkiewicz, definition of tropism, 
 56; effect of concentration of me- 
 dium on reactions, 284; 354; reac- 
 tions of Crustacea to color, 355- 
 358. 
 
 Mitsukuri, reversal in reactions, 286. 
 
 Modifiability, in behavior, 264-301; 
 '368; chemical regulation of, 370. 
 See Acclimatization. 
 
 Mollusks, 233; 243; reactions to 
 shadows, 247; acclimatization, 
 248; 257; 259; 260; 343. 
 
 Monkey, color vision, 360. 
 
 Morse, reactions of Gonionemus, 164. 
 
 Mosquito larvae, reactions to 
 shadows, 247; 249; 257; 260. 
 
 Moths, flight of. into flame, 227, 228; 
 adaptation, 237; change in reac- 
 tions, 280; 346. 
 
 IMotor reaction, defined, 45. 
 
 Motor reflex, 17; defined, 45; 240. 
 
 IMouse, dancing, color vision, 360. 
 
 Movement, random, as a factor in 
 orientation, 50, 51, 157; rate of, 
 in: swarm-spores and Volvox, 100, 
 loi, Euglena, 102, no, Hydra, 
 151, Musca larvae, 184-189, 
 earthworms, 199, Planaria, 208; 
 random, in Musca larvae, 189-192, 
 196; random, as a factor in orien- 
 tation of earthworms, 203, 232; 
 effect of random, on aggregation, 
 239-241, 245; periodic, in plants, 
 286; periodic, in Littorina, etc., 
 286; cause of change in, 366. 
 
 Muller, H., 13; 354. 
 
 Miiller, J., vitalism, general physi- 
 ology, 8, 9; 52. 
 
 Muller, N. J. C, 60; change in sense 
 of reactions (seedhngs), 265; re- 
 actions of seedlings in spectrum, 
 
 317,319- 
 Musca larvae, 50; orientation in 
 light from two sources, 87, 177, 
 197; 88; orientation according to 
 Loeb, 175; locomotion, 176; ac- 
 curacy of orientation, 177; orien- 
 tation perpendicular to the rays, 
 180-183; distribution of sensitive 
 tissue, 183, 184, 188; effect of light 
 intensity on rate of locomotion, 
 184-189, 197; random movements, 
 189-192, 196; acclimatization of, 
 189, 190, 197; mechanics of orien- 
 tation, 189-197, 232; orientation 
 compared with that in Euglena 
 
INDEX 
 
 403 
 
 and Stentor, 195, 197; trial and 
 error, 196, 197; circus movements, 
 216; 257-260; 270; change in re- 
 actions, 285; 346. 
 
 Mustard. Sec Sinapis. 
 
 Myxomycetes, 74; 229; 233; adapta- 
 tion, 237. 
 
 Nageli, early observations on move- 
 ment of flagellates and swarm- 
 spores, 14; 15; effect of light in- 
 tensity on rate of movement, 100. 
 
 Nagel, 58; 247; on reactions to 
 shadows and acclimatization, 248; 
 250; 251; effect of different rays 
 on reactions, 302. 
 
 Narcotics, effect of, on reversal of 
 reactions, 279-283, 300. 
 
 Natural selection and adaptation, 
 238, 239, 272. 
 
 Nemertean, 355. 
 
 Nernst glower, 61; 86; 92. 
 
 Newcombe, transmission of stimuli, 
 
 59- 
 Nichols, distribution of energy in 
 
 spectrum, 304, 305. 
 
 Oats. See Avena. 
 
 Oedogonium swarm-spores, orienta- 
 tion in light from two sources, 87; 
 description of, 123, 124; collection 
 of material, 124; locomotion of, 
 124; reversal in sense of orienta- 
 tion, 125; mechanics of orienta- 
 tion, 126-128; aggregation of, 127, 
 241; 229. 
 
 Oltmanns, experiments on ray direc- 
 tion and intensity difference, 39, 
 40; 42; 60; 63; 265; 290. 
 
 Optimum intensity, effect of, on 
 aggregation, 242; variation in, 
 288-297, 301. 
 
 Orchestia, reversal in reactions, 271, 
 284, 300. 
 
