The Animal Mind: A Textbook of Comparative Psychology

It is a well-known fact that when a human being with normal vision looks at the band of spectral colors, the band appears brightest to him in the region of the yellow. Yellow rays, that is, produce most effect on the normal human retina. They are also the most intense rays in sunlight. Now if a totally color-blind human being looks at the spectrum, he sees it as a band of different grays, the brightest gray being not in the yellow region but in the yellow-green; that is, it has been shifted towards the

violet end. He also sees in place of the red a gray darker than the brightness of red to the normal eye would lead one to expect. This altered distribution of brightness in the spectrum occurs for the normal eye also, under very faint illumination: in twilight the spectrum looks to the normal eye just as it does to the totally color-blind eye, a band of grays brightest in the yellow-green region. If we had no other means of deciding whether or not a man was color-blind, we should take as evidence of colorblindness the fact that for him the brightest region of the spectrum lay in the yellow-green rather than the yellow, in ordinarily bright light. It is therefore of some importance to the problem of color vision in the lower animals to find how strongly the light rays of various wave-lengths affect them. But we must bear in mind that for the lower animals it is impossible to conclude color-blindness from the fact that the brightness values, that is, the effective intensities, of the different colors are what they would be for a color-blind human being. Just this unsafe inference is, however, drawn by certain authorities.

In plants, the maximum effect of colored light is exerted by the rays at the violet end: violet, indigo, or blue. The problem has been investigated for microscopic animals by an arrangement such that two beams of light fall on the organism at right angles to each other. Now if the organism has a tendency either to seek or to avoid light, and if the two beams are of equal intensity, the animal will move on a diagonal between the two beams. If either ray has a stronger effect than the other, the course of an animal which seeks light will be inclined towards the more effective beam; that of an animal which avoids light, towards the less effective beam. If the two beams are of different colors, it will thus be possible to test the stimu-

lating efficiency of differently colored rays. Mast (474), using this method, found that some animal forms, such as the larve of the blowfly, are most strongly stimulated by that region of the spectrum which acts most strongly on the color-blind human being; others, such as the earthworm and the larve of the worm Arenicola, were most responsive to blue, as plants are. We shall later note the significance which Loeb ascribes to resemblances between plant and animal responses to light.

Amoeba, which as we have seen reacts to a change of light intensity by a checking of movement at the point affected, appears when tested by light passed through differently colored filters to react in the majority of cases most markedly to blue, although there are individual variations: some individuals respond most definitely to violet, others to green or yellow, and still others to red (467). The difference is merely in degree of response, and we can infer nothing about a qualitative differentiation of conscious accompaniments. In Hydra, which comes to rest in moderately illuminated regions, blue and green light seem to be a better substitute for white light than are red and yellow (791). Schmid (661) says that red and yellow affect the sea-anemone Cereactis aurantiaca differently from blue and green, but does not state wherein the difference consists.

Graber (266) attempted to test the color discriminations of a great many different animal forms by observing their preferences for differently colored lights. As we have already seen, where an animal displays no preference or choice between two stimuli, it by no means follows that the stimuli are not discriminated: they may produce differsubject of tests by the Preference Method, it was found that if a choice was offered between two compartments, one illuminated with diffuse daylight, the other dark, and if the number of worms in each compartment was counted at the end of every hour, those in the darkness were on the average 5.2 as many as those in the light. If ground glass was substituted for the dark screen, making the compartment under it about half as light as the other, the number in the lighter compartment was about .6 of the number in the darker, though still moderately light, compartment, showing that the worms were sensitive to comparatively small differences in intensity. When colored glasses were placed over the compartments, the following results were obtained: the worms preferred red to blue even when the former was much lighter than the latter to the human eye; they preferred green to blue under similar conditions, and red to green. They emphatically preferred white light from which the ultra-violet rays had been subtracted to ordinary white light, 6.7 times as many being found in a compartment covered by a screen impervious only to ultra-violet rays. It would thus appear that in determining avoidance, blue light is the most effective ; on the other hand, Yung (833) finds the effect of colored rays on the earthworm to be proportional to their intensity, the green and yellow regions of the spectrum being most effective.

