MONOGRAPHS ON EXPERIMENTAL BIOLOGY EDITED BY JACQUES LOEB, Rockefeller Institute T. H. MORGAN, Columbia University W. J. V. OSTERHOUT, Harvard University VOLUME I FORCED MOVEMENTS, TROPISMS, AND ANIMAL CONDUCT BY JACQUES LOEB, M.D.,' PH.D., Sc.D. ON EXPERIMENTAL PUBLISHED VOLUME I FORCED MOVEMENTS, TROPISMS, AND ANIMAL CONDUCT By JACQUES LOEB, Rockefeller Institute IN PREPARATION THE CHROMOSOME THEORY OF HEREDITY By T. H. MORGAN, Columbia University INBREEDING AND OUTBREEDING: THEIR GENETIC AND SOCIOLOGICAL SIGNIFICANCE By E. M. EAST and D. F.JONES, Bussey Institution, Harvard University PURE LINE INHERITANCE By H. S. JENNINGS, Johns Hopkins University THE EXPERIMENTAL MODIFICATION OF THE PROCESS OF INHERITANCE By R. PEARL, Johns Hopkins University LOCALIZATION OF MORPHOGENETIC SUBSTANCES IN THE EGG By E. G. CONKLIN, Princeton University TISSUE CULTURE By R. G. HARRISON, Yale University PERMEABILITY AND ELECTRICAL CONDUCTIVITY OF LIVING TISSUE By W. J. V. OSTERHOUT, Harvard University THE EQUILIBRIUM BETWEEN ACIDS AND BASES IN ORGANISM AND ENVIRONMENT By L. J. HENDERSON, Harvard University CHEMICAL BASIS OF GROWTH By T. B. ROBERTSON, University of Toronto THE ELEMENTARY NERVOUS SYSTEM By G. H. PARKER, Harvard University COORDINATION IN LOCOMOTION By A. R. MOORE, Rutgers College OTHERS WILL FOLLOW MONOGRAPHS ON EXPERIMENTAL BIOLOGY FORCED MOVEMENTS, TROPISMS, AND ANIMAL CONDUCT BY JACQUES LOEB, M.D., PH.D., Sc.D. MEMBER OF THE ROCKEFELLER INSTITUTE FOR MEDICAL RESEARCH PHILADELPHIA AND LONDON J. B. LIPPINCOTT COMPANY LIBRARY COPYRIGHT, IQl8, BY J. B. LIPPINCOTT COMPANY Eleetrotyped and Printed by J. B. Lippincott Company The Washington Square Press, Philadelphia, U.S.A. EDITORS' ANNOUNCEMENT THE rapidly increasing specialization makes it im- possible for one author to cover satisfactorily the whole field of modern Biology. This situation, which exists in all the sciences, has induced English authors to issue series of monographs in Biochemistry, Physiology, and Physics. A number of American biologists have decided to provide the same opportunity for the study of Experimental Biology. Biology, which not long ago was purely descriptive and speculative, has begun to adopt the methods of the exact sciences, recognizing that for permanent progress not only experiments are required but that the experi- ments should be of a quantitative character. It will be the purpose of this series of monographs to emphasize and further as much as possible this development of Biology. Experimental Biology and General Physiology are one and the same science, by method as well as by contents, since both aim at explaining life from the physico-chemical constitution of living matter. The series of monographs on Experimental Biology will therefore include the field of traditional General Physiology. JACQUES LOEB, T. H. MOKGABT, W. J. V. OSTERHOUT. 676386 AUTHOR'S PREFACE ANIMAL conduct is known to many through the roman- tic tales of popularizers, through the descriptive work of biological observers, or through the attempts of vital- ists to show the inadequacy of physical laws for the explanation of life. Since none of these contributions are based upon quantitative experiments, they have led only to speculations, which are generally of an anthropo- morphic or of a purely verbalistic character. It is the aim of this monograph to show that the subject of animal conduct can be treated by the quantitative methods of the physicist, and that these methods lead to the forced movement or tropism theory of animal conduct, which was proposed by the writer thirty years ago, but which has only recently been carried to some degree of com- pletion. Many of the statements, especially those con- tained in the first four chapters of the book, are familiar to those who have read the writer's former publications, but so much progress has been made in the last few years that a new and full presentation of the subject seemed desirable. Chapters V to XIII and Chapter XVI are partly or entirely based on new experiments. Only that part of the literature has been considered which contributes to or prepares the way for quantitative experiments. 8 AUTHOR'S PEEFACE The writer is under obligation for valuable criticism to his wife, to Professor T. H. Morgan, and to Lieutenant Leonard B. Loeb, who were kind enough to* read the manuscript. J. L. The Rockefeller Institute for Medical Research, New York. March, 1918. CONTENTS / I. INTRODUCTION 13 v II. THE SYMMETRY RELATIONS OF THE ANIMAL BODY AS THE START- ING POINT FOR THE THEORY OF ANIMAL CONDUCT 19 III. FORCED MOVEMENTS 24 IV. GALVANOTROPISM 32 i/' V. HELIOTROPISM. THE INFLUENCE OF ONE SOURCE OF LIGHT... 47 1. General Facts 47 2. Direct Proof of the Muscle Tension Theory of Heliotrop- ism in Motile Animals 52 3. Heliotropism of Unicellular Organisms 62 4. Heliotropism of Sessile Animals 63 VI. AN ARTIFICIAL HELIOTROPIC MACHINE 68 VII. ASYMMETRICAL ANIMALS 70 VIII. Two SOURCES OF LIGHT OF DIFFERENT INTENSITY 75 IX. THE VALIDITY OF THE BUNSEN-ROSCOE LAW FOR THE HELIO- TROPIC REACTIONS OF ANIMALS AND PLANTS 83 X. THE EFFECT OF RAPID CHANGES IN INTENSITY OF LIGHT 95 XI. THE RELATIVE HELIOTROPIC EFFICIENCY OF LIGHT OF DIFFER- ENT WAVE LENGTHS 100 XII. CHANGE IN THE SENSE OF HELIOTROPISM 112 XIII. GEOTROPISM 119 XIV. FORCED MOVEMENTS CAUSED BY MOVING RETINA IMAGES: RHEOTROPISM : ANEMOTROPISM 127 XV. STEREOTROPISM 134 XVI. CHEMOTROPISM 139 XVII. THERMOTROPISM 155 XVIII. INSTINCTS 156 XIX. MEMORY IMAGES AND TROPISMS 164 LITERATURE 173 9 ILLUSTRATIONS FIG. PAGE 1. Forced Position of Larva of the Dragon Fly whose Left Cerebral Ganglion is Destroyed 30 2. Forced Position of Shrimp when Galvanic Current Goes from Head to Tail 34 3. Forced Position of Shrimp when Positive Current Goes from Tail to Head 35 4. Position of Legs of Shrimp when Current Goes Sideways through the Animal 37 5-6. Show Same Effects of Current on the Common Crawfish as Those on Shrimp in Figs. 2 and 3 38 7. Diagram Indicating the Orientation of the Neurons for Flexor and Extensor Muscles of the Right and Left Legs 39 8-9. Diagram Indicating Orientation of Neurons for Flexor and Extensor Muscles of Third and Fifth Pairs of Legs 40 10. Forced Position of Amblystoma Larva Under Influence of Galvanic Current Going Through Animal from Head to Tail. . . . 41 11. Forced Position of Amblystoma Larva When Current Goes from Tail to Head 41 12. Tentacles and Manubrium of Jellyfish Under Influence of Galvanic Current ; 42 13. Strip of Jellyfish Under Influence of Galvanic Current 42 14. Paramcedum Under Normal Conditions 43 15. Current Going Through Paramoecium 44 16. Showing that Positively Heliotropic Animals Will Move from Sun- light into Shade if Illumination of Both Eyes Remains the Same.. . . 50 17. Position of Water Scorpion When Right Eye is Towards the Light . . 53 18. Positions of Ranatra When Light is in Front and Behind Animal .... 54 19. Robber Fly Under Normal Conditions 55 20. Robber Fly with Right Eye Blackened 56 2 1 . Position of Robber Fly when Lower Halves of Both Eyes are Blackened 57 22. Position of Robber Fly when Upper Halves of Both Eyes are Blackened 58 11 12 ILLUSTRATIONS FIG. PAGE 23. Diagram Showing Position of the Flagellum as Seen in a Viscid Medium 62 24. Tube Worms in Aquarium 63 25. Same Animals After Position of Aquarium was Reversed 64 26. Polyps of Eudendrium all Growing Towards Source of Light 66 27. Fly with Right Eye Blackened 72 28. Diagram of Apparatus Used to Produce Differential Bilateral Light Stimulation 76 29. Diagram to Show Method of Measuring Trails 77 30. Diagram for Constructing Direction of Motion of Larvae 80 31. Method for Proving Validity of Bunson-Roscoe Law 90 32. A Glass Plate 91 33. Difference in Gathering Places of Animals 96 34. Method of Determining the Relative Heliotropic Efficiency of Two Different Parts of the Spectrum . . .• 107 35. Geotropic Curvature of Stems of Bryophyllum calycinum 120 36. All Stems were Originally Straight and Suspended Horizontally 121 37. When the Size of the Leaf is Reduced by Cutting Out Pieces from the Middle 120 38. Effect of Cutting off Lateral Parts of the Leaves 121 39. Influence of Motion of the Hand on a Swarm of Sticklebacks in an Aquarium 132 40. The Regenerating Polyp of Tubularia in Contact with Glass Wall of Aquarium 137 41. Reactions of Chilononas to a Drop of -fa per cent. HC1 145 42. Method of Proving the Paramcecia are not Positive to Acid of Low Concentration.. . 146 FORCED MOVEMENTS, TROPISMS, AND ANIMAL CONDUCT CHAPTER I INTRODUCTION THE analysis of the mechanism of voluntary and instinctive actions of animals which we propose to under- take in this volume is based on the assumption that all these motions are determined by internal or external forces. Our task is facilitated by the fact that the over- whelming majority of organisms have a bilaterally sym- metrical structure, i.e., their body is like our own, divided / into a right and left half. The significance of this symmetrical structure lies in the fact that the morphological plane of symmetry of an animal is also its plane of symmetry in physiological or dynamical respect, inasmuch as under normal con- ditions the tension in symmetrical muscles is the same, and inasmuch as the chemical constitution and the velocity of chemical reactions are the same for symmetrical ele- ments of the surface of the body, e.g., the sense organs. Normally the processes inducing locomotion are equal in both halves of the central nervous system, and the ten- sion of the symmetrical muscles being equal, the animal moves in as straight a line as the imperfections of its 13 14 TEOPISMS locomotor apparatus permit. If, however, the velocity of chemical reactions in one side of the body, e.g., in one eye of an insect, is increased, the physiological symmetry of both sides of the brain and as a consequence the equality ,pf tension of the symmetrical muscles no longer exist. The muscles connected with the more strongly illuminated eye are thrown into a stronger tension,a and if now impulses for locomotion originate in the~central nervous system, they will no longer produce an equal response in the symmetrical muscles, but a stronger one in the muscles turning the head and body of the animal to the source of light. The animal will thus be compelled to change the direction of its motion and to turn to the source of light. As soon as the plane of symmetry goes through the source of light, both eyes receive again equal illumination, the tension (or tonus) of symmetrical muscles becomes equal again, and the impulses for locomotion will now produce equal activity in the symmetrical muscles. As a conse- quence, the animal will move in a straight line to the source of light until some other asymmetrical disturbance once more changes the direction of motion. What has been stated for light holds true also if light is replaced by any other form of energy. Motions caused by light or other agencies appear to the layman as expres- sions of will and purpose on the part of the animal, whereas in reality the animal is forced to go where carried by its legs. For the conduct of animals consists of forced movements. The term forced movements is borrowed from brain physiology, where it designates the fact that certain ani- mals are no longer able to move in a straight line when a We are speaking of positively heliotropic animals exposed to only one source of light. INTRODUCTION 15 certain parts of the brain are injured, but are compelled to deviate constantly toward one side, which is (accord- ing to the species and the location of the injury in the brain) either the side of the injury or the opposite side. The explanation of these forced movements is that on account of the one-sided injury of the brain the tension of the symmetrical muscles is no longer the same. As a consequence, the impulses1 for locomotion which are equal for symmetrical muscles will cause greater contraction certain muscles of one side of the body than in the symmetrical muscles of the other side, and the animal will no longer move in a straight line.! The only difference between the forced movements induced by unequal illu- mination of the two eyes and by injury to the brain is that in the latter case the forced movements may last for days or throughout the whole life, while in the former case they last only as long as the illumination on the two sides of the body is unequal. If we bring about a per- manent difference in illumination in the eyes, e.g., by blackening one eye in certain insects, we can also bring about permanent circus motions. This shows that animal conduct may be justly designated as consisting of forced movements. The idea that the morphological and physiological symmetry conditions in an animal are the key to the understanding of animal conduct demanded that the same principle should explain the conduct of plants, since plants also possess a symmetrical structure. The writer was able to show that sessile animals behave toward light exactly as do sessile plants ; and motile animals like motile plants. The forced orientations of plants by outside sources of energy had been called tropisms; and the theory of animal conduct based on the symmetrical struc- 16 TROPISMS ture of their body was, therefore, designated as the tropism theory of animal conduct. We started with symmetrical animals since in their case the analysis jof conduct is comparatively simple ; the results obtained in the study of these symmetrical organisms allow us also to understand the conduct of asymmetrical animals. We shall see that the principles underlying their conduct are the same as in the case of symmetrical animals, the asymmetry of the body altering only the geometrical character of the path in which the animal is compelled to move, not, however, the mechanism of conduct. While a perfectly symmetrical organism, possessed of positive heliotropism, moves in a straight line to the source of light, the path deviates from the straight line in the case of an asymmetrical organism and may in some cases, as, e.g., in Euglena, be a spiral around the straight line as an axis. Some authors have tried to use asymmetrical organisms as a starting point for the analysis of conduct, but since it is impossible to understand the conduct of the asymmetrical organisms unless it is based upon that of the symmetrical animals, these authors have been led to anthropomorphic specula- tions, such as " selection of random movements " which, as far as the writer can see, cannot even be expressed in the language of the physicist. Although the tropism theory of animal conduct was • offered thirty years ago 285> 286> 287 its acceptance was delayed by various circumstances. In the first place, the majority of the older generation of biologists did not realize that not only the methods of the physicist are needed but also the physicist's general viewpointpon- cerning the nature of scientific e'$MiiaHoiLs*^irmany cases the problem of animal conduct is treated in a way INTRODUCTION 17 which corresponds more to the viewpoint of the^ intro- spective psychologist than to that of the physicist. The attempts to explain animal conduct in terms of " trial and error " or of vague "physiological states'7 may serve as examples. None of these attempts have led or can lead to any exact quantitative experiments in the sense of the physicist. Other biologists have still more openly adopted an anthropomorphic method of explana- tion. If pleasure and pain or curiosity play a role in human conduct, why should it be otherwise in animal conduct! The answer to this objection is that typical forced movements when produced in human beings, as, J e.g., in Meniere 's disease or when a galvanic current goes through the brain, are not accompanied by sensations of pleasure or pain, and there is no reason to attribute the circus movements of an animal, after lesion of the brain or when one eye is blackened, to curiosity or thrills of delight. An equally forcible answer lies in the fact that plants show the same tropisms as animals, and it seems somewhat arbitrary to assume that the bending of a plant to the window or the motion of swarmspores of algae to the window side of a vessel are accompanied or deter- mined by curiosity or by sensations of joy or satisfaction. 'And finally, since we know nothing of the sentiments and sensations of lower animals, and are still less able to meas- ure them, there is at present no place for them in science. The second difficulty was created by the fact that the Aristotelian viewpoint still prevails to some extent in biology, namely, that an animal moves only for a pur- pose, either to seek food or to seek its mate or to under- take something else connected with the preservation of 2 18 TROPISMS the individual or the race.5 The Aristotelians had ex- plained the processes in the inanimate world in the same teleological way. Science began when Galileo overthrew this Aristotelian mode of thought and introduced the method of quantitative experiments which leads to mathe- matical laws free from the metaphysical conception of purpose. The analysis of animal conduct only becomes scientific in so far as it drops the question of purpose and reduces the reactions of animals to quantitative laws. This has been attempted by the tropism theory of ani- mal conduct. i> Tills view is still held, especially among authors, who lean more or less openly to vitalism, e.g., v. Uexkiill, Jordan, Franz, Bauer, Budden- brock, and others. CHAPTER II THE SYMMETEY EELATIONS OF THE ANIMAL BODY AS THE STARTING POINT FOE THE THEOEY OF ANIMAL CONDUCT THE starting point for a scientific and quantitative analysis of animal conduct is the symmetry relations of the animal body. (Jhe existence of these symmetry rela- tions reduces the analysis to a comparatively simple problem. ; Organisms show(two forms of symmetry, radial sym- metry, for which jellyfish and the stems and roots of most plants Vffer a well-known example, and lateral sym- metry, such as exists in man and most animalsyln radial symmetry the(peripheral elements are distributed equally about an axis of symmetry^ in the case of lateral sym- metry the ^peripheral elements are distributed equally to the right and left of the plane of symmetry^ (or the median plane) by which the body is divided into a right and left half. The importance of this symmetrical struc- ture lies in the fact that the morphological plane of sym- metry is also the dynamical plane of symmetry of the organism. ^Symmetrical spots of the surface of an animal are chemically identical, having the same chemical con- stitution and also the same quantity of reacting masses. ) Thus the two eyes are symmetrical organs, each contain- ing the same photochemical substances in equal quantity. In the eye itself each element is to be considered as chemically identical with the symmetrical point in the other eye. Hence, if the two eyes are illuminated equally, 19 20 TEOPISMS the photochemical reaction products produced in the same time will be equal in both eyes. What is true for the eyes is true for all symmetrical elements of the surface of an animal. The median plane is also the plane of symmetry for the muscles and the muscular activity of the body. Sym- metrical muscles possess under equal conditions equal tension and symmetrical muscles are antagonistic to each other in regard to moving the body to the right or left. We say that impulses go from the central nervous system to the muscles ; and from the surface of the body to the central nervous system. According to our present knowledge that which is called a/nervous_ impulse seems to consist of a wave of chemical reaction traveling along a nerve fiber.^ The central nervous system is also sym- metrical and, moreover, we may conceive a projection of the elements of the surface of the body upon the ganglion cells and from here tt) the muscular system of the body. The complications in this system of projections consti- tute the difficulties in our understanding of the structure of the brain. This idea of a projection of the sense organs or the surface of the body upon the brain will explain why the morphological plane of symmetry of an organism is also its plane of symmetry in a dynamical sense. When symmetrical elements of the eyes are struck by light of the same wave length and intensity, the velocity of photo- chemical reactions will be the same in both eyes. Sym- metrical spots of the retina are connected with symmetri- cal elements in the brain and these in turn are connected with symmetrical muscles. As a consequence of the equal photochemical reactions in the symmetrical spots of the retina equal changes are produced in the symmetrical brain cells with which the are connected, and SYMMETRY RELATIONS 21 changes in tension will be produced in the symmetrical muscles on both sides of the body with which the active brain elements are connected.a On account of the sym- metrical character of all the changes no deviation from the original direction of motion will occur. If, however,s~ one eye is illuminated more than the other eye, the influ- ence upon the tension of symmetrical muscles will no longer be the same and the animal will be forced to deviate from the original direction of motion. We have thus far considered only the relation between right and left. Aside from this symmetry relation we il have polarity relations, between apex or head and base ' or tail end. Just as we found that the morphological plane of symmetry is also a dynamical plane of symmetry, we find that with the morphological polarity head-tail is connected a dynamic polarity of motion, namely, forward and backward. This will become clear in the next chapter on forced movements. Physiologists have long been in the habit of studying not the reactions of the whole organism but the reactions of isolated segments (the so-called reflexes). While it may seem justifiable to construct the reactions of the a Physiologists assume that stimulations are constantly sent from the brain to the muscles and that this maintains their tension, v. Uexkiill uses the term that "tonus " is sent out to the muscle and that the brain is a reservoir of " tonus " as if the latter were a liquid. The writer wonders whether it might not be wiser to substitute for such metaphors hypotheses in terms of chemical mass action. -'Constant illumination causes a sta- tionary process in photosensitive elements of our eye, in which the mass of the reaction product is determined by the Bunsen-Roscoe law. We assume, moreover, that in proportion to this photochemical mass action correspond- ing chemical reactions take place in the brain elements with which the eyes are connected; and that as a consequence corresponding chemical reactions take place in the muscles by which the tension of the latter is determined. These processes in the muscles may possibly consist in the establishment of a definite hydrogen ion concentration. Such hypotheses! have the advantage over the " stimulation " hypothesis that they can be tested by physico- chemical measurements. 22 TROPISMS organism as a whole from the individual reflexes, such an attempt is in reality doomed to failure, since reactions produced in an isolated element cannot be counted upon to occur when the same element is part of the whole, on account of the mutual inhibitions which the different parts of the organism produce upon each other when in organic connection13; and^t is, therefore, impossible to express the conduct of a whole animal as the algebraic sum of the reflexes of its isolated segments/) E. P. Lyon320 has shown that if the tail in a normal shark be bent to one side (without changing the position of the head) the eyes of the animal move as promptly as compass needles in association with the bent tail around the same axis in which the bending occurs, but in an opposite sense. On the convex side of the animal, the white of the eye is more visible in front, on the concave side it is more visible behind ; hence the former has moved backward, the latter forward. This was observed not only in the normal fish but also when the optic and audi- tory nerves were cut. (The central nervous system acts as one unit.) R. Magnus 332 and his fellow-workers have shown that an alteration in the position of the head of a dog inevitably alters the tone of the muscles of the legs.c These and other associations and mutual inhibitions make possible that simplification which allows us to treat the b When the stem of a plant (e.g., Bryophyllum] is cut into as many pieces as there are nodes, each node will under the proper conditions give rise to one or two shoots. If we leave them in connection, only the buds at the apical end will grow out, the rest of the buds remaining dormant. The whole stem acts as though it consisted of only the bud situated at the apex. c The problem of coordination will form the subject of another volume in this series by Professor A. R. Moore, and for this reason a fuller discussion of work on coordination, such as that by Sherrington and by v. Uexkull, may be reserved for Professor Moore's volume. SYMMETRY RELATIONS 23 organism as a whole as a mere symmetry machine, a simplification which forms the foundation of the tropism theory of animal conduct. It would, therefore, be a misconception to speak of tropisms as of reflexes, since tropisms are reactions of the organism as a whole, while reflexes are reactions of isolated segments. Reflexes and tropisms agree, how- ever, in one respect, inasmuch as both are obviously of a purely physico-chemical character. CHAPTER III FOKCED MOVEMENTS (WHEN we destroy or injure the brain on one side we paralyze or weaken the muscles connected with this side.N CA.S a consequence the morphological plane of symmetry ceases to be the dynamical plane of symmetry) and the animal has a tendency to move in circles instead of in a straight line. Suppose a fish swimming forward by motions of its tail fin. Normally the stroke occurs with equal energy to the right and to the left, and the rudder action of the tail is equal in both directions, but after the lesion of one side of the brain the stroke and the rudder action cease to be the same in both directions, it is weakened in one direction. Hence the animal instead of swimming in a straight line is forced to deviate contin- ually toward one side from the straight line of locomo- tion. We speak in such a case of a forced motion. (When we destroy the ventral portion of the left optic lobe in a shark (Scyllium, canicula), the fish no longer swims in straight lines but in circles to the right (when the right optic lobe is destroyed it swims in circles to the left).) After the destruction of the left optic lobe, the muscles on the left side of the tail are weakened or semi- paralyzed) and they no longer produce the same rudder action as the muscles on the right side. Hence the im- pulses (or nerve processes) which flow in equal intensity to the muscles on both sides will no longer produce equally energetic rudder action of the tail to the right and to the left, but the muscles turning the tail to the right will 24 FORCED MOVEMENTS 25 contract more powerfully than those turning it in the opposite direction. The outcome of this greater rudder action of the tail when moving to the right is that the fish instead of swimming in a straight line moves in a circle to the right.290 It is often the case that the body of such a fish even when quiet is no longer straight but bent in a circle, the left side forming the convex side ; and when such a fish dies and rigor mortis sets in it may become stiff in this position. These latter observations (prove that the (circus movements to the right are due to the lowering of the tension of the lateral muscles of the body on the left side of the fish.^ This is the fundamental fact for the theory of forced movements — namely, that a lesion in one side ^ of the brain lessens the tension of the muscles on one side /\ of the body ; as a consequence the motions of the animal become difficult or impossible in one direction and become—- easy in the opposite direction. In many cases the motions of an animal depend upon . a cooperative activity of two sets of appendages, e.g.,* Y~£ the pectoral fins of a fish or the legs of an animal. Such cooperative or associated action is determined by the fact that the same nerve center supplies antagonistic muscles of the two organs (e.g., the lateral fins). Thus the same nerve impulse causes both our eyes to move simultaneously to the right or to the left. When we look to the right, the same impulse which causes the contrac- tion of the rectus externus muscle in the right eye causes a contraction of the rectus internus muscle in the left eye. These two muscles then are associated. In a fish like the shark the position and innervation of the eyes differ from that of the human being. In the shark the eyes are not in front but on the side, and the 26 TROPISMS muscles which lift the eye on one side are associated with those which lower it on the other side of the head. A similar association exists in regard to the pectoral fins, the muscles which lift the right pectoral fin are associated with those which lower the left one, and vice versa. When a normal shark swims the two pectoral fins work equally and the fish swims without rolling over to the right or to the left. If wre destroy in a shark the left side of the medulla oblongata forced changes in the position of the two eyes and the two pectoral fins will follow.290 (There are in addition correlated changes in the other fins and the rest of the body which we will omit in order to simplify the presentation of the subject.) When a shark, whose left medulla is cut, is kept in a horizontal position, its left eye looks down and the right eye looks up. This change of position of both eyes indicates that the relative tension between the muscles of the eyes has changed. In the left eye the tension of the lowering muscles predominates over that of their antagonists, in the right eye the reverse is the case. The pectoral fins likewise show associated changes of position. The left fin is raised up dorsally, the right is bent down ventrally. Since we know that the destruction of the central nervous system causes a paralysis of muscles and not the reverse we must con- clude that the(9estruction of the left side of the medulla in a shark causes a weakening or partial paralysis of the muscles which lower the left fin and of those which raise the right fin. Hence the muscles which press down on the water will press harder in the right than in the left fin. When such an animal swims rapidly, it will come under the influence of a couple of forces which must produce a rolling movement around the longitudinal axis of its FORCED MOVEMENTS 27 body toward the left. These rolling motions are another well-known type of forced movements. When such an animal swims slowly, it will roll more than a normal fish, but it will not roll completely around its longitudinal axis. These are the same motions which were observed in dogs by Magendie and Flourens155 after an operation in the medulla or pons.. We can state, ^heref ore, that the rolling motions are caused by the weakening of one group of (associated) muscles while their antagonists are not weakened. ) It is of interest to consider the nature of forced move- ments after injury of the cerebral hemispheres in a dog. When in a dog one of the cerebral hemispheres is injured the animal immediately after the operation no longer moves in a perfectly straight line, but deviates from the straight line toward that side where the brain is in- jured.178 When the left hemisphere is injured circus motions toward the left ensue. If one offers a dog which was operated in the left cerebral hemisphere a piece of meat, removing it as fast as the dog approaches, the dog will move at first a certain distance in a straight line ; it will then suddenly turn to the left and describe a com- plete circle, moving afterward for a little while in a straight line toward the meat and turning again through an angle of 360° to the left, and so on.284 The explanation is the same as for the foregoing cases. The lesion of the left cerebral hemisphere caused a weakening or partial paralysis of the muscles which turn the body to the right. Hence the animal will, when following the meat, deviate to the left, and this causes a displacement of the retina image in the same direction and an apparent motion of the object to the right. We shall see in a later chapter on 28 TROPISMS the orienting effect of moving retina images that this deviation of the retina image to the left causes a forced motion of the animal to the right which compensates its tendency to deviate to the left due to the effect of the brain lesion. Hence the animal approaches the meat in an approximately straight line. But it does so with diffi- culty and sooner or later tiring of this effort it moves in the usual automatic way, whereby equal impulses reach the muscles on both sides. This results in a complete circus movement to the left on account of the weakening (caused by the operation) of muscles which turn the body to the right. The retina image of the meat again induces a straight motion and the whole process described is repeated. When the injury to the brain was less severe the animal may follow the meat for long distances without turning in a circle. When such a dog is offered simultaneously two pieces of meat, one in front of the left, the other in front of the right eye, it invariably moves toward the one on the left side. The equal flow of impulses caused by the sym- metrically located pieces of meat results in a stronger contraction in the muscles on the left than on the right side of the body, since as a consequence of the lesion the tension of the former muscles is greater than that of the latter. When two pieces of meat are simultaneously offered to the dog, but both pieces are in front of the left eye, the dog tries to get the piece nearest to its mouth, but its effort carries it a little too far to the left and then it takes the other piece of meat which is situated farther to the left.284 Some time after the operation these disturbances may become less and may ultimately disappear. If now the FOECED MOVEMENTS 29 dog is operated on the other, e.g., the right hemisphere, circus motions to the right appear. We do not wish to exhaust the chapter on forced move- ments but may perhaps for the sake of completeness point out the following facts. We have seen that if one cerebral hemisphere is injured the dog shows a tendency to circus movements to the operated side. When both hemispheres are injured, e.g., both occipital lobes are removed, the dog can hardly be induced to move forward and it is impos- sible to cause it to go downstairs, while it is willing to go upstairs. Its front legs are extended and its head is raised high, giving the impression as if such a dog had a tendency to move backward rather than forward or that the forward movement was difficult. If the two anterior halves of the cerebral hemispheres are removed the re- verse happens. The animal runs incessantly as if driven by a mad impulse ; its head is bent down and it is in every respect the converse of the animal operated in the occipi- tal lobes. These two types of forced movements corre- spond to the morphological polarity tail-head. This corresponds to the idea of a projection of the surface elements upon the brain either directly or by crossing. These three types of forced movements: the circus motions, the tendency to go backward, and the irresistible tendency to move forward will appear in the form of the tropistic reactions to be described in this volume. Since we shall deal in this volume chiefly with inverte- brates, it may be of importance to show that forced move- ments can also be produced in this group of animals by lesion of one side of the cerebral ganglion, and that these forced movements depend also upon the fact that as a consequence of the operation the tension of sym- metrical muscles (which is equal under normal condi- 30 TBOPISMS tions) becomes unequal. Fig. 1,5, gives the change in posi- tion of the body and of the legs in the larva of a dragon fly (^Eschna) after the left half of the cerebral ganglion has been destroyed (Matula 541). Such an animal moves in a circle to the right. The longitudinal muscles con- necting the segments of the body are under higher tension on the right side of the body than on the left and the body FIG. 1. — B, forced position of larva of the dragon fly (dZschna) whose left cerebral ganglion is destroyed. The body is convex on the left side, due to a relaxation of the muscles connecting the segments on the left side. The position of the legs is such that the animal can only move in circles to the right. This asymmetry disappears again when both ganglia are destroyed, C. A, normal animal. (After Matula.) is bent with its convex side to the left. The normally symmetrical position of the legs (Fig. 1, A) is now changed in such a way (Fig. 1, B) that the animal is no longer able to move in a straight line, but is forced to move in circles to its right. We shall see later that similar changes in the position of the legs are produced in a posi- tively heliotropic insect when the left eye is blackened and in a negatively heliotropic insect when the right eye FOECED MOVEMENTS 31 is blackened. Circus motions after destruction of one cephalic ganglion in an insect are a general occurrence and have been known for a long time. The importance of these forced movements caused by lesion of the brain for the explanation of the conduct of normal animals lies in the fact that the latter is essen- tially a series of forced movements. The main difference between the forced movements after brain lesion and the conduct of a normal animal lies in the fact that the former are more or less permanent ; while in the normal animal conduct the changes in the relative tone of sym- metrical muscles leading to a temporary forced movement are caused by a difference in the velocity of chemical^ reactions in symmetrical sense organs or other elements of the surface. \ CHAPTER IV GALVANOTBOPISM WHEN we send a galvanic current lengthwise through a nerve, at the region near the anode the irritability of the nerve is diminished, while it is increased near the cathode. The condition of decreased irritability near the anode is called aneiectrotonus and the increased irrita- bility near the cathode is called catelectrotonus. When a current is sent through an animal, those nerve elements which lie in the direction of the current will have an ane- lectrotonic and a catelectrotonic region ; while the nerves through which the current goes at or nearly at right angles are not affected. Ganglia or nerve tracts in the anelectro- tonic condition will, therefore, act as if they were tem- porarily injured, and hence we need not be surprised to find that the galvanic current causes forced movements which last as long as the current lasts, and which cease with the current. Hermann reported in 1885 204 that when a current is sent, through a trough containing tadpoles of a frog, the tadpoles orient themselves in the direction of the current curves putting their heads to the anode.a Blasius and Schweizer 523 found soon afterwards that a large number of animals when put into a trough with water through which a galvanic current passes have a tendency to go to the anode. The explanation given by Hermann and by Blasius and Schweizer is not correct. They a The writer has never been able to repeat this observation. 