 Orientation in light, in plants and 
 animals compared (Loeb), 24, 25; 
 plumules of Zea mays: 63-69, dis- 
 cussion of, 70-71; in leaves, 71-73; 
 in Amoeba: 76-79, discussion of, 
 80; in Euglena (Jennings), 83, 84; 
 from two sources (Euglena, etc.), 
 86-89, 219-221, 224; accuracy of, 
 in Euglena, 92; mechanics of, in 
 Euglena crawling, 92-99; discus- 
 sion of, in Euglena, 99-102; in 
 negative Euglena, 99; mechanics 
 of, in Euglena swimming, 102-104; 
 mechanics of, in Oedogonium 
 swarm-spores, 126-128; in Trache- 
 lomonas: accuracy of, 129, me- 
 chanics of, 129, 130; mechanics of, 
 in Chlamydomonas, 132, 133; me- 
 chanics of, in Volvox, 137-144, 
 231; in Volvox compared with 
 that in Euglena and Stentor, 142; 
 in Eudorina and Pandorina, 147, 
 231; in Hydra, 157, 231; in Euden- 
 drium planulae, 161-163; in Eu- 
 dendrium hydranths, 163, 164; 
 in Arenicola larvae, 167-171, 232; 
 in Musca larvae, 189-197, 232; 
 in earthworms, 198-205, 232; in 
 Echinoderms, 21 1-2 13; in Bufo, 
 219-223; in Caprella, 224, 225; 
 in Corethra larvae, 225; in lobster 
 larvae, 226; in zoeae of Brachyura 
 and Caridea, 226; mechanics of 
 (general), 233-235; effect of, on 
 aggregation, 241-244; fundamen- 
 tal cause of, 243; in Daphnia, 
 264. 
 
 Oscillaria, reactions of, in spectrum, 
 323, 326, 327, 362; distribution of 
 stimulating efficiency in specti um, 
 
 332. 
 Ostwald, 265. 
 Oxygen, effect of, on reactions, 279, 
 
 323-325, 362, 374- 
 
404 
 
 INDEX 
 
 Pagurus, reactions to shadows, 247; 
 249; modilication in reactions of, 
 296. 
 Palaemon, 264. 
 
 Pandorina, orientation in light from 
 two sources, 87, 147; function of 
 eye-spot, 109, 147; 136; structure 
 of, 146; locomotion of, 147; orien- 
 tation and change in sense of, 
 147, 231; 171; aggregation of, 242. 
 Papaver, 288; 313, 
 
 Paramecium, 17; 45; 82; reactions 
 to light, 134, 135, 361; 142; 324; 
 344; 350; 366. 
 Paramecium bursaria, method of 
 aggregation (Engelmann), 16; 45; 
 aggregation of, 240; effect of oxy- 
 gen on reactions of, 279; reactions 
 in spectrum, 323, 324; 374. 
 
 Parker, definition of tropism, 56; 
 orientation in earthworms, 198; 
 201; 202; 214; on circus move- 
 ments, 216; 221; 223; effect of 
 size of illuminated area on reac- 
 tions, 227; 244; 250; 265; 274; 
 effect of contact on reactions, 
 284. 
 
 Patten, 247. 
 
 Payer, reactions of seedlings in spec- 
 trum, 314, 317, 319. 
 
 Pearse, orientation in holothurians, 
 211; 221; 247; decoration in crabs, 
 357. 
 
 Peckham, 354. 
 
 Pecten, 247. 
 
 Pelomyxa palustris, 74. 
 
 Perichaeta bermudensis, 199. 
 
 Pfeffer, 12; 17; 21; 59; 60; 94; 100; 
 warming-stage, 274; sleep move- 
 ments in plants, 286; effect of dif- 
 ferent rays on reactions, 303; 317. 
 
 Phacus, aggregation of, 241; 274. 
 
 Phagocata gracilis, 207. 
 
 Phalaris, 19. 
 
 Photokinesis, defined, 35. See Unter- 
 schicdscmpfindlichkeit. 
 
 Photopalhy, defined by Davenport, 
 40, 41; 55; 56; 241; defined, 253- 
 256; compared with phototaxis, 
 254; 302. 
 
 Photosynthesis, 311, 312, 361. 
 
