The chromatic function--and I use this term to designate the faculty of changing color according to surroundings--is possessed by a number of the lower animals. The chameleon is the best known of all the tinctumutants (_tinctus_, color, and _mutare_, to change), though many other animals possess this faculty in a very marked degree. In order to understand the manner in which these changes or modifications of color take place, one must know the anatomy of the skin, in which structure these phenomena have their origin. The frog is a tinctumutant, and a microscopic study of its skin will clearly demonstrate the structural and physiological changes that take place in the act of tinctumutation.

The skin of a frog consists of two distinct layers. The epidermis or superficial layer is composed of pavement epithelium and cylindrical cells. The lower layer, or _cutis_, is made up of fibrous tissue, nerves, blood-vessels, and cavities containing glands and cell elements.

The glands contain coloring matter, and the changes of color in the frog's skin are due to the distribution of these pigment-cells, and the power they have of shrinking or contracting under nerve irritation. The pigment varies in individuals and in different parts of the body. Brown, black, yellow, green, and red are the colors most frequently observed.

The color-cells are technically known as _chromatoph.o.r.es_. If the web of a frog's foot be placed on the stage of a microscope and examined with an achromatic lens, the chromatoph.o.r.es can readily be made out.

Artificial irritation will immediately occasion them to contract, or, as is frequently the case, when contracted, will occasion them to dilate, and the phenomena of tinctumutation may be observed _in facto_. Under irritation the orange-colored chromatoph.o.r.es, when shrunk, become brown, and the contracted yellow ones, when dilated, become greenish yellow.

When all the chromatoph.o.r.es are dilated, a dark color will predominate; when they are contracted, the skin becomes lighter in color. Besides the pigment-cells just described, Heincke discovered another kind of chromatoph.o.r.e, which was filled with iridescent crystals. They were only visible, as spots of metallic l.u.s.tre, when the cells were in a state of contraction. He observed these latter chromatoph.o.r.es in a fish belonging to _Gobius_, the cla.s.sical name of which is _Gobius ruthensparri_.[94] I have seen this kind of color-cell in the skin of the gilt catfish, which belongs to a family akin to _Gobius_. The skin of this fish retains its vitality for some time after its removal from the body of the living animal, and the chromatoph.o.r.es will respond to artificial irritation for quite a while. In making my observations, however, I prefer to dissect up the skin and leave it attached to the body of the fish by a broad base. A few minims of chloroform injected hypodermatically rendered the animal anaesthetic, and I could then proceed at my leisure, without being inconvenienced by its movements. The causation of tinctumutation is now definitely known. The theory that light acts directly on the chromatophoric cells has been proved to be incorrect. Even the theory that light occasions pigmentation is no longer tenable. I have, time and again, reared tadpoles from the eggs in total darkness, yet they differ in no respect from those reared in full daylight. The chromatoph.o.r.es were as abundant and responded to irritation as promptly in the one as in the other. The distinguished Paul Bert declared that the young of the axolotl could not form pigment when reared in a yellow light. Professor Semper, on the contrary, declares Bert's axolotls to be albinos, and states that albinism is by no means infrequent in the axolotl; also that Professor Kolliker, of Wurtzburg, reared a family of white axolotls in a laboratory where there was an abundance of light, and that he (Semper) never succeeded in rearing an albino, though there was less light in his laboratory than in that of Kolliker, and his axolotls came from the same stock. Bert made the mistake of confounding albinism with the phenomenon of etiolation in plants; in fact, he gives the name "etiolation" to the albinism noticed in his axolotls.[95]

[94] Semper, _Animal Life_, p. 93.

[95] _Ibid._, p. 88 _et seq._

There is a marked difference between the functions of the chlorophyll bodies found in plants and the chromatoph.o.r.es found in animals. The former play one of the most important roles in the drama of plant life, inasmuch as they subserve a vital function, while the latter act a minor part, because they serve only as an instrument or means of protection.

Light is of great importance in its influence on chlorophyll, which is a microscopic, elementary body on which the vital strength of the plant depends, while it is not at all necessary to the chromatoph.o.r.es,--cell bodies secreting pigmentary matter for the purpose of protection. Of course, when animals are subjected to darkness for very long periods of time, the chromatoph.o.r.es are modified, and, sometimes, are wholly obliterated. They follow a well-known natural law, which declares that, when a function of an organ is no longer of any use to an animal, both organ and function become rudimentary, and finally disappear.

Many animals live for generations in total darkness before losing their pigment. I, myself, have seen black beetles in Mammoth Cave, Kentucky, in the neighborhood of Gorin's Dome, which is far within the depths of the cave. As beetles rarely range over a hundred yards from their place of birth, these insects must have been born in the cave and reared in the dark.