32 GALVANOTROPISM 33 assumed that the current, acting upon the central nervous system, causes sensations of pain when it goes in the direction from tail to head in the animal; while it has a soothing or hypnotizing effect when it goes in the opposite direction, namely from head to the tail. In the latter case the head is directed toward the anode. The authors assume that the animals choose the position with least pain, i.e., with their heads to the anode. This assumption is wrong, since we know that when a galvanic current is sent through the head of a human being automatic motions comparable to those observed in animals occur which are not voluntary and which are unaccompanied by any pain sensation. Thus when a galvanic current is sent laterally through the head, the person falls toward the anode side but has no feeling of pain. Mach noticed the same effect of falling to the side of the anode when a galvanic current was sent sidewise through fishes.830 /These galvanotropic motions are in reality forced move- ments/and this has been proved by direct observations. It was shown by/Loeb/and Maxwell 307/in experiments on crustaceans/and by Loeb and Garrey 306 on salaman- aersihat when we send a galvanic current through ani- mals which go to the anode, changes in the position of the legs occur comparable to the changes in the position of fins and eyes mentioned in the previous chapter, and that these changes are of such a character as to. make it easy for the animal to move in the direction of the anode and difficult if not impossible to move in the opposite direction. In all these experiments it is of importance to choose the proper density of the current. For the experiments on the shrimp (Palamonetes)™1 the animals were put into a 3 34 TBOPISMS square trough, two opposite sides of which were formed of platinum electrodes. The cross section of the fresh water in the trough was 1,400 mm.2 and the intensity of the current about 1 milliarnpere or a little less. We found it advisable to increase the intensity very gradually by increasing slowly the resistance of a rheostat in* a short circuit until the phenomenon of galvanotropism appeared most strikingly. When the current is too strong or too weak the phenomena are no longer clear. The com- mon shrimp (Palcemonetes) is a marine crustacean which FIG. 2. — Forced position of shrimp (Palcemonetes) when galvanic current goes from head to tail. Tension of extensor muscles of tail fin prevails over that of flexors. Animal can swim forward (to anode), but not backward. (After Loeb and Maxwell.) lives also in brackish water and which 'can stand fresh water long enough for the purpose of these experiments. The animal can swim forward as well as backward; in forward swimming the extensor muscles of its tail fin work more strongly than the flexors (Fig. 2) ; in swim- ming backward the flexors work energetically (Fig. 3) and thus produce a powerful stroke forward, while the ex- tensors contract with less energy. When we put a Palce- monetes in a trough through which a current goes and if we put the animal with its head toward the anode the tail is stretched out (Fig. 2). This means that the tension of the extensor muscles prevails over that of the flexors and since the forward swimming is due to the stroke of GALVANOTROPISM 35 the extensors, and since it is antagonized by the tension of the flexors, the animal can swim forward but not backward, or only with difficulty; if we put the animal with its head toward the cathode the tail is bent ventrally (Fig. 3), which means that the tension of the flexors is stronger than that of the extensors. As a consequence the animal can swim backward but not forward, or only FIG. 3. — Forced position of shrimp when positive current goes from tail to head. Tension of flexors of tail fin prevails over that of extensors. Animal can swim backward (to anode), but not forward. (After Loeb and Maxwell.) with difficulty. In both cases the result will be a swim- ming of the animal to the anode, in the former case by swimming forward in the latter by swimming backward. We can further show that the tension of the muscles of the legs of Palcemonetes is always altered in such a sense by the galvanic current that motion toward the anode is facilitated, while that toward the cathode is rendered difficult or impossible. The animal uses the third, fourth, and fifth pair of legs for its locomotion (Fig. 2). The third pair pulls in the forward movement 36 TKOPISMS and the fifth pair pushes. The fourth pair acts like the fifth and requires no special discussion. If a current be sent through the animal longitudinally from head to tail and the intensity be increased gradually, a change soon takes place in the position of the legs. In the third pair the tension of the flexors predominates (Fig. 2), in the fifth the tension of the extensors. The animal can thus move easily by pulling of the third and by pushing of the fifth pair of legs, that is to say, the current changes the tension of the muscles in such a way that the forward motion is facilitated, while the backward motion is ren- dered difficult. Hence it can easily go toward the anode but only with difficulty toward the cathode. If a current be sent through the animal in the opposite direction, namely from tail to head, the third pair of legs is extended, the fifth pair bent (Fig. 3) ; i.e., the third pair can push, the fifth pair can pull backward. The animal can thus go backward with ease but forward only with difficulty. This again will lead to a gathering of such animals at the anode, this time, however, by walking backward. The phenomena thus far described recall the forced movements mentioned in the third chapter, where certain injuries of the brain accelerate forward motion while other lesions in the opposite parts of the brain make forward motion difficult if not impossible. Palcemonetes can also walk sidewise. This movement is produced by the pulling of the legs on the side toward which the animal is moving (contraction of the flexors), while the legs of the other side push (contraction of ex- tensors). If a current be sent transversely, say from left to right, through the animal, the legs of the left side assume the flexor position, those of the right side the aALVANOTROPISM 37 extensor position (Fig. 4). The transverse current thus makes it easy for the animal to move toward the left— the anode — and prevents it from moving toward the right — the cathode. If a galvanic current flows transversely Fio. 4. — Position of legs of shrimp when current goes sidewise through the animal, from left to right. In the legs on the left side the tension of the flexors, in those of the right side the tension of the extensors predominates. The animal can easily go to the left (anode), but not to the right. (After Loeb and Maxwell.) through the animal, it creates the analogue of the circus motions produced by injury of one side of the brain. Figs. 5 and 6 show that the current produces similar effects in the crayfish as those produced in the shrimp (Figs. 2 and 3). 38 TEOPISMS It is not difficult to suggest by aid of a diagram the arrangement of the elements in the central nervous system required to bring about the phenomena of galvanotropism just described for Palcemonetes. We take it for granted that the regular phenomena of anelectrotonus and cate- lectrotonus of motor nerve elements suffice for the ex- planation of these phenomena. We assume that if the cell body of a neuron is in the state of catelectrotonus Fia. 5. FIGS. 5 and 6. — Show the same effects of current on the common crayfish as those on Palcemonetes in Figs. 2 and 3. its activity is increased, when it. is in anelectrotonic con- dition activity is diminished. Neurons having the same orientation will always be affected in the same sense by the current. Fig. 7 is a diagram of the arrangement of neurons giving rise to the bending of the legs on the side of the anode and to the extension of the legs on the side of the cathode when the current goes sidewise through the ani- mal. This diagram assumes that the nerves innervating the extensors come from the opposite side of the central GALVANOTROPISM 39 nervous system, while those innervating the flexors are on the same side. This diagram corresponds to reality, ac- cording to the histological work of Allen. When the cur- rent goes from right to left through the crustacean the cell bodies of the neurons on the right side are in catelectro- tonus, those on the left side in anelectrotonus. The for- mer are, therefore, in a state of increased "irritability," the latter in a state of diminished ' ' irritability. ' ' Hence the flexors of the right leg are contracted and the exten- sors relaxed, while the flexors of the left leg are relaxed and the extensors contracted. FIG. 7. — Diagram indicating the orientation of the neurons for flexor and extensor muscles of the right and left legs to explain changes of position of legs under influence of galvanic current. (After Loeb and Maxwell.) Another crustacean Gelasimus307 shows the same effect of the current when it goes sidewise through its body. When the thoracic ganglion from which the nerves of the legs originate is cut longitudinally in the middle, all the legs assume permanently a bent position, confirming our assumption that the extensor nerves cross over while the flexors originate from the same side of the ganglion on which their muscles are. It, therefore, looks as if our diagram were the expression of the actual condition. In the same way we can explain the results of a gal- vanic current when it goes through the animal length- wise. We only need to assume that the cell bodies which send their fibers to the flexors of the third pair of legs 40 TROPISMS have the same orientation as the cell bodies which send their fibers to the extensors of the fifth pair of legs (Fig. 8). Hence when the positive current goes from head to tail through the animal (Fig. 8), the flexors of the third pair of legs and the extensors of the fifth pair must be thrown into greater activity, since the cell bodies of both these nerves are in a condition of catelectrotonus, i.e., increased activity. Fio. 8. Fia. 9. FIGS. 8 and 9.— Diagram indicating orientation of neurons for flexor and extensor muscles of third and fifth pairs of legs to explain galvanotropic reaction. (After Loeb and Maxwell.) When the current goes from tail to head the cell bodies of the extensors of the third and of the flexors of the fifth pair of legs are in catelectrotonus. This possibility is expressed in the diagram Fig. 9. In this way the theory of the galvanotropic reaction of those animals which go to the anode seems complete. What has been demonstrated for Palcemonetes holds not only for many crustaceans but for vertebrates also. Loeb and Garrey 306 have shown that when a current GALVANOTEOPISM 41 is sent through a trough containing larvae of a salamander (Amblystoma) the legs and head of the larvae assume definite positions depending upon the direction of the current. When the current goes from head to tail the legs are pushed backward and the head is bent (Fig. 10) ; FIG. 10. — Forced position of Amblystoma larva under influence of galvanic current going through animal from head to tail. Head down, body convex on dorsal side. Legs backward. (After Loeb and Carrey.) when the current goes from tail to head the opposite position is observed (Fig. 11). The analogy with the observations on Palcemonetes is obvious. Galvanotropic reactions are found throughout the whole animal kingdom and the following observations made by Bancroft on a jellyfish (Poly orchis penicillata) Fia. 11. — Forced position of Amblystoma larva when current goes from tail to head. Head raised, legs pushed forward, tail raised. (After Loeb and Garrey.) are of especial interest.16 Strips containing tentacles and the manubrium were cut out from the animal and put into a trough through which a current flowed of 25 to 0.200 m. a. for 1 square mm. of the cross section of the liquid (dilute sea water) in the trough. 42 TEOPISMS If a meridional strip passing from the edge on one side through the center of the bell to the other edge be prepared and the current passed through transversely, tentacles and manubrium turn and point toward the cathode (Fig. 12). A reversal of the current initiates a turning of these organs in the opposite direction, which is usually com- pleted in a few seconds. This can be repeated many times and the tentacles continue to respond after hours of activity. The manubrium, however, tires sooner and fails to re- spond. If the strip is placed with its subumbrella surface upward and ex- tended in a straight line parallel to the current lines, the making of the current causes the tentacles at the anode end to turn through an angle of 180° and point toward the cathode. The ten- tacles at the cathode end become more crowded together, reminding one of the tip of a moistened paint brush, and also point more directly toward the cathode (Fig. 13). The experiment may be varied in still other ways by cutting smaller or larger pieces from the edge of the swimming bell, but the response is always the same. The tentacles, wherever pos- sible, and to a less extent the manubrium, bend so as to point toward the cathode. The response depends in no way upon the con- nection of these organs with the swimming bell, muscles, or nerve ring, for it is obtained equally well with isolated tentacles and pieces of tentacles. Isolated tentacles when placed transversely to FIG. 12. — Tentacles T and manu- brium M of a jellyfish (Poly orchis) under influence of galvanic current are turned to the negative pole. (After Bancroft.) FIG. 13. — Strip of jellyfish showing that under the influence of galvanic current tentacles on both ends point towards cathode. (After Bancroft.) the current lines curve so as to assume a more or less complete U-shape, with their concave side toward the cathode. When placed parallel to the current, the tentacles do not curve.19 The latter observation shows the fact that the whole reaction is due merely to an increase in the tension of the muscles on the cathode side of the organ. Phenomena of galvanotropism can be observed also in infusorians. Thus Verworn493 observed that when GALVANOTROPISM 43 a current goes through a trough containing Paramcecia the animals will all move toward the cathode. The mech- anism of the reaction was discovered by Ludloff.817 The locomotion of Paramcecium is brought about by cilia. As a rule the cilia are directed backward (Fig. 14), and in their normal movement they strike powerfully backward and are retracted with less energy to their normal posi- tion. Since their powerful stroke is back- ward the animal is pushed forward in the water. Ludloff and Bancroft 17> 18 show that if a Paramcecium is struck sidewise by the current, the position of the cilia on the cathode side is reversed inasmuch as they are now turned forward. On the anode side they continue to be directed backward (Fig. 15, a). Instead of striking symmetrically on both sides of the animal, the cilia on the cathode side strike for- ward powerfully while those on the anode side strike powerfully backward. The animal is thus under the influence of a couple of forces which turn its oral pole toward the cathode side. As soon as it is in this condition the symmetrical cilia are struck at the same angle by the parallel current lines and they must assume a symmetri- cal position which is as in Fig. 15, b, namely the cilia are pointed forward toward the cathode at the oral end, and backward toward the anode at the aboral end. As long as the current is not too strong, the oral region, where the cilia are pointing forward, is rather small and there- fore the action of those cilia prevails which are in the majority and which are pointed backward. As a result the organism moves slowly forward to the cathode. 44 TROPISMS A similar mechanism of galvanotropic conduct exists in Volvox a spherical, unicellular organism which is sur- rounded by cilia on its' whole surface. A definite pole of the organism is always foremost in all locomotions. This organism usually swims to the anode when in a galvanic field. Bancroft made the action of the cilia of Volvox visible with the aid of india ink and was able to show that the current made the cilia on the anode side stop, while those on the cathode side continue to beat.20 FIG. 15. — a, current going from left to right through Paramascium, the position of cilia on the cathode side is now reversed, their free ends pointing forward. The animal when swimming is automatically turned with its oral end toward the cathode. 6, current going through Paramcecium from aboral to oral end. Cilia symmetrical on both sides but pointing forward at oral end and backward at aboral end. Since the backward stroke is always the effective one the organism is thus carried automatically toward the anode. Terry478 found that Volvox can be made to move toward the anode as well as toward the cathode. It moves to the anode after having been kept in the dark for two or three days, while after exposure to light it swims to the cathode. Volvox contains chlorophyll and the change in the sense of reaction is therefore connected with the formation of a product of chlorophyll activity. Bancroft found that when Volvox was made cathodic by exposure to sunlight, the cilia stop on the cathode side. GALVANOTROPISM 45 While the locomotor mechanism of unicellular organ- isms, like Paramcecia and Volvox, is as simple as that of higher organisms, the locomotion of microorganisms possessing only one flagellum, like Euglena, is more com- plicated. It was generally assumed that the flagellum acted like a single oar and that it was directed forward, but this is not correct. It is shaped like a U and its free end is directed backward; and Bancroft has emphasized that it acts by the formation of a loop which moves like a wave from the base of the flagellum to its free tip. The same author discovered that Euglena are galvanotropic when raised in acid media, On account of the asymmetry of their locomotor apparatus they are compelled to swim in a spiral, in most cases to the cathode, exceptionally to the anode. Bancroft showed that the orientation of these \ organisms by the galvanic current is identical with that by light.21 All the phenomena of galvanotropism are, therefore, reduced to changes in the tension of associated muscles or contractile elements, as a consequence of which the motion of the organism toward one pole is facilitated, while the motion toward the opposite pole is rendered difficult. Galvanotropism is, therefore, a form of forced motions produced by the galvanic current instead of by * injury to the brain. There remains then the question of how a galvanic current can bring about those changes which result in the anelectrotonic and catelectrotonic condition mentioned at the beginning. Currents can pass through tissues only] in the form of ions whose progress is blocked by mem- branes which are more permeable for certain salts than for others. Those salts which go through the membrane carry the current through the tissue elements, those 46 TBOPISMS v which do not go through will increase in concentration at the surface of the membrane. It is the latter,, which cause the elect rotonic effects ; according to Lq^b and Budgett 304 by secondary chemical reactions at thp boun- dary. Nernst has pointed out that a stationary;, condition must arise at the surface of the membrane due to the fact that the increase in concentration of ions by the electric current gives rise to a current of diffusion of salt- in the opposite direction away from the membrane. ' ' The aver- age change of concentration at the membrane depends, therefore, upon the antagonistic effects of the current and of the diffusion/'524 'This must be kept in mind since otherwise the effect of the constant current should increase constantly with its duration, which is not the case, on account of the establishment of a condition of equilibrium between the increase of the concentration of ions at the boundary with the duration of the current and the diminution of this concentration by the diffusion of the ions in the opposite direction due to osmotic pressure. CHAPTER V HELIOTROPISM THE INFLUENCE OF ONE SOURCE OF LIGHT 1. GENERAL FACTS THE fact that certain animals go to the light had, of course, been known for hundreds of years, but this was explained in an anthropomorphic way. Thus Lubbock, and Graber,180 had taken it lor granted that certain animals went to the light or away from it on account of fondness for either light or darkness, and their experi- ments were calculated to demonstrate this fondness. Animals were distributed in a box, one-half of which was covered with common window glass, the other with an opaque body or with colored glass, and after a while the number of animals in each half was counted. When the majority of animals were found in the dark part the animals were believed to have a preference for darkness; when in the light part they were believed to be fond of light. The same method was used to decide whether animals preferred blue to red or vice versa. The writer attacked the problem from the physical viewpoint, assum- ing that the animals are "fonoT" neither of light nor of " darkness, " but that they are oriented by the light in a similar way as plants are ; being compelled to bend or — as in the case djf motile algae — move automatically either to a source of light-, or away from it.285' 287 In the case. of unequal illumination of the two eyes the tension of the. symmetrical muscles in an animal becomes 47 48 TEOPISMS unequal. In this condition the equal impulses of locomo- tion will result in an unequal contraction of the muscles on both sides of the animal. As a consequence the animal will turn automatically until its plane of symmetry is in the direction of the rays of light. As soon as this happens the illumination of both eyes and the tension of sym- metrical muscles become equal again and the animal will now move in a straight line — either to or from the source of light. What appeared to the older authors as the expression of fondness for light or for darkness was according to the writer's theory the expression of an influence of light upon the relative tension of symmetrical muscles. ^ Animals which are compelled to turn and move to the source of light the writer called positively heliotropic, those which are compelled to turn and move in the oppo- site direction he called negatively heliotropic. The designation heliotropism (or phototropism) was chosen to indicate that these reactions of animals are of the same nature as the turning of plants to the light; and the writer was indeed able to show that sessile animals bend to the light as do plants which are raised near a win- dow;288 while motile animals move to (or from) a source of light as do the motile swarmspores of algae or motile algae themselves. We will first discuss positively heliotropic motile ani- mals. The positively heliotropic caterpillars of Porthesia chrysorrhcea 288 or the winged plant lice of Cineraria 288 or the newly hatched larvae of the barnacle 183 were used by the writer in his earliest experiments and they may serve as examples. The larvae of Porthesia must be used after hibernation before they have taken food. When about 50 or 100 of such larvae are put into a test tube and HELIOTEOPISM 49 the latter is placed at right angles against a window, all the animals begin to move to the window in as straight a line as the imperfections of their locomotion and col- lisions permit. As soon as they reach the window side of the test tube they remain there permanently, unless the test tube is turned around. If we turn the test tube around an angle of 180° the animals go at once to the window again. They react in this way whether the source of light is sunlight, diffused daylight, or lamp light ; and this can be repeated indefinitely. The animals are slaves of the light. These experiments are typical for posi- tively heliotropic motile animals. When the animals have reached the window end of the test tubes they remain there, since the light prevents them from going back. But in staying there they may assume any kind of orientation, thus proving that the light orients them only as long as they are in motion. The light affects the tension of the muscles and we shall see later that when, the animals are not moving, the change in the tension of the muscles manifests itself by changes in the position of the legs, which is noticeable in organisms with comparatively large appendages. That these animals do not go to the light because they prefer light to darkness but because the light orients them is proved by the fact that they will go from light into the shade if by so doing they remain oriented with their heads toward the source of light.287 Let direct sunlight S fall upon a table through the upper half of a window (W, Fig. 16), the diffused daylight D through the lower half. A test tube ac is placed on the table in such a way that its long axis is at right angles with the plane of the window; and one-half ab is in the direct sunlight, the other half in the shade. If at the beginning of the 4 50 TBOPISM'S experiment the positively heliotropic animals are in the direct sunlight at a, they promptly move toward the win- dow, gathering at the window end c of the tube, although by so doing they go from the sunshine into the shade. This experiment shows also that it is not the intensity Fia. 16. — Showing that positively heliotropic animals will move from sunlight into shade if in so doing the illumination of the two eyes remains the same. gradient of light in the dish which makes positively helio- tropic animals move to the light, but that difference in intensity on both sides of the animal which is caused by the screening effect of the animal's own body. The same holds true for chemotropism. Thus far we have discussed positively heliotropic HELIOTBOPISM 51 animals only. In the case of unequal illumination of the two eyes or of the two sides of the body of a negatively heliotropic animal the tension in the muscles turning the animal to the source of light is diminished. The impulses^ for locomotion which are equal for the muscles of both sides of the body will, therefore, result in turning the head of the animal away from the source of light. As soon as the plane of symmetry of the animal goes again through the source of light, the symmetrical photosen- sitive elements of the head receive again equal illumina- tion, and the animal will now continue to move in a straight line away from the source of light. The fully grown larvae of the housefly when they are ready to pupate show this negative heliotropism. Negatively heliotropic animals, e.g., the fully grown larvae of the blowfly, can be made to move from weak light to strong light, e.g., from the shade into direct sunlight, if in so doing the illumination on the two sides of the body remains equal.287 This was shown by the writer by a'fa arrangement similar in principle to the one de- scribetl above. Thus the idea that the intensity gradient of light determines the direction of motion was disproved also for negatively heliotropic animals. Thus far we have shown only that a heliotropic animal^ is oriented in such a way to a source of light that its plane of symmetry goes through the source of light. This does * not yet explain why a positively heliotropic animal cannot go away from the source of light, since in going to or going away from the source of light both sides of the animal receive equal illumination. The fact that a posi- tively heliotropic animal cannot go away from the light finds its explanation by observations of Holmes 228 and Garrey,177 showing that when light falls from behind 52 TEOPISMS and above on a positively lieliotropic animal its progres- sive motions are stopped, and in some cases a tendency to turn a somersault backward may even arise. The case is similar to that of galvanotropism when the current goes through an animal lengthwise (see previous chapter). We must conclude from the observations of Holmes and Garrey, which will be discussed farther on, that if the mead of a positively heliotropic animal is turned to a /source of light its forward motions are facilitated and the backward motions rendered difficult ; while in the case of a negatively heliotropic animal it is just the reverse. ' If the animal now moves to the right or to the left the illumination of the two eyes or of the two sides of the body becomes different again, causing a forced movement, whereby the plane of symmetry of the moving animal is caused to go through the source of light again ; with the head toward the source of light when the animal is posi- tively heliotropic or away from it when it is negatively heliotropic. 