 Phototaxis, defined by Davenport, 
 40; 55; defined by Hadley, 56; 
 217; defined, 253-256; compared 
 with photopathy, 254; 302. See 
 Ihiiotropism. 
 
 Phototropism. See Ilcliotropism. 
 
 Phycomyces, 265; 318. 
 
 Physiological states, dependence of 
 reactions upon, 49; effect of, on 
 reversal in reactions, 284-287; 
 Jennings on, 372-375. 
 
 Pilobolus, 318. 
 
 Pinnularia, 323. 
 
 Pisa, 355. 
 
 Planaria, accuracy of orientation, 
 206; locomotion and rate of loco- 
 motion, 207-210; method of aggre- 
 gation, 210; 254; 257; 261; 270; 
 346; 366; maculata, 207; gono- 
 cephala, 207; torva, 255. 
 
 Plants, theory of orientation in: 
 (Darwin), 18-21, (Loeb), 30; re- 
 actions of, in light, 59-73; sensi- 
 tiveness of, 288, 313, 319; effect 
 of different wave-lengths on re- 
 actions of, 310, 313-320, 361; 
 effect of mixed rays on reactions 
 of, 362. See Table of Contents. 
 
 Planulae of Eudendrium, 87. See 
 
 Eudendrium. 
 Plateau, 354. 
 
 Plumules, orientation of, 63-69. 
 Poggioli, effect of different rays on 
 
 reactions of plants, 314, 321. 
 Pollock, transmission of stimuli in 
 roots, 31, 59; theory of root curva- 
 ture, 71. 
 
INDEX 
 
 405 
 
 Polygordius larvae, 266; reversal in 
 sense of reaction, 273, 279, 283, 
 300; 346. 
 
 Porter, 360. 
 
 Porthesia chrysorrhoea, 31; 336; 346; 
 
 371- 
 Potamilla oculifera, 247. 
 
 Pouchet, 247. 
 
 Pouillet. Sec Dutrochet. 
 
 Prawns, 249; 257; 260. 
 
 Preference method, 10; 343. 
 
 Preyer, 10. 
 
 Pringsheim, protoplasmic streaming, 
 
 75- 
 Problems, in reactions to light, 
 
 statement of, i, 2, 3, 57, 58. 
 Protozoa, 51. See Table of Contents. 
 Protula intestimum, 247. 
 Psychic phenomena, in organisms, 
 
 9, 10, 27; in Euglena, 102; in frogs 
 
 and toads, 223, in ants, 350, in 
 
 bees, 353. 
 Psychoid, defined, 377. 
 Purkinje's phenomenon, 305. 
 
 Raccoon, color vision, 360. 
 
 Radl, theory of orientation (pres- 
 sure of light), 42, 43; cause of 
 change in sense of orientation, 
 43; definition of tropism, 55; 214; 
 on circus movements, 216, 217; 
 234; 264; 265. 
 
 Rana, 260; 273. 
 
 Ranatra, 217; 218; 226; 260; reversal 
 in reactions, 271, 273, 280, 284, 
 300; modification in reactions of, 
 296; 367. 
 
 Raphanus, 314. 
 
 Rawitz, 247. 
 
 Ray, explanation of movement in 
 plants, 7; orientation in plants, 12. 
 
 Ray direction, as used by Sachs, 14; 
 effect of, on orientation in animals 
 (Loeb), 24-26; compared with dif- 
 
 ference of intensity, 27; Oltmanns 
 on, 39, 40; function of, in orienta- 
 tion: plumules, 68, 69, Euglena, 
 83, 87, no, III, Stentor, 114, 118, 
 Hydra, 150, Musca larvae, 180- 
 182; 137; 144; 241; Davenport on, 
 254-256. 
 
 Reactions to light, distribution of, i; 
 preference method, 10; problems in, 
 1-3, 57, 58; in various organisms. 
 See Table of Contents; classification 
 of, 253-256; reclassification of, 256- 
 262; evolution of, 262, 263. 
 