It is thus clear that when an animal discriminates between rays of different colors, the discrimination may be based merely on the intensity of the rays, either in themselves or in the effect which they have on the organism, rather than on their wave-length or color. Minkiewicz offers as evidence of true color discrimination in a Nemertean worm, Lineus ruber, the fact that he could alter its reactions to colored light while its response to white light remained unchanged. When placed in diluted sea water, the animal would, after a day, direct itself toward violet rays, although still negative in response to white light. On the fourth day the ordinary “‘chromotropism” was restored ; that is, the worm sought red rays. After two or three weeks of life in the diluted sea water, on being restored to ordinary sea water the worm again showed inverted chromotropism, becoming “‘ positive” to the violet rays, while still “ negative”’ to white light. Moreover, intermediate stages in the passage from the red- to the violet-seeking phase were observed ; a stage where, still positive to red, the animal ceased to distinguish red from yellow, and others where it sought violet, but had become indifferent to green and yellow. These stages lasted for several hours, but corresponding ones were not observed during the passage from the violet phase back to the red phase: perhaps they occurred too rapidly to be noted (493).

Hess (314), on the other hand, concludes the total colorblindness of the marine worm Serpula from the fact that when tested by the direction in which it turned when subjected to light passing through differently colored glass, it showed evidence that the yellow-green had most effect, and that the effectiveness diminished rapidly towards red, slowly towards violet: in other words, that the brightness effect of the colors was like that shown in the case of a color-blind human being.

Hess (306) also studied the comparative effectiveness of different colored rays on the eyes of cephalopod mollusks by measuring with a special instrument the degree of expansion or contraction of the pupil produced by the various colors. He found that the yellow and green rays produce much more effect than the red and violet rays. Since this is true also of the color-blind human eye, he argues that the animals tested are totally color-blind. He holds, in fact, that all invertebrate animals are totally color-blind, on the same evidence. The feet of starfish belonging to the Astropectinide are, he says, very sensitive to light: red light has little effect on them, blue and green light, even when much darker than red to normal human vision, decidedly more effect, as they would have for a totally colorblind human being. The same results appear in the case of the sea-urchin. qi A human being’s Fic. 10.— Daphnia. at, antenna; aél, antennule; sensitiveness to oc, eye. After Yerkes.

light is increased when he remains in darkness for some time. This effect, called darkness adaptation, appears according to Hess in the Astropectinide (313). The problem as to whether light of different colors produces different sensations in the crustacean consciousness was the subject of experiments a number of years ago, in which the Preference Method was used. Lubbock (442, 443) arranged to have a sunlight spectrum thrown on a long trough containing Daphnias, tiny crustaceans belonging to the lowest subclass, that of the Entomostraca (Fig. 10). Daphnia is ordinarily positive in its response to light, that is, it seeks light. At the end of ten minutes glass partitions were slipped across the trough at the approximate dividing lines between the spectral colors. The number of animals in each compartment was then counted. The experiment was repeatedly performed, and the greatest number was always found in the yellow-green region. Bert

obtained similar results with the use of an electric light spectrum; but besides throwing all the colors at once upon the vessel, he allowed each color to act separately through a narrow opening, and noted the speed of the positive response produced. That the “preference” shown for yellow-green light is not a matter of color vision, but of response to the greater intensity of the light in this region of the spectrum, was suggested by Bert (46), and Merejkowsky showed that the larve of Balanus and Dias longiremis manifested no color preference when the colors were made of equal intensity (484). Lubbock attempted to prove the existence of qualitative as distinguished from intensive discrimination by various modifications of the experiment, but without entirely conclusive results (444, pp. 221 ff.). Yerkes, working on Simocephalus, a form closely related to Daphnia, found that when a gaslight spectrum was used, the animals collected in the red-yellow region, that of greatest intensity for such light; and that if this region had its intensity diminished by a screen of India ink or paraffine paper, the crustaceans moved out of it (799). This seemed strong evidence that the apparent color reactions of these animals were really responses to differences in the intensity of the light. Hess (306), studying the movements of the eyes of Daphnia when subjected to light of different colors, finds another case of total colorblindness, and Erhard (207) gets similar results on Simocephalus when the light is reflected from colored surfaces. Nevertheless, there is evidence that colored rays have an effect on these crustaceans that is not wholly dependent on their intensity ; evidence, that is, in favor of color vision.