When speaking of light, if not otherwise specified, I mean diffused daylight which carries no heat rays. I believe that heat is a prominent factor in the production of color; the discussion of this point, however, does not properly belong to the subject under consideration.

Some experiments on newts, made by myself several years ago, show that the absence of light does not influence pigmentation,--that is, through several generations. My animals were kept under observation from the extrusion of the eggs until full maturity had been reached, and great care was taken to make experiments as accurate and as conclusive as possible.

Those reared in total darkness or in a red light were always dark-colored; those reared in a yellow light[96] were almost but not quite as dark; while those reared in white ironstone crocks and in diffused daylight were very much lighter, being pearl-gray in color.

This apparent (for the microscope showed that it was only apparent) absence of color in the last-mentioned specimens was due to tinctumutation.

[96] Vide Dewar, "The Physiological Action of Light," _Nature_, p.

433, 1877; quoted also by Semper, _loc. cit. ante_, Notes, p. 423. I do not think that the absence of the slight amount of color in the animals reared under the yellow light was due to the "optic current"

of Dewar. The microscope showed that the chromatoph.o.r.es were just as large and just as numerous, and that they contained as much pigment, as those reared under the red light. The apparent absence of color was due to tinctumutation.--W.

In most viviparous animals the embryo is developed in almost or absolutely total darkness, yet when it is born it has bright colors. Kerbert has found in the cutis of the embryonic chick, about the fifteenth day, certain pigment-cells. These cells have entirely disappeared by the twenty-third day. It is probable that little, if any, light can reach the chick through the sh.e.l.l and membranes, yet pigment-cells develop and disappear again.[97]

[97] Karl Semper, _Animal Life_, p. 422.

A b.u.t.terfly emerges from the coc.o.o.n arrayed in all the colors of the rainbow; yet it was developed, while in the _pupa_ state, in total darkness. It is not necessary to mention further instances; we readily see that pigmentation in animals is not necessarily dependent on light.

Neither is tinctumutation the result of the direct influence of light on the chromatoph.o.r.es. Light, however, if not the direct, is the indirect cause of this phenomenon. Lister, in 1858, showed that animals with imperfect eyesight were not good tinctumutants, notwithstanding the fact that they had the chromatophoric function. He showed, by his experiments on frogs, that the activity of the chromatoph.o.r.es depended entirely on the healthy condition of the eyes,--that is, so far as the phenomenon of tinctumutation was concerned. So long as the eyes remained intact and connected with the brain by the optic nerve, the light reflected from the surrounding objects exerted a powerful influence on the chromatoph.o.r.es.

As soon as the optic nerve was severed, the chromatoph.o.r.es ceased to respond to the influence of light and color, no matter how bright and varied they were. The deductions drawn from these experiments are not to be controverted or denied. The chromatoph.o.r.es are influenced by light reflected from objects and transmitted _via_ the optic nerve to the brain; from this organ the impression or irritation goes to the nerve governing the contractile fibres of these pigment-holding glands.[98]

[98] Karl Semper, _Animal Life_, p. 95.

Pouchet followed Lister, and confirmed his conclusion by experiments on fishes and crabs. He remarked that the plaice--a fish with a white under-surface and a party-colored back--had the chromatophoric function highly developed. Among a number of specimens which appeared pale on the white, sandy bottom, he met "one single dark-colored fish, in which, of course, the chromatoph.o.r.es must have been in a state of relaxation; and this specimen was as distinct from its companions as from the bottom of the aquarium. Closer investigation proved that the creature was totally blind,[99] and thus incapable of a.s.suming the color of the objects around it, the eyes being unable to act as a medium of communication between them and the chromatoph.o.r.es of the skin."[100] Thus far Pouchet had only confirmed Lister's observations, although it is highly probable that he was unaware of Lister's experiments. But he went a step further.

There are two ways in which cerebral impressions may be transmitted from the brain to the skin: one, by way of the spinal cord and the pairs of nerves arising from it and known as spinal nerves; the other, by two nerves running close to the vertebral column--the sympathetic nerves.

[99] Mr. Gordon Rett has recently called my attention to a blind "angel fish" which shows, most conspicuously, a lack of tinctumutation. This fish was made blind for experimental purposes.--W.

[100] Karl Semper, _Animal Life_, pp. 95, 96.

Pouchet cut the spinal cord close to the brain, yet the chromatoph.o.r.es still responded to light impression, showing that they did not receive the message through the cord and spinal nerves. He then divided the sympathetic nerves, and the chromatoph.o.r.es lost at once the power of contraction; he thus demonstrated that the sympathetic nerves were the transmitters of the optical message, and not the cord.

This discovery of Pouchet is, psychologically, of great importance, though he failed to recognize it as such. He was satisfied with its anatomical and physiological significance.

When we remember that the actions of the sympathetic nerves are almost, if not