 Reactions, to shadows, 247-250; to 
 sudden increase of light intensity, 
 250, 251; to continued illumina- 
 tion, 252, 253, 257; to change of 
 intensity, 257; of questionable 
 cause, 258; caused by direct effect 
 of light, 258; caused by indirect 
 effect, 258-260; caused by what 
 light represents, 260; fundamental 
 cause of, 258-260, 278, 279, 298, 
 300, 320, 363, 366; reversal in, 
 264-267, 355; cause of reversal in, 
 267-287, 299; extent of reversal 
 in, 271; effect of light on reversal 
 in, 265-272, 299; effect of tem- 
 perature on reversal in, 272-279, 
 300; effect of chemicals on re- 
 versal in, 279-283, 300; effect of 
 concentration of medium and me- 
 chanical stimuli on reversal in, 
 283, 284, 300; effect of internal 
 changes on reversal in, 284-287; 
 variability and modifiability in: 
 general, 288-297, 361, 362, in ants, 
 351, in bees, 352-354, in higher 
 Crustacea, 355-358, in fishes, 358- 
 360; effect of mixed colors on, 310, 
 
 316, 335, 340, 362, 363- 
 Regulation, in behavior, 50, 377; 
 Chapters 13, 14, and 20. See 
 
 Jennings. 
 
4o6 
 
 INDEX 
 
 Reighard, color vision in fishes, 359, 
 360. 
 
 Reinke, chlorophyll-absorption band, 
 312. 
 
 Reptiles, t,j^7,. 
 
 Rhizopods, 74; 79; 229; 233; aggre- 
 gation of, 242; 258. 
 
 Romanes, 10; 24; 211; 235; 251. 
 
 Roots. See Plants. 
 
 Root-tip, function of, in reactions, 
 19, 20. 
 
 Rothert, 12; 21; 72. 
 
 Ryder, 247. 
 
 Sabella microphthalmia, 247. 
 Sachs, J. von, ray-direction theory 
 of orientation in plants (heUo- 
 tropism, geotropism), 13-16; ag- 
 gregation of organisms due to 
 currents, 15; criticism of Darwin's 
 idea of function of root-tip in 
 movement of roots, 21; 24; 25-30; 
 Sy, 34; 38; 39; 40; 42; definition 
 of tropism, 53, 54; 57; 60; 70; 80; 
 86; ray-direction theory criticized, 
 87; 112; ray-direction theory ap- 
 plied to Vol vox, 137, 144; 149; 
 ^i2>\ 314; effect of different colors 
 on reactions of seedlings, 317; 319; 
 See Ray Direction atid Criticism. 
 Sagartia, 48. 
 
 Sand fleas, adaptation, 237, 238. 
 Sarsia, 251. 
 Scapholeberis armata, 214; 271; 277; 
 
 280; 300. 
 Schafer, cause of invisibility of infra- 
 red and ultra-violet, 362. 
 Schreckbewegung, origin and mean- 
 ing of term, 17; in Euglena, 82; 
 no; 161; 233; 240; 246; 257; 259; 
 in bacteria, 325. 
 Scystosiphon lomentarium, 274. 
 Sea anemone, 258; 299. 
 Sea squirt, 250. 
 
 Sea urchin. See Echitwderms. 
 
 Seedlings, orientation in light, 59- 
 73; change in sense of orientation, 
 265; reactions in spectrum, 314. 
 See Plants. 
 
 Semotilus atromaculatus, 358. 
 
 Sensation. See Psychic phenomena. 
 
 Sensibilite differcntielle, compared 
 with motor reflex, 17; 243. 
 
 Sensitiveness, with different surfaces 
 exposed: in Euglena, 104-106, in 
 Stentor, 114, 115, ng; variation 
 in, 288-297, 301; in plants, 288, 
 
 313,319- 
 Serpula, 247; 254; 258. 
 
 Serpula uncinata, 247. 
 Setaria italica, 21. 
 Shadows, reactions to, 247-250. 
 Sharroc, 7, 
 
 Shock-movement. See Schreckbewe- 
 gung. 
 
 Shrimps, 249; 257; 260. 
 
 Sigesbeckia, 286. 
 
 Sign, reaction to, 243; 250; 259-263; 
 286; 292-295; 298. 
 
 Simocephalus sima, 265; reactions 
 in spectrum, 337, 341-343, 362. 
 
 Sinapis alba, reactions in spectrum, 
 316, 317- 
 
 Smith, orientation in earthworms, 
 198; 201. 
 
 Spaulding, modification in reactions 
 of hermit crabs, 296. 
 