When Daphnias have been kept for some time in light of a certain intensity, an increase in the intensity makes them avoid light, while a decrease in intensity makes them seek light. But if a blue screen is interposed between the light and the animal, in spite of the fact that the intensity is thereby diminished, the Daphnias avoid it; if yellow light is added to white light, in spite of the fact that the intensity of the light is thereby increased, the Daphnias seek it. These results were obtained by Von Frisch (248), who is as determined to find color vision in invertebrates as Hess is to disprove it. Ewald (213) reports that of the Daphnias under his observation one group sought the light, which was in this case most effective in the green-yellow regions, but that another group avoided light, and for these the most effective region was the blue-violet, so the effect of colored rays was independent of intensity. He reports, however, that certain colored rays could be replaced by colorless rays without affecting the responses of the Daphnias; these rays were the red and green ones. He therefore concludes that Daphnia is not totally color-blind, but red-green blind. This, as we shall see, is Von Frisch’s belief with regard to certain other invertebrates. Ewald thinks he has also evidence in the case of Daphnia of simultaneous contrast and successive contrast, such as human - vision shows. The successive effect (negative after-images) occurs for both color and brightness stimuli, and is shown by the fact that the animals reverse their reaction to the same white light according as they have been exposed previously to white (or blue) light, or to darkness (or yellow light). Simultaneous contrast Ewald concludes from the observation that when the region surrounding a constant stimulus light is brightened, the reaction of the animals tends to become positive, that is, they move towards the light; darkening the surroundings makes them move towards the same light.

This effect, it is argued, is due to the stimulation of the side regions of the eye: now, since colors have no special influence in producing these simultaneous contrast phenomena, Ewald concludes that the side regions of Daphnia’s eye, like those of our own eye, are totally color-blind. All of which seems a heavy weight of inference to depend from rather slender evidence. That Daphnia seeks a region affected by the ultra-violet rays of the spectrum in preference to darkness, although the two look alike to our eyes, was shown by Lubbock (444). On the other hand, Loeb (431) and Moore (so02) report that the action of ultra-violet rays instantly makes Daphnia avoid the light containing them, and it appears that these rays, which are without effect on the human eye, exert their influence through the eye of Daphnia, since individuals without eyes are unaffected by them (300).

Adaptation to darkness apparently takes place in the eye of Daphnia, for individuals which have been a long time in darkness will respond to a lower intensity of light than those which have been long exposed to illumination (174). Experiments on the effect of intermittent and continuous lights of equal intensity on the movements of the Daphnia eye indicate that the Talbot-Plateau Law, according to which such lights are identical in effect, - holds for the vision of this crustacean as for the human eye (212). Itis this law which enables us to measure the grey produced by a rapidly revolving disk of black and white sectors as equal in brightness to the amount of light reflected by the sectors at rest.

Experiments on the reactions of the crayfish, which has a moderately strong tendency to avoid light, show that when light falls vertically through red glass the animal seeks it in preference to darkness: no marked preference is indicated when the light is passed horizontally through red glass (40). No clear evidence of color discrimination appears here. Hess (306), of course, holds that all crustaceans are totally color-blind, arguing from his results on the relative stimulating effect upon them of different spectral colors. Minkiewicz (494, 495), on the other hand, believes he has evidence of color discrimination in certain crabs. The hermit crabs, for instance, are naturally attracted to light, but when subjected to colored lights they do not seek them in the order of their intensity. Green is the most attractive color, violet next; then the order is “‘blue, yellow, red, and black.” He finds it possible with crabs, as with worms (see page 147), to reverse the response to white light without reversing the response to color (493). Minkiewicz’s most remarkable observations were made on certain crabs (Maia) which have the instinct possessed by many crab species of attaching to their shells foreign objects, bits of seaweed and the like, serving the purpose of making them less conspicuous in their ordinary environment. When these crabs are kept for some time in an aquarium lined with a certain color, their subsequent behavior is modified in two ways. (1) On being given bits of paper some of which are colored like the aquarium, while others are of a different color, the crabs select for decorative purposes the bits that match their surroundings. (2) When placed in another aquarium offering a choice between two compartments, one with walls matching those of the tank they have left, the other with differently colored walls, the crabs choose the former. Two American investigators have performed experiments similar to these. Pearse (567) fails to get any evidence that when crayfishes, spider crabs, crab spiders and caddis fly larve are kept in colored boxes they develop any tendency to choose later an environment of the same color. On the other hand, Stevens (693), working with a Pacific coast crab which has

the decorating instinct, finds that it does acquire such a tendency, but that it does not learn to decorate itself with colors matching its surroundings. The acquired ‘‘chromotropism,” or tendency to seek a certain color, in crabs might be interpreted as merely a response to the brightness of the colors, not to their color as such; that is the crabs may after all be totally color-blind, seeing the colors as grays. Stevens found indications that green comes nearest to white light in its effect on the animals, by noting the promptness and accuracy with which they faced the light.