2. DIRECT PROOF OF THE MUSCLE TENSION THEORY OF HELIOTROPISM IN MOTILE ANIMALS The fact that light causes forced movements, like those described in the case of brain lesions and of galvano- tropism, has been proved by many observers, and espe- cially clearly by Holmes and Garrey. Holmes worked on the positively heliotropic water scorpion Ranatra.228 When this animal is illuminated from the right side, the legs on the right side of the body are bent and those on the left side extended (Fig. 17). This effect is identical with the one observed in Palcemonetes, when a galvanic current goes sidewise through the animal. Hence Ranatra HELIOTBOPISM 53 can easily move to the source of light on its right side but with difficulty or not at all in the opposite direction. When the light is placed behind the animal, the body is raised up in front and the head held high in the air (Fig. 18). The opposite attitude is assumed, when the light is placed in front, the body being lowered and the head bent down (Fig. 18). These effects resemble the FIG. 17. — Position of the water scorpion Ranatra when the right eye is toward the light. (After Holmes.) galvanotropic effects observed in the position of the head of Amblystoma when the current goes forward or back- ward through the animal. These latter observations of Holmes explain, as already mentioned, why a positively heliotropic animal cannot move away from the light and why a negatively heliotropic animal cannot move to a source of light. The progressive motions of the negatively heliotropic animal will be stopped when the light strikes it in front ; while 54 TEOPISMS these motions of the positively heliotropic animal will be facilitated when the light is in front and will be rendered impossible when the light is behind. The writer had observed long ago that when the con- vexity of one eye is cut off in the housefly it will no longer go in a straight line but will make circus movements, the normal eye being directed toward the center of the circle.286 It was shown by Parker that blackening of one FIG. 18. — The lower figure represents the position of Ranatra when the light is behind the body. The upper figure represents the position assumed when the light is in front jof the animal. (After Holmes.) eye of the positively heliotropic butterfly Vanessa antiopa calls forth circus movements, with the unblackened eye toward the center of the circle.398 Holmes, Kadi,447 Axenfeld, Garrey, m and many other authors have since made similar observations. In the positively heliotropic Ranatra, Holmes described the effect of blackening one eye as follows : If one eye of Ranatra is blackened over or destroyed the insect in most cases no longer walks in a straight line but performs more or less decided circus movements toward the normal side. Under the stimulus of light the insect assumes a peculiar attitude; the body leans over toward the normal side and the head is tilted over in the same direction.228 HELIOTROPISM 55 This is the combination of circus movements with roll- ing movements familiar to those who have experimented on the brain of fish, where a destruction of one side of the midbrain calls forth rolling motions as well as circus motions toward the same side. Holmes 's observations FIG. 19. — Robber fly under normal conditions seen from above. (After Carrey.) were extended by Garrey's experiments on a large num- ber of insects. Garrey found that the robber fly (Procta- canthus) (Fig. 19), which is positively heliotropic, is an unusually good object for the demonstration that the heliotropic reactions of animals are of the type of forced movements. When one eye of this fly is blackened the legs on the side of the unblackened eye are flexed and the 56 TBOPISMS legs on the side of the blackened eye are more extended than normally and spread farther apart.3- The body may tilt as far toward the side of the unblackened eye as to press the legs to the table (Fig. 20) . There is sometimes a FIG. 20. — Robber fly with right eye blackened, seen from above as in Fig. 19. The body tilts over to the left side so that only the right eye is visible from above. Position of legs changed in such a way as to make motion toward left possible, toward right impossible. (After Garrey.) tendency on the part of the body of the animal to become slightly concave toward the side of the unblackened eye. Garrey found also that the same changes take place when one eye receives a stronger illumination than the a Figs. 19 to 22 and 27 were drawn from photographs kindly given to the writer for this purpose by Professor Garrey. The draughtsman was unfortunately not familiar with the anatomy of insects, which accounts for shortcomings in the drawings, which, however, have no bearing on the prob- lem for which the drawings are intended. HELIOTBOPISM 57 other. Bringing one eye into the bright beam of light directed through the objective of the optical system of the string galvanometer, while the other eye is illuminated only by the subdued light of the optical room, promptly produced the same changes in the position of the legs and body which were observed when one eye was black- ened, the more weakly illuminated eye acting like the blackened eye in the former experiment. When the illu- FIG. 21. — Position of robber fly when the lower halves of both eyes are blackened. Head tilted up. (After Carrey.) mination on one side of such animals is stronger than on the other the legs on the more strongly illuminated side of the animal are bent, those on the opposite side are ex- tended ; and the head has a tendency to bend toward the light. When an impulse to move originates in the animal, it can turn easily to the light but with difficulty in the opposite direction. As soon as its head is turned to the source of light and both eyes receive the same illumination the difference in tension of the legs on the two sides of the body disappears and now the animal moves or is carried in a straight direction toward the light. By these experiments the proof of the writer's muscle tension theory of heliotropism is made complete.177 58 TEOPISMiS Garrey observed that when the lower halves of the eyes of the robber fly are blackened the position of the legs of the two sides is symmetrical, but the anterior and middle pairs of legs are extended forward to the maximal extent, producing a striking posture in which the anterior end of the robber fly is pushed up and back from the sur- face of the table. The body is in opisthotonus with the abdomen concave on the dorsal side, while the head is tilted far up and back (Fig. 21). FIG. 22. — Position of robber fly when upper halves of both eyes are blackened. Head down, body convex above. (After Garrey.) When walking these robber flies gave the impression of trying to climb up into the air. The wings are frequently somewhat spread and the animal may push itself up and back until poised vertically on the tips of the wings and abdomen. The tendency to fly is very pronounced in this condition and upon the slightest disturbance the fly soars upward and backward, striking the top of a confining glass dish or completing a circle by " looping the loop " backward. If it falls upon its back it rights itself by turning a backward somersault. Unequal blackening of the lower parts of the two eyes results in a combination of the effects just described, with those described for blackening one eye, for the animal also performs circus motions. With the upper halves of the eyes blackened the attitude is the reverse of that described in the preceding section (Fig. 22). The an- terior and middle pairs of legs are flexed. The anterior and posterior HELIOTROPISM 59 ends of the body bend ventrally with the body in emprosthotonus. The head is bent far down. The animal may actually stand on its head, but the abdomen retains its ventral curvature, leaving a considerable angle open between its dorsuni and the wings which normally rest on it. In both walking and flying it continually keeps close to the table, and upon encountering an obstacle it frequently does a forward somer- sault. If it gets on its back it rights itself with greatest difficulty as its efforts simply result in bending the tail and head ventrally until they may form a complete ring. In galvanotropism the same general picture is presented by Palcemonetes and Amblystoma when the anode is at the head end, the tonus changes involved being identical in the two conditions (Garrey1"). These experiments leave no doubt that the primary | effect of light consists in changes in the tension of muscles and that the heliotropic reactions which appeared to the older observers as voluntary acts are in reality forced movements. in the chapter on forced movements after brain lesion the fact was mentioned that a dog which had shown circus movements to the left after lesion of the left cerebral hemisphere shows circus motions to the right when after- ward the right hemisphere is injured symmetrically; in- stead of being a physiologically symmetrical animal again after the second operation. The explanation is that the new operation is more effective than the old one whose effect has partly worn off. Garrey has made an obser- vation on heliotropism which shows some analogy with this experiment on the brain. He found177 that "robber flies with one eye black- ened show the postural conditions in the most pronounced way in the early morning or after being kept for some hours in the dark. Constant exposure to the light pro- duces considerable fatigue of the eye with recovery in the dark. These facts among others suggested the possi- 60 TEOPISMS bility of producing a different sensitiveness of the two eyes and corresponding differences in the muscle tonus with asymmetry of position, and in physiological action of the muscles of the two sides of the body when the \ two eyes were equally illuminated. Such an experiment I constitutes a crucial test of the tonus theory of helio- 1 tropism. It succeeded beyond our greatest expectations. Asphalt black was applied to the right eye of several specimens of P root acanthus. In two or three days the paint had formed a brittle shell. During this time the blackened eye had become 'dark adapted. ' When such a fly is exposed to light, it tilts and circles to the left. If now the brittle shell is cracked off the right eye by care- fully pinching with fine forceps, the exposure of this very sensitive eye to light results in a reversal of the whole picture; the fly circles toward the side from which the black was removed. Although the illumination of the two eyes is of equal intensity, what was the normal eye now becomes relatively a darkened eye owing to its lesser sensitiveness. A differential effect results, probably due to a difference in the rate of photochemical change in the two eyes. This reversal of the muscle tonus and of forced motions may persist for an hour or two or even longer, until the two eyes become, as they ultimately do, of equal sensitiveness and the fly behaves like a normal animal. " These experiments are not only incompatible with any l avoidance' idea, for after removal of the black there is nothing to avoid, but they are also incompatible with the conception of * habit formation,' for ' habit' in the per- formance of the circling movements is of no avail when light is admitted to the darkened eye — the animals circle to that side because the tonus of the muscles is such that they are forced to do so. / HELIOTROPISM 61 "All the experiments show that the muscle tone is de- pendent upon the intensity of the light and that the postures assumed depend upon the relative difference in the light stimulus to the eyes. In animals with one eye completely covered the radii of the circles in which they moved were shorter the more intense the illumination of the normal eye. With one eye partially covered the circles were larger than when completely covered, and in the same way the circles were larger when one eye was covered by a film of collodion or of brown shellac, which admits some light, than when subsequently covered by opaque asphalt black. When one eye was partially cov- ered by central application of the black paint the tilting and circling to the opposite side were abolished or re- versed by brilliant illumination of the partially blackened eye. These results explain why a positively heliotropic animal with one eye blackened approaches a light by a series of alternating small and large circles, the former being executed when the good eye is illuminated from the source of light, the larger when it is in the shadow." We have thus far discussed chiefly positively helio- tropic animals, i.e., animals which are compelled to move toward the source of light. The difference between these and negatively heliotropic animals is that the legs on the illuminated sida_j^L a negatively heliotropic animal are extended, while those on the opposite side are in flexed position. This has been directly observed by Holmes, who also made sure of the fact that negatively heliotropic animals, when one eye is blackened, turn in circles with the blackened eye toward the center of the circle 228 ; while positively heliotropic animals turn in circles with the unblackened eye toward the center of the circle. 62 TKOPISMS 3. HELIOTROPISM OF UNICELLULAR ORGANISMS In unicellular organisms, where cilia act as locomotor organs, it can easily be shown that the orientation by light is of the nature of changes in the position of cilia; this is for instance the case in respect to V'cfyox. Holmes226 states for the heliotropic reactions of this organism, that they are due to differences in the activity of the cilia on both sides of the organism and this ex- planation agrees with the actual observations of Bancroft on the galvanotropic reactions of Volvox. In flagellates, the mechanism of locomotion is very complicated and does not consist in an oar-like action of a flagellum as was formerly assumed. Bancroft has shown that in Euglena, as already stated, the flagellum inserted at the anterior end of the organism is bent backward in the form of an inverted U, and that locomotion is brought about by the formation of a loop which travels from the base of the flagellum toward the free end (Fig. 23). The path of the organism which results from this action is a spiral with continual rotation of the organ- ism around its longitudinal axis. Bancroft has shown that the behavior of the organism under the influence of light is identical with that in a constant galvanic field.21 One-sided illu- mination as well as a current going transversally through such an organism cause changes in the position of cilia comparable with those observed in the legs of crustaceans, insects, and vertebrates. V FIG. 23. — Diagram showing the position of the flagellum as seen in a viscid medium, a, when Euglena is swimming forward in a narrow spiral; 6, when swerving sharply toward the dorsal side; c, when moving backward. (After Bancroft.) HELIOTROPISM 63 4. HELIOTROPISM OF SESSILE ANIMALS When we study the effects of light on sessile animals we find that they behave in a similar manner to sessile plants. When illuminated from one side they bend their heads to the source of light until their axis of symmetry goes through the source of light. In this case the sym- metrical photosensitive elements receive equal illumina- tion and the symmetrical muscles are under equal tension. Hence the animal remains in this orientation. These sessile animals were the first examples by which the FIG. 24. — Tube worms in aquarium, all bending toward light. muscle tension theory of animal heliotropism was proved.288 Spirographis spallanzani (Fig. 24) is a marine annelid from 10 cm. to 20 cm. long, which lives in a rather rigid yet flexible tube. The latter is formed by a secretion from glands at the surface of the animal. The tube is attached by the animal with its lower end to some solid body, while the other end projects into the water. The worm lives in the tube and only the gills, which are arranged in a spiral at the head end of the worm, project from the tube. The gills, however, are quickly retracted, and the worm withdraws into the tube when touched or if a shadow is cast upon it. 64 TBOPISMS When such tubes with their inhabitants are put into an aquarium which receives light from one side only, it requires, as a rule, a day or more until the foot end of the tube is again attached to the bottom of the aquarium. As soon as this occurs, the anterior end of the tube is raised by the worm until the axis of symmetry of the gills falls into the direction of the rays of light (Fig. 24) which enter through the window into the aquarium.288 When the animal has once reached this position it retains it as Fia. 25. — The same animals after the position of the aquarium to the window was reversed. long as the position of the aquarium and the direction of the rays of light remain the same. When, however, the aquarium is turned 180°, so that the light falls in from the opposite direction, the animal bends its tube during the next twenty-four or forty-eight hours in such a way that the axis of symmetry of its circle of gills is again in the direction of the rays of light (Fig. 25). When the. light strikes the aquarium from above, the animals assume an erect position, like the positively heliotropic stems of plants when they grow in the open. In these phenomena the mechanical properties of the tube play a role. When the animal is taken out of the bent tube, the latter retains its form. This permanent HELIOTEOPISM 65 change of form of the tube is apparently caused through the secretion of new layers on the inside of the tube. The youngest layers of the secretion are more elastic than the old layers, and, moreover, have at first a powerful tend- ency to shorten. If such a secretion occurs on one side of the tube only, or more so than on the opposite side, the former must become shorter than the latter, and the result must be a curvature of the tube, that side becoming con- cave where the new secretion has occurred. On this assumption the process of heliotropic curva- ture is in this case as follows : when the light strikes' the circle of gills from one side only, in these elements certain photochemical reactions occur to a larger extent, than on the opposite side. This results in corresponding altera- tions of the sensory nerve endings, the sensory nerves and the corresponding motor nerves, and their muscles. The sense of these changes is such as to throw the muscles connected with the nerves of the gills on the light side into a more powerful tonic or static contraction than the muscles on the opposite side of the body. The consequence is a bending of the circle of tentacles, or the head, toward the source of light, which will continue until the axis of symmetry of the circle of tentacles falls into the direction of the rays of light. When this happens, symmetrical tentacles are struck at the same angle (or in other words, with equal intensity) by the rays of light, and therefore the tone of the antagonistic muscles is the same. The result is that the circle of tentacles becomes fixed in this position. The bending of the head produces an increased pressure and friction of the animal against that side of the tube which is directed toward the light, and this pres- sure and friction lead to an increased secretion and the formation of a new layer inside the tube. 5 66 TBOPISMS Heliotropic curvature of sessile animals can be shown equally well in a hydroid, Eudendrium. It is necessary to cut off the old polyps at once when the animal is brought into the laboratory and to put the stem into fresh, clear, FIG. 26. — Polyps of Eudendrium, all growing toward source of light. The arrow indi- cates the direction of the rays of light, which in one case fall in from above, in the ]other from the left side. sea water. In a day or two new polyps are formed by regeneration and these polyps will bend toward the light until their axis of symmetry is in the direction of the rays of light (Fig. 26). The region at the base of the polyps is contractile and when light strikes the polyps from one HELIOTBOPISM 67 side only, the stem on the side of the light contracts more than on the other side, and this results in a bending of the stem, whereby the polyp is put into the direction of the rays of light. As soon as the axis of the polyp is in the direction of the rays of light (provided there is only one source of light), the tension of the contractile elements is the same all around, and there is no more reason for the organism to change its orientation. It, therefore, re- mains in this orientation to the light. The muscle tension theory of animal heliotropism is, therefore, proved for all classes of the animal kingdom, infusorians, hydroids, annelids, crustaceans, etc. It would be wrong to state that the theory holds only for insects. CHAPTER VI AN AETIFICIAL HELIOTEOPIC MACHINE THE reader will have perceived that in the preceding analysis animals are treated as machines whose appar- ently volitional or instinctive acts, as e.g., the motion toward the light, are purely physical phenomena. The best proof of the correctness of our view would consist in the fact that machines could be built showing the same type of volition or instinct as an animal going to the light. This proof has been furnished by the well-known inventor, Mr. John Hays Hammond, Jr. The following is a descrip- tion of the machine given by one of Mr. Hammond's fellow-workers who cooperated with him in the develop- ment of the machine, Mr. B. F. Miessner. This " Orientation Mechanism " consists of a rectangular box, about 3 feet long, 1% feet wide, and 1 foot high. This box contains all the instruments and mechanism, and is mounted on three wheels, two of which are geared to a driving motor, and the third, on the rear end, is so mounted that its bearings can be turned by solenoid electro-magnets in a horizontal plane. Two 5-inch condensing lenses on the forward end appear very much like large eyes. If a portable electric light, such as a hand flashlight, be turned on in front of the machine it will immediately begin to move toward the light and, moreover, will follow that light all around the room in many complex manoeuvres at a speed of about 3 feet per second. The smallest circle in which it will turn is about 10 feet diameter; this is due to the limiting motion of the steering wheel. Upon shading or switching off the light the " dog " can be stopped immediately, but it will resume its course behind the moving light so long as the light reaches the condensing lenses in sufficient intensity. In- deed, it is more faithful in this respect than the proverbial ass behind the bucket of oats. To the uninitiated the performance of the pseudo dog is very uncanny indeed. The explanation is very similar to that given by Jacques Loeb, of reasons responsible for the flight of moths into a flame. . . . 68 HELIOTEOPIC MACHINE 69 The orientation mechanism here mentioned possesses two selenium cells corresponding to the two eyes of the moth, which when influenced by light effect the control of sensitive relays instead of controlling nervous apparatus, as is done in the moth. The two relays (500 to 1,000 ohm polarized preferred) controlled by the selenium cells in turn control electro-magnetic switches, which effect the following operations: When one cell or both are illuminated the current is switched on to the driving motor; when one cell alone is illuminated an electro-magnet is energized and effects the turning of the rear steering wheel. The resultant turning of the machine will be such as to bring the shaded cell into the light. As soon and as long as both cells are equally illuminated in sufficient intensity, the machine moves in a straight line toward the light source. By throwing a switch, which reverses the driving motors, the machine can be made to back away from the light irra most surprising manner. When the intensity of the illumination is so decreased by the increasing distance from the light source that the resistance of the cells approach their dark resistances, the sensitive relays break their respective circuits and the machine stops. The principle of this orientation mechanism has been applied to the " Hammond Dirigible Torpedo " for demonstrating what is known as attraction by interference. That is, if the enemy tries to interfere with the guiding station's control the torpedo will be attracted to him, ete.a Nothing seems to have been published beyond these meagre details, but the writer understands that the active machine has been demonstrated in a number of places in this country. It seems to the writer that the actual con- struction of a heliotropic machine not only supports the mechanistic conception of the volitional and instinctive actions of animals but also the writer's theory of helio- tropism, since this theory served as the basis in the con- struction of the machine. We may feel safe in stating that there is no more reason to ascribe the heliotropic reac- tions of lower animals to any form of sensation, e.g., of brightness or color or pleasure or curiosity, than it is to ascribe the heliotropic reactions of Mr. Hammond's machine to such sensations. & Electrical Experimenter, September, 1915, 202. CHAPTER VII ASYMMETRICAL ANIMALS IT was necessary for us to begin our analysis with symmetrical animals since as the result of this analysis the conduct of asymmetrical organisms offers no difficulty. The result of the asymmetry consists merely in a change in the geometrical character of the path in which an animal is compelled to move to or from the source of -energy. While this path is a straight line in a symmetrical and positively heliotropic organism it is a spiral around this straight line as an axis in an asymmetrical organism, like Euglena. Suppose a positively heliotropic animal to have slightly asymmetrical appendages which give it a tendency to deviate to the left. Let us suppose that the plane of symmetry of the animal goes at the beginning of the experiment through the source of light and that the animal is swimming toward the light. After a few strokes the head of the organism will have deviated slightly to the left on account of the asymmetry in the activity of the appendages. As soon as the median plane of the animal deviates to the left, the left eye is less illuminated than the right one. As a consequence, a difference in the ten- sion of the muscles on the two sides of the animal will be produced which will compensate the natural lack of sym- metry in the muscles and the animal will cease to deviate further to the left; and this compensating effect of the unequal illumination of the two eyes will continue until the animal is actually oriented in the right way again, i.e., 70 ASYMMETRICAL ANIMALS 71 until its plane of symmetry goes through the source of light. All that the inherited or accidental asymmetry does is to cause the animal to move in a path which is not a mathematically straight line; but this deviation will be marked only in a case of very pronounced or excessive asymmetry. We have already described the behavior of a dog whose left cerebral hemisphere has been injured and who has a tendency to deviate to the left. When such a dog is shown a piece of meat it moves toward it in a fairly straight line, its tendency to deviate to the left being compensated by the orienting effect of the retina image of the piece of meat. If the dog deviates to the left, the piece of meat is apparently dislocated to the right of the dog and this dislocation alters the tension of the muscles on the two sides of the animal in such a way as to make it turn back to the right. In this way the dog reaches the piece of meat in a fairly straight line, though with a greater amount of labor, since the tendency to deviate to the left is constantly compensated automatically by a stronger contraction of the muscles turning the animal to the right. The writer showed many years ago that many insects have a tendency to creep upward, and that this is due to an orienting effect of gravity upon the animal. When a perfectly symmetrical insect is put on a vertical stick it walks upward in a straight line. What will happen when such an animal is made asymmetrical1? Garrey has per- formed this experiment by using flies in which one eye was blackened. As we have seen, such organisms are rendered asymmetrical not only in regard to the eyes but also in regard to their apparatus of locomotion, since in one side of the body the tension of the flexors, in the 72 TROPISMS legs of the other side the tension of the extensors pre- vails. As a consequence the fly has a tendency to move in circles with the intact eye toward the center. Grarrey has shown that when a fly with one eye black- ened is put on a vertical stick, it still walks upward, but in spirals around the stick (Fig. 27), instead of in a straight line. The asymmetry of loco- motion changes only the geo- metrical nature of the path in which the animal moves, from a straight line to a spiral, but does not alter the forced move- ment character of the reaction. Bancroft has pointed out that when in a positively helio- tropic amphipod one eye is blackened and the legs of the same side are cut off, the ani- mal's path would be a combina- tion of a circus motion induced by the blackening of the eye and of a rolling motion around its longitudinal axis. Both effects combined would result in the animal swimming in a spiral path, and if the animal is positively heliotropic it would swim in such a path toward the light. This is the path which aquatic, asymmetrical posi- tively heliotropic organisms, such as the flagellate Euglena, describe in their motions to the light. FIG. 27. — Fly with one (right) eye blackened can creep only in a spiral on a vertical stick, while normally it creeps in a straight line. (After Garrey.) ASYMMETRICAL ANIMALS 73 But this locomotor mechanism (of Euglena) is imperfect, it forces the organism to move in a spiral, and always to turn toward a structurally determined side. There, are many organisms which swim in spirals and become oriented by turning toward a structurally defined side. Jen- nings and Mast include all such orientations under "trial and error" and contrast them with the direct orientation of such animals as the amphipods in which the turning may be either toward the left or the right. Let us now consider whether the orientation of Euglena is more like the selection of random movements, (which we would all agree may justifiably be called " trial and error"), or whether it is more like the orientation of the terrestrial amphipods studied by Holmes. I think that all students of behavior including Jennings and Mast believe that in the case of these amphipods we have direct heliotropic orientation. If the right eye of such a positively heliotropic amphipod be covered with asphalt varnish it will execute circus movements towards the left. The usual explanation is that the main nervous connection is between the eye on one side and the legs on the opposite side of the body. The light shining on the uncovered eye brings about a condition of in- creased muscular tonus in the legs of the opposite side, which is not present in the legs connected with the covered eye. Consequently the right legs push more strongly and the amphipod turns towards the left. Suppose now we remove some or all of the left legs from an amphipod of this kind so that it will always turn toward the left, and transfer it to water in which it must be supposed to swim in a spiral path. We will then have an organism which would become oriented in essentially the same way that Euglena does. The animal would always swerve toward the left. But, when the spiral course brings it into such a posi- tion that the light shines directly on the left eye, the muscular tonus of the right legs would be increased and the swerving toward the light would increase. Thus orientation would be effected in just the same way that it is in Euglena. While these hypothetical changes that must be made in the amphipod, to make it react like Euglena, are considerable, they concern only the details. The fundamental nature of the photochemical substances, the nature of their stimulation and the character of their connection with the locomotor organs have none of them been modified. All that has been done is to make an asymmetrical organism swimming in a spiral out of a bilateral one.a These changes are much less fundamental than a Swimming in a straight line. 74 TEOPISMS those which we would have to imagine in order to make an amphipod orient to light by the selection of random movements. In order to bring about this latter change the whole nature of the photochemical sub- stances and their relations to the leg muscles would have to be modified. In the one case the required changes are all of a mechanical nature and so simple that the experiment might possibly succeed. In the other case the required changes are largely chemical, and so complex that we have no data for even imagining what ought to be done in order to bring them about (Bancroft 21 ). The asymmetry of organisms only modifies the geo- metrical character of the path but not the mechanism of the reaction. CHAPTER VIII TWO SOUKCES OF LIGHT OF DIFFERENT INTENSITY THE writer observed that if heliotropic animals are exposed to two equidistant lights of equal intensity they I move in a line perpendicular to the line connecting the' two lights.287' 294 This has been confirmed by numerous observers, e.g., Bohn on Littorina, by Parker and his pupils, especially by Bradley M. Patten on the larvae of the blowfly, by Loeb and Northrop on the motions of the larvae of Balanus, and by others. The question arises : In which line will an animal move when the intensity of the two lights differs ? . When the animal is positively heliotropic it should cease to move in a line at right angles to the line connecting the two lights but should move in a line which deviates toward the stronger of the two lights ; if the animal is negatively heliotropic it should deviate toward the weaker of the two lights. When the two eyes are illuminated by two lights of different intensity, the illumination in both eyes can become approximately equal only when the eye struck by the weaker light is exposed at a larger angle than the eye struck by the stronger light. Under such conditions, the animal should be com- pelled to move in a straight line which, however, is no longer at right angles to the line connecting the two lights, but which deviates to an extent determined by the differ- ence in the intensity of the two lights. The case was 75 76 TBOPISMS worked out quantitatively by Bradley M. Patten on a negatively heliotropic animal, the full grown larva of the blowfly.412,413 The source of light was at G (Fig. 28) (one or more Nernst lamps of measured candle power), a portion of light from these lamps passed through the screens d and d' to the mirrors M and M', set at a definite FIG. 28. — Diagram of apparatus used to produce differential bilateral light stimulation. O, five 220-volt Nernst glowers; M and M' , mirrors; / and /', central point of mirrors; O, center of observation stage; dotted lines, central ray of beam of light from the glowers reflected to O by the mirrors; d and d', screens with rectangular openings; s and «', light shields; a and 6, 2 c.p. orienting light with screens. (After Patten.) angle so that the rays were reflected to the observation point 0. The two beams of light reaching 0 were of the same intensity. With the means of one of the lights at a or b the animal was first caused to move across the field 0 at right angles to the rays reflected from the mir- rors M and M'. The animals first started in this direction, then came suddenly under the influence of the light re- TWO SOURCES OF LIGHT 77 fleeted by M and M'. In order to make the ratio of inten- sities of light from M and M' different, the observation stage 0 was put at un- equal distance from M and M'. The larvae were made to record their trails while moving under the influence of two lights and the devi- ation of this trail from the perpendicular upon the line connecting the two sources of light M and M' was measured with a goniometer (Fig. 29). The result of the measurements of 2,500 trails, showing the pro- gressive increase in an- gular deviation of the larvae (from the per- pendicular upon the line connecting the two lights) with increasing differences between the lights, are given in Table I. Since the devi- ation or angular deflec- tion was toward the weaker of the two lights (the animal being negatively heliotropic) the deviation is marked negative. Fio. 29. — Diagram to show the method of measuring trails. The lines xy and x'y' are drawn through the trails at the points reached — marked by the arrows — when the side lights were turned on. The angle of deflection from this line is measured by a protractor, P. The small figures near the arrows indicate the number of wig-wag movements made when the side lights were turned on; 1st and 2nd refer to the sequence in which the trails were run. (After Patten.) 78 TBOPISMS TABLE I. Percentage difference in the intensity of the two lights Average angular deflection of the two paths of the larvae toward the weaker light Per cent. 0 8% 16% 25 331/3 50 831/3 100 Degrees - 0.09 - 2.77 - 5.75 - 8.86 -11.92 -20.28 -30.90 -46.81 -77.56 Patten also investigated the question whether the same difference of percentage between two lights would give the same deviation, regardless of the absolute intensities of the lights used (Weber's law). The absolute intensity was varied by using in turn from one to five glowers. The relative intensity between the two lights varied in succes- sion by 0, 8 1/3, 16 2/3, 25, 33 1/3, and 50 per cent. Yet the angular deflections were within the limits of error identical for each relative difference of intensity of the two lights, no matter whether 1, 2, 3, 4, or 5 glowers were used. Table II gives the results. TABLE II A TABLE BASED ON THE MEASUREMENTS OF 2,700 TRAILS SHOWING THE ANGULAR DEFLECTIONS AT FIVE DIFFERENT ABSOLUTE INTENSITIES Number Difference of intensity between the two lights of glowers 0 per cent. 8^ per cent. 16^ per cent. 25 per cent. 33^ per cent. 50 per cent. 1 2 3 4 5 -0.55 -0.10 +0.45 -0.025 -0.225 Defle -2.32 -3.05 -2.60 -2.98 -2.92 ction in de -5.27 -6.12 —5.65 -6.60 -5.125 grees -9.04 -8.55 -8.73 -9.66 -8.30 -11.86 -11.92 -13.15 -11.76 -10.92 -19.46 -22.28 -20.52 -19.88 -19.28 Average. . . -0.09 -2.77 -5.75 -8.86 —11.92 -20.28 TWO SOURCES OF LIGHT 79 On the writer's theory the following explanation of these deviations should be given. The muscles moving the head of the animal to the side of the weaker illumina- tion, having a higher tension than their antagonists, bring about a deflection of the animal toward the side of the weaker light. As soon as its two photosensitive areas in the head — the animal has no eyes — which are not parallel, but inclined to each other are deflected from the perpen- dicular upon the line connecting the two lights, the photo- sensitive areas of the animal will no longer be struck by the lights at the same angle, but on the side of the weaker light the area will be struck at an angle nearer to 90° than the photosensitive area exposed to the stronger light. In this way the change in angle will compensate the dif- ference in intensity of the two lights until the orientation of the animal is such that the compensation is complete and both photosensitive areas receive the same illumina- tion. The animal will then continue to move in this direction. Patten has computed the angle of the photosensitive surfaces for these animals from the angle of their orienta- tion under varying inequalities of illumination. This angle has been computed for the blowfly larva, using the "angu- lar deflections " already ascertained. The magnitude of the angle may bear no direct relation to the actual angle at which the sensitive areas are located in the body of the animal, because of the many factors which may modify the direction of the rays before they fall on the sensitive surfaces. The significant test of the hypothesis would be the constancy of the angle when computed from experimental data obtained under varying conditions. The method of constructing such an 'angle is shown in Fig. 30, in which the opposing lights are assumed to be of a two-to-one ratio of intensity. The line AB is drawn perpendicular to the direction of the rays of light. On the line AB, construct angle BO C equal to the actual average angular deflection of the larva? at a two-to-one ratio of lights. 