 Spectrum, energy distribution in, 
 304, 305; brightness distribution 
 in, 305-308; actinic effect, dis- 
 tribution in, 308-312, 360; reac- 
 tions: of plants in, 313-320, of 
 unicellular forms in, 321-332, of 
 Amoeba in, 327-332; distribution 
 of stimulating efficiency in: plants, 
 314-320; swarm-spores, 321, 322; 
 diatoms and Oscillaria, 323, 326, 
 327, ciliates, t,2t„ flagellates, 324, 
 
INDEX 
 
 407 
 
 bacteria, 325, Amoeba, 327, 330- 
 332, Hydra, 334, 335, Daphnia, 
 337-341, Simocephalus, 341-343, 
 worms, spiders, insects, mollusks, 
 fishes, amphibia, reptiles, birds, 
 mammals, 343^34^, ants, 348-352, 
 bees, 352-355, higher Crustacea, 
 355-358, fishes, 358-360, birds and 
 mammals, 360, general, 361, 362, 
 367; for fishes and dancing mouse, 
 
 359, 360. 
 Sphinx, 346. 
 Spider, water, reversal in reactions, 
 
 277, 300; 343; 354; color vision 
 
 in, 364. 
 Spirographs spallanzani, 33; 247; 
 
 254; 255; 258. 
 Stahl, 74; light reactions in Euglena, 
 
 82; 265. 
 
 Starfish. See Eckinoderms. 
 
 Stenorynchus, 355; 358. 
 
 Stentor coeruleus, 42; orientation in 
 light from two sources, 87, 115, 
 116; distribution of sensitive tissue 
 in, 114, 119-121; mechanics of 
 orientation, 114-119, 122; reac- 
 tions when attached, 115; aggre- 
 gation of, 121, 241; activity in 
 different intensities of light, 123; 
 124; 126; 127; 132; 137; 156; 
 orientation compared with that 
 in Musca larvae; 195; 215; 229; 
 230; adaptation, 237; 250; 257; 
 259; 261; 263; 270; 277; 280; 
 variation in sensitiveness, 291, 
 301, 368; 297; 300; variation in 
 reactions, 374. 
 
 Stentor viridis, aggregation of, 240; 
 effect of oxygen on reaction, 279. 
 
 Stimulation, differential response to 
 localized, in: Euglena, 83, loi, 
 Stentor, 121, Oedogonium swarm- 
 spores, 128, Volvox, 137, Hydra, 
 158, Eudendrium, 162, medusae, 
 
 164, 165, Arenicola larvae, 174, 
 fly larvae, 195, earthworms, 200, 
 201, 205, Planaria, 206, 210, vari- 
 ous species, 214, Caprella, 225, 
 arthropods, 226; general, 48; 
 Summary of, 230-235. 
 
 Stimulus, transmission of, in plants, 
 12, 21; fundamental cause of 
 (Jennings), 50; 59; orienting, 
 cause of in: Euglena, 94, 98, 
 Stentor, 118, Oedogonium swarm- 
 spores, 127, Paramecium, 134, 
 135, Volvox, 144, Hydra, 157, 
 Eudendrium, 163, Arenicola lar- 
 vae, 1 71-174, Musca larvae, 194, 
 195, earthw^orms, 204, 206, Plan- 
 aria, 208, 210, Echinoderms, 212, 
 Ranatra, 218, frogs and toads, 
 219, 223, gramineae, myxomy- 
 cetes, rhizopods, and various other 
 organisms, 229-235; a sign as a 
 cause of, 243; cause of, in reac- 
 tions to shadows, 250; cause of 
 (constant intensity), 252, 253; 
 cause of, in reactions to light 
 (Davenport), 254-256; character 
 of, 256-258. 
 
 Stobbe, on reversible photochemical 
 reactions, 308-312; 336. 
 
 Strasburger, reactions of swarm- 
 spores to light, 15, 123; 16; 27; 
 40; effect of light intensity on 
 rate of movement, loc; 124; 
 reversal in reactions of swarm- 
 spores, etc., 265, 272-274; change 
 in optimum, <?88; 290; reactions of 
 swarm-spores in spectrum, 221, 
 222. 
 