Experiments have been made on color discrimination in spiders: some by the Preference Method, where the spiders showed an inclination for red when offered a choice of compartments illuminated through red, green, blue, and yellow glass (570); others by attempting to form an association between paper of a certa‘n color and the spider’s nest. This latter, containing eggs, was surrounded with colored paper, and when a spider had become accustomed to going in and out over the paper, another color was substituted, and a false nest made in another place, surrounded by the original strips of paper. The spider under these circumstances showed some confusion and tendency to go to the false nest It is obvious that this method takes no account of the possibility that the spider was reacting only to the intensity of the colored rays and not to their color as such (571).

On the color sense of insects there are, first, the old experiments of Graber by the Preference Method, whose most definite result was to show that positively phototropic, that is, light-seeking, insects prefer colors containing the ultra-violet rays, while the negatively phototropic or lightavoiding ones prefer red, from which these rays are absent. No proof that the discriminations were made on the basis of color proper rather than brightness was forthcoming (267). Similar observations were made by Lubbock on ants, which in their underground life are negatively phototropic, the eggs and larve apparently needing darkness in order to develop, but on their foraging expeditions are comparatively indifferent to light. They showed a preference for red when tested, and a tendency to avoid the ultra-violet rays, so marked that they preferred bright daylight from which these rays had been extracted by chemical screens, to darkness that contained the ultra-violet rays (441, pp. 207 ff.). Graber suggested that the ultraviolet rays produce a skin sensation in the ants; but Forel agrees with Lubbock that the effect is visual, because he found that varnishing the eyes made the ants indifferent to ultra-violet (231). Ants of the family Lasius seem to be normally insensitive to these rays (235). It is just possible, then, that a visual sensation of quality wholly foreign to our experience may accompany the action of ultra-violet rays on insects. Loeb has noted that the relative effect of violet and ultra-violet vibrations, as compared with that of the rest of the spectrum, is greater, the less developed the visual organ (419). Termites, which seek darkness, prefer red to blue colored glass (6). Lubbock’s experiments on the color sense of bees are more to the point than those on ants, for they were made not by the Preference Method, but by associating a color with food. No precaution, however, was taken against the brightness error. He found that bees which had eaten honey from blue paper would pick out the blue pieces from a number of differently colored papers, whose positions were altered during the experiments (441). Forel got similar results, and reports that a bumblebee thus trained selected all the blue objects in the room for special ex-

amination (231). Lubbock’s tests with wasps gave negative results. Lovell (439) and Turner (725) also infer color vision in the honey-bee from its ability to pick out objects of the same color as that on which it has recently found food: the former takes no account of the brightness error, while the latter holds that it has been sufficiently eliminated by the fact that the color identifications were made by the bees under varying lights and shades out of doors. This, however, is probably an inadequate precaution. Von Frisch (246) offers more convincing evidence of color vision in the bee, and thinks he has indications that bees are red-green color-blind. His experiments were performed in the open air. Having trained the bees to come to strips of yellow paper, on which food was placed, he mingled such strips, without food, among strips of thirty different shades of gray. The bees, he reports, were able to make the discrimination, and to do equally well when blue was used: they failed, however, with red, confusing red-violet with blue, and dark red with dark gray. In another article (247) he says that a certain bluish green also was confused with gray. This general method, where a large range of grays is used and an animal proves capable of discriminating a color from any or all of them, is the best way of eliminating the brightness error. The use of colored papers in experiments on color vision in animals is open to criticism unless some precaution is taken against the possibility that something in the surface texture or grain of the papers may help the animal to distinguish. This possibility Von Frisch guarded against by varnishing the papers, a proceeding which did not affect the behavior of the bees.

Hess (312, 315), anxious to defend his theory of the total color-blindness of all invertebrates, repeated Von Frisch’s experiments and could not confirm his results. Using colored lights of measured intensities and studying their effect in causing the bees to collect under them, he compared these effects with the influence of the various colors on the reflex contraction of the pupil in human beings. He found that the smallest differences in intensity to which the bees reacted were those just perceptible to the human eye, and that the relative effect of different colors was like their relative effect on a color-blind human being. He reports similar results with butterflies. Hess thinks this method, which deals with reflexes, superior to any method which, like Von Frisch’s, involves learning on the part of the animals. But again we may remind ourselves that it does not follow that because a human being who finds the yellowgreen, rather than the yellow, the brightest spectral region, is totally color-blind, therefore an animal, especially an invertebrate animal, the chemical substances in whose © eye may have no resemblance to those in the human eye, is color-blind if it shows these reactions to the different regions of the spectrum. MHess’s method is defective just because it deals with reflexes whose stimuli are intensity differences. If an animal is capable of distinguishing both intensity differences and color differences, the use of reflexes that depend on the former is a poor way of studying the capacity to discriminate the latter.