80 TEOPISMS The problem now resolves itself into the construction of an angle about 0(7 as a bisector, which shall be of such a magnitude that equal dis- Fia. 30. — Diagram for constructing direction of motion of larvae under influence of two lights of different intensity. (After Patten.) tances on its opposite sides shall have projections on the line AB of the ratio of two to one. Construction : From a point D on the line OC draw Dh perpendicular TWO SOURCES OF LIGHT 81 to AB. Lay off on AB distances hx and hy, such that hy — 2hx. From x and y erect lines perpendicular to AB, they will intersect 0(7 at / and e respectively. Bisect the line ef, and at its middle point, g, con- struct a line kl perpendicular to OC. From the point of intersection of kl and yy' (M), draw a line to D. From the intersection of kl and xx' (N), draw a line to D. The angle MDN is the desired angle. Proof: eg — gf (construction). Angle egM = angle fgN (construction). Angle Meg = angle Nfg (alternate int. angles of parallel lines, yy' and xx' being parallel by construction). Therefore triangle Meg = triangle Ngf (side and two adjacent angles being equal). Ng — gM (similar sides of equal triangles). gD = gD (identical). Therefore triangle NgD = triangle MgD (rt. triangles, altitude and base equal). Therefore angle gDM = angle gDN and side DM = side DN. Now by construction hx is the projection of DN on AB and hy the projection of MD on AB, and by construction hy = 2hx. This fulfills all the conditions of construction. The equal lines MD and DN represent equal bilateral sensitive areas inclined to each other at such an angle, MDN, that the surface represented by MD intercepts an area of light twice as great as the surface repre- sented by DN, its projection on the perpendicular to the light rays being twice as great (hy = 2hx). But the light falling on DN is of twice the intensity of the light falling on DM, so that the total amount of light received by each of the equal areas is the same. By this method of construction, the average angle of sensitiveness was computed for four intensity differences, using as a basis the angular deflection of the larva? as determined by experiment. The magnitude of the angles is almost identical in all four cases.412 Experiments by a somewhat different method, to be discussed in the next chapter, on the positively heliotropic larvae of the barnacle show that these results of Patten are more general. We may, therefore, say that the migration of animals to or from the light is of the nature of a forced movement determined by the effect of light on the photosensitive elements of the body. Unequal illumination of symmetri- 6 82 TEOPISMS cal photosensitive elements on the two sides of the body alters the tension of symmetrical muscles, and as a con- sequence the animal is, when moving, compelled to change its direction of motion until it is oriented in such a way to the light that symmetrical elements receive the same illumination. In this case the tension of symmetrical muscles is equal again and the animal is compelled to move in this direction. It has been suggested by the anthropomorphic inter- preters of animal conduct that the motion of an animal to- a source of light is the same phenomenon as when a human being who has lost his way in the dark is attracted by an illuminated human habitation. As Bonn pointed out, the definite path in which a positively heliotropic animal moves when under the influence, of two lights, shows that the anthropomorphic interpretation is as erroneous in this as in any other case. A human being would go to one of two illuminated houses and not toward a point between them, determined by the relative intensity of the two lights.66 CHAPTER IX THE VALIDITY OF THE BUNSEN-EOSCOE LAW FOB THE HELIOTEOPIC EEACTIONS OF ANIMALS AND PLANTS WE have thus far said little about the identity of the heliotropism of plants and animals. Yet the two phe- nomena are essentially alike. When we keep positively heliotropic sessile plants and sessile animals near a win- dow, both will bend toward the source of light, though the mechanism of bending may not be the same in all details, the bending being produced in the case of the plant (and possibly in certain animals like Eudendrium) by unequal growth in length of the plant on the illuminated and shaded sides; while in the case of higher animals, e.g., Spirographis, it is produced by differences in the tension of the muscles on the illuminated and shaded sides of the animal. Motile plant organisms like Volvox, are driven to the source of light, owing to differences in the tension of the contractile organs on the shaded and illuminated side, and the same is true for animals like insects. A further point of coincidence lies in the validity of the photochemical law of Bunsen and Eoscoe for the heliotropism of animals and plants. The law of Bunsen and Eoscoe says that within certain limits the chemical effect produced by light increases in proportion with the product of intensity into the duration of illumination, e.g., Effect = Kit, where i is intensity, t duration of illumination, and K a constant. This is true 83 84 TEOPISMS for the blackening of photographic paper by light, and it can be shown that the same law holds for heliotropic reactions of plants as well as animals. Blaauw46*47 established this fact for the etiolated seed- lings of Avena saliva. These organisms were exposed to lights of a definite candle power for some time and then left in the dark. After a certain time the seedlings began to bend, becoming concave on that side which had pre- viously been illuminated. By varying the candle power of light (i) and the duration of illumination (£), he found that the value of it required to cause 50 per cent, of the seedlings to bend was always the same. Table III gives TABLE III Time required for different intensities of light to produce heliotropic curvatures in 50 per cent, of the seedlings of Avena Candle-meter Duration of illumination Candle-meter-seconds 0.00017 43 hours 26.3 0.000439 13 hours 20.6 0.000609 10 hours 21.9 0.000855 6 hours 18.6 0.001769 3 hours 19.1 0.002706 100 minutes 16.2 0.004773 60 minutes 17.2 0.01018 30 minutes 18.3 0.01640 20 minutes 19.7 0.0249 15 minutes 22.4 0.0498 8 minutes 23.9 0.0898 4 minutes 21.6 0.6156 40 seconds 24.8 1.0998 25 seconds 27.5 3.02813 8 seconds 24.2 5.456 4 seconds 21.8 8.453 ' 2 seconds 16.9 18.94 1 second 18.9 45.05 2/5 seconds 18.0 308.7 2/25 seconds 24.7 511.4 1/25 seconds 20.5 1,255 1/55 seconds 22.8 1,902 1/100 seconds 19.0 7,905 1/400 seconds 19.8 13,094 1/800 seconds 16.4 26,520 I/ 1000 seconds 26.5 BUNSEN-ROSCOE LAW 85 the time required for different intensities of light varying from 0.00017 to 26,520 candle power to cause 50 per cent, of the seedlings to show heliotropic curvatures. As can be seen, the product it is always approximately 20. Ewald and the writer 300> 305 tested the validity of the law of Bunsen and Roscoe for the heliotropic curvatures of Eudendrium. A number of stems of Eudendrium, from which the polyps had been cut off, were put upright into a trough with parallel walls, containing sea water. As soon as the new polyps had regenerated they were exposed to light of a certain intensity for a short time and then kept in the dark. In the dark the bending of the polyps in the direction of the former source of light occurred. The purpose was to find the minimum time of exposure required for a given light (40 candle power) to induce 50 per cent, of the polyps to bend to the light (Table IV). TABLE IV Percentage of polyps bending toward the former source of light Duration of Distance of the polyps from the light in meters illumination 0.25 0.50 1.00 1.50 2.00 10 651 15 68 20 74 30 42 35 40 56 45 60 50 60 60 90 120 65 30 150 48, 50 180 240 300 85 40 360 40 (15) 420 57 1 Very young, abnormally sensitive polyps. 86 •TROPISMS If we calculate from this the value of the product it for different intensities of light we find that it obeys the Bunsen-Boscoe law (Table V). TABLE V Distance of polyps from Time required to call forth heliotropic curvature in 50 per cent, of the polyps Observed Calculated according to the Bunsen-Roscoe law Meters 0.25 0.50 1.00 1.50 Minutes 10 between 35 and 40 180 between 360 and 420 Minutes 40 160 360 The material varies considerably so that it is not always possible to induce 50 per cent, of the polyps to undergo heliotropic curvature. For this reason Loeb and Wasteneys 312 repeated these experiments by a some- what different method. We confined our experiments to three intensities of light by putting the specimens at distances of 25, 37.5, and 50 cm. from a Mazda incandescent lamp, of about 33 Hefner candles. The times of exposure were adjusted so that on the assumption of the applicability of the Bunsen- Eoscoe law the same effect, i.e., the same percentage of polyps bending towards the light should be produced. Thus in some experiments the exposure for the three dis- tances given was 10, 22.5, and 40 minutes respectively, in others, 7, 15.75, and 28 minutes, and so on. The ratios of the percentage of polyps bending toward the light for the three distances should be as 1 : 1 : 1. Since the material differed widely in different experiments and in different dishes, it was necessary to compute the averages of a large number of experiments. The colonies, immersed in sea water, were arranged BUNSEN-KOSCOE LAW 87 in a row in rectangular glass dishes, the stems being in- serted in holes made in a layer of paraffin mixed with lamp black as in the previous experiments. The rear side of the dish was also coated with the paraffin lamp black mixture in order to prevent reflection of light from the slightly uneven back surface of the dish. Table VI gives a summary of the results. The first three columns give the times of exposure for the three TABLE VI Times of exposure in minutes Ratio of per cent, of hydranths bending towards light 25 cm. 37.5 cm. 50 cm. 25 cm.: 37.5 cm. 25cm.: 50 cm. 37.5 cm.: 50 cm. 15 60 1.50 20 80 1.30 10 22.5 40 1.20 (3.08) (2.56) 10 22.5 40 0.94 1.47 1.55 10 22.5 40 1.57 (2.30) (2.43) 10 22.5 40 1.43 1.04 0.94 10 22.5 40 0.76 1.09 1.47 10 22.5 40 1.05 1.13 0.90 0.96 10 22.5 40 1.15 0.99 7 15.75 28 0.84 0.62 0.74 7 15.75 28 1.70 0.49 0.58 7 15.75 28 0.85 1.25 1.35 7 15.75 28 (2.09)1 0.99 1.08 7 15.75 28 1.14 1.15 0.55 7 15.75 28 0.44 0.92 0.44 7 15.75 28 1.52 0.80 0.61 7 15.75 28 0.59 0.36 0.70 7 15.75 28 0.48 1.07 0.31 7 15.75 28 1.00 0.48 1.80 7 15.75 28 0.69 1.09 0.81 7 15.75 28 1.26 0.85 1.09 7 15.75 28 0.86 1.38 0.85 7 15.75 28 0.70 1.07 1.59 7 15.75 28 0.77 1.24 7 15.75 28 0.60 Mean 1.02 0.99 1.02 Probable error . . ±0.01 ±0.01 ±0.01 1 Bracketed values being extreme variates are excluded from calculations of the means and probable errors. 88 TKOPISMS distances of the source of light, selected, as stated, on the assumption that the Bunsen-Eoscoe law holds. On that assumption the ratio of percentage bent in any two or all three dishes on any one day should equal 1.0. These ratios for each pair of distances of the source of light are given in the three other columns of the table. The per- centage bending was only compared in dishes containing material regenerated and exposed on any one day, since only in this case was there any likelihood that the material was in any way uniform, and since only in this case the experiments were carried on at the same temperature and the same conditions of regeneration. The result was that the observed ratios were as 1.02 : 0.99 : 1.02 (with a probable error of =t 0.01) while the values calculated on the assumption of the validity of the Bunsen-Eoscoe law were as 1:1:1; i.e., the results showed as great an approximation between observed and calculated values as one could expect. There is a second method for testing the validity of the Bunsen-Eoscoe law, based on the use of two sources of light of equal intensity. If it is true that the heliotropic efficiency of light is determined by the product of intensity, i, into duration of illumination, t, we can alter this product by varying t as well as by varying i. Eadl had shown that the position of the eye of the fresh water crustacean, Daphnia, is determined by the position of a source of light,447 and Ewald 145 found that by exposing the eye to two different sources of light simultaneously the eye is put into a position determined by the relative intensity of the two lights. When one light remained constant and the intensity of the other light was lowered the position of the eye changed. He now BUNSEN-BOSCOE LAW 89 could show that when the duration of illumination of one eye was altered by a rotating opaque disk with one sector cut out, the heliotropic effect on the eye of Daphnia was the same as when the intensity i of the same light was reduced to an amount corresponding to the Bunsen- Eoscoe law. Under the influence of two constant lights of equal intensity heliotropic animals move in a direction at right angles to the line connecting the two lights. If the law of Bunsen and Eoscoe holds the effect of a constant light should be diminished if a rapidly rotating opaque disk with one sector cut out be put in front of the light, and the diminution should be equal to the fraction of the arc of the sector. Thus a sector of 90°, which reduces, the total duration of illumination to one-fourth, should also reduce the heliotropic effect of the light to one-fourth, and the ani- mal should deviate from the old direction in the direction toward the light without a disk before it. If, however, we lower the intensity of the latter light to one-fourth by doubling its distance we also reduce its heliotropic effect to one-fourth, and now the animal should move again in a line at right angles to the line connecting the two lights. The following experiments carried out by Loeb and Northrop 309 on the larvae of the barnacle are perhaps the best proof for the validity of the Bunsen-Eoscoe law for animal heliotropism. These animals are small and can be obtained in large numbers. They were made to collect in the corner of a dish with a little sea water and were then sucked up into a pipette ef, Fig. 31, which was blackened with the exception of the opening. When such a pipette is put into a glass dish with parallel walls whose bottom is black (by putting paraffin black- ened with lampblack at the bottom of the dish) the larvae will flow out in a fine stream and swim when they are positively heliotropic in a straight line toward the source of light. They thus form a rather narrow white trail on the dark bottom and it is possible to measure the angle of this 90 TEOPISMS trail with the line connecting the two lights. In this way in each observa- tion the average trail of thousands of individuals is measured. By using one constant and one intermittent source of light and comparing the results with those obtained by two constant lights we can test the validity of the Bunsen-Roscoe law. The method of the experiments was as follows : abed (Fig. 31) is a square dish of optical glass with blackened bottom and containing a FIG. 31. — Method for the proof of the validity of Bunsen-Roscoe law for the positively heliotropic larvae of the barnacle. (After Loeb and Northrop.) layer of sea water. A and B are two lights, the intensity of which is determined by a Lummer-Brodhun contrast photometer. In front of each light is a screen with a round hole permitting a beam .of light to go to the dish. The lights and the dish abed are so adjusted that the two beams of light striking the sides ab and be at right angles cross each other in the middle of the dish. The light A is fixed while the light B is movable on an optical bench. The experiment is made in BUNSEN-KOSCOE LAW 91 a dark room and the lights A and B are enclosed in a box. At the beginning of the experiments the pipette is filled with a dense suspension of larvae in sea water and then put with its point touching the bottom of the dish. The animals flow out in a fine stream which is narrow at the opening of the pipette and widens slightly, owing probably to the negative stereotropism of the animals. A glass plate (Fig. 32) tiikl, which has a strong red line no and a fine parallel line pq (cut with a diamond), is then put on the dish and so adjusted that pq is in the middle of the stream fg of the ani- mals. Then the angle a which pq makes with the perpendicular from A on ab is measured. This perpendicular is marked in the form of a red line on the black base on which the glass vessel abed stands. The angle a is measured with a goniometer. When the lights are equal in intensity a should be 45° ; if the two lights have different intensities and if A be the stronger light a should become smaller with increas- ing difference in intensity. The individual measurements vary com- paratively little, as long as the difference in the intensity of the two lights is not too great; for this reason our observations do not go beyond a wider ratio of the two lights than 10 : 1, though 4 : 1 is perhaps the limit for good results. Table VII gives the results. A is always the stronger light. Each table is the average of from 40 to 60 individual observations, each being the average of the path of many thousands of animals. TABLE VII FIG. 32. Value of a for different ratios of intensities of the two lights Ratio of the two lights . 1: 1 2:1 4: 1 10: 1 Value of a (direction of path) 45.6° 40° 34.4° 28.8° In the next series of experiments an opaque rotating disk with one sector cut out was placed before light B. In one set of experiments the sector cut out was 90°. The rate of rotation (by an electric motor) was 1,500 to 2,500 revolutions per minute. The other light was constant and its distance was chosen on the assumption of the validity of the Bunsen-Roseoe law for these cases. Thus when the two lights without sector were equal at a given distance of A} by putting 90° sector before 92 TKOPISMS B, it was assumed that the ratio of effects would be the same as if, with constant light, B had been placed at the double distance and the ratio of intensities of the two lights had been 4 : 1. Going on such a calculation we should expect the same values for a as in Table VII. As one sees from Table VIII, the observed values are slightly smaller but practically identical with the values obtained when the two lights are constant. The deviation is probably due to the well established fact that the photochemical efficiency of an intermittent light is a trifle less than that calculated on the basis of the Bunsen-Roscoe law. TABLE VIII Ratio of the two lights 1 1 2: 1 4: 1 Value of a. . 44 9° 383° 34. 1° Value of a when one light is inter- mittent (90° sector) and the othar constant, and the efficiency of the two lights is calculated on the basis of the validity of the Bunsen-Roscoe photochemical law We carried out some experiments with a sector of 144°. When the efficiency of both lights was equal on the assumption of the validity of the Bunsen-Roscoe law a. was found to be 44.9° (instead of 45°), and for the ratio 2 : 1 a was found to be 38.8°. The values are, within the limits of error, identical with the values in Tables VII and VIII.309 Bradley M. Patten also showed that for the heliotropic reactions of the negatively heliotropic larvae of the fly the law of Bunsen and Eoscoe holds. Photochemical processes have a very small tempera- j ture coefficient and it agrees with this that lowering of temperature within the limits compatible with the motility j of animals does not affect the heliotropic response ; on the contrary, we shall see that in certain crustaceans (e.g., DapJinia) lowering of the temperature may enhance posi- tive heliotropism.296 We must, therefore, conclude that the light produces in an eye or an element of the photosensitive skin a chemi- BUNSEN-KOSCOE LAW 93 cal reaction which results in the formation of a certain mass of a reaction product. This mass acts on the periph- eral nerve endings and brings about an as yet unknown change in the brain elements with which these nerve end- ings are connected. This change in turn affects the tone or tension of the muscles with which the brain elements are connected. When the rate of photochemical reaction is the same in both eyes or in the photosensitive elements on both sides of the body, the change of tone in the sym- metrical muscles of both sides of the body is the same and no change in the position or direction of motion of the organism should occur. If the rate of illumination is dif- ferent in both eyes, differences in the relative tension of the symmetrical muscles occur, which make the motion to the source of light easy and in the opposite direction more difficult when the animal is positively heliotropic. For the negatively heliotropic animal the opposite effect will be brought about. These experiments, therefore, show that the tropism theory not only allows us to predict the nature of the , / animal reactions but allows us to predict them quantita- " tively. Thus far the tropism theory is the only one which satisfies this demand of exact science. The degree of directness with which a heliotropic ani- mal goes to or from a source of light depends, aside from the degree of perfection of its locomotor apparatus, upon the intensity of the light and the relative sensitiveness of the animal. Animals which in strong light will move in approximately straight lines to or from the source of light may in weak light reach their goal in a more or less irregular zigzag line. This is easily understood. When 94 TEOPISMS an animal by chance gets its median plane too far out of the direction of the rays of light (we assume them to be parallel), the rate of photochemical reaction will be- come different in both eyes. As soon as the difference between the photochemical reaction products in both eyes exceeds a certain limit the animal will automatically put its plane of symmetry again into the direction of the rays of light. The weaker the light and the less sensitive the animal, the longer it will take until this happens, and the greater the freedom of the animal to deviate from the straight line. CHAPTER X THE EFFECT OF EAPID CHANGES IN INTEN- SITY OF LIGHT IT may prove necessary to make a similar assumption for the effect of a constant illumination as was made by Nernst for the theory of the action of galvanic currents, namely that there are two antagonistic processes going on, one being the photochemical effect of light and the second either a process of diffusion of the substances formed or a chemical reaction of the opposite character as that caused by the action of the light. Many animals which are oriented by constant illumination react by a quick, jerky movement when the intensity of light is either rapidly increased or diminished. In this case the effect is determined by the rapidity of the change in the intensity, ^ , and not by the product of intensity into dura- tion of illumination, it.297 These twitching or jerking effects caused by a rapidly changing intensity of light are comparable to the twitching brought about in a muscle by a rapid increase or decrease in the intensity of a cur- rent. The writer described such reactions first for tube worms like Serpula, which withdraws suddenly into its tube when a shadow passes over it or when the intensity of light is suddenly diminished in some other way. The anthropomorphists, of course, declare this reaction to be induced by the instinctive fear of an enemy, oblivious of the fact that if they were consistent they would have to give the same explanation for the twitching of a muscle upon rapid changes in the intensity of a current. The 95 96 TROPISMS problem to be solved is in both cases a purely physico- chemical one. It was also found that the motions of certain animals stop when they come suddenly from strong light into weak light. This was observed in planarians which as a consequence collect in greater density in spots °f the space where the intensity of light is a relative minimum.291 The difference in the conduct of helio- tropic organisms like Daphnia which go to or from the light and animals like planarians which come to rest where the intensity of light is a rela- tive minimum can be demonstrated by putting them into a circular vessel ( Fig. 33 ) . The positively heliotropic animals collect at a, the negatively heliotropic at &, while the planarians collect at c and d where the intensity of light is a minimum. Reactions de- termined by the value Jj do not lead to phenomena of orientation, though such (improperly called) "fright reactions "a occur in many helio- tropic animals; they may lead, however, to collections of animals. Jennings has maintained that all reactions of unicel- lular organisms are due to "fright" or "avoiding reac- aThe reader should notice the difference in tlie treatment of animal conduct from the point of view of the physicist and of the introspective psy- chologist. What the physicist expresses correctly by the term — the an- thropomorphic biologist explains in terms of human analogy as " avoiding reaction " or " fright reaction," a term which not only assumes the existence of sensations without any adequate proof, but removes the problem from the field of quantitative experimentation. FIG. 33. — Difference in place of gathering be- tween heliotropic animals and animals which come to rest when reaching a relative minimum in the intensity of light. In a circular vessel a c b d and W W representing the window, positively helio- tropic animals will collect at a, negatively heliotropic animals at 6, and animals which come to rest where the intensity of light is a relative minimum at c and d. CHANGES IN INTENSITY 97 tions," and it seems as if at one time lie even intended to deny the existence of tropisms and to maintain that all animals were influenced only by rapidly changing intensi- ties of light. It is needless to discuss such an idea (which he probably no longer holds) in view of the contents of the preceding chapters. He seems, however, to cling to it as far as asymmetrical unicellular organisms are concerned. When moving about Paramcecia often reverse the direc- tion of their progressive motion for a moment, but then do not return in the old direction, moving sidewise, on account of the asymmetry in the arrangement of their cilia. Jennings is probably right in assuming that this factor can lead to collections of such infusorians, since it may prevent their leaving a drop and going into the sur- rounding medium. When, e.g., at the boundary of the two media such a reversal of the action of the cilia occurs, the organisms are prevented from crossing from one medium into the other. But Jennings goes too far in this attempt, when he tries to explain the heliotropic reactions of certain uni-i cellular organisms, e.g., Euglena, in this way. He main- tains 253 that unicellular organisms like Euglena go to the light on account of shock movements produced by the shading of the photosensitive region of the animal. Euglena moves with a constant rotation around its longi- tudinal axis and Jennings assumes that in a certain phase of the rotation a photosensitive element (the eye spot) of the organism is shaded. This he thinks causes a shock movement, whereby the animal is swerved to the light again during the next half of the spiral revolution, and so on. Similarly in negatively heliotropic Euglena the swerving away from the light is, according to Jennings, the shock movement caused by the increased illumination 98 TROPISMS of the photosensitive end of the animal produced by swerv- ing toward the light during the previous half of the spiral revolution. Bancroft 21 showed that Jennings 's theory was based upon incomplete facts. According to Jennings's view positive heliotropism is conditioned by and should be accompanied by shock movements produced by sudden shading (—shading reaction), and negative heliotropism should always be accompanied by shock movements produced by sudden illumination. It has been found, however, that this usual association of shock movements with tropism is not a necessary one, but that it can be destroyed if the proper means be taken. Consequently the view that the heliotropic swerving is a shock movement must fall. When Euglence from Culture B were placed in the rays of the arc light, at a distance of four or five feet from the light, they were strongly positively heliotropic and gave the shading reaction. When, however, they were gradually brought nearer to the light a point was reached at which the heliotropism disappeared but the shading reaction per- sisted. When moved still closer to the light they became negatively heliotropic but still without any change of the shading reaction. When moved still closer to the light, there was a short time when no shock movements could be obtained, but soon the illumination reaction ap- peared. At the same time the negative heliotropism became more prompt and precise. Finally, when the light was still further increased and allowed to act for a considerable time, even the illumination reactions frequently disappeared completely, and a most pronounced and compell- ing negative heliotropism held full sway. . . . It is very evident, then, that the invariable correlation of positive heliotropism with the shading reaction, which is required by Jennings's theory, does not exist. Both kinds of heliotropism may be associated with either the shading or the illumination reaction. Accordingly, it must be concluded that the heliotropic mechanism does not depend upon J the mechanism for the shock movements, but that the two mechanisms ! are independent.21 The simplest method of determining whether or not the orientation of flagellates depends upon rapid changes in intensity of light or upon constant illumination can be furnished with the aid of intermittent light. We know that a striped muscle contracts only when a current is CHANGES IN INTENSITY 99 made or broken, but not while the constant current lasts. Henc6 a rapidly alternating current throws the muscle into tetanus, while the constant current has no effect. If it is the rapid change in the intensity of light which causes the swimming of a positively heliotropic Euglena to the light, an intermittent light, of a sufficient number of alternations per second, should be much more efficient than a constant light; while in case the positive helio- tropism is determined by constant illumination, this should not be the case and the Bunsen-Eoscoe law should hold. Mast348 has recently published experiments on the relative efficiency of the various parts of the spectrum by a method based on the assumption of the validity of the Bunsen-Eoscoe law for the heliotropic orientation of these organisms. If his assumption b is correct, it con- tradicts the theory which Jennings and Mast have de- fended now for more than fifteen years ; if his assumption is wrong, his experiments on the relative efficiency of various parts of the spectrum cannot be correct. Since, however, Mast's results with this method coincide with those by Loeb and Wasteneys312 obtained by a direct method, it is very probable that the law of Bunsen and Roscoe holds for the heliotropic reactions of Euglena and unicellular flagellates in general, and, if this is true, the heliotropic reactions of unicellular algae (Euglena in- cluded) are determined by light of constant intensity. t> He does not seem to have noticed that his method was based on this assumption. CHAPTER XI THE EELATIVE HELIOTBOPIC EFFICIENCY OF LIGHT OF DIFFEBENT WAVE LENGTHS 1. The validity of the Bunsen-Boscoe law for the helio- tropic reactions of animals and plants leaves no doubt that these reactions are determined by the rate of photo- chemical processes. /Heliotropic reactions depend, how- ever, not only upon the intensity but also upon the wave length of light. Photochemistry shows that the most efficient wave length varies with the nature of the photo- chemical substance and that comparatively slight changes in the constitution of a molecule may bring about con- siderable changes in the relative efficiency of different wave lengths. The search for differences in the helio- tropic effect of different wave lengths can be of service in detecting the nature of the photochemical substances responsible for heliotropic reactions. The investigations on the relative heliotropic efficiency of different wave lengths have generally been undertaken for a different purpose, namely, to get information con- cerning the color sensations of animals. Graber gave it as the result of his observations that all animals which were fond of light were also fond of blue, and animals which were fond of dark were also fond of red.180 He put animals into a box half of which was covered with trans- parent glass and half with an opaque object, and then counted the relative numbers of organisms in both halves of the box. He then replaced these screens by colored glasses and obtained the above-mentioned result. The 100 WAVE LENGTH writer showed that the animals are neither fond of blue nor of red but are oriented by the light in the same way as are plants, and the statement that animals which were "fond" of light also were "fond" of blue and those which were "fond" of "dark" were "fond" of red the writer explained in a simpler way, namely that the light filtered through red glass had a smaller orienting effect than the light filtered through blue glass.287 Hence red glass acted like an opaque, blue glass like a trans- parent screen. This had already been known to be true for the heliotropic reactions of plants for which Sachs had shown that they occur behind a blue glass in the same way as behind common window glass, while behind red glass heliotropic reactions do not occur at all or occur very slowly as if the light were weak. The writer was able to show that the same is true for animals.267 When positively heliotropic animals are put into a box covered with blue glass they go as rapidly to the window side as when the box is uncovered; while when it is covered with red glass the animals will go to the window but more slowly and irregularly. Behind a red screen they behave therefore as if they were exposed to weak light. Blue glass is permeable not only for blue but also for rays which produce the sensation of green. Paul Bert 38 had already made experiments with positively heliotropic Daphnia in a solar spectrum and found that the animals "accouraient beaucoup plus rapidement au jaune ou au vert qu' a toute autre couleur."a Bert concluded from this that to the eye of a Daphnia those parts of the spec- trum appear brightest which also appear brightest to the human eye. Bert's aim was to find out whether the sensa- a This method of ascertaining the most efficient part of the spectrum is not reliable and has been replaced by other methods. 