 Swarm-spores, 87; rate of movement 
 in different intensities of light, 
 100; aggregation of, 241; 257; 259; 
 change in sense of reaction, 265; 
 272; 274; reactions of, in spec- 
 trum, 321, 322, 361. 
 
4o8 
 
 INDEX 
 
 Sylvius, founder of iatrochemical 
 
 school, 6. 
 System, harmonious equipotential, 
 
 375- 
 
 Talorchestia, 271. 
 
 Temora longicornis, 284; 300. 
 
 Temperature, effect of, on reversal 
 in reactions, 272-279, 300; effect 
 of, compared with that of light, 
 276, 367; extent of effect of, on 
 reversal in reactions, 277. 
 
 Tenebrio, 346. 
 
 Theory, of orientation in plants: 
 De CandoUe, 12, Sachs, 13-16, 
 Darwin, 18-21; Loeb: first theory 
 of orientation (ray direction), 24, 
 25, 34, second theory (angle of 
 rays), 28, 29, 35, third theory 
 (intensity difference), 29-31, 35; 
 of orientation in animals (Daven- 
 port), 40-42; Radl's^ 42, 43; Jen- 
 nings' (trial and error), 46-49; 
 Torrey's, 84, 85; Holmes', 218; 
 Graber's, 218; of local response 
 to local stimulation, 86. See 
 Criticism. 
 
 Threshold, in Euglena, 104-106; in 
 Stentor, 114, 115, 119; in plants, 
 288; in Vol vox, 289-291. 
 
 Thyone briareus, 212; reactions to 
 shadows, 247. 
 
 Tissue, sensitive, distribution of: as 
 a factor in orientation, 3, in 
 plants, 21, 59, 71, in Hydra, 156, 
 157, in Arenicola larvae, 172, in 
 Musca larvae, 183, 184, 188, 197, 
 in earthworm, 201, 205; effect of, 
 in reactions, 261, 262. 
 
 Toads. Sec Bufo and Frogs. 
 
 Torelle, 214; reactions of frogs to 
 light and shadow, 218, 219; 222; 
 223; 244; reversal in reactions 
 (frog), 273. 
 
 Torrey, definition of tropism, 56; 57; 
 criticism of Jennings on Euglena, 
 84, 85; 89; 100; 101; 104; 112; 205. 
 
 Towle, effect of mechanical stimu- 
 lation on reaction, 284. 
 
 Trachelomonas, orientation in light 
 from two sources, 87; function of 
 eye-spot, 109, 130; description of, 
 128; structure of eye-spot, 128, 
 129; accuracy of orientation, 129; 
 mechanics of orientation, 129, 130; 
 146; 229; 230; aggregation of, 241; 
 
 257; 259. 
 Trembley, 33, 34; observations on 
 
 movements of Hydra, 148, 149. 
 Trial and error, orientation by, 46; 
 
 in Euglena, 99; in Stentor, 123; 
 
 in Volvox, 142, 145; in Euden- 
 
 drium, 162; in Musca larvae, 196; 
 
 in earthworms, 198, 199, 203, 206; 
 
 214; defined by Jennings, 215; 
 
 Engelmann on, 240. 
 Titicum vulgare, 66. 
 Trochophores, Hydroides, 87. 
 Tropaeolum, 59; 68; 72. 
 Tropism, introduction of term and 
 
 original meaning, 11, 23, 52; de- 
 fined, 53-57; 83. 
 
 Uca pugnax. Sec Fiddler Crab. 
 
 Uexkiill, von, 212; 247; 248. 
 
 Ulothrix, 274. 
 
 Ultra-violet, effect of, on: Paramecia, 
 134, 135, Daphnia, 339, 343, chemi- 
 cal reactions, 310, 311, 360, ants, 
 349, seedlings, 314, 315, 3^9, 
 Lumbricus, 345; cause of invisi- 
 bility of, 362; 368. 
 
 Ulva lactua, 274. 
 
 Unterschiedsempfindlichkeit, com- 
 pared with motor reflex, 17; 27; 
 
 33; 85; 94; 114; lis; 241; 243; 
 compared with heliotropism, 254- 
 256; 258. 
 
INDEX 
 
 4CX) 
 
 \'anessa antiopa, 216; reactions to 
 light, 227; aggregation of, 244; 
 260. 
 