We have already noted the dispute as to how far visual sensations in general are involved in the reactions of bees to flowers, and have seen that Plateau maintains their relative unimportance in this connection, as compared to smell. Besides the experiments which we have quoted on pp. 104f., he adduces the facts that he could never persuade insects to alight upon artificialflowers, though these were not distinguishable by human eyes from real ones (600-

602); that bees show no preference for flowers of any particular color (603); and that they often make errors, in alighting on closed buds, seed pods, and wilted flowers, which indicate defective vision (605). But Josephine Wéry and others have noted that bees do seek artificial flowers (778). Even Plateau does not deny that an insect may perceive flowers from a distance, ‘‘whether because it sees the color in the same way that we do, or because it perceives some kind of contrast between the flowers and their surroundings” (603).

Von Buttel-Reepen (114) gives one or two instances to show that the color perception of bees is sometimes influential in helping them to recognize their own hives. He reports a case where a stock of bees had been driven from their hive and scattered. The front of the hive was blue. Some of the bees tried to find their way into other hives, and selected for their efforts those which had blue doors. It will be remembered that Loeb is convinced that the relative effect of the different regions of the spectrum on invertebrate animals is identical with the effect on plants; that is, strongest for the violet end of the spectrum. This position has no significance for the problem of color discrimination, but obviously Loeb and Hess are sharply opposed as to the facts. Recently Gross (271) has used colored spectral lights of carefully equated intensity, and a method which permits measurement of the exact amount of light effect, in deflecting an animal from its course of movement. Adult blowflies, fruit flies, and moths, as well as larve, were used as subjects. All the lights were made of equal intensity, whereas in the ordinary daylight spectrum the yellow region is most intense and differences of intensity exist all along the line. Under these conditions, Loeb’s contention was confirmed

and Hess’s overthrown for all the subjects except the larve of the blowfly; that is, the insects were more strongly affected by blue than by yellow or green. But the blowfly larva was more strongly affected by green than by any other colored light. It responded, in other words, as Hess and his pupil Weve (779) had found it to do. The vertebrate eye differs in origin and structure from any form of invertebrate eye, the most striking difference in structure being the location of the pigmented layer of the retina behind the nerve fibre layer, a location which is responsible for the existence of the blind spot in the vertebrate eye, where the trunk of the optic nerve breaks through the retinal layers. Another point of unlikeness consists in the fact that the invertebrate optic nerves do not cross on their way to the brain, while in the vertebrates there is either total or partial crossing of the fibres.

The reactions of Amphioxus to light offer as chief evidence that they are accompanied by a specific sensation quality the fact that they may be fatigued independently of other reactions. The only structures suggesting a visual function are pigment spots on the back near the head, and other pigment spots distributed down the back. Amphioxus makes negative responses to light, especially when the light, from which heat rays have been extracted by passing it through water, is directed at any point on the back, the most sensitive region lying just behind the eye-spot (406, 543). Fatiguing the light reactions had no effect on response to other forms of stimulation (543). Attempts to test the color “preferences” of Amphioxus by illuminating

different parts of a trough with differently colored lights gave negative results (406). Hess (306) found that the maximal effect on the activity of Amphioxus was exerted by the yellow and green rays, the red and violet being much less effective; hence he concluded that as in all invertebrates, so in this rudimentary vertebrate, total color-blindness exists. True skin sensitiveness to light has been observed in larval lampreys, which will give negative reactions even when the optic nerves are cut (540), and in cave-dwelling blind fish (zor). Parker, however, finds no other fish in which it exists, although it is quite common in amphibians. He therefore reaches the conclusion that in vertebrates, skin sensitiveness is not a primitive form of visual sensibility, from which vision by the eye has been derived, but a ‘“‘secondarily acquired peculiarity.”” He points out that the fish and amphibians which show it are freshwater animals, whereas the primitive vertebrates were certainly marine (545).