102 TEOPISMS tions caused by light in lower animals are the same as those caused in a human being. But even if the relative efficiency of the various parts of the spectrum were the same for the sensations of brightness in human beings and for the heliotropic reactions in lower animals, it would not prove that the latter also have sensations of brightness. For we have no guarantee that the heliotropic reactions of lower animals are due to or accompanied by sensations of brightness. If the yellow-green rays are the most efficient in causing heliotropic reactions in an organism, it suggests only that in such an organism the photosensi- tive substance responsible for the heliotropic response is most easily decomposed by the yellow-green part of the spectrum. A similar error of reasoning as that by Bert has recently been made by Hess. Hess corroborated what the writer had already pointed out, that the red rays of the visible solar spectrum are the least efficient, and he found, moreover, as Bert had found for Daphnia, that for the heliotropic reactions of a number of animals, from the fishes down, the yellow-green region of the solar spectrum is the most efficient. Now it happens that to a totally color blind human being, to whom the different parts of the solar spectrum appear only as shades of gray, the region A = 540 w in the yellow-green appears to be the brightest; while the red part of the spectrum gives a very faint sensation of brightness. From this similarity or apparent identity between the relative effects of dif- ferent wave lengths upon the heliotropic effects of certain lower animals and upon the sensations of brightness of a totally color blind human being, Hess draws the con- clusion that these animals are totally color blind. In our opinion the only conclusion which Hess has a right to WAVE LENGTH 103 draw is that the photosensitive substance which causes sensations of brightness in the eye of the color blind human being is either identical with or is affected in a similar way by light waves as is the substance giving rise to heliotropic reactions in certain animals. This assumption is entirely adequate and harmonizes better with the facts than the assumption made by Hess. The substance responsible for the sensations of brightness in the eyes of the totally color blind human being is visual purple which is bleached most rapidly by light of A = 540 w. That our objection is justified is proved by the experiments of v. Frisch on bees. Frisch168 has shown by very ingenious and care- ful experiments that bees can be trained to discriminate between blue and yellow but not between different shades of gray. On a table were put square cardboards of dif- ferent shades of gray and among them one blue piece of cardboard. On each gray square was put a watch crystal containing water, while the watch crystal on the blue con- tained sugar water. The bees of an observation hive visiting the sugar crystal were marked with a fine paint brush. After a sufficient period of training it was found that the marked bees always went directly to those crys- tals which were on a blue piece of cardboard, whether they contained sugar water or pure water; and when there was no sugar water on the blue cardboard they alighted on any blue object, e.g., a blue pencil. The crystals and cardboard pieces were always renewed in different tests, to avoid any influence of odor. It was never possible to train the bees to select a piece of cardboard with a definite shade of gray among cardboards of different shades of gray. Hess had shown that for the heliotropic reaction of 104 TEOPISMS bees the yellowish-green part of the spectrum is most efficient, and he concluded that bees are totally color blind. To a totally color blind person the blue cardboard appears only like a shade of gray, and such a person is unable to learn to discriminate between a blue and gray piece of cardboard, v. Frisch's experiments support the con- clusion that it is unjustifiable to use experiments on heliotropism to draw conclusions concerning light or color sensations, v. Frisch and Kupelwieser 169 have also demonstrated selective effects of different light waves for Daphnia which differ from those found for the eye of a totally color blind person, and their observations have been confirmed by Ewald.147 2. Hess's conclusions are in conflict with another group of facts. For many plants the blue region of the solar spectrum is the most efficient. Hess is, therefore, compelled to conclude that the heliotropism of such plants is different from that of animals, since it would seem preposterous to assume that swarmspores of plants should go to the light because they have sensations of color. He, therefore, assumes that plants are heliotropic — in the sense of the mechanistic theory — but that positively helio- tropic animals go to the light on account of their love for brightness which is exactly the old viewpoint of Graber. It can be shown, however, that the difference in the helio- tropism of animals and plants, which Hess assumes, is contrary to the facts, since there are heliotropic animals for which the blue rays are the most efficient, as for most plants ; and there are green algae for which the yellowish- green rays are most efficient, as for animals. For many if not most plants the blue rays are the most efficient for inducing heliotropic curvatures. Blaauw47 proved this in the following way: He exposed a row of WAVE LENGTH 105 seedlings of Av ena to a carbon arc spectrum for a certain time. The seedlings were then placed in the dark and after the proper time it was ascertained which part of the spectrum had induced heliotropic curvatures. By vary- ing the duration of time of exposure to the spectrum it was found that with a minimal time of exposure only certain blue rays, namely, those of a wave length of 478 w caused heliotropic bending, while with longer exposure longer waves also became efficient. In this way the mini- mum duration of exposure for various parts of the spec- trum was ascertained. Table IX gives his results. TABLE IX Duration of illumination, in seconds Location of threshold in the spectrum, in micra 6.300 534 [L[i 1,200 510 WA 120 499^ 15 491 w 5 487 w* 4 478 (jm 3 4 466 WJL 6 448ixtx The red and yellow parts of the spectrum were ineffec- tive for the intensity and time limits used and the optimum of efficiency was in the blue, in the region between 466 and 478 w. A shorter series of experiments was made on the fruit bearers of Phycomyces f with the following results : 44 to 47 per cent, of the Phycomyces showed heliotropic curvatures after 192 seconds of illumination at 615 fip after 192 seconds of illumination at 550 up after 16 seconds of illumination at 495 pp after 32 seconds of illumination at 450 ^ after 64 seconds of illumination at 420 /,/, 106 TROPISMS The number of experiments was limited but they indi- cate an optimum between 495 and 450 ^, in this respect agreeing with the results on Avena. The fact then exists that for the heliotropic reactions of certain plants the blue rays are most efficient, while for the heliotropic reactions of a number of animals the yellowish-green rays are most efficient. But this state- ment cannot be generalized. Loeb and Wasteneys determined the most efficient wave length of light for various lower organisms with the result that there are heliotropic animals for which the blue rays are as efficient as they are for plants ; and that for different unicellular green organisms the opti- mum lies in different parts of the spectrum. They found, by a method similar to that used by Blaauw, that for the heliotropic curvature of the animal Eudendrium the most efficient part of the spectrum lies in the blue A = approxi- mately 473 /u/x.811 The same was found by them for the larvae of the marine worm Arenicola. On the other hand, on investigation of two closely related forms of green flagellates, Euglena and Chlamy- domonas, it was found311 that they behave differently. For Euglena viridis 'the blue rays A — 470 to 480 w are especially efficient, while for Chlamydomonas pisiformis the most efficient part was in the region of A = 534 ^, in the yellowish-green.b For another green algae, Pan- do rina, Loeb and Maxwell had already found the greatest efficiency in the greenish-yellow. bThis would lead us, on the basis of the reasoning of Hess, to the conclusion that the unicellular plant Chlamydomonas has sensations of brightness, suffers from total color blindness (although it has no eyes), that it is not heliotropic, and that it is an animal ; while its unicellular cousin, Euglena, has a highly developed color sense, has no sensations of brightness, is heliotropic, and is a plant. WAVE LENGTH 107 The method used for these experiments by Loeb and Wasteneys is as follows : A carbon arc spectrum, about from 18 to 23 cm. wide, was thrown on a black screen 88 (see Fig. 34) with two slits a and b in the two different parts of the spectrum which were to be compared in regard to their heliotropic FIG. 34. — Method of determining the relative heliotropic efficiency of two different parts of the spectrum. (After Loeb and Wasteneys.) efficiency. The two beams of light passing through the slits are reflected by the two mirrors M and M^ into the square glass trough cdfe in such a way as to strike the same region g of the back wall of the trough. The glass trough is surrounded by black paper except at R and R19 where the two beams of light enter from the mirrors. Be- fore the experiment begins, all the organisms are collected in the spot g with the aid of an incandescent lamp. As 108 TEOPISMS soon as the spectrum is turned on, these organisms are simultaneously exposed to two different beams of light which come from the two mirrors M and M^. When one type of light, e.g., that from M, is much more efficient than the other coming from M19 practically all the organisms are oriented by the light from M and move toward this mirror, collecting in the region R. When the relative effi- ciency of the two types of light is almost equal, the organ- isms move in almost equal numbers to R and R^. By using as a standard of comparison the same region of the spectrum and successively altering the position of the other slit in the spectrum, we were able to ascertain with accuracy the relative efficiency of the different parts of the spectrum for the two forms of organisms. When the two parts of the spectrum which are to be compared are very close to each other, it is necessary to deflect the beams with the aid of deflecting prisms, before they reach the two mirrors.311 Experiments on the newly hatched larvae of Arenicola, a marine worm, showed that the most efficient part of the spectrum was in the bluish-green of about A = 495 /«/*, while for the larvae of Balanus eburneus the most efficient part of the spectrum was found by Loeb and Maxwell, by Hess, and by Loeb and Wasteneys in the region of yellow and yellowish-green.311 Mast 348 made similar experiments on these organisms with a method in which the organisms were exposed to two beams of light of different wave length crossing each other at right angles. One light was kept constant while the other was made intermittent by a disk with a sector cut out rotating in front of the light. The size of the sector was varied until the organisms moved at an angle of 45° to the two beams. When this happened the heliotropic WAVE LENGTH a Q s p *• I OSOSOsOSOSCn CnCnCn rfi. CO tO i— k O CO 00 -. rfi.rfi.rfi. Cn Cn Cn Cn Cn Cn rfi. Cn rfi. CO tO i— ' O CO rfi. rfi. rf>. rfi. CO CO CO rfi. rf^. rfi. rfi. rfi. rfi. rfi. 00 1— k >— k i— k O CO OS tO "o ° V, g£££ O CnOO Cn C*> CO i— k Cn !' lit .^ O tO CnCOCO CO i— k os ,_» L_l |_1 ,_l to oo o co co co os k— k t-^ t—l OS CO O M Cn 1 If tO O Cn O O Cn O O ggS^Sgg S3S88 m II f 110 TEOPISMS efficiency of the two beams was considered equal. Mast's results, which are given in Table X, agree with those of Loeb and Wasteneys. The error which Hess makes is of epistemological in- terest inasmuch as it shows the danger of false analogy. The real analogy for heliotropic reactions are forced movements and other tropisms, e.g., galvanotropism or geotropism. Since forced movements (e.g., in Meniere's disease) and galvanotropic reactions caused by a constant current through our head are not determined or accom- panied by special sensations, the same may be true in regard to heliotropic reactions. This is not an idle assumption, since we know that the contraction of the iris of our eye under the influence of light is not accom- panied by any sensation of brightness or color and such contractions occur also under the influence of light when the iris is excised. Hess ignores not only this analogy, but the whole existence of forced movements and of other tropisms, and he uses the color and light sensations of human beings, who are not heliotropic, to explain helio- tropism in animals about whose sensations we know noth- ing. He fails to see that by this false analogy he dodges the real problem of heliotropism, namely, why the tension of symmetrical muscles changes upon one-sided illumina- tion of an animal. For the explanation of this problem, we find assistance in the field of forced movements and of galvanotropism and of geotropism, but not in the behavior of totally color blind human individuals who show no trace of heliotropism. The adoption of the false analogy between visual sen- sations and heliotropism makes it impossible for Hess to admit that bees should be heliotropic and at the same time be able to discriminate between blue and gray ; while WAVE LENGTH 111 if we take cognizance of the analogy between heliotropism and the other tropisms we realize that the heliotropism of the bees and their reactions to blue are separate and independent phenomena, which need not be mutually ex- clusive and which in all probability depend upon different parts of the brain. When in certain cases the relative heliotropic efficiency of the various parts of the spectrum is identical with the curve for its apparent relative bright- ness to a totally color blind person, we may conclude that the photosensitive substances responsible for the two groups of phenomena behave similarly or may even be identical, but not that the sensations of brightness of the color blind and the heliotropic reactions of insects are' identical or analogous phenomena. Many mutants of Drosophila differ in regard to the pigments of the eye. It was natural to raise the question whether or not such hereditary variations of pigmentation of the eye influence the reaction of the flies to monochro- matic light. McEwen investigated this possibility with the following result : ' ' Colored lights which may be con- veniently described as violet, green and red, are effective in the order named upon the insects whose eye color is lighter than the red eye of the wild fly. In the case of wild flies, and flies whose eyes are of a still darker shade called sepia, red is more effective than green" (McEwen549). CHAPTER XII CHANGE IN THE SENSE OF HELIOTROPISM WE have stated that while in a positively heliotropic animal a one-sided illumination increases the tension of the muscles which turn the animal toward the source of light, in the negatively heliotropic animal the one-sided illumination must result in the opposite effect, namely, in a diminution of tension in the same muscles. As a conse- quence, the negatively heliotropic animal can turn more easily away from the light than toward the light. Groom and Loeb 183 noticed that the larvae of the bar- nacle upon hatching go directly to the light and gather at the light side of a dish, but that sooner or later their positive heliotropism may give way to an equally pro- nounced negative heliotropism. The stronger the light the more rapidly the larvae are transformed into negatively heliotropic organisms. Later the reversibility of the sense of heliotropism was observed and studied in a number of organisms.291 In a summary of the subject 30° (p. 470) the writer pointed out that this reversion was due either to a modification of photochemical processes or to an effect upon the nervous system. That an influence on the nervous system can indeecl bring about a change in the sign of the reaction is very strikingly demonstrated in the following observation of A. E. Moore on starfish.525 Ordinarily, when a starfish which is moving in an aquarium is touched, it stops immediately and clings tenaciously to the surface of the vessel with its tube feet, so that it is impossible to remove the animal without injury to the tube feet. This normal response to sudden contact can be completely reversed by the administration of strychnine, so that when touched the animal loosens its hold on the bottom completely. 112 HELIOTBOPIC TEANSFOEMATION 113 The starfish poisoned with strychnine upon sudden touch withdraws all the tube feet, so that it can be moved about like an inert object. For this purpose 1 or 2 c.c. o±' a 0.5 per cent, solution of strychnine sulfate were injected into a starfish of medium size. If the stretching out of the tube feet is due to an in- crease in the tone of the ring muscles (and a decrease in the tension of the longitudinal muscles) the drawing in is due to an increase in the tone of the longitudinal muscles of the tube feet. We therefore see that the same " stimulus, " namely, a sudden touch, which causes one set of muscles to contract in a normal animal causes the antagonists of these muscles to contract in an animal poisoned with strychnine. We shall see that a number of cases of reversal of heliotropism may well find their explanation on this basis. On the other hand, the phe- nomena of solarization known in photography indicate that the sign of heliotropic response may also be changed by an excessive action of light on the photochemical sub- stance. This effect, of course, may in the last analysis also result in an influence upon the central nervous system, such as that brought about by strychnine in Moore 's ex- periment. We will now consider some cases more in detail. The writer found296 that certain fresh water crusta- ceans, namely Californian species of Daphnia, copepods, and Gammarus when indifferent to light can be made intensely positively heliotropic by adding some acid to the fresh water, especially the weak acid C02. When car- bonated water (or beer) to the extent of about 5 or 10 c.c. is slowly and carefully added to 50 c.c. of fresh water containing these Daphnia,, the animals will become in- tensely positive and will collect in a dense cluster on the 8 114 TKOPISMS window side of the dish. Stronger acids act in the same way but the animals are liable to die quickly. Esters, e.g., ethylacetate, act also like acids and the addition of 1 c.c. of a grammolecular solution of ethylacetate to 50 c.c. fresh water also makes all the organisms positively helio- tropic. Alcohols act in the same way. In the case of Gammarus the positive heliotropism lasts only a few seconds, while in DapJmia it lasts from 10 to 50 minutes and can be renewed by the further careful addition of some C02. The following table gives the minimal con- centration of various acids and alcohols for the production of positive heliotropism in certain California species of fresh water copepods, and Daphnia: For Copepods For Daphnia Formic acid 0.006 N Acetic acid 0.006 N Propionic acid 0.005, N Butyric acid 0.004 N Valerianic acid 0.004 N Capronic acid 0.002 N 0.6 N Ethyl alcohol 0.19 N 0.2 N Propyl alcohol 0.054 N 0.05 to 0.1 N Normal butyl alcohol 0.019 N Isobutyl alcohol 0.04 N Amyl alcohol 0.011 N As far as alcohols are concerned each higher alcohol is about three times as efficient as the previous one, with the exception of amyl alcohol. This order of relative efficiency is also characteristic for the surface tension effects of these alcohols.299 It was of importance to find means of making these organisms negatively heliotropic. Moore368 found that caffein makes the heliotropically indifferent fresh water crustacean Diaptomus intensely negatively heliotropic. It required the addition of 1.2 c.c. of a 1 per cent, solution of caffein to 50 c.c. of water to bring about this intense HELIOTEOP1C TRANSFORMATION 115 negativation. In two minutes all the animals are col- lected in a dense cluster on the negative side which lasts for about 35 minutes. A weak negative collection could also be obtained by adding 0.1 c.c. of a 0.5 per cent, solu- tion of strychnine nitrate. Moore found that if the Diap- tomus were first made positively phototropic by the addi- tion of alcohol or acids, it was impossible to alter their response by the action of caffein, strychnine, or atr opine. On the other hand, animals which had formed a negative collection under the influence of caff ein if treated with car- bonated water at once changed their response and swim- ming to the light side of the dish formed a positive gathering. What causes these effects? The fact that alcohols make the organisms positively heliotropic suggested the possibility of a " narcotic " effect; the writer found, how- ever, that narcosis requires a concentration of alcohols three times as high as the one required to produce positive heliotropism. He tried the effect of temperature on the reversal of the sign of heliotropism in Daphnia and found that lowering of the temperature enhanced the effect of acids in making the animals positive.296 The writer had found previously that in marine crus- taceans and in larvae of a marine annelid, Polygordius, the sense of heliotropism can be reversed by changes of tem- perature as well as by changes in the osmotic pressure of the sea water.291 Increase in the osmotic pressure of sea water (by adding about 1 gm. of NaCl or its osmotic equivalent of other substances to 100 c.c. of sea water) made the negative animals positively heliotropic, and lowering of the concentration by adding 30 to 60 c.c. dis- tilled water to 100 c.c. sea water made positive organisms negative. Negative larvae of Polygordius or negative 116 TEOPISMS marine copepods could be made positive by lowering the temperature, and positive larvae could be made negative by slowly raising the temperature. Since in the latter case the animals suffered from the high temperature the results were not so striking as in the case of the positivat- ing effect of lowering the temperature. The same effect of the concentration of sea water and of temperatures was observed by Ewald for the larvae of Balanus perfor- alus. He found, moreover, the interesting fact that a change of the ratio ~~ in the sea water affected the sign of heliotropism of barnacle larvae. An increase of Na made them more positive, an increase in Mg more negative.144 The larvae of Porthesia are strongly positively helio- tropic before they have eaten, while they lose their helio- tropism almost completely after they have eaten.287 The writer observed that male and female winged ants are strongly positively heliotropic but as soon as they lose their wings their heliotropism ceases.287 McEwen 549 has found that when Drosophila is deprived of its wings its heliotropism ceases. Holmes found that terrestrial amphipods are posi- tively, while the aquatic amphipods are negatively helio- tropic. By putting a terrestrial amphipod into water it became negatively heliotropic.225 That a reversal in the sense of heliotropism may be due to a nervous effect is suggested by an observation by Miss Towle 485 that a certain ostracod, Cypridopsis, can be made positively heliotropic by mechanical shock, and the writer noticed that indifferent fresh water Gam- marus can be made negatively heliotropic by shaking them. In both cases the heliotropism lasts only a short time. HELIOTEOPIC TEANSFOEMATION 117 The attempt to explain all these reversals on the assumption of a change in the central nervous system meets with the difficulty that such reversals occur also in unicellular organisms which have no central nervous system. Thus the writer observed that Volvox, which occurred in the same ponds in California from where Daphnia came, could also be made positive by C02.296 In swarmspores of algae reversals of heliotropism are a common phenomenon. While these unicellular organisms have no central nervous system they may have synapses such as exist between different neura of metazoa. The writer is not sufficiently familiar with the behavior of synapses in higher animals to suggest that this condition is responsible for the changes in the sense of heliotropism. We may finally discuss briefly a possible solarization effect. The writer found that it is possible to make ani- mals generally negatively heliotropic with the aid of ultraviolet light.296 If once rendered negative such ani- mals will be negative not only to ultraviolet rays but also to the light of an incandescent lamp. A. E. Moore366 found that the ultraviolet rays having such an effect have a wave length shorter than 3341 A.U. Oltmanns had ob- served that Phycomyces is positively heliotropic in weak light, indifferent in somewhat stronger light, and nega- tively heliotropic in still stronger light. Blaauw found that when the illumination was strong the seedlings of Avena became negatively heliotropic.47 He suggests the analogy with solarization effects in photography. The discovery of photodynamic effects by v. Tappeiner477 adds to the possibilities which should be considered in this connection. While Drosophila is usually positively heliotropic, McEwen has recently described a mutant of this species 118 TEOPISMS which is not heliotropic. This lack of heliotropic response is linked with a peculiar color — "tan" — by which the mutant is characterized. The character "tan" is sex linked. The daughters inherit the factor for the character from their fathers but do not show the character, while the sons inherit the factor from their mothers and do show the character. The lack of heliotropic reaction in this mutant is apparently not due to any structural defect in the eye (McEwen549). Keeping successive generations of flies in the dark does not influence their heliotropism. F. Payne 550' 551 raised sixty-nine successive generations of Drosophila in the dark, but the reaction of the insects to light (as well as their eyes) remained entirely normal. CHAPTER XIII GEOTEOPISM 1. When the stem of certain plants is placed in a horizontal position, the apex grows vertically upward and the root downward. The downward growth of the root is called positive, the upward growth of the apex nega- tive geotropism. The writer has observed a similar phe- nomenon in a hydroid, Antennularia antennina 294' 30° and his observations were confirmed by Miss Stevens.553 Animals as well as plants, therefore, show the phenomenon of geotropism. These phenomena have given rise to a strange dis- cussion, namely : What constitutes the ' ' stimulus ' ' in the case of geotropism? When a galvanic current is sent through a motor nerve the muscle answers with a con- traction only when the current is made or broken, but not while a constant current is flowing through the nerve. The older physiologists were not able to form a mental picture of what happened in this case, and they cut the knot by invoking a verbalism, namely by calling the mak- ing or breaking of a current a ' l stimulus. ' ' This perhaps innocent verbalism then led to the less harmless dogma that only a rapid change could act as a " stimulus. ' ' Thus Jennings 253 and Mast 346 took it for granted that phe-^ nomena __pf _orientatiQii_ by Jight could only be produced by rapid changes in the intensity of light and not by constant illumination, since they had the a priori convic- tion that only a rapid change in the intensity of a gal- vanic current or of light is a " stimulus. " The same diffi- 119 120 TEOPISMS culty arose in regard to the action of gravity upon orien- tation, since it was contrary to the definition of a i l stimu- lus ' ' that the mere permanent lying in a horizontal posi- tion should cause the apex of a stem to bend upward. All these difficulties disappear if we take the law of chemical mass action into consideration. Light acts not as a "stimulus" but acts by increasing the mass of certain chemical compounds, and it is the mass of these products which is responsible for the effect of light. Now, mass action is not proportional to the rapidity of the change of acting masses but to the acting mass itself. When two sides of an organism are struck by light of different intensity the quantity of photochemical products on both sides becomes unequal. In galvano- tropism the galvanic current alters the distribution of the mass of certain ions along the nerve elements. It can be shown that gravitation acts by influencing | the distribution of chemical substances in an organism. '! When the stem of a plant is put into a horizontal position ! certain chemical substances gather in greater concen- tration on the lower side of the stem ; and this causes a difference in the velocity of chemical reactions between < the lower and the upper side. As a result of this we •notice the bending. In the normal upright position of the plant the same substances were distributed equally about the axis of symmetry. The following facts may be offered as a proof for this statement.526 When we put a piece of the stem of Bryo- phyllum, calycinum in a horizontal position it soon bends and gradually assumes the form of a U with the concave side above (Fig. 35). This bending is due to the fact that the cortex on the under side of the stem grows in length while the cortex on the upper side remains unaltered FIG. 35. — Geotropic curvature of stems of Bryophyllum calycinum. These stems were originally straight and suspended in a horizontal position. In about ten days they bent, becoming concave on the upper side. - The black rings, made with india ink, which were originally parallel, remain unaltered on the upper side of the stems, while their distance \ V« increases on the lower side, indicating that the curvature is due to an increase in growth ) " on the lower side (of the cortex) of the stem. cp a £»• « cr 1 = — c &? if CD ^ QJ g O l| > 0) CS.C ** N O •5,4 ° S 322>326 has shown that the phenomena which were formerly described as rheotropism in fish are due to the orienting effect of moving retina images. The reader is familiar with the fact that many fish when in a lively current have a tendency to swim against the current. This phenomenon was believed to be due to the friction of the water. Lyon showed that fish orient themselves just as, well when they are put into a closed glass bottle, which is dragged through the water, although in this case they are not under the influence of any fric- tion from the current. When the bottle is not moved the fish swim in any direction inside the bottle. It is obviously the motion of the retina images of the objects on the bank of the brook which causes the "rheotropic" orientation of fish. When driven backward by the current or when dragged backward in a bottle through the water, the objects on the bank of the river seem to move in the opposite direction. The animal being compelled to keep 132 TBOPISMS the same object fixed, an apparent forward motion of the fixed object changes the muscles of the fins in such a sense as to cause the animal to follow the fixed object automatically. When such rheotropic fishes were kept in an aquarium and a white sheet of paper with black stripes was moved constantly in front of the aquarium the fish oriented them- 1 fi FIG. 39. — Influence of motion of the hand of an observer on the direction of the motion of a swarm of sticklebacks in an aquarium. The arrows indicate the direction in which the hand was moved. The swarm of fish moves always in the opposite direction in which the hand is moved. (After Garrey.) selves against the direction in which the paper and its stripes moved. The phenomenon was more marked in young than in older specimens. All the phenomena of rheotropism ceased in the dark or when the fish were blind. Wheeler 5Q8 has observed a phenomenon of anemotrop- ism, namely that certain insects have a tendency to put the axis of their body in the direction of and against the wind. He considers this analogous to the phenomenon of rheotropism in fishes. The cause is also in all probability the tendency toward fixation of the moving retina image. A very pretty demonstration of the orienting effect of moving retina images was discovered by Garrey in BHEOTKOPISM 133 sticklebacks.176 When a swarm of such fish was kept in an aquarium it was noticed that all the fish were oriented with the long axes parallel and that the whole school swam in a course parallel, but in a direction opposite, to that of the moving observer. If the observer remains station- ary opposite the aquarium and moves an object, prefer- ably white, which is held in the hand, the little fish at once respond by moving slowly and oppositely to that of the moving object. They can be thus made to move up or down or to the right or left (Fig. 39). By experiments which space forbids us to report in detail Garrey has reached the conclusion that the motion of a near object causes an apparent motion of the whole horizon in the opposite direction and this apparent motion the fish tries to compensate by the motions of its body. This brings the observations on the stickleback into har- mony with the general influence of moving retina images, consisting in a compensatory motion of the fish. We have already referred to the fact that the influence of a moving retina image is capable of compensating the forced movement of a dog after a one-sided lesion of the cerebral hemispheres. CHAPTER XV STEBEOTROPISM OUK orientation in space is determined by three groups of tropistic influences, two of which we have already dis- cussed, light and gravitation. The third one is pressure on certain nerve endings of the skin. When the tactile influences on the skin of the soles of the feet are weakened (as is the case in locomotor ataxia), the patient finds it difficult to stand and walk in the dark. When he can use his eyes the difficulty is diminished, since the orienting effect of the retina image can compensate the tactile de- ficiency; just as we have seen that the effect of the loss of the ears in crustaceans can be compensated by the orienting influence of the eyes. The role of tactile influences on the orientation of ani- mals is most clearly demonstrable in starfish, flatworms, and many other animals, when put on their backs. The animals "right" themselves, i.e., they turn around until the ventral surfaces or their feet are pressed against solid objects again. As the writer pointed out long ago,293 gravitation has nothing to do with the phenomenon, since starfish will stick to solid surfaces with their tube feet even if by so doing their backs are permanently turned to the center of the earth. Unless the nerve endings on the sole of their tube feet are pressed against a solid surface the animals are restless and the arms move about until the feet are again in contact with solid bodies. This phe- nomenon of orientation the writer called stereotropism. Quantitative investigations of this form of tropism are 134 STEREOTROPISM 135 still lacking and we must be satisfied with a few descriptive remarks. Certain animals show a tendency to bring their body completely into contact with solid bodies, e.g., by creeping into crevices. Without further experimental test this might appear as an expression of negative heliotropism, but it can be shown that this assumption would be wrong. Amphipyra is a positively heliotropic butterfly which, in spite of its positive heliotropism, shows the peculiarity that it creeps into crevices when given an opportunity. Such animals were kept in a box at the bottom of which was a square glass plate resting with its four corners on supports just high enough to allow the animals to creep under the glass plate. After some time every Amphipyra was found under the glass plate. This happened also when the glass plate was exposed to full sunshine, while the rest of the box was in the shade.287 The same stereotropism is found in female ants at the time of sexual maturity. When such animals are put into a box containing folded pieces of paper or of cloth, after some time every individual is found inside the folds. This happens also when the boxes are kept in the dark.287 The same form of stereotropism is found in many species of worms. When earthworms are kept in jars with vertical walls they are found creeping in the corners where their body is as much as possible in contact with solid bodies. It is this tropism which compels the animals to burrow into the ground. Maxwell349 kept Nereis, a form of marine worms, which burrows in sand, in a porcelain dish free from sand. Into the dish glass tubes were put, whose diameter was of the order of that of the worms. After 24 hours every tube was inhabited by a worm who made it its permanent 136 TEOPISMS abode. They even remained in the tube when exposed to sunlight which rapidly killed them. We find the opposite, negative stereotropism, in many pelagic animals, e.g., larvae of the barnacle or of other crustaceans, which avoid contact with solids. The phenomenon is liable to interfere with heliotropic experiments. The importance of stereotropism in animals was first pointed out by the experiments of Dewitz on the sperma- tozoa of the cockroach.120, 121 He noticed that when a drop of salt solution containing the spermatozoa was put under a cover glass resting on low supports on a slide, the spermatozoa collect at the solid surfaces of the slide arid cover glass, while the liquid between remains free from spermatozoa. When a small glass bead is put into the liquid the spermatozoa will also swim on the surface of the bead, never leaving it again. Dewitz is of the opin- ion that this stereotropism is of assistance in securing the entrance of a spermatozoon into the egg. The egg of the cockroach is rather large and the spermatozoon can enter it only through a micropyle. When the egg is laid it passes by the duct of the seminal pouch in which the female keeps the sperm after copulation. On passing the duct some spermatozoa reach the egg. Dewitz points out that these cannot leave the surface of the egg any more but are compelled to move incessantly on the surface of the egg until one of the spermatozoa by chance gets into the micropyle. It is an important fact that different organs of the same organism react differently. We have already men- tioned the tendency of starfish or flatworms to right them- selves, i.e., their ventral surface is positively their dorsal negatively stereotropic. The stolons of hydroids stick STEEEOTEOPISM 137 to solid bodies, while the polyps bend and continue to grow away at right angles from solid bodies with which they come in contact. Thus the stem of Tubularia mesem- bryanthemum, a marine hydroid, grows in a straight line. When such stems, after their polyp is cut off, are put with one end in sand, the free end forms a new polyp and the stem continues to grow in a vertical direction upward. When, however, the stem is put near the glass wall as soon as the polyp grows out it bends away from FIG. 40. — The regenerating polyp of Tubularia when in contact with the glass wall of an aquarium bends at right angles to the glass wall. the solid wall, and the stem will now continue to grow at right angles to the vertical wall (Fig. 40). This phenomenon raises the question whether or not the law of chemical mass action underlies phenomena of stereotropism. We have seen that this law dominates the phenomena of heliotropism, inasmuch as the Bunsen- Eoscoe law is the expression of the influence of light on the mass of the photochemical reaction product. We have also been able to show that in the case of the geo- tropic curvature of Bryopliyllum the mass of the apical 138 TEOPISMS leaf determines the rate of geotropical curvature of a horizontally placed stem. The only way in which the mass of the leaf could have such an influence is through the mass of substances it sends into the stem, so that this case of geotropism is a function of mass action. Tnere are indications that the way contact with a solid in- fluences the behavior of living matter is also through the influence on the rate of certain chemical reactions. The writer observed that the stolons of a hydroid, Aglao- phenia, have a tendency to adhere to solid surfaces and not to leave them any more if they once reach them, and that as soon as such a stolon reaches a solid surface, e.g., a piece of a glass slide, its growth is accelerated con- siderably. It was very astonishing to notice how much more rapid the growth of roots of Aglaophenia was when they were in contact with a solid body than when they grew in sea water. The rate of growth is the function of a chemical mass action (Loeb 543). CHAPTER XVI CHEMOTEOPISM 1. When we create a center of diffusion in water or in air we may theoretically expect orienting effects. Thus when a fine capillary tube containing a solution of a salt, e.g., sodium malate, is put into a drop of water containing motile organisms, and the right side of an organism is turned to the source of diffusion, the diffusing molecules will collect in increasing concentration on that side. On the left side of the organism, no such increase in the con- centration of molecules will occur. If now the molecules collecting on the right of the organism in increasing den- sity are able to produce some chemical or some concen- tration chain effect, the two sides of the organism will be acted upon unequally and the tension of the symmetrical motile organs will no longer be the same. As a conse- quence the organism will turn until the mass of molecules or ions striking the organism in the unit of time will be the same for both sides. These effects only take place when the organism is close to the opening of the capillary tube, since the diffusion from the tube is slow. It is obvious, however, that it is difficult to provide experimental conditions which give exact chemotropic reactions. First of all, if the diffusion is rapid the differ- ences in concentration of the effective chemotropic sub- stance on two sides of an organism are too slight to result in a turning movement. A second condition which is liable to vitiate the result are the unavoidable convection cur- rents due to changes or differences of temperature. In 139 140 TBOPISMS order to get clear results a method must be used which prevents a rapid diffusion of the substance; and, more- over, the current of diffusion must be confined to an almost straight line. It is possible that Pfeffer's method satisfies this condition.424.425 He introduced the sub- stance to be tested for its chemotropic effect into a capil- lary tube, the end of which was then sealed. The other end was pushed into a drop of water containing the sus- pension of the organisms whose chemotropism was under investigation. From this capillary the diffusion was ex- tremely slow. Moreover, the current of diffusion was approximately linear at the orifice. Hence the test for the existence of positive chemotropism was perhaps pos- sible. When an organism, struck sidewise by the line of diffusion near the opening of the capillary tube, turns toward the tube going into it, some probability of positive chemotropism exists ; and when all the organisms coming near the orifice of the tube are thus compelled to go into it, the probability may become certainty, provided that the substance used does not paralyze the organism and therefore act as a trap, allowing the organisms to come in but not to go out. The capillary tubes used were of 10 to 15 mm. length and of a width of about 0.1 mm. Pfeffer and his pupils found that the spermatozoa of ferns go in large numbers into a capillary tube containing sodium malate in a concentration of 0.01 per cent, (a solu- tion ten times as diluted is still slightly active). This effect of the malate is specific in this case and this indi- cates that either a definite chemical action of the malate ion or a specific permeability of the organism for it is the source of the chemotropism. Such specific chemotropic effects are not rare, since Pfeffer found that Bacterium termo and Spirillum undula are positively chemotropic CHEMOTEOPISM 141 to a liquid containing 0.001 per cent, of peptone or of meat extract. It is stated that cholera bacilli are strongly attracted by potato sap. Pf effer found also that the sper- matozoa of certain mosses are positively chemotropic to cane sugar solution in dilutions of 0.1 per cent. Pfeffer's work preceded the discovery of electrolytic dissociation, and his pupils Buller89 and Shibata465 made some of the additions required by the theory, namely, that it is the malate anion which acts in the case of the spermatozoa of the ferns, and that when the anion is offered in the form of malic acid the H ion counteracts the effect of the malate anion. Shibata made extensive experiments on the chemotrop- ism of the spermatozoa of Isoetes 465 which he found posi- tively chemotropic for the malate anion, and also for the succinate, tartrate, and fumarate anion, when offered in the form of their neutral salts. The anion of the stereo- isomere of fumaric acid, namely of maleic acid, was with- out effect. This indicates a high degree of specificity of these reactions. Neutral sodium malate acted best in dilutions from m/100 to m/1000, but some action could still be discovered in m/20,000 solutions. When malic acid was used no positive chemotropism could be discovered in solutions of m/100 or above on account of the contrary effect of the hydrogen ion, and the spermatozoa of Isoetes did not even go into capillary tubes containing m/1000 malic acid. When any acid other than malic was added to sodium malate the motion of the spermatozoa into the tube was prevented, even a m/6000 HC1 solution still had such an effect. Shibata studied especially the mode by which the spermatozoa are oriented chemotropically by malates and found that the reaction consists always in a turning of 142 TBOPISMS the axis of the body of the spermatozoa toward the capil- lary tube containing malates or succinates, as the tropism theory demands. When the capillary tube and the surrounding medium contain the same solute for which the organisms are posi- tively chemotropic, they will not go into the tube unless the concentration in the tube is a definite multiple of the concentration of the outside solution. Thus Pf effer found that the concentration of sodium malate in the capillary must be at least thirty times as great as in the outside solu- tion to induce the spermatozoa of fern to move into it, and in the case of Bacterium termo the solution of meat ex- tract in the tube had to be at least four times as great as the outside solution. In the case of Isoetes spermatozoa Shibata found the ratio of about 400 to 1. This constancy of the ratio is known as Webpr's law, which therefore holds for chemotropic phenomena. Lidforss281 found with the aid of Pf effer 's method that the spermatozoa of Marchantia are positively chemo- tropic to certain proteins, especially egg albumin, vitellin from the egg yolk, hemoglobin, and mucin of the sub- maxillary gland ; blood albumin, casein, and legumin were less effective. The lowest concentration for hemoglobin solutions and for egg albumin was 0.001 per cent. ! It may also be stated that Lidforss found a chemo- tropic effect of proteins upon the direction of growth of pollen tubes.280 Bruchmann81 found that the spermatozoa of Ly co- podium were positively chemotropic to the watery extract in which pieces of the prothallium had been boiled. Pfef- fer's capillary method was used. They showed also posi- tive chemotropism to the citrate anion. Thus, sodium citrate was efficient in a 0.1 to 0.5 per cent, solution. The CHEMOTEOPISM 143 lower limit was a little above a 0.001 per cent, solution. The effect of the free citric acid was a mixed one since the spermatozoa were negative to H ions and positive to the citrate anion. Instead of being able to use a 0.1 per cent, solution, as in the case of the sodium salt, a 0.01 per cent, solution was the highest concentration to which they were positively chemotropic. This means that the hydrogen ion of citric acid solutions above m/1000 repel the spermatozoa, while when solutions of m/2000 or below are used the hydrogen ion effect no longer in- hibits the positive effect of the citrate anion. In addition the validity of Weber 's law could be demonstrated. The spermatozoa were indifferent to malates, oxalates, and many other salts, as well as to sugar and proteins. 2. While all the botanical observers, from Buller on, had found that the hydrogen ion has only a preventive effect upon the positive chemotropism of lower organisms, Jennings tried to show that acids have a positive effect, especially when in low concentrations.250 But his con- centrations are not quite as low as he seems to assume, since a 1/50 per cent, (m/180) HC1 solution, toward which he believes to have proven positive chemotropism of Para- m&cia, is a deadly concentrations Jennings 's interest in the problem was aroused by a phenomenon of aggregation, not infrequently found in the suspensions of infusorians. It is well known that when certain infusoria are left undisturbed they do not remain scattered, but gather in more or less dense groups. Thus, if they are mounted on a slide in a thin layer of water, soon dense aggregations will be formed in certain areas, while the remainder of the a The cells of the stomach resist a much higher concentration of HC1 but this is an exception. Infusorians, fish, and organisms in general are killed in a short time in m/180 HC1 or in a much lower concentration of acid. Thus Fundulus does not live more tham one hour in m/3000 HC1 or HN03. (Loeb, J., and Wasteneys, H., Biochem Z., 1911, xxxiii, 489; 1912, xxxix, 167.) 144 TBOPISMS slide will be nearly deserted. One of the first investigators to describe this phenomenon was Pfeffer. He observed its occurrence in Glaucoma scintillans, and less markedly in Colpidium colpoda, Stylonychia mytilus, and Paramcecium. Pfeffer was inclined to believe that these aggregations were due, partly at least, to a contact stimulus, resulting from a striking of the organisms against small solid bodies, and especially against each other.250 This conclusion of Pfeffer may after all be correct, since it has been shown that sea water containing jelly from the egg of a sea urchin causes spermatozoa to stick together for some time when they impinge upon each other. This agglutination no longer occurs when the spermatozoa are immobilized. Jennings came to the con- clusion that these aggregations of infusorians are due to the fact that they can go into a weak concentration of acid, while they cannot escape from such a weak concen- tration; and since Paramcecia themselves produce C02 he assumed that the C02 produced by themselves acts as a center of attraction for other Paramcecia. In order to prove this he used the following method : The organisms were studied in a thin layer of water, by mounting them on a slide covered with a large cover glass supported near its ends by slender glass rods. Their reactions were tested by introducing with a capillary pipette a drop of the substance in question beneath the cover glass, or in some cases by allowing it to diffuse inward from the side of the cover glass.250 Thus Jennings introduced a drop of 1/50 per cent. (m/180) HC1 on a slide containing Chilomonas. Very soon a somewhat denser ring of these individuals was formed around the drop (Fig. 41). A 1/50 per cent. HC1 solution paralyzes (and .soon kills) Chilomonas or Para- mcecia and hence the surface of the drop must act like a trap into which the organisms will steadily swim, with- out being able to swim back. This will naturally increase CHEMOTKOPISM 145 the density of organisms around the drop and may give rise to a ring formation around a high concentration of HC1 although the organisms are not positive to the acid. Jennings found, however, that when such organisms are in a drop of weak acids which do not paralyze the organ- isms quickly, e.g., 1/50 per cent, acetic or in C02 solutions, they become negative to the surrounding neutral medium (H20 or hay infusion) and stay in the acid. He, therefore, assumes that the organisms are positive to weak acid, and FIG. 41. — Reaction of Chilomonas to a drop of 1/50 per cent. HC1. a, preparation immediately after the introduction of the drop (no organisms either within or gathered about the drop), b, the same preparation a few minutes later. (After Jennings.) negative to strong acid as well as to their natural neutral or faintly alkaline medium. This negativity to their natural surroundings when in weak acid as well as to strong acid when in weak acid Jennings does not interpret in terms of the tropism theory, and in this he is probably correct. He interprets both phenomena as a trap action due to the asymmetry of certain infusorians ; a sudden change in the concentration of a solution causes a reverse of the stroke of their cilia by which the organism is driven back. When the old nor- mal stroke of the cilia is resumed the direction of the locomotion is changed on account of the asymmetrical arrangement of the cilia. This happens when the organ- isms go from weak into strong acid or from weak acid into 10 146 TROPISMS a neutral medium. In this way a collection of the organ- isms at the surface of a drop of acid may be brought about. This phenomenon is not tropistic in the strict sense of the word, and as a matter of fact Paramcecium is not positively chemotropic to acid of any strength. Barratt24 investigated the chemotropism of Para- mcecia for varying concentrations of different acids with Distilled Water HCL 0,0001n NaOH 0,001n Hay Infusion Fia. 42. — Method of proving that Paramoecia are not positive to acids of low concentration. (After Barratt.) Pfeffer's method of capillary tubes, counting the number of individuals going into the tube containing acid and comparing it with the number going simultaneously into a control tube containing only distilled water free from C02 (Fig. 42). b The acids used varied from 0.001 N to 0.0001 N. The results were unequivocal. Toward solu- tions of 0.001 N the Param&cia are negative and possibly t> In addition two other controls accompanied the test, namely, one tube containing hay infusion (the natural medium of the organisms) and one alkali. CHEMOTBOPISM 147 also slightly negative to acids as weak as 0.0001 N. In no case, not even with the weakest acid, was it possible to prove the existence of positive chemotropism for acid (or base). The number of Paramcecia which went into a tube containing, e.g., 0.00002 N acid, was on the average not greater than that which went into the control tubes. The tubes were sufficiently wide so that the Paramcecia could and did move into the tubes. Barratt, therefore, concludes that acids have only a repelling action upon Paramcecia which, however, diminishes or disappears when the hydrogen ion concentration approaches that of distilled water. The observations of Barratt contradict the statement that Paramcecia are positive to weak acid. We have seen that when spermatozoa or swarmspores are positive to malates this can be elegantly shown by Barratt 's method. The same method has shown that when even a trace of acid is added to the neutral malates this positivity disappears. By testing systematically all concentrations of different acids within the range to be considered, Barratt found no trace of any positivity to or any trap action by weak acid for Paramcecia. It may be true, however, that when the organisms are in very dilute acid neutral or faintly alka- line water repels them in the way described by Jennings. Barratt states also that there is nothing to support Jen- nings 's assertion that the C02 given off by the Paramcecia causes the aggregation in their natural medium, since they are not positive to low concentrations of hydrogen ions. The natural aggregations of infusorians may be due, as Pfeffer suggested, to transitory agglutinations when Paramcecia impinge upon each other, and the sticki- ness or tendency to agglutinate may possibly be increased 148 TBOPISMS by certain substances produced and excreted by the organ- isms themselves, e.g., C02. 3. The results obtained with the spermatozoa of ferns and mosses by Pfeffer and other botanists led some authors to the tacit assumption that the spermatozoa of animals were positively chemotropic toward substances contained in or secreted by the eggs of the same species. Some accepted this assumption without test, others made tests which they considered adequate but which seem doubtful, and it may be of some interest to discuss the subject, since far-reaching conclusions might be based on these experiments. Pfeffer's method of testing for chemotropism with the aid of the capillary tube has proved satisfactory and the application of this method has shown that the spermatozoa of certain animals, e.g., of sea urchins, are not chemotropic toward substances contained in or given off by the egg. Thus Buller, who had worked in Pfeffer's laboratory on the chemotropism of the sper- matozoa of ferns, investigated carefully and extensively the question whether or not the spermatozoa of echino- derms are positively chemotropic for egg substances.90 His results were entirely negative. Thoroughly washed, ripe unfertilized eggs of Arbacia (Naples) were put into a small volume of sea water for from 2 to 12 hours. Capillary glass tubes, about 12 mm. long and 0.1 to 0.3 mm. internal diameter, and closed at one end, were then half filled with the (super- natant) sea water (which had contained the eggs) by means of an air pump. The tubes were then introduced into a large open drop of sea water, in which fresh, highly motile spermatozoa were swimming. If the eggs excrete an attracting substance it was argued that it should be present in the tubes, and the spermatozoa should collect there. . . . No attraction into the tube could be observed. Except for a surface- contact phenomenon to be further discussed, they went in and out with indifference. Apparently, therefore, the water which had contained the eggs exercised no directive stimulus on the spermatozoa whatever. CHEMOTROPISM 149 I then attempted to find some substance which could give a ehemo- tactic stimulus to spermatozoa. The substances tested were such as are known to give a directive chemical stimulus to many protozoa, the sper- matozoa of ferns, pollen-tubes, etc. The following solutions were tried by the capillary tube method: distilled water; meat extract 1 per cent.; KNOs 10 per cent., 2 per cent. ; NaCl 5.8, 2.9, 0.58 per cent. ; K* malate 1, 0.1 per cent. ; asparagin 1 per cent. ; glycerine 5 per cent. ; grape sugar 18, 9, 4.5, 2.25 per cent. ; peptone 1 per cent. ; alcohol 50, 25, 10 per cent. ; diastase 1 per cent.; oxalic acid 0.9, 0.09, 0.009 per cent.; nitric acid 1, 0.1, 0.01 per cent. No definite chemotactic reaction — neither attraction nor repulsion — was observed in any case. Into tubes containing the weaker solutions the spermatozoa went in and out with apparent indifference. » . . On coming into contact with strong acid solutions (oxalic acid 0.9, 0.09 per cent.; nitric acid 1, 0.1 per cent.) the spermatozoa were killed, and thus formed slight collections. They were thus not able to avoid acids by means of a negative chemotactic reaction.90 Other authors, e.g., Dewitz and the writer, have also reached the conclusion that the egg of the sea urchin con- tains no substance for which the spermatozoon of the same species is positively chemotropic, and that Buller's conclusions that positive chemotropism plays no role in the entrance of the spermatozoon of sea urchins into the egg is correct. F. Lillie has recently expressed the opposite view, namely that the egg of the sea urchin contains a substance to which the spermatozoa are positively chemotropic and to which he gave the name ' l f ertilizin. ' ' 283 He first tried Pfeffer's correct method with capillary tubes with negative result, just as Buller and the rest of the obser- vers. Instead of concluding that the spermatozoa are not chemotropic he discarded the method and used Jen- nings 's method, stating that it gives "incomparably more delicate results than Pfeffer's method of using capillary tubes " (p. 533). Lillie found with this method that the spermatozoa of Arbacia are positively chemotropic to 150 TEOPISMS H2S04 of a concentration as high as N/10 and that they are never negatively chemotropic, not even to the highest concentrations of the strongest acid. It seems to the writer that Lillie 's observations are more naturally ex- plained on the assumption that when an acid is sufficiently strong and concentrated, e.g., N/10 HN03 or H2S04, it will paralyze and kill the spermatozoa, and that when a drop of such acid is introduced in sea water containing spermatozoa, a somewhat denser ring of the organisms will be formed around the surface of the drop on account of this action of the acid. With the same method Lillie tried to prove that the spermatozoa of Nereis and Arbacia are positively chemo- tropic to extracts of their own eggs.283 He proceeded as follows: A suspension of Arbacia sperm, freshly made, was put under a raised cover slip and a drop of the super- natant sea water which had been standing over eggs (as in Buller's experiments) was introduced under the cover slip. Observation with the naked eye showed that around this drop of egg-sea water immediately a dense ring of spermatozoa formed and behind this a clear external zone was formed about 1.2 to 2 mm. wide. The dense ring then broke up into small agglutinated masses. In Lillie 's opinion the formation of this dense ring of spermatozoa at the periphery of the egg-sea water is the expression of a positive chemotropism of the spermatozoa for a sub- stance contained in the egg-sea water, the ' l f ertilizin. ' ' He assumes that the spermatozoa near the drop of egg- sea water all swim to the egg-sea water, leaving a clear space behind them. While this explanation of the ring formation might be true — if supported by a direct chemo- tropic method like Pf effer 's — it can be shown that the ring formation is in all probability due to an entirely different CHEMOTROPISM 151 phenomenon which has no relation to chemotropism or any other tropism. , Buller had already observed that the supernatant sea water of sea urchins contains a substance which causes the agglutination of spermatozoa.90 A drop of sea water in which eggs had been deposited was placed upon a slide and a drop containing spermatozoa near it. On joining the drops a large number of small balls were formed in a very few seconds. When very numerous spermatozoa were present the balls became 0.1 mm. in diameter, containing many thousands of spermatozoa packed together in a dense mass. Buller explains the phenomenon as being due to small bits of egg jelly floating in the sea water so small that they will (like spermatozoa) pass through ordinary filter paper and, so transparent that one cannot directly see them. A few spermatozoa become attached to each piece of jelly, the presence of which may be inferred from the manner in which the small groups of sperma- tozoa move about. Owing to the length of the spermatozoon, although its head may be imbedded in a jelly particle, the tail may remain partly free. The little collections of spermatozoa thus move about hither and thither in no particular direction. When two such groups come by accident into contact they fuse. Certain of the spermatozoa adhere to both little masses of jelly and lock them together. The fused mass combines with other simple and fused masses, and so on.c The writer was able to show that when the jelly of the egg of Strongylocentrotus purpuratus is dissolved by an acid treatment the eggs when washed and transferred to sea water no longer give off agglutinating substances, while the acid sea water containing the dissolved jelly, when rendered neutral through the addition of alkali, will cause the agglutination of sperm.302 While all the jelly can be washed off with an acid treatment in the egg of purpuratus, the same is not true for the egg of Arbacia c This explanation of the fusion of two clusters to a larger one is per- haps not correct. The writer is inclined to ascribe it to the adhesion or agglutination of the spermatozoa of two neighboring clusters with each other, due to a sticky surface on the sperm head, 152 TEOPISMS of Woods Hole. Here the acid treatment does not as a rule dissolve all the jelly, or possibly some new jelly may be given off by the egg. While Buller may be correct in assuming that micro- scopic pieces of the egg jelly form the center of these sperm clusters, the writer reached the conclusion that the dissolved mass of the jelly makes the surface of the spermatozoa transitorily sticky, so that if they impinge against each other they will stick together for some time, until the sticky compound formed by the jelly on the sperm head is dissolved by the sea water, which occurs after a short time. This agglutinating effect of the egg-sea water upon the sperm of Arbacia gives rise to that ring formation which Lillie considers a proof of positive chemotropism. When a drop of egg-sea water is put into a sufficiently dense suspension of spermatozoa, the spermatozoa at the surface of the drop will agglutinate into practically one dense ring around it, and through the diffusion of some of the dissolved jelly through this ring numerous little clusters will form at the external periphery of the ring, and these clusters will fuse with the ring. In this way the clear region behind the ring originates. The process of fusion continues inside the ring with the result that the latter breaks up into numerous bead-like spherical clusters as Lillie described. In a former paper the writer has pointed out the analogy between the phenomena of transitory sperm agglutination (under the influence of egg-sea water) and surface tension phenomena, inasmuch as two small clusters upon coming in contact fuse into one larger one and inasmuch as elongated clusters break up into two or more spherical clusters. The ring formation described by Lillie has, therefore, CHEMOTKOPISM 153 in the opinion of the writer no connection with positive chemotropism.d 4. The method of Pfeffer cannot well be used for larger organisms. Barrows 25 has devised an apparatus which allowed him to test quantitatively the chemotropic reactions of Drosophila. The flies which are positively heliotropic were allowed to go to the light inside of a narrow hollow groove. At a certain spot of the groove two glass bottles were inserted with their openings oppo- site each other, one of which contained the substance to be tested for chemotropic efficiency, while the other served as a control. The number of flies which on their path were deviated by the bottle containing the substance to be tested were counted and their number compared with that going into the control bottle. The collection of odor- ous matter in the groove was removed by suction. In this way it was possible to ascertain that the flies are posi- tively chemotropic to ethyl and amyl alcohol, acetic and lactic acid, and to ether. The chemotropic effect of alcohol was increased through the admixture of traces of an ester, e.g., methyl acetate. > ; In describing the manner of reaction of these flies, Barrows makes the statement that when the odor is weak the fruit fly i ' attempts first to find the food by the method of trial and error, but as the fly passes into an area of greater stimulation, these movements give way to a direct orientation. This orientation is a well defined tropism response." A similar statement had been made by d Lillie also assumes that it is the intensity gradient which determines the direction of motion in tropistic reactions. This is not correct, since posi- tively heliotropic animals go to the light even if by so doing they have to go from strong into weak light ( see page 50 ) . The direction of motion in tropistic reactions is determined by differences in the mass of chemical substances on both sides of a symmetrical animal. 154 TBOPISMS Harper for the heliotropism of certain worms, namely that in strong light the animals move by heliotropism, in weak light by " trial and error." These statements are as erroneous as the assertion that while a stone falls under the influence of gravity a feather finds its way down by the method of ' ' trial and error. ' ' Barrows and Harper overlook the role of mass action and reaction velocity. When an animal is struck on one side only by light or by a chemically active substance emanating from a center of diffusion, the mass of this substance or of the photochemical reaction product in- creases on this side. These substances react with some substance of the nerve endings and as soon as the mass of the reaction product reaches a certain quantity the automatic turning, the tropistic reaction, occurs. When the light is strong or when the animal is near the center of diffusion, this happens in a short time and the tropistic character of the reaction is striking, since the animal is quickly put back into its proper orientation if it deviates from it. When the light is weak or when the animal is at some distance from the center of diffusion it will take a longer time before this critical value of the reaction prod- uct is reached, and in this case the animal can deviate considerably out of the correct orientation before it is brought back into the right orientation. CHAPTER XVII -V THEEMOTROPISM UNDER the name of thermotropism M. Mendels- sohn 352-355 has described the observation that Paramcecia gather at a definite end of a trough when these ends have a different temperature. The organisms were put into a flat trough resting on tubes through which water was flowing. When the water in the tube had a temperature of 38° at one end of the trough, while the tube at the opposite end was perfused by water of 26° the organisms all gathered at the latter end. If then the temperature of the water in the two tubes was reversed the organisms went to the other end of the trough. If one end had the temperature of 10° the other of 25°, all went to the latter end. In this case we are in all probability not dealing with a tropistic reaction but with a collection of organisms due to the mechanism of motion described for Param&cium by Jennings. When these organisms come suddenly from a region of a moderate temperature to one of lower tem- perature the activity of their cilia is transitorily reversed, but owing to the asymmetrical arrangement of their cilia they do not go back in the old direction but deviate to one side. This can lead to a collection of Paramczcia such as Mendelssohn described. - 155 CHAPTER XVIII INSTINCTS THE teleological way of analyzing animal conduct has predominated to such an extent that there has been a tendency to connect all animal reactions with the preser- vation of the individual and the species. Instincts are considered to be such reactions of the organism as a whole which lead to the nutrition of the individual, the mating of the two sexes, and the care of the offspring. "If the tropism theory of animal conduct is justified it must be possible to show that instincts are tropistic reactions. We have insisted in previous chapters that animals indifferent to light can be made strongly positively or negatively heliotropic by certain chemicals or vice versa (e.g., the experiments on certain fresh water crustaceans with acids or alcohol and caffein) . We know that the body itself produces at various periods- of its existence definite hormones and such hormones can act similarly as the acids or the caffein in the experiments on crustaceans, since it makes no difference whether such substances as acid are introduced into the blood from the outside or from certain tissues of the animal's own body. We know through F. Lillie 's observations that in the blood of the male cattle embryo substances circulate which inhibit the develop- ment of secondary sexual characters of the female embryo, and we know through Steinach's experiments that the in- termediate tissue from the sexual gland of one sex when introduced into the castrated organism of the opposite sex may impart to the latter the sexual instincts of the 156 INSTINCTS 157 former. Hormones produced by definite tissues, there- fore, influence the instincts. We want to show that this influence is due to a modification of tropistic reactions by the hormones. Mating in certain fish, like Fundulus, consists in the male pressing that part of its body which contains the opening of the sperm duct against the corresponding part of the female body. The latter responds by pressing back, and the pressure of the body is maintained by both sexes through motions of the tail. During this mutual pressure or friction both sexes shed their sexual cells, sperm and eggs, into the water, and since the openings of the cloaca of the male and female, through which the sex cells are shed, are brought almost in contact with each other, sperm and eggs mix at the moment they are shed. This act of mating is due to a stereotropism which exists only during the spawning season and which is supposedly due to cer- tain hormones existing at this time in the animal. The existence of such hormones is also indicated by certain colorations which develop and exist in the male during this period. This stereotropism is to some extent specific since it is exhibited by the contact between the two sexes. The specificity of this stereotropism is of importance and needs further experimental analysis, but that it is in reality a type of common stereotropism is evidenced by the fact that if during the spawning season we keep females isolated from males in an aquarium the females will go through the motions of mating and shed the eggs every time they come in contact with the glass walls of the aquarium. When they are kept permanently isolated from the male they repeat this non-specific purely stereotropic mating throughout the season. The eggs which they shed they quite frequently devour. 158 TEOPISMS These manifestations of a highly developed stereo- tropism in the segments of the reproductive organs are probably widespread in the animal kingdom. The late Professor Whitman told the writer that male pigeons when kept in isolation will try to go through the motions of mating with any solid object in their field of vision, e.g., glass bottles, and even with objects which give only the optical impression of a solid, namely, their own shadow on the ground. In ants, the winged males and females become intensely positively heliotropic at the time of mating. Copulation occurs in the air, in the so-called nuptial flight. At a cer- tain time — in the writer's observation toward sunset, when the sky is illuminated at the horizon only — the whole swarm of males and females leave the nest and fly in the direction of the glow. The wedding flight is a heliotropic phenomenon287 presumably due to substances produced in the body during this period. After copulation the female loses its wings and also its positive heliotropism.a It becomes now intensely stereotropic. When kept in a dark box with pieces of cloth in folds the wingless female will now be found in the folds where its body is as closely as possible in contact with the solids. This positive stereotropism leads the queen to begin a subterranean existence which marks the founding of a new nest. Helio- tropism and stereotropism are, therefore, the controlling factors in mating and the starting of a new nest in these ants.287 V. L. Kellogg 265 has made observations which show that the nuptial flight in bees is also due to an outburst of positive heliotropism as in the ant. a It has already been mentioned that artificial removal of the wings of the fruit fly will also abolish its heliotropism. INSTINCTS 159 In the course of some experiments on the sense-reactions of honey- bees, I liave kept a small community of Italian bees in a glass-sided, narrow, high observation hive, so made that any particular bee, marked, which it is desired to observe constantly, can not escape this obser- vation. The hive contains but two frames, one above the other, and is made wholly of glass, except for the wooden frame. It is kept covered, except during observation periods, by a black cloth jacket. The bees live contentedly and normally in this small hive, needing only occasional feeding at times when so many cells are given up for brood that there are not enough left for sufficient stored food supplies. Last spring at the normal swarming time, while standing near the jacketed hive, I heard the excited hum of a beginning swarm and noted the first issuers rushing pellmell from the entrance. Interested to see the behavior of the community in the hive during such an ecstatic condition as that of swarming, I lifted the cloth jacket, when the excited mass of bees which was pushing frantically down to the small exit in the lower corner of the hive turned with one accord about face and rushed directly upward away from the opening toward and to the top of the hive. Here the bees jammed, struggling violently. I slipped the jacket partly on; the ones covered turned down; the ones below stood undecided; I dropped the jacket completely; the mass began issuing from the exit again; I pulled off the jacket, and again the whole community of excited bees flowed — that is the word for it, so perfectly aligned and so evenly moving were all the individuals of the bee current — up to the closed top of the hive. Leading the jacket off permanently, I prevented the issuing of the swarm until the ecstasy was passed and the usual quietly busy life of the hive was resumed. About three hours later there was a similar performance and failure to issue from the quickly unjacketed hive. On the next day another attempt to swarm was made, and after nearly an hour of struggling and moving up and down, depending on my manipulation of the black jacket, most of the bees got out of the hive's opening and the swarming came off on a weed bunch near the laboratory. That the issuance from the hive at swarming time depends upon a sudden extra-development of positive heliotropism seems obvious. The ecstasy comes and the bees crowd for the one spot of light in the normal hive, namely, the entrance opening. But when the covering jacket is lifted and the light comes strongly in from above — my hive was under a skylight — they rush toward the top, that is, toward the light. Jacket on and light shut off from above, down they rush; jacket off and light stronger from above than below and they respond like iron filings in front of an electromagnet which has its current suddenly turned on. 160 TBOPISMS Finally there are indications of the role of chemo- tropism in mating. It has been observed for a long time that if a female butterfly is kept hidden from sight in a not too tightly closed box, male butterflies of the same species will be attracted by the box and settle on it. The female apparently gives off a substance to which the male is positively chemotropic. All these observations should be worked out more systematically. The data suffice, however, to indicate that what the biologist and psycholo- gist call instinct are manifestations of tropisms. The fact that eggs are laid by many insects on material which serves as a nutritive medium for the offspring is a typical instinct. An experimental analysis shows again that the underlying mechanism of the instinct is a positive chemotropism of the mother insect for the type of sub- stance serving her as food ; and when the intensity of these volatile substances is very high, i.e., when the insect is on the material, the egg-laying mechanism of the fly is auto- matically set into motion. Thus the common housefly will deposit its eggs on decaying meat but not on fat; but it will also deposit it on objects smeared over with asafotida, on which the larvae cannot live. Aseptic banana flies will lay their eggs on sterile banana, although the banana is only an adequate food for the larvae when yeast grows on it. It seems that the female insect lays her eggs on material for which she is positively j3J3£motropic, and this is generally material which she also eats. The fact that such material serves as food for the coming genera- tion is an accident. Considered in this way, the mystic aspect of the instinctive care of insects for the future generation is replaced by the simple mechanistic concep- tion of a tropistic reaction. yln this case natural selection plays a role since species whose females would too fre- INSTINCTS 161 quently lay their eggs on material on which the larvae cannot thrive would be liable to die out. ^) As an illustration of the role of tropisins in the instinc- tive self-preservation the writer wishes to apologize for selecting an example which he has used so often in pre- vious discussions, namely the role of heliotropism in the preservation of the life of the caterpillars of Porthesia chrysorrhcea.287 This butterfly lays its eggs upon a shrub, on which the larvae hatch in the fall and on which they hibernate, as a rule, not far from the ground. As soon as the temperature reaches a certain height, they leave the nest ; under natural conditions this happens in the spring when the first leaves have begun to form on the shrub. (The larvae can, however, be induced to leave the nest at any time in the winter, provided the temperature is raised sufficiently). After leaving the nest, they crawl directly upward on the shrub where they find the leaves on which they feed. If the caterpillars should move down the shrub they would starve, but this they never do, always crawl- ing upward to where they find their food. What gives the caterpillar this never-failing certainty which saves its life and for which the human being might envy the little larva! Is it a dim recollection of experiences of former generations, as Samuel Butler would have us believe ! It can be shown that this instinct is merely posi- tive heliotropism and that the light reflected from the sky guides the animals upward. The caterpillars upon waking from their winter sleep are violently positively heliotropic, and it is this heliotropism which makes the animals move upward. At the top of the branch they come in contact with a growing bud and chemical andjactile influences set the mandibles of the young caterpillar into activity. If we put these caterpillars into closed test tubes which lie 11 162 TBOPISMS with their longitudinal axes at right angles to the window they will all migrate to the window end where they will stay and starve, even if we put their favorite leaves into the test tube close behind them. These larvae are in this condition slaves of the light. The few young leaves on top of a twig are quickly eaten by the caterpillar. The light which saved its life by mak- ing it creep upward where it finds its food would cause it to starve could the animal not free itself from the bondage of positive heliotropism. After having eaten it is no longer a slave of light but can and does creep down- ward. It can be shown that a caterpillar after having been fed loses its positive heliotropism almost completely and permanently. If we submit unfed and fed caterpillars of the same nest to the same artificial or natural source of light in two different test tubes the unfed will creep to the light and stay there until they die, while those that have eaten will pay little or no attention to the light. Their positive heliotropism has disappeared and the ani- mal after having eaten can creep in any direction. The restlessness which accompanies the condition of starva- tion makes the animal leave the top of the branches and creep downward — which is the only direction open to it— where it finds new young leaves on which it can feed. The wonderful hereditary instinct upon which the life of the / animal depends is its positive heliotropism in the unfed condition and the loss of this heliotropism after having eaten. The chemical changes following the taking up of the food abolish the heliotropism just as C02 arouses positive heliotropism in certain Daphnia. Mayer and Soule have shown that negative geotropism and positive heliotropism keep the caterpillars of Danais plexippus on its plant (the milk-weed). The chemical INSTINCTS 163 nature of the leaf starts the eating reactions, but "once the eating reaction be set into play, it tends to continue, so that the larva may then be induced to eat substances which it would never have commenced to eat in the first instance. "3fil These few examples may suffice to show that the theory of tropisms is at the same time the theory of instincts if due consideration is given to the role of hormones in producing certain tropisms and suppressing others. A systematic analysis of instinctive reactions from the view- point of the theory of tropisms and hormones will prob- ably yield rich returns. As an example we may quote the fact that diurnal depth migrations of aquatic animals, consisting in an upward motion during the night and a downward motion during the day, are in all probability determined by a periodic change in the sense of heliotropism.183' 30° CHAPTER XIX MEMOKY IMAGES AND TROPISMS WHEN a muscle is stimulated several times in succes- sion, the effect of the second or third or later stimulation may be greater than that of the first. A consistently anthropomorphic author should draw the inference that the muscle is gradually learning to react properly. What seems to happen is that the hydrogen ion concentration is raised by the first stimulations to a point where the effect of the stimulation becomes greater. When the stimula- tions continue and the hydrogen ion concentration be- comes still greater, the response of the muscle declines and finally becomes zero ; the hydrogen ion concentration has now become too high. The writer observed that when winged plant lice of a Cineraria were taken directly from the plant, they did not react as promptly as after they had gone through several heliotropic experiments. There is nothing to indicate that this is a case of " learning, " since it may also be the result of a change in the hydrogen ion concentration or of some other reaction product. It may also be the result of some purely mechanical obstacle to rapid locomotion being removed. We can speak of learning only in such organisms in which the existence of associative memory can be proved. By associative memory we mean that mechanism, by which a stimulus produces not only the direct effects determined by its nature, but also the effects of entirely different stimuli which at some former period by chance attacked the organism at the same time with the given 164 MEMORY IMAGES 165 stimulus. Thus the image or the odor of a rose may call up the memory of persons or surroundings which were present on a former occasion when the image or odor of the flower impressed us. Brain physiology shows that this type of associative memory is the specific function of definite parts of the brain, e.g., the cerebral hemispheres which exist only in definite types of animals. We see also that certain species among vertebrates, insects, crus- tacea, and cephalopods possess associative memory, while to the knowledge of the writer no adequate proof for its existence has ever been given for worms, starfish, sea urchins, actinians, medusae, hydroids, or infusorians.293 Claims for the existence of such memory in these latter groups of animals have frequently been made, but such claims are either plain romance or due to a confusion of reversible physiological processes with the irreversible phenomena of associative memory. The less a scientist is accustomed to rigid quantitative experiments, the more ready he is to confound the reversible after effects of a stimulus — e.g., the after effects due to an increase in hydrogen ion concentration — with indications of associa- tive memory. Learning is only possible where there exists v a specific organ of associative memory, the physical mechanism of which is still unknown. The manifestations of associative memory are gener- \/ ally discussed by the introspective psychologists, who as a rule are not familiar with or do not appreciate the methods of the physicist. There have been made repeated attempts to develop methods for the analysis of associa- tive memory, among which thus far only one satisfies the demands of quantitative science, namely Pawlow's method. As is well known even to the layman, eating causes a flow of saliva. The quantity of saliva excreted 166 TROPISMS by the parotid (one of the salivary glands) in the dog can be collected and measured. The earlier physiological workers had observed that in a dog which had often been used for the study of the influence of eating upon the flow of saliva, the saliva began to flow whenever the prepara- tions for feeding were made before the eyes of the dog, even when no food was given. Pawlow made use of this fact to study quantitatively the "strength" of such asso- ciative phenomena, which he terms "conditioned re- flexes" (to escape the terminology and interpretations of the introspective psychologist).537 A fistula a of the duct of the par otic gland allows the saliva to flow outside the cavity of the mouth. This fistula is connected with a long manometer which by a special air chamber arrange- ment gives a considerable change in the height of the meniscus for the secretion of as little as one drop of saliva, The variations of the height of the column of liquid in the manometer are observed outside of the room where the dog is. For each dog which is to serve for such experiments the meal is preceded by a certain signal, the sounds of a metronome of definite rhythm, or a definite musical sound, or a definite optical signal, and so forth, which is to form the special conditioned reflex for this dog. After a certain number of repetitions the association is established and from now on the flow of saliva commences from the dog's parotid when the typical signal is given. It was found that the quantity of saliva excreted by the signal changes in a definite sense and quantity when the signal varies or when other conditions accompanying the signal vary. a The writer is indebted for the details of Pawlow's method to a short review by Dr. Morgulis.^ 533 MEMOBY IMAGES 167 Thus in one dog "by persistent training a conditioned reflex has been established to the stimulation with 100 oscillations per minute of the metronome. The stimu lation of intermittent sounds of such frequency called forth 6 to 10 drops of saliva every time. The interval between successive oscillations was then modified, the moment of the disappearance of the conditioned salivary reflex indicating the lowest limit of differentiation. With- out going into any details of this most interesting investi- gation or quoting actual data, I will say that the dog could sharply distinguish the shortening of the interval by less than 1/40 to 1/43 of a second. Indeed with the well-developed reflex to the stimulation of 100 beats per minute a change of the rate to either 96 or 104 beats was immediately reacted upon by a marked diminution or even complete cessation of the flow of saliva." This example will give an indication how sensitive is this method of measuring the effect of a memory association. It is not our purpose to give the details of Pawlow's results — they have only been published in Russian and are^ therefore not accessible to the writer — but to show that the influence of an associative memory image is as exactly measurable as, e.g., the direct illumination of the eye; and moreover that what we call a memory image is not a " spiritual' ' but a physical agency. We there- fore need not be surprised to find that such memory images or "conditioned reflexes " can vary and multiply the number of possible tropistic reactions. We have mentioned in the previous chapter that the stereotropism in the mating instinct includes apparently an element of species specificity inasmuch as naturally only males and females of the same species mate. The 168 TEOPISMS late Professor Wliitman has shown by experiment that this specificity is, in pigeons at least, not inherited but effect of memory images (a " conditioned reflex " in the sense of Pawlow). Whitman took the eggs or young of wild species, giving them to the domestic ring-dove to foster, with the result, that the young reared by the ring- doves ever after associated with ring-doves and tried to mate with them. Passenger pigeons when reared by ring- doves refuse to mate with their own species but mate with the species of the foster parents.539 This shows inciden- tally that racial antagonism is not inherited but acquired. We have mentioned the fact that the mating instinct is determined by tropisms aroused by specific internal secretions, and that in isolated male pigeons any solid body can arouse the mating reaction. Craig540 raised male pigeons in isolation so that they never came in con- tact with other pigeons until they were adult. One pigeon was hatched in July and isolated in August. Throughout the autumn and early winter this bird cooed very little. But about the first of February there began a remarkable development of voice and social behavior. The dove was kept in a room where several men were at work, and he directed his display behavior toward these men just as if they belonged to his own species. Each time I put food in his cage he became greatly excited, charging up and down the cage, bowing-and-cooing to me, and pecking my hand whenever it came within his cage. From that day until the day of his death, Jack continued to react in this social manner to human beings. He would bow-and-coo to me at a distance, or to my face when near the cage ; but he paid greatest attention to the hand — naturally so, because it was the only part with which he daily came into direct contact. He treated the hand much as if it were a living bird. Not only were his own activities directed toward the hand as if it were a bird, but he received treatment by the hand in the same spirit. The hand could stroke him, preen his neck, even pull the feathers sharply, Jack had absolutely no fear, but ran to the hand to be stroked or teased, showing the joy that all doves show in the attentions of their companions. MEMORY IMAGES 169 When this pigeon was almost a year old it was put into a cage with a female pigeon, but although the female aroused the sexual instinct of the formerly isolated male the latter did not mate with her, but mated with the hand of his attendant when the hand was put into the cage, and this continued throughout the season. Thus the memory images acquired by the bird at an impressionable age and period perverted its sexual tropisms. It is perhaps of more importance to show that memory images may have a direct orienting influence. The chemo- tropic phenomenon of an insect laying its egg on a sub- stance which serves as food (for both mother and off- spring) and for which the mother is positively chemo- tropic, may be modified by an act of associative memory, e.g., when a solitary wasp drags the caterpillar on which it lays its eggs to a previously prepared hole in the ground. The essential part of the instinct, the laying of the eggs on the caterpillar, does, perhaps, not differ very much from the fly laying its eggs on decaying meat; and the solitary wasp may be strongly positively chemotropic for the caterpillar on which it lays the eggs, although this has not yet been investigated. But the phenomenon is complicated by a second tropism, which we will call the orienting effect of the memory image. As is well known, the wasp before "going for" the caterpillar digs a hole in the ground to which it afterwards drags the caterpil- lar, often from a distance. The finding of this previously prepared hole by the returning wasp, the writer would designate as the tropistic or orienting effect of the memory image of the location of this hole ; meaning thereby that the memory image of the location of this hole makes the animal return to this location. The conduct of these wasps 170 TROPISMS is familiar to many readers and the writer may be par- doned for quoting from a formerly published observation. Ammophila, a solitary wasp, makes a small hole in the ground and then goes out to hunt for a caterpillar, which, when found, it paralyses by one or several stings. The wasp carries the caterpillar back to the nest, puts it into the hole, and covers the latter with sand. Before this is done, it deposits its eggs on the caterpillar which serves the young larva as food. An Ammophila had made a hole in a flower bed and left the flower bed flying. A little later I saw an Ammophila running on the sidewalk of the street in front of the garden, dragging a caterpillar which it held in its mouth. The weight of the caterpillar prevented the wasp from flying. The garden was higher than the sidewalk and separated from it by a stone wall. The wasp repeatedly made an attempt to climb upon the stone wall, but kept falling down. .Suspecting that it might have a hole prepared in the garden, I was curious to see whether and how it would find the hole. It followed the wall until it reached the neigh- boring yard, which had no wall. It now left the street and crawled into this yard, dragging the caterpillar along. Then crawling through the fence which separated the two yards, it dropped the caterpillar near the foot of a tree, and flew away. After a short zigzag flight it alighted on a flower bed in which I noticed two small holes. It soon left the bed and flew back to the tree, not in a straight line but in three stages, stopping twice on its way. At the third stop it landed at the place where the caterpillar lay. The caterpillar was then dragged to the hole, pulled into it, and the hole was covered with tiny stones in the usual way.293 It is not enough to say that the animal possesses associative memory and returns to the hole; we must add that the brain image of the region of the hole becomes the source of a forced orientation of the animal — of an added special tropism — compelling the animal to return to the region corresponding to the image. And the same may be said in regard to the return of the wasp to the caterpillar which had been temporarily deposited at the foot of the tree. This example, which might be easily multiplied, will MEMORY IMAGES 171 show the addition necessary to the tropism theory to make it include the endless number of reactions in which associa- tive memory is involved. The psychiatrist would find it easy to supply numerous examples of this type of forced movements toward certain objects which have left a memory image. Since the writer has not investigated this subject sufficiently he is not in a position to give more than a suggestion for the direction of further work. He is inclined to believe that with this enlargement the trop- ism theory might include human conduct also if we realize that certain memory images may exercise as definite an orienting influence as, e.g., moving retina images or sex hormones. This tentative extension of the forced movement or tropism theory of animal conduct may explain why higher animals and human beings seem to possess freedom of will, although all movements are of the nature of forced movements. The tropistic effects of memory images and the modification and inhibition of tropisms by memory images make the number of possible reactions so great that prediction becomes almost impossible and it is this impossibility chiefly which gives rise to the doctrine of free will. The theory of free will originated and is held not among physicists but among verbalists. We have shown that an organism goes where its legs carry it and that the direction of the motion is forced upon the organ- ism. When the orienting force is obvious to us, the motion appears as being willed or instinctive ; the latter generally when all individuals act alike, machine fashion, the former when different individuals act differently. When a swarm of Daphnia is sensitized with C02 they all rush to the source of light. This is a machine-like action, and many 172 TBOPISMS will be willing to admit that it is a forced movement or an instinctive reaction. After the C02 has evaporated the animals become indifferent to light, and while formerly they had only one degree of freedom of motion they now can move in any direction. In this case the motions appear to be spontaneous or free, since we are not in a position to state why Daphnia a moves to the right and Daphnia &, to the left, etc. As a matter of fact, the motion of each individual is again determined by something but we do not know what it is. | The persistent courtship of a human male for a definite individual female may appear as an example of persistent will, yet it is a complicated tropism in which sex hormones and definite memory images are the determining factors. Eemoval of the sex glands abol- ishes the courtship and replacing the sex glands of an individual by those of the opposite sex may lead to a complete reversal of the sex instincts. ( What appears as persistent will action is, therefore, essentially a tropistic reaction. The production of heliotropism by CO2 in Daphnia and the production of the definite courtship of the male A for the female B are similar phenomena differ- ing only by the nature of the hormones and the additional tropistic effects of certain memory images in the case of courtship. Our conception of the existence of ' ' free will ' ' in human beings rests on the fact that our knowledge is often not sufficiently complete to account for the orienting forces, especially when we carry out a "premeditated" act, or when we carry out an act which gives us pain or may lead to our destruction, and our incomplete knowl- edge is due to the sheer endless number of possible com- binations and mutual inhibitions of the orienting effect of individual memory images. LITERATURE 173 LITERATURE * 1 ABBOTT, J. F., and LIFE, A. C. : Galvanotropism in Bacteria. Am. J. Phijsiol, 1908, xxii, 202-206. 2 ADAMS, G. 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B., and LEWIS, F. T. : Phototaxis of Daphnia. Science, 1899, ix, 368. 116 DAVENPORT, C. B., and PERKINS, H. : A Contribution to the Study of Geotaxis in the Higher Animals. /. Physiol, 1897, xxii, 99-110. "7 DAY, E. C.: The Effect of Colored Light on Pigment Migration in the Eye of the Crayfish. Bull Mus. Comp. Zool, 1911, liii, 303- 343. 118 DELAGE, Y. : Etude experimentale sur les illusions statiques et dyna- miques de direction pour servir a determiner les fonctions des eanaux semicirculaires de Foreille interne. Arch. Zool exper. et gener., 1886, (2) iv. 119 DELAGE, Y. : Sur une fonction nouvelle des otocystes comme organcs d'orientation locomotrioe. Arch. Zool. exper. et gener. , 1887, (2) v, 1-26. 1 20 DEWITZ, J. : Ueber die Vereinigung der Spermatozoen mit dem Ei. Arch. ges. Physiol, 1885, xxxvii, 219-223. 121 DEWITZ, J. : Ueber Gesetzinassigkeit in der Ortsveranderung der Spermatozoen und in der Vereinigung derselben mit dem Ei. Arch, ges. Physiol, 1886, xxxviii, 358-385. 122 DEWITZ, J. : Ueber den Rheotropismus bei Tieren. Arch. Physiol, 1899 (Suppl.), 231-244. 1 23 DOLLEY, W. L., JR. : Reactions to Light in Vanessa antiopa, with Special Reference to Circus Movements. J. Exp. Zool, 1916, xx, 357-420. 124 DRIESCH, H. : Heliotropismus bei Hydroidpolypen. Zool Jahrb., 1890, v, 147-156. 125 DRIESCH, H. : Die taktische Reizbarkeit der Mesenchymzellen von Echinus microtuberculatus. Arch. Entwcklngsmech., 1896, iii, 362- 380. 126 DRIESCH, H. : Die organischen Regulationen. Leipzig, 1901, pp. 228. 1 27 DUBOIS, R. : Sur le mecanisme des fonctions photodermatique et photogenique dans le siphon du Pholas dactylus. Compt. rend. Acad. Sc., 1889, cix, 233-235. LITEEATURE 181 1 28 DUBOIS, R. : Sur Faction des agents modificateurs de la contraction photodermatique chez le Pholas dactylus. Compt. rend. Acad. Sc., 1898, cix, 320-322. !29 DUBOIS, R. : Sur la perception des radiations lumineuses par la peau, chez les Protees aveugles des grottes de la Carniole. Compt. rend. Acad. Sc., 1890, ex, 358-361. 1 3° DUBOIS, R. : Note sur 1'action de la lumiere sur les echinodermes (oursin). Commun. 9me. Cong, internat. Zool., Monaco, 1913, (1), 8-9. 131 DUSTIN, A. P.: Le role des tropismes et de I'odogenese dans la re- generation du systeme nerveux. Arch. BioL, 1910, xxv, 269-388. 132 ENGELMANN, T. W. : Ueber Reizung kontraktilen Protoplasmas durch plotzliche Beleuchtung. Arch. ges. Physiol, 1879, xix, 1-7. 133 ENGELMANN, T. W. : Ueber Licht- und Farbenperzeption niederster Organismen. Arch. ges. Physiol., 1882, xxix, 387-400. 134 ENGELMANN, T. W. : Bacterium photometricum. Ein Beitrag zur ver- gleichenden Physiologie des Licht- und Farbensinnes. Arch. ges. Physiol, 1882, xxx, 95-124. 13s ENGELMANN, T. W.: Ueber die Funktion der Otolithen. Zool. Anz., 1887, x, 591, 664. 1 36 ENGELMANN, T. W. : Die Purpurbakterien und ihre Beziehungen zum Licht. Bot. Ztg., 1888, xlvi, 661-669, 677-689, 693-701, 709-720. 137 ENGLISCH, E. : Ueber die Wirkung intermittierender Belichtungen auf Bromsilbergelatine. Arch. wiss. Phot., 1899, i, 117-131. 138 ENGLISCH, E. : Ueber den zeitlichen Verlauf der durch das Licht verursachten Veranderungen der Bromsilbergelatine. Arch. wiss. Phot., 1900, ii, 131-134. 139 ERHARD, H. : Beitrag zur Kenntnis des Lichtsinnes der Daphniden. BioL Centr., 1913, xxxiii, 494-496. 140 ESTERLY, C. O. : The Reactions of Cyclops to Light and Gravity. Am. J. Physiol, 1907, xviii, 47-57. 141 EWALD, J. R. : Physiologische Untersuchungen iiber das Endorgan des Nervus octavus. Wiesbaden, 1892. 14 2 EWALD, J. R. : Ueber die Wirkung des galyanischen Stroms bei der Langsdurchstromung ganzer Wirbeltiere. Arch. ges. Physiol., 1894, Iv, 606-621 (Berightigung, 1894, Ivi, 354). 143 EWALD, W. E. : Ueber Orientierung, Lokomotion und Lichtreaktionen einiger Cladoceren und deren Bedeutung fur die Theorie der Trop- ismen. BioL Centr., 1910, xxx, 1-16, 49-63, 379-399. 144 EWALD, W. E. : On Artificial Modification of Light Reactions and the Influence of Electrolytes on Phototaxis. J. Exp. Zool, 1912, xiii, 591-612. 182 TEOPISMS 145 EWALD, W. E. : The Applicability of the Photochemical Energy Law to Light Reactions in Animals. Science, 1913, xxxviii, 236-237. 146 EWALD, W. E. : 1st die Lehre vom tierischen Phototropismus wider- legt? Arch. Entwcklngsmech., 1913, xxxvii, 581-598. 147 EWALD, W. E. : Versuche zur Analyse der Licht- und Farbenreak- tionen eines Wirbellosen (Daphnia pulex) . Z. Sinnesphysiol, 1914, xlviii, 285-324. 148 EYCLESHYMER, A. C. : The Reactions to Light of the Decapitated Young Necturus. J. Comp. Neurol. and Psychol., 1908, xviii, SOS- SOS. 149 FAUVEL, P., and BOHN, G. : Le rythme des marees chez les diatomees littorales. Compt. rend. Soc. BioL, 1907, Ixii, 121-123. 150 FIGDOR, W. : Ueber Helio- und Geotropismus der Gramineenblatter. Per. bot. Ges., 1905, xxiii, 182-191. 151 FIGDOR, W. : Experimentelle Studien iiber die heliotropische Empfind- lichkeit der Pflanzen. Wiesner Festschrift, Wien, 1908. 152 FIGDOR, W. : Heliotropische Reizleitung bei Begonia-Slattern. Ann. Jardin bot. Buitenzorg., 1910 (Suppl.), iii, 453-460. 153 FIGDOR, W. : Ueber thigmotropische Empfindlichkeit der Asparagus- Sprosse. Sitzngsb. Akad. Wiss. Wien. mathem.-naturw. Kl. Abt. I, 1915, cxxiv, 353. 154 FITTING, H. : Untersuchungen iiber den geotropischen Reizvorgang. Jahrb. wiss. Bot., 1905, xli, 221-398. 155 FLOURENS, P. : Recherches experimentales sur les proprietes et les fonctions du systeme nerveux dans les animaux vertebres. Paris, 1842, pp. xxviii + 516. 156 FORSSMAN, J. : Ueber die Ursaehen, welche die Wachstumsrichtung der peripheren Nervenfasern bei der Regeneration bestimmen. Beitr. path. Anat., 1898, xxiv, 56-100. 157 FORSSMAN, J. : Zur Kenntnis des Neurotropismus. Beitr. path. Anat., 1900, xxvii, 407-430. 158 FRANDSEN, P. : Studies on the Reactions of Limax maximus to Direc- tive Stimuli. Proc. Am. Acad. Arts and Sc., 1901, xxxvii, 185-227. 159 FRANZ, V. : Phototaxis und Wanderung. Nach Versuchen mit Jung- fischen und Fischlarven. Int. Eev. ges. Hydrobiol. u. Hydro graphic, 1910, iii, 306-334. 16<> FRANZ, V.: Beitrage zur Kenntnis der Phototaxis. Nach Versuchen an Siisswassertieren. Int. Eev. ges. Hydrobiol. u. Hydrographie, Biol. Suppl. (2), 1911, 1-11. 161 FRANZ, V.: Weitere Phototaxisstudien. I. Zur Phototaxis bei Fischen. II. Phototaxis bei marinen Crustaceen. III. Phototak- tische Lokomotionsperioden bei Hemimysis. Int. Eev. ges. Hydro- biol. u. Hydrographie, Biol. Suppl. (3), 1911, 1-23. LITERATURE 183 162 FRANZ, V. : Zur Frage der vertikalen Wanderimgen der Planktontiere. Arch. Hydrobiol. u. Planktonkunde, 1912, vii, 493-499. 163 FRANZ, V. : Die phototaktischen Erscheintmgen im Tierreiche und ihre Rolle im Freileben der Tiere. Zool. Jahrb., 1913, xxxiii, 259- 286. 164 v. FRISCH, K. : Ueber farbige Anpassung bei Fischen. Zool. Jahrb., 1912, xxxii, 171-230. 165 v. FRISCH, K.: Sind die Fische farbenblind? Zool. Jahrb., 1912, xxxiii, 107-126. 166 v. FRISCH, K. : Ueber die Farbenanpassung des Crenilabrus. Zool. Jahrb., 1912, xxxiii, 151-164. i67v. FRISCH, K. : Weitere Untersuchungen iiber den Farbensinn der Fische. Zool. Jahrb., 1913, xxxiv, 43-68. 168 v. FRISCH, K. : Der Farbensinn und Formensinn der Biene. Zool. Jahrb., 1914, xxxv, 1-182. 169 v. FRISCH, K., and KUPELWIESER, II.: Ueber den Einflnss der Licht- farbe auf die phototaktischen Reaktionen niederer Krebse. Biol. Centr., 1913, xxxiii, 517-552. ITO FROHLICH, F. W. : Vergleichende Untersuchungen liber den Licht- und Farbensinn. Deutsch. med. Wchnschr., 1913, xxxix, 1453-1456. 171 FROSCHEL, P. : Untersuehung iiber die heliotropische Prasentations- zeit. I. Sitzngsb. Akad. Wiss. Wien. mathem.-naturw. Kl., 1908, cxvii, 235-256. 172 FROSCHEL, P.: Untersuchung iiber die heliotropische Prasentations- zeit. II. Sitzngsb. Akad. Wiss. Wien. mathem.-naturw. Kl., 1909, cxviii, 1247-1294. 173 FUCHS, R. F. : Der Farbenwechsel und die chromatische Hautf unk- tion der Tiere. Winterstein's Handb. vergl. Physiol., 1914, iii, I. Halfte 2, 1189-1656. 1™ GALIANO, E. F. : Beitrag zur Untersuchung der Chemotaxis der Para- m&cien. Z. allg. Physiol., 1914, xvi, 359-372. 175 GARRET, W. E.: The Effect of Ions Upon the Aggregation of Flagel- lated Infusoria. Am. J. Physiol., 1900, iii, 291-315. 176 GARREY, W. E. : A Sight Reflex Shown by Sticklebacks. Biol. Bull, 1905, viii, 79-84. 177 GARREY, W. E. : Proof of the Muscle Tension Theory of Heliotropism. Proc. Nat. Acad. Sc., 1917, iii, 602-609. 178 GOLTZ, F. : Ueber die Verrichtungen des Grosshirns. I-V. Arch. ges. Physiol, 1876, xiii, 1-44; 1877, xiv, 412-443; 1879, xx, 1-54; 1881, xxvi, 1-49; 1884, xxxiv, 450-505. 179 GRABER, V.: Fundamental versuche iiber die Helligkeits- und Farben- empfindlichkeit augenloser und geblendeter Tiere. Sitzngsb. Akad. Wiss. Wien, 1883, Ixxxvii, 201-236. 184 TEOPISMS 180 GRABER, V. : Grundlinien zur Erforschung des Helligkeits- und Far- bensinnes der Tiere. Leipzig, 1884, pp. vii +322. 181 GRABER, V. : Ueber die Helligkeits- und Farbenempfindlichkeit einiger Meertiere. Sitzngsb. Akad. Wiss. Wien., 1885, xci. 182 GRABER, V.: Thermische Experimente an der Kiichenschabe (Peri- planeta orientalis). Arch. ges. Physiol, 1887, xli, 240-256. 183 GROOM, T. T., and LOEB, J. : Der Heliotropismus der Nauplien von Balanus perforatus und die periodischen Tiefenwanderungen pelag- ischer Tiere. Biol. Centr., 1890, x, 160-177. 184 GROSS, A. 0. : The Reactions of Arthropods to Monochromatic Lights of Equal Intensities. J. Exp. ZooL, 1913, xiv, 467-514. 185 HABERLANDT, G. : Ueber die Perzeption des geotropischen Reizes. Ber. bot. Ges., 1900, xviii, 261-272. 186 HADLEY, P. B.: The Relation of Optical Stimuli to Rheotaxis in the .American Lobster (Homarus americanus>) . Am. J. Physiol, 1906, xvii, 326-343. 187 HADLEY, P. B. : Galvanotaxis in Larva? of the American Lobster (Homarus americanus). Am. J. Physiol., 1907, xix, 39—52. 188 HADLEY, P. B. : The Reaction of Blinded Lobsters to Light. Am. J. Physiol., 1908, xxi, 180-199. 189 HADLEY, P. B. : Reactions of Young Lobsters Determined by Food Stimuli. Science, 1912, xxxv, 1000-1002. 190 HARPER, E. H. : Reactions to Light and Mechanical Stimuli in the Earthworm, Perichata bermudensis (Beddard). Biol. Bull, 1905, x, 17-34. 101 HARPER, E. H. : Tropic and Shock Reactions in Perichata and Lum- bricus. J. Comp. Neurol. and Psychol., 1909, xix, 569-587. 192 HARPER, E. H. : The Geotropism of Paramcecium. J. Morphol., 1911, xxii, 993-1000. 193 HARPER, E. H. : Magnetic Control of Geotropism in Paramcecium. J. Animal Behav., 1912, ii, 181-189. 194 HARRINGTON, N". R., and LEAMING, E. : The Reaction of Amoeba to Lights of Different Colors. Am. J. Physiol, 1899, iii, 9-18. 195 HASEMAN, J. D. : The Rhythmical Movements of Littorina littorea Synchronous with Ocean Tides. Biol. Bull, 1911, xxi, 113-121. 196 HAUSMANN, W. : Die photodynamische Wirkung des Chlorophylls und ihre Beziehung zur photosynthetischen Assimilation der Pflan- zen. Jahrb. wiss. Bot., 1909, xlvi, 599-623. 197 HELMHOLTZ, H. : Handbuch der physiologischen Optik. Hamburg, 1909-11, 3. Ed. 198 HENRI, MME. V., and HENRI, V. : Excitation des organismes par les rayons ultra-violets. Compt. rend. Soc. Biol, 1912, Ixxii, 992-996; Ixxiii, 326-327. LITERATURE 185 1 j>: The Chemical Basis of Regeneration and Geotropism. Science, 1917, xlvi, 115-118. LOEB^ J. : Influence of the Leaf upon Root Formation and Geo- tropic Curvature in the Stem of Bryophyllum calycinum and the Possibility of a Hormone Theory of These Processes. Bot. Gaz., 1917, Ixiii, 25-50. LoEB, J. : The Chemical Mechanism of Regeneration. Ann. Inst. Pasteur, 1918, xxxii, 1-16. 304 LOEB, J., and BUDGETT, S. P. : Zur Theorie des Galvanotropismus. IV. Ueber die Ausscheidung electropositiver lonen an der ausseren Anodenflache protoplasmatischer Gebilde als Ursache der Abwei- chungen vom Pfliiger'schen Erregungsgesetz. Arch. ges. Physiol., 1897, Ixv, 518-534. 305 LOEB, J., and EWALD, W. F.: Ueber die Giiltigkeit des Bunsen- Roscoe'schen Gesetzes f iir die heliotropische Erscheinung bei Tieren. Centr. Physiol., 1914, xxvii, 1165-1168. 306 LOEB, J., and GARREY, W. E. : Zur Theorie des Galvanotropismus. II. Versuche an Wirbeltieren. Arch. ges. Physiol., 1896, Ixv, 41-47. 30 7 LOEB, J., and MAXWELL, S. S. : Zur Theorie des Galvanotropismus. Arch. ges. Physiol., 1896, Ixiii, 121-144. 3°s LOEB, J., and MAXWELL, S. S. : Further Proof of the Identity of Heliotropism in Animals and Plants. Univ. Col. Pub. Physiol., 1910, iii, 195-197. 30 9 LOEB, J., and NORTHROP, J. H. : Heliotropic Animals as Photometers on the Basis of the Validity of the Bunsen-Roscoe Law for Helio- tropic Reactions. Proc. Nat. Acad. Sc., 1917, iii, 539-544. 310 LOEB, J., and WASTENEYS, H. : On the Identity of Heliotropism in Animals and Plants. Proc. Nat. Acad. Sc., 1915, i, 44r-47; Science, 1915, xli, 328-330. 311 LOEB, J., and WASTENEYS, H. : The Relative Efficiency of Various Parts of the Spectrum for the Heliotropic Reactions of Animals and Plants. J. Exp. Zool, 1915, xix, 23-35; 1916, xx, 217-236. 312 LOEB, J., and WASTENEYS, H. : A Re-examination of the Applicability of the Bunsen-Roscoe Law to the Phenomena of Animal Heliotrop- ism. /. Exp. Zool., 1917, xxii, 187-192. 313 LOHNER, L. : Untersuchungen iiber den sogenannten Totstellreflex der Arthropoden. Z. allg. Physiol., 1914, xvi, 373-418. 314 LUBBOCK, J. : On the Sense of Color Among Some of the Lower Animals. I and II. J. Linn. Soc. (Zool), 1881, xvi, 121-127; 1882, xvii, 205-214. 315 LUBBOCK, J. : On the Senses, Instincts and Intelligence of Animals, with Special Reference to Insects. Internat. sc. Series, London, 1899. 192 TROPISMS 316 LUBBOCK, J.: Ants,' Bees and Wasps. New York, 1904, xiii+435. 31 7 LUDLOFF, K. : Untersuchungen iiber den Galvanotropismus. Arch. ges. Physiol, 1895, lix, 525-554. 318 LYON, E. P.: The Functions of the Otoeyst. J. Comp. Neurol. and Psychol, 1898, viii, 238-245. 319 LYON, E. P. : A Contribution to the Comparative Physiology of Com- pensatory Motions. Am. J. PhysioL, 1899, iii, 86-114. 320 LYON, E. P. : Compensatory Motions in Fishes. Am. J. Physiol., 1900, iv, 77-82. 321 LYON, E. P.: On Rheotropism. I. Rheotropism in Fishes. Am. J. Physiol., 1904, xii, 149-161. 322 LYON, E. P. : Rheotropism in Fishes. Biol. Bull, 1905, viii, 238-239. 323 LYON, E. P. : On the Theory of Geotropism in Paramacium. Am. J. Physiol., 1905, xiv, 421-432. 324 LYON, E. P. : Note on the Geotropism of Arbacia Larva3. Biol. Bull., 1906, xii, 21-22. 325 LYON, E. P. : Note on the Heliotropism of Palcemonetes Larvae. Biol. Bull, 1906, xii, 23-25. 326 LYON, E. P. : On Rheotropism. II. Rheotropism of Fish Blind in One Eye. Am. J. Physiol, 1909, xxiv, 244-251. 326a LYON, E. P. : Note on the Geotropism of Paramcecium. Biol. Bull., 1918, xxxiv, 120. 326b McCLENDON, J. F. : Protozoan Studies. J. Exp. Zool, 1909, vi, 265- 283. 32? MACCURDY, H. : Some Effects of Sunlight in the Starfish. Science, 1913, xxxvi, 98-100. 327a McEwEN, R. S. : The Reactions to Light and to Gravity in Droso- phila and its Mutants. J. Exp. Zool, 1918, xxv, 49-106. 328 McGiNNis, M. 0. : Reactions of Branchipus serratus to Light, Heat and Gravity. J. Exp. Zool, 1911, x, 227-240. 329 MACH, E. : Physikalische Versuche iiber den Gleichgewichtssinn des Menschen. Sitzngsb. Akad. Wiss. Wien., 1873, Ixviii; 1874, Ixix. 330 MACH, E. : Grundlinien der Lehre von den Bewegungsempfindungen. Leipzig, 1875, pp. 127. 331 MACH, E. : Beitrage zur Analyse der Empfindungen. Jena, 1902. 332 MAGNUS, R. : Welche Teile des Zentralnervensystems miissen fiir das Zustandekommen der tonischen Hals- und Labyrinthreflexe auf die Korpermuskulatur vorhanden sein? Arch. ges. Physiol, 1914, clix, 224-250. 333 MAGNUS, R., and DE KLEIJN, A. : Die Abhiingigkeit des Tonus der Extremitatenmuskeln von der Kopfstellung. Arch. ges. Physiol, 1912, cxlv, 455-548. LITERATURE 193 334 MAGNUS, R., and D'E KLEIJN, A.: Die Abhangigkeit des Tonus der Nackenmuskeln von der Kopfstellung. Arch. ges. Physiol, 1912, cxlvii, 403-416. 335 MAGNUS, R., and DE KLEIJN, A.: Die Abhangigkeit der Korperstel- lung vom Kopfstande beim normalen Kaninchen. Arch. ges. Physiol., 1913, cliv, 163-177. 336 MAGNUS, R., and DE KLEIJN, A.: Analyse der Folgezustande einsei- tiger Labyrinthexstirpation mit besonderer Berucksichtigung der Rolle der tonischen Halsreflexe. Arch, ges. Physiol., 1913, cliv, 178-306. 337 MAGNUS, R., and VAN LEEUWEN, W. S. : Die akuten und die dauernden Folgen des Ausfalles der tonischen Hals- und Labyrinthreflexe. Arch. ges. Physiol., 1914, clix, 157-217. 338 MAGNUS, R., and WOLF, C. G. L. : Weitere Mitteilungen iiber den Einfluss der Kopfstellung auf den Gliedertonus. Arch. ges. Phys- iol., 1913, cxlix, 447-461. 339 MARCHAL, P. : Le retour au nid chez le Pompilus sericeus V. d. L. Compt. rend. Soc. Biol., 1900, lii, 1113-1115. 340 MASSART, J. : Recherches sur les organismes inferieurs. I. La loi du Weber verifiee pour Pheliotropisme du champignon. Bull. Acad. Roy. Belg., 1888, (3) xvi, 590. 341 MASSART, J. : Sur Firritabilite des spermatozoides dans Poeuf de la grenouille. Bull. Acad. Roy. Belg., 1888, (3) xv; 1889, xviii. 542 MASSART, J. : La sensibilite tactile chez les organismes inferieurs. J. Soc. Roy. Sc. med. et nat., Bruxelles, 1890. 343 MASSART, J. : Recherches sur les organismes inferieurs. III. La sensi- bilite a la gravitation. Bull. Acad. Roy. Belg., 1891, (3) xxii, 158-167. 344 MASSART, J. : Essai de classification des reflexes non-nerveux. Ann. Inst. Pasteur, 1901, xv, 635-672. 345 MASSART, J. : Versuch einer Einteilung der nichtnervosen Reflexe. Biol Centr., 1902, xxii, 9-23. 346 MAST, S. 0.: Light and the Behavior of Organisms. New York, 1911, pp. 410+xi. 347 MAST, S. 0. : Behavior of Fire-flies (Pliotinus pyralis?) with Special Reference to the Problem of Orientation. /. Animal Behav., 1912, ii, 256-272. 348 MAST, S. 0. : The Relation between Spectral Color and Stimulation in the Lower Organisms. J. Exp. Zool., 1917, xxii, 471-528. .-usa MATULA, J. : Untersuchungen iiber die Funktionen des Zentral- nervensystems bei Insekten. Arch. ges. Physiol., 1911, cxxxviii, 388-456. 13 194 . TEOPISMS 349 MAXWELL, S. S. : Beitrage zur Gehirnphysiologie der Anneliden. Arch. ges. Physiol, 1897, Ixvii, 263-297. 350 MAXWELL, S. S. : Experiments on the Functions of the Internal Ear. Univ. Cal. Pub. Physiol., 1910, iv, 1-4. 351 MAYER, A. G., and SOULE, C. G. : Some Reactions of Caterpillars and Moths. /. Exp. Zool., 1906, iii, 415-433. 352 MENDELSSOHN, M. : Ueber den Therm otropismus einzelliger Organ- ismen. Arch. ges. Physiol., 1895, Ix, 1-27. 353 MENDELSSOHN, M. : Recherches sur la thermotaxie des organismes unicellulaires. J. Physiol. et Path, gener., 1902, iv, 393-409. 354 MENDELSSOHN, M. : Recherches sur ^interference de la thermotaxie avec d'autres tactismes et sur le mecanisme du mouvement thermo- tactique. J. Physiol. et Path, gener., 1902, iv, 475-488. 355 MENDELSSOHN, M. : Quelques considerations sur la nature et le role biologique de la thermotaxie. J. Physiol. et Path, gener., 1902, iv, 489-496. 356 MENKE, H. : Periodische Bewegungen und ihr Zusammenhang mit Licht und Stoffweehsel. Arch. ges. Physiol., 1911, cxl, 37-91. 357 MEREJKOWSKY, C. DE: Les crustaces inferieurs distingnent-ils les couleurs? Compt. rend. Acad. Sc.., 1881, xciii, 1160-1161. 3 58 MILLER, F. R. : Galvanotropism in the Crayfish. J. Physiol, 1907, xxxv, 215-229. 359 MINKIEWICZ, R. i Sur le chromotropisme et son inversion artificielle. Compt. rend. Acad. Sc., 1906, cxliii, 785-787. 360 MINKIEWICZ, R. : Le role des phenomenes chromotropiques dans Fetude des problemes biologiques et psycho-physiologiques. Compt. rend. Acad. Sc., 1906, cxliii, 934-935. 361 MINKIEWICZ, R. : Une experience sur la nature du chromotropisme chez les nemertes. Compt. rend. Acad. Sc., 1912, civ, 229-231. 362 MITSUKURI, K. : Negative Phototaxis and Other Properties of Lit- torina as Factors in Determining Its Habitat. Annotationes Zoologies Japonenses, 1901, iv, 1-19. s<*3 MOLISCH, H. : Untersuchungen tiber den Hydrotropismus. Sitzngsb. Akad. Wiss. Wien. mathem.-naturw. KL, 1883. 364 MOORE, ANNE : Some Facts Concerning Geotropic Gatherings of ParameEcia. Am. J. Physiol, 1903, ix, 238-244. 365 MOORE, A. R. : On the Righting Movements of the Starfish. Biol Bull, 1910, xix, 235-239. 366 MOORE, A. R. : Concerning Negative Phototropism in Daphnia pulex. J. Exp. Zool, 1912, xiii, 573-575. 367 MOORE, A. R. : Negative Phototropism in Diapt&mus by Means of Strychnine. Univ. Cal Pub. Physiol, 1912, iv, 185-186. LITERATURE 195 368 MOORE, A. R. : The Negative Phototropism of Diaptomus Through the Agency of Caffein, Strychnine, and Atropin. Science, 1913, xxxviii, 131-133. 369 MOORE, A. R. : The Mechanism of Orientation in Gonium. J. Exp. Zool., 1916, xxi, 431-432. 369a MOORE, A. R. : The Action of Strychnine on Certain Invertebrates. J. Pharm. and Exp. Tlierap., 1916, ix, 167-169. 370 MOORE, A. R., and KELLOGG, F. M.: Note on the Galvanotropic Response of the Earthworm. Biol. Bull, 1916, xxx, 131-134. 371 MOORE, B. : Observations of Certain Marine Organisms of (a) Variations in Reaction to Light, and (b) a Diurnal Periodicity of Phosphorescence. Biochem. J., 1909, iv, 1-29. 3710 MORGAN, C. L. : Animal Behavior. London, 1900. 37i&MoRGULis, S.: The Auditory Reactions of the Dog Studied by the Pawlow Method. J. Animal Behav., 1914, iv, 142-145. 371CMORGUUS, S.: Pawlow's Theory of the Function of the Central Nervous System and a Digest of Some of the More Recent Con- tributions to This Subject from Pawlow's Laboratory. J". Animal Behav., 1914, iv, 362-379. 372 MORSE, M. W. : Alleged Rhythm in Phototaxis Synchronous with Ocean Tides. Proc. Soc. Exp. Biol. and Med., 1910, vii, 145-146. 373 MILLER, H. : Ueber Heliotropismus. Flora, 1876, lix, 65-70, 88-95. 37 4 MULLER-HETTLINGEN, J. : Ueber galvanische Erseheinungen an kei- menden Sameu. Arch. ges. PhysioL, 1883, xxxi, 193-212. 375 MURBACH, L. : The Static Function in Gonionemus. Am. J. Physiol., 1903, x, 201-209. 376 MURBACH, L. : Some Light Reactions of the Medusa Gonionemus. Biol. Bull., 1909, xvii, 354-368. 377 MUSSET, CH. : Selenotropisme. Compt. rend. Acad. Sc., 1890, ex, 201-202. 378 NAGEL, W. A. : Beobachtungen iiber den Lichtsinn augenloser Museh- eln. Biol. Centr., 1894, xiv, 385-390. 379 NAGEL, W. A. : Ein Beitrag zur Kenntnis des Lichtsinnes augenloser Tiere. Biol. Centr., 1894, xiv, 810-813. 379a NAGEL, W. A. : Experimentelle sinnesphysiologische Untersuchungen an Coelenteraten. Arch. ges. Physiol., 1894, Ivii, 495-552. 380 NAGEL, W. A. : Ueber Galvanotaxis. Arch. ges. Physiol., 1895, lix, 603-612. 381 NAGEL, W. A. : Der Lichtsinn augenloser Tiere. Jena, 1896, pp. 120. 382 NAGEL, W. A. : Phototaxis, Photokinesis und Unterschiedsempfind- lichkeit. Bot. Ztg., 1901, lix, 298-299. . 196 TEOPISMS 383 NAGEL, W. A. : Methoden zur Erforschung1 des Licht- und Farben- sirmes. Tigerstedt's Handb. physiol. Methodik, 1909, iii, Abt. 2, Sinnesphysiologie, ii, 1-99. 384 NATHANSOHN, A., and PRINGSHEIM, E. : Ueber die Summation inter- mittierender Lichtreize. Jahrb. wiss. Bot., 1908, xlv, 137-190. 385 NihMEC, B. : Ueber die Wahrnehmung des Schwerkraftreizes bei den Pflanzen. Jahrb. wiss. Bot., 1901, xxxvi, 80-178. 38 5a NERNST, W., and BARRATT, J. 0. W. : Ueber die elektrische Nerven- reizung durch Wechselstrome. Z. Electrochem., 1904, x, 664-668. 386 NEUBERG, C. : Chemische Umwandlungen durch Strahlenarten. Bio- chem. Z., 1908, xiii, 305-320; 1909, xvii, 270-292. 387 NUEL, J. P. : La vision. Paris, 1904, pp. 376. 388 NYBERGH, T. : Studien iiber die Einwirkung der Temperatur auf die tropistische Reisbarkeit etiolierter J.i;ewa-Keimlinge. Ber. bot. Ges., 1912, xxx, 542-553. 389 OLTMANNS, F. : Ueber die photometrischen Bewegungen der Pflanzen. Flora, 1892, Ixxv, 183-266. 390 OLTMANNS, F. : Ueber positiven und negativen Heliotropismus. Flora, 1897, Ixxxiii, 1. 391 OSTWALD, Wo.: Ueber eine neue theoretische Betrachtungsweise in der Planktologie, insbesondere iiber die Bedeutung des Begriffs der " inneren Reibung des Wassers " fiir dieselbe. Forsch.-ber. biol. Station Plon, 1903, pt. 10, 1-49. 392 OSTWALD, Wo. : Zur Theorie der Richtungsbewegungen schwimmen- der niederer Organismen. Arch. ges. Physiol., 1903, xcv, 23-65; 1906, cxi, 452^472; 1907, exvii, 384-408. 393 OSTWALD, Wo. : Ueber die Lichtempfindlichkeit tierischer Oxydasen und iiber die Beziehungen dieser Eigenschaft zu den Erscheinungen des tierischen Phototropismus. Biochem. Z., 1908, x, 1-130. 394 PAAL, A. : Ueber phototropische Reizleitungen. Ber. bot. Ges., 1914, xxxii, 499-502. 395 PARKED, G. H. : Photomechanical Changes in the Retinal Pigment Cells of Palamonetes, and Their Relation to the Central Nervous System. Bull. Mus. Comp. Zool., 1897, xxx, 273-300. 3 96 pARKER> G. H.: The Photomechanical Changes in the Retinal Pig- ment of Gammarus. Bull. Mus. Comp. Zool., 1899, xxxv, 141-148. 397 PARKER, G. H. : The Reactions of Copepods to Various Stimuli and the Bearing of This on Daily Depth-migrations. Bull. U. S. Fish Comm., 1901, 103-123. 398 PARKER, G. H. : The Phototropisin of the Mourning-cloak Butterfly, Vanessa antiopa Linn. Mark Anniversary Vol., 1903, 453-469. LITEEATURE , 197 399 PARKER, G. H. : The Skin and the Eyes as Receptive Organs in the Reactions of Frogs to Light. Am. J. Physiol, 1903, x, 28-36. 400 PARKER, G. H. : The Stimulation of the Integumentary Nerves of Fishes by Light. Am. J. Physiol., 1905, xiv, 413-420. 401 PARKER, G. H. : The Reactions of Amphioxus to Light. Proc. Soc. Exp. Biol. and Med., 1906, iii, 61-62. 4°2 PARKER, G. H. : The Influence of Light and Heat on the Movement of the Melanophore Pigment, Especially in Lizards. J. Exp. Zool., 1906, iii, 401-414. 403 PARKER, G. H. : The Sensory Reactions of Amphioxus'. Proc. Am. Acad. Arts and Sc., 1908, xliii, 415-455. 404 PARKER, G. H. : The Integumentary Nerves of Fishes as Photore- ceptors and Their Significance for the Origin of the Vertebrate Eyes. Am. J. Physiol, 1909, xxv, 77-80. 405 PARKER, G. H. : Mast's "Light and the Behavior of Organisms." J. Animal Behav., 1911, i, 461-464. 406 PARKER, G. H., and ARKIN, L. : The Directive Influence of Light on the Earthworm Allolobophora fcetida (Sav.). Am. J. Physiol, 1901, v, 151-157. 407 PARKER, G. H., and BURNETT, F. L. : The Reactions of Planarians, With and Without Eyes, to Light. Am. J. Physiol, 1900, iv, 373- 385. 408 PARKER, G. H., and METCALF, C. R. : The Reactions of Earthworms to Salts: a Study in Protoplasmic Stimulation as a Basis of In- terpreting the Sense of Taste. Am. J. Physiol, 1906, xvii, 55-74. 4°9 PARKER, G. H., and PARSHLE.Y, H. M. : The Reactions of Earthworms to Dry and to Moist Surfaces. /. Exp. Zool, 1911, xi, 361-363. 410 PARKER, G. H., and PATTEN, B. M.: The Physiological Effect of In- termittent and of Continuous Lights of Equal Intensities. Am. J. Physiol, 1912, xxxi, 22-29. 4 11 PARMLEE, M.: The Science of Human Behavior. New York, 1913, \/ xvii+443. 41 2 PATTEN, B. M. : A Quantitative Determination of the Orienting Reaction of the Blowfly Larva (Calliphora erythrocephala Meigen), J. Exp. Zool, 1914, xvii, 213-280. 413 PATTEN, B. M. : An Analysis of Certain Photic Reactions with Reference to the Weber-Fechner Law. I. The Reactions of the Blowfly Larva to Opposed Beams of Light. Am. J. Physiol, 1915, xxxviii, 313-338. 414 PATTEN, B. M. : The Changes of the Blowfly Larva's Photosensitivity with Age. J. Exp. Zool, 1916, xx, 585-598. 198 TEOPISMS 415 PATTEN, B. M. : Reactions of the Whip-tail Scorpion to Light. J Exp. Zool., 1917, xxiii, 251-275. 416 PAYNE, F. : The Reactions of the Blind Fish, Amblyopsis spelceus, to Light. Biol Bull, 1907, xiii, 317-323. 4i6apAYNE, p>: Forty-nine Generations in the Dark. Biol Bull., 1910, xviii, 188-190. 416& PAYNE, F. : Drosophila ampelophila Loew Bred in the Dark for Sixty-nine Generations. Biol Bull, 1911, xxi, 297-301. 417 PEARL, R. : Studies on Electrotaxis. I. On the Reactions of Certain Infusoria to the Electric Current. Am. J. Physiol, 1900, iv, 96-123. 418 PEARL, R. : Studies on the Effects of Electricity on Organisms. II. The Reactions of Hydra to the Constant Current. Am. J. Physiol, 1901, v, 301-320. 419 PEARL, R. : The Movements and Reactions of Fresh-water Planarians : a Study in Animal Behavior. Quart. J. Micr. Sc., 1902-03, xlvi, 509-714. 4 20 PEARL, R., and COLE, L. J.: Thei Effect of Very Intense Light on Organisms. Third Rep. Mich. Acad. Sc., 1901, 77-78. 421 PEARSE, A. S. : The Reactions of Amphibians to Light. Proc. Am. Acad. Arts and Sc., 1910, xlv, 161-208. 422 PEREZ, J. : Notes zoologiques. De ^attraction exercee par les odeurs et les couleurs sur les insects. Acta Soc. Linn., Bordeaux, 1894, vii, 245-253. 423 PFEFFER, W. : Locomotorische Richtungsbewegungen durch chemische Reize. Ber. bot. Ges., 1883, i, 524-533. 424 PFEFFER, W. : Locomotorische Richtungsbewegungen durch ehemische Reize. Unters. Bot. Inst. Tubingen, 1884, i, 363-482. 425 PFEFFER, W. : Ueber chemotaktische Bewegungen von Bakterien, Flagellaten und Volvocineen. Unters. Bot. Inst. Tubingen, 1888, ii, 582-661. 426 PHIPPS, C. F. : An Experimental Study of the Behavior of Amphipods with Respect to Light Intensity, Direction of Rays, and Metabolism. Biol Bull, 1915, xxviii, 210-223. 427 PLATEAU, F. : Recherches sur la perception de la lumiere par les myriopodes aveugles. J. Anat. et Physiol, 1886, xxii. 42« PLATEAU, F. : Nouvelles recherches sur les rapports entre les insectes et les fleurs. Mem. Soc. Zool France, 1899, xii. 429 PLATEAU, F. : La choix des couleurs par les insectes. Mem. Soc. Zool France, 1899, xii, 336-370. 4so PLATEAU, F. : Experiences sur 1'attraction des insectes par les etoffes colorees et les objets brillants. Ann. Soc. Ent. Belgique, 1900, xliv. LITERATURE 199 431 PLATT, J. B. : On the Specific Gravity of Spirostomwm, Paramcecium, and the Tadpole in Relation to the Problem of Geotaxis. Am. Nat., 1899, xxxiii, 31-38. 432 POLIMANTI, 0. : Ueber eine beim Phototropismus des Lasius niger L. beobachtete Eigentiimlichkeit. Biol. Centr., 1911, xxxi, 222-224. 433 POLIMANTI, 0. : Sul reotropismo nelle larve dei batraci (Bufo e Eana). Biol. Centr., 1915, xxxv, 36-39. 434 PORODKO, TH. M. : Vergleichende Untersuchungen iiber die Tropis- men. I. Das Wesen der chemotropen Erregung bei den Pflanzen- wurzeln. Ber. bot. Ges., 1912, xxx, 16-27. 435 PORODKO, TH. M. : II. Thermotropismus der Pflanzenwurzeln. Ber. bot. Ges., 1912, xxx, 305-313. 436 PORODKO, TH. M. : IV. Die Giiltigkeit des Energiemengengesetzes fiir den negativen Chemotropismus der Pflanzenwurzeln. Ber. bot. Ges., 1913, xxxi, 88-94. 437 PORODKO, TH. M. : V. Das mikroskopische Aussehen der tropistisch gereizten Planzenwurzeln. Ber. bot. Ges., 1913, xxxi, 248-256. 438 POWERS, E. B. : The Reactions of Crayfishes to Gradients of Dis- solved Carbon Dioxide and Acetic and Hydrochloric Acids. Biol. Bull., 1914, xxvii, 177-200. 439 PRENTISS, C. W.: The Otocyst of Decapod Crustacea: Its Structure, Development, and Functions. Bull. Mus. Comp. Zool., 1901, xxxvi, 165-251. 440 PRINGSHEIM, E. G. : Die Reizbewegungen der Pflanzen. Berlin, 1912, viii+326. 441 PRINGSHEIM, E. G. : Das Zustandekommen der taktischen Reaktionen. Biol. Centr., 1912, xxxii, 337-365. 442PRZiBRAM, K. : Ueber die ungeordnete Bewegung niederer Tiere. Arch. ges. Physiol., 1913, cliii, 401-405. 443 PUTTER, A. : Studien iiber Thigmotaxis bei Protisten. Arch. Anat. u. Physiol., Physiol. Abt., 1900, Suppl., 243-302. 444 RADL, E.: Ueber den Phototropismus einiger Arthropoden. Biol. Centr., 1901, xxi, 75n86. 44 s RADL, E.: Untersuchungen iiber die Lichtreaktion der Arthropoden. Arch. ges. Physiol., 1901, Ixxxvii, 418-466. 446 RADL, E. : Ueber die Lichtreaktionen der Arthropoden auf der Dreh- scheibe. Biol. Centr., 1902, xxii, 728-732. 447 RADL, E. : Untersuchungen iiber den Phototropismus der Tiere. Leip- zig, 1903, viii+188. 448 RADL, E. : Ueber die Anziehung des Organismus durch das Licht. Flora, 1904, xciii, 167-178. 449 RADL, E.: Einige Bemerkungen und Beobachtungen iiber den Photo- tropismus der Tiere. Biol. Centr., 1906, xxvi, 677-690. 200 TROPISMS 450 REAUMUR: Memoires pour servir a 1'histoire des insectes. Paris, 1740. 451 REESE, A. M. : Observations on the Reactions of Cryptobranchus and Necturus to Light and Heat. Biol Bull, 1906, ad, 93-99. 452 RILEY, C. F. C. : Observations on the Ecology of Dragon-fly Nymphs : Reactions to Light and Contact. Ann. Ent. Soc. Am., 1912, v, 273- 292. 453 ROMANES, G. J. : Animal Intelligence. New York, 1883, pp. 520. 454 ROMANES, G. J. : Jelly-fish, Star-fish and Sea-urchins. New York, 1893, x+323. 455 ROTHERT, W. i Ueber Heliotropismus. Beitr. Biol Pftanzen, 1894, vii, 1. 456 ROTHERT, W. : Beobachtungen und Betraehtungen iiber taktische Reizerscheinungen. Flora, 1901, Ixxxviii, 371-421. 457 Roux, W. : Ueber die Selbstordnung (Cytotaxis) sich " beriihrender " Furchungszellen des Froscheies durch Zusammenfiigung, Zellen- trennung und Zellengleiten. Arch. Entwcklngsmech., 1896, iii, 381-468. 458 ROYCE, J.: Outlines of Psychology. New York, 1903, pp. 417. 459 RUCHLADEW, N. : Untersuchungen zur Kritik der Methodik chemotak- tischer Versuche und zur Biologic der Leukozyten. Z. Biol., 1910, liv, 533-559. 460 S'CHAFER, K. L. : Ueber den Drehschwindel bei den Tieren. Z. Psy- chol. u. Physiol. Sinnesorg., 1891. 461 SCHAEFFER, A. A. i Reactions of Ameba to Light and the Effect of Light on Feeding. Biol. Bull, 1917, xxxii, 45-74. 462 SCHMID, B. : Ueber den Heliotropismus von Cereactis aurantiaca. Biol. Bull, 1911, xxxi, 538-539. 462a SCHNEIDER, G. H. : Der tierische Wille. Leipzig, 1880. 4626 SCHNEIDER, K. C. : Tierpsychologisches Praktikum in Dialogfonn. Leipzig, 1912, pp. 719. 462c SCHNEIDER, K. C. : Vorlesungen iiber Tierpsychologie. Leipzig, 1909. 463 SCHOENICHEN, W. i Die Empfindlichkeit der Nachtschmetterlinge gegen Lichtstrahlen. Prometheus, 1904, xvi, 29-30. 464 SCHOUTEDEN, H. : Le phototropisme de Daphnia magna Straus (Crust.). Ann. Soc. Ent. Belgique, 1902, xlvi, 352-362. 465 SHIBATA, K. : Studien iiber die Chemotaxis der Zsoe'tes-Sperrnato- zoiden. Jahrb. wiss. Bot., 1905, xli, 561-610. 466 SHOHL, A. T. : Reactions of Earthworms to Hydroxyl Ions. Am. J. Physiol, 1914, xxxiv, 384-404. LITERATURE 201 467 SMITH, A. C. : The Influence of Temperature, Odors, Light, and Contact on the Movements of the Earthworm. Am. J. Physiol., 1902, vi, 459-486. 468 SMITH, G.: The Effect of Pigment-migration on the Phototropism of Gammarus annulatus S. I. Smith. Am. J. Pliysiol., 1905, xiii, 205-216. 469 SOSNOWSKI, J. : Untersuchungen iiber die Veranderungen des Geo- tropismus bei Param&cium aurelia. Bull. Internal. Acad. Sc. Cracovie, 1899, 130-136. 470 STATKEWITSCH, P. : Ueber die Wirkung der Induktionschlage auf einige Ciliata, Le Physiologists Russe, 1903, iii, 41-45. 471 STATKEWITSCH, P.: Galvanotropismus und Galvanotaxis der Ciliata. Z. allg. Physiol., 1904, iv, 296-332; 1905, v, 511-534; 1907, vi, 13-43. 472 STRASBURGER, E. : Wirkung des Lichtes und der Warme auf Schwarm- sporen. Jenaische Z. Naturwiss., 1878, (N.F.) xii, 551-625. Also separate, Jena, pp. 75. 473 SZYMANSKI, J. S. : Ein Versuch, das Verhaltnis zwischen modal ver- schiedenen Reizen in Zahlen auszudriicken. Arch. ges. Physiol., 1911, cxxxviii, 457-486. 474 SZYMANSKI, J. S. : Aenderung des Phototropismus bei Kiichenschaben durch E'rlernung. 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Zool, 1917, xxiv, 211-217. 532 CLAPAREDE, E.: Les tropismes devant la psychologic. J. Psychol. u. Neurol, 1908, xiii, 150-160. 533NAGEL, W. A.: Experimentelle sinnesphysiologiehe Untersuchungen an Coelenteraten. Arch. ges. Physiol, 1894, Ivii, 495-552. 534 SCHNEIDER, G. H. : Der tierische Wille. Leipzig, 1880. 535 SCHNEIDER, K. C. : Tierpsychologisches Praktikum in Dialogform. Leipzig, 1912, pp. 719. 536 SCHNEIDER, K. C. : Vorlesungen iiber Tierpsychologie. Leipzig, 1909. 53? MORGULIS, S. : The Auditory Reactions of the Dog Studied by the Pawlow Method. /. Animal Behav., 1914, iv, 142-145. 538 MORGULIS, S. : Pawlow's Theory of the Function of the Central Ner- vous System and a Digest of Some of the More Recent Contributions to This Subject from Pawlow's Laboratory. J. Animal Behav. , 1914, iv, 362-379. 639 CRAIG, W. : The Voices of Pigeons Regarded as a Means of Social Control. Am. J. Sociology, 1908, xiv, 86-100. 54<> CRAIG, W. : Male Doves Reared in Isolation. J. Animal Behav., 1914, iv, 121-133. 541 MATULA, J. : Untersuchungen iiber die Funktionen des Zentralnerven- systems bei Insekten. Arch. ges. Physiol., 1911, cxxxviii, 388-456. LITERATURE 205 542 LOEB, J. : Influence of the Leaf upon Boot Formation and Geotropic Curvature in the Stem of Bryophyllum calcycinum and the Possi- bility of a Hormone Theory of These Processes. Bot. Gaz., 1917, Ixiii, 25-50. 543 LOEB, J. : Untersuchungen zur physiologischen Morphologie der Tiere. I. Heteromorphose. II. Organbildung und Wachstum. Wiirzburg, 1891-1892. 544 LOEB, J. : The Chemical Mechanism of Regeneration. Ann. Inst. Pasteur, 1918, xxxii, 1-16. 545 CRAIG, W. : Appetites and Aversions as Constituents of Instincts. Biol Bull, 1918, xxxiv, 91-107. 546 KANDA, S. : Further Studies on the Geotropism of Paramtecium can- datum. Biol. Bull., 1918, xxxiv, 108-119. 547 LYON, E. P. : Note on the Geotropism of Paramacium. Biol. Bull., 1918, xxxiv, 120. 54« MCCLENDON, J. F. : Protozoan Studies. /. Exp. ZooL, 1909, vi, 265- 283. 549 McEwEN, R. S. : The Reactions to Light and to Gravity in Drosophila and its Mutants. J. Exp. Zool., 1918, xxv, 49-106. 550 pAYNE, F.: Forty-nine Generations in the Dark. Biol. Bull., 1910, xviii, 188-190. 551 PAYNE, F. : Drosophila ampelophila Loew Bred in the Dark for Sixty- nine Generations. Biol. Bull, 1911, xxi, 297-301. 552 MORGAN, C. L. : Animal Behavior. London, 1900. 653 STEVENS, N. M. : Regeneration in Antennularia. Arch. Entwcklngs- mech., 1910, xxx, pt. 1, 1-7. 554 MAXWELL, S. S'. : On the Exciting Cause of Compensatory Move- ments. Am. J. Physiol, 1911-12, xxix, 367-371. INDEX , 30 Aglaophenia, 138 Allen, 39 Amblystoma, 41, 53, 59 Ammophila, 170 Amphipyra, 135 Anelectrotonus, 32ff. Anemotropism, 132 Antennularia antennina, 119, 125 Arbacia, 148 ff. Arenicola, 106, 108, 109 Aristotelian viewpoint of animal conduct, 17, 18 Asymmetrical animals, 70 ff. Avena sativa, 84, 105, 106, 117 "Avoiding reactions," 96 Axenfeld, D., 54 Bacterium termo, 140, 142 Balanus eburneus, 75, 108 perforatus, 116 Bancroft, F. W., 41 ff. 62, 72, 74, 98 Barratt, J. 0. W., 146, 147 Barrows, W. M., 153, 154 Bauer, V., 18 Bees, heliotropic reactions of, 103 ff. 159 Bert, P., 101, 102 Blaauw, A. H., 84, 104, 106, 117 Blasius, E., 32 Blowfly, 51, 76, 109 Bohn, G., 75, 82 Brain lesions in fish, 24 ff. in dogs, 27 ff. in sEschna, 30 Bruchmann, H., 142 Bryophyllum calycinuni, 22, 120, 125, 137 Buddenbrock, W., 18 Budgett, S. P., 46 Buller, A. H. R., 141, 143, 148 ff Bunsen-Roscoe law, 21, 83 ff., 99, 100, 137 Butler, S., 161 Catelectrotonus, 32 ff. Centrifugal force, 125, 126 Chemotropism, 139 ff., 160 Chilomonas, 144, 145 Chlamydomonas pisiformis, 106, 109 Cineraria, 48, 164 Circus movements, fish, 24 ff., dogs, 27 ff., fflschna, 30 housefly, 54, Ranatra, 54, Proct acanthus, 60, 61, 72, Euglena, 72 ff., Vanessa Antiopa, 54 Color sensations, 100 ff. Colpidium colpoda, 144 Compensatory motions, 126, 128 ff. "Conditioned reflexes," 166 ff. Craig, Wv 168 Crayfish, 38 Cucumaria cucumis, 125 Cypridopsis, 116 Danais plexippus, 162 Daphnia, 88, 89, 92, 96, 101, 102, 104, 113 ff, 162, 171, 172 Delage, Y., 123, 124 Dewitz, J., 136, 149 Diaptomus, 114, 115 Dragon fly larva, 30 Drosophila, 111, 116, 117, 153 Eudendrium, 66, 73, 83, 85, 106 Euglena, 16, 45, 62,' 70, 72 ff., 97 ff., 106, 109 Ewald, W. F., 85, 88, 104, 116 "Fertilizing 149, 150 Flourens, P., 27 Forced movements, 24 ff. Franz, V., 18 "Fright reactions," 96 v. Frisch, K.., 103, 104 Fundulus, 143, 157 Galileo, 18 Galvanotropism, 32 ff. Gammarus, 113, 114, 116 Garrey, W. E., 33, 40, 41, 51 ff., 71 72, 132, 133 .Gelasimus, 39, 124 Geotropism, 119, ff. 207 208 INDEX Glaucoma scintillans, 144 Gonium, 109 Graber, V., 47, 100, 104 Groom, T. T., 112 Hammond, J. H. Jr., 68 Harper, E. H., 154 Heliotropic machine, 68 ff. Heliotropism, 47 ff. Hering, E., 127 Hermann, L., 32 Hess, C., 102 ff. Holmes, S. J., 51 ff., 73, 116 Instincts, 156 ff. "Irritability," 39 Isoetes, 141, 142 Jellyfish, 41, 42 Jennings, H. S., 73, 96 ff., 119, 125, 143 ff., 155 Jordon, H., 18 Kellogg, V. L., 158 Knight, 125 Kreidl, A., 124 Kupelwieser, H., 104 Lidforss, B., 142 Lillie, F., 149 ff., 15,6 Littorwa, 75 Lizard, nystagmus in, 126, 129, 130 Lubbock, J., 47 Ludloff, K., 43 Lumbricus, 109 Lummer-Brodhun photometer, 90 Lycopodium, 142 Lyon, E. P., 22, 125, 128, 131 McEwen, R. S., Ill, 116 ff. Mach, E., 33 Magendie, 27 Magnus, R., 22 Marchantia, 142 Mast, S. O., 73, 99, 108, 109, 119 Matula, J., 30 Maxwell, S. S., 33 ff., 106, 108, 126, 135 Mayer, A. G., 162 Mazda lamp, 86 Memory images, 164 ff. Mendelssohn, M., 155 Me'nifcre's disease, 17, 110 Miessner, B. F., 68 Moore, A. R., 22, 112, 115, 117 Morgulis S., 166 Muscle tension, 20 ff.; after brain lesions in fish, 24 ff., dogs, 27 ff.; under influence of, galvanic cur- rent, 32 ff., one source of light, 47 ff., two sources of light, 75 ff., changes in intensity of light, 95 ff. Nereis, 135, 150 Nernst, W., 46, 95 Nernst lamps, 76 Neurons, orientation of, 38 ff. Northrop, J. H., 75, 89, 90 Nystagmus, 126, 129, 130 Oltmanns, F., 117 Palcemon, 124 Palcemonetes, 33 ff., 52, 59 Pandorina, 106, 109 Paramcecium, 43 ff., 97, 125, 143, 144, 164 ff., 155 Parker, G. H., 54, 75 Patten, B. M., 75 ff., 92 Pawlow, 165 ff. Payne, F., 118 Pfeffer, W., 140 ff. Phacus Triqueter, 109 Phrynosoma, 126, 129, 130 Pliy corny ces, 105, 117 Platyonichus, 124 Polygordius, 115 Poly orchis penicillata, 41, 42 Porthesia chrysorrhoea, 48, 116, 161 Proctacanthus, 55 ff. Radl, E., 54, 88, 128 Ranatra, 52 ff. Reflexes, 21 ff., 166 Retina images, 127 ff. Reversal of helitropism, 112 ff. Rheotropism, 131 ff. Robber fly, 55 ff., 72 Sachs, 101. Salamander larvae, galvanotropism of, 41 Schweizer, F., 32 Scyllium canicula, 24 Serpula, 95 Shark, forced movements in, 22, 24 ff. Sherrington, 22 INDEX 209 Shibata, K, 141, 142 Shock movements, 97, 98 Shrimp, galvanotropism in, 34 ff. Soule, C. G., 162 Spirillum undula, 140 Spirographis spallanzani, 63, 83 Spondylomorum, 109 Steinach, 15.6 Stereotropism, 134 if., 157 Stevens, N. M., 119 Sticklebacks, 132 Strongylocentrotus purpuratus, 151 Stylonychia mytllus, 144 Symmetry relations of animal body, 19 ff. v. Tappeiner, H., 117 Terry, O. P., 44 Thermotropism, 155 Towle, E. W., 116 Trachelomonas euchlora, 109 "Trial and error," 17, 73, 153, 154 TubuJaria mesembryanthemum, 137 v. Uexkiill, J., 18, 21, 22 Vanessa antiopa, 54 Verworn, M., 42 Vitalism, 18 Volvox, 44, 45, 62, 83, 117 Wasteneys, H., 86, 99, 106, 108, 143 Wave lengths, heliotropic efficiency of, 100 ff. Weber's law, 78, 142, 143 Wheeler, W. 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