 Variability, in reactions, 264-301; 
 367; chemical regulation of, 370. 
 Sec ModiJlabilUy. 
 
 Vaucheria, 31; 265. 
 
 Vermes, aggregation, 242. See 
 Earthworm. 
 
 X'ertebrates, 2T)T,. 
 
 N'ervvorn, 11; 35; theory of orienta- 
 tion (tropism), 36-38; same com- 
 pared with Loeb's theory, 38, 39; 
 general application of theory, 39; 
 42; 52; 53; definition of tropism, 
 55; 56; 57; 70; 80; 104; 112; 113; 
 122; 168; 171; theory of orienta- 
 tion applied to: Arenicola larvae, 
 173, earthworms, 205; 229; 234; 
 265; on effect of different colors 
 on reactions, 303; reactions of 
 unicellular forms in spectrum, 326, 
 327. See Criticism. 
 
 Vicia sativa, 315, 316. 
 
 Vierordt, brightness in spectrum, 
 
 305-307- 
 Vision, as a factor in orientation, 
 
 218, 219, 223, 224, 233; color: 
 in Daphnia, 339, in ants, 350, in 
 bees, 354, in fishes, 358-360, in 
 birds, 360, in monkeys, 360, in rac- 
 coon, 360, in dancing mouse, 360, 
 in general, 364, 365. 
 
 \'isual purple, 307. 
 
 \'italism, origin and early ideas on, 
 8; 52; theory of (Driesch), 374- 
 
 378. 
 
 Vitis, 265. 
 
 Voechting, 71. 
 
 Volvox, Oltmann's experiments on, 
 39; orientation in light from two 
 sources, 87; rate of movement in 
 difTerent intensities of light, 100, 
 loi; function of eye-spots, T09; 
 
 structure of, 136; locomotion, 136; 
 distrilnilion of sensitive tissue, 137 ; 
 mechanics of orientation, 137-144, 
 231; orientation of segments, 142, 
 143; orientation in negative col- 
 onies. 143; change in sense of ori- 
 entation, 145, 267-270, 280, 2S7, 
 299, 300; orientation in light com- 
 pared with orientation in a gal- 
 vanic current, 145, 146; 148, 
 orientation compared with that in 
 Arenicola larvae, 169, 171; ada)!- 
 tation, 236; aggregation of, 24-'; 
 255; 257; 259; 263; 265; chan^'e 
 in sensitiveness and optimum, 
 289-292, 301; 367; 370; 374- 
 
 Wager, eye-spot in Euglena, 81, 82, 
 
 106. 
 Walter, definition of tropism, 55; 
 
 reactions of Planaria, 207, 26$. 
 Washburn, definition of tropism. 56; 
 
 on movement in Hydra, 150; 
 
 211. 
 Washburn and Bentley, color vision 
 
 in fishes, 358, 359. 
 Wasps, modification in reactions of, 
 
 296, 297. 
 Watson, 360. 
 Weber, 9; law of. 363. 
 Wheeler, definition of tropi .m, 56. 
 Whitman, reactions of Clepsine to 
 
 shadows, 249. 
 Wiesner, 19; 60; 265; effect of ditTer- 
 
 ent rays on reactions of plants, 
 
 310, 314-317, 322, 362. 
 Willow borer, adaptation in cater- 
 pillar of, 238. 
 Wilson, reactions of Hydra to light, 
 
 150; 265; effect of different rays 
 
 on reactions of Hydra, 310, 362; 
 
 reactions of Hydra in spectrum, 
 
 333-335- 
 Wohler, 9. 
 
4IO 
 
 INDEX 
 
 Yerkes, Ada W., reactions of Hy- 
 droides and modification in, 249, 
 
 292, 293. 
 Yerkes, R. M-, definition of tropism 
 (phototaxis, photopathy), 55; 63; 
 reactions of Gonionemus, 164; 214; 
 251; classification of reactions to 
 light, 255, 256; 266i 274i reactions 
 
 of Simocephalus in spectrum, 341- 
 343; 360. 
 
 Zc^ mays, orientation of plumules, 
 
 59-73. 229. 
 Zoeae, orientation in light from 
 two sources, 87; 226; 243; 277i 
 280. 
 
' -'' INDIANA 
 
;;?i{^;.-£^rn>;>o. ;'.•':.:•