Among the many animals whose supposed color preferences Graber tested were two species of fish, but no con- _vincing proof of their powers of color discrimination was obtained (267). Bateson (25) placed food on differently colored tiles, and observed that the fish picked it off most readily from white and pale blue, and least readily off dark red and dark blue; which establishes little save that the bait was probably more conspicuous on the white and pale blue. Professor Bentley and the writer (757) got good evidence that the common brook chub could distinguish between red and green paints, by training it to bite at forceps to which red sticks were attached, and to refrain from biting at similar forceps carrying green sticks. The possibility of guidance by smell or by the position of the for-

ceps was ruled out, and the fish could identify the red .forceps whether_they were to the human eye darker or lighter than the green. It is not, however, a sufficient guard against the brightness error to use human judgments of brightness as a standard. Reighard (631), similarly, trained the gray snapper to avoid minnows dyed in certain colors and select those dyed in other ‘colors, several brightnesses differing to the human eye being used, but the brightness error not being more fully eliminated. Bauer (27) believes he has secured evidence that fish discriminate colors, and that certain fish are afraid of red, but the general character of his methods and conclusions does not inspire confidence. Hess (304, 308) is convinced that the spectrum is seen by fishes with the same distribution of brightnesses that is characteristic of the color-blind human eye, and makes the inference, which we have previously challenged, that total color-blindness must exist in such a case. Von Frisch (245), on the other hand, argues that fish possess color vision. He has shown their ability to pick out a color from a whole series of grays. He points to the fact that in the spawning season many fish assume bright colors and patterns; these, he urges, must have some influence in bringing the sexes together (243). Hess (310) in opposition to this points out that such colors would not be visible below a certain depth of water; Von Frisch replies that most of the fish which show them spawn in shallower waters. One can hardly, however, infer color vision from the existence of such colors, for they may be only incidental effects of the physiological state of the ’ animals, and without any influence on their behavior. A more persuasive line of argument is derived from the way in which various flatfish change their markings and colors to suit the ground on which they lie. Von Frisch (240)

studied this phenomenon in the case of the fish Phoxinus laevis. If two fish whose skins are, at the time, of equal brightness, are placed one on a yellow ground, the other on a gray ground, and these grounds are properly chosen as to brightness, the fish will not alter their own brightnesses, although if either ground is made lighter or darker a corresponding change occurs in the skin of the fish lying on the altered ground. When the fish remain at the same brightness, then, it may be inferred that the “‘brightness values” of the two grounds are identical. But after a few hours, it will be found that the fish on a yellow ground shows a yellow stripe which does not appear on the other fish (see also 277). Mast (475) has made a very thorough study of this phenomenon in the case of the flounders Paralichthys and Ancylopsetta. These fishes become strikingly bluish on blue grounds, greenish on green grounds, and so forth, adapting themselves to blue, green, yellow, orange, pink, and brown, and less successfully to red. The color changes are brought about by certain pigment-controlling mechanisms in the skin, which are connected with the sympathetic nervous system. But the color stimulus acts through its effect on the eyes: the changes do not occur if the eyes are covered. Moreover, the effect of the stimulus received by one eye is modified by that of the stimulus received from the other eye: if one eye is on a black ground and the other on a white ground, the skin becomes gray. Mast succeeded in showing that the rate at which alternating black and white sectors must follow each other in order to fuse into a continuous gray is the same for the eye of the flounder as for that of the human being: he placed the fish over a rotating black and white disk and noted the speed of rotation required for the fish to become gray instead of mottled black and white.

Watson (771) does not think these observations sufficiently prove color discrimination on the part of the fish. He says, ‘Ordinarily we mean when we say that an animal is sensitive to difference in wave-length that such stimuli play a réle in the adjustment of the animal to food, sexual objects, shelter, escape from enemies, etc. z.e., that such stimuli initiate activity in arcs which end in the striped muscles.” Because the changes of color are produced not by such arcs, but by the sympathetic nervous system, Watson thinks color vision not proved ; ‘‘we can easily conceive,” he says, “‘of mimicry of this kind taking place in an animal whose retina does not contain the physico-chemical substances . . . necessary to initiate response to differences in wave-length.” Since the changes of color are induced by differences in wave-length and induced through the retina, we may reply that it does not seem easy, or in fact at all possible, to conceive the absence of such photochemical substances from the fish’s retina. Moreover, Mast finds that fish which have thus become adapted to a given color will seek that color: this is an activity involving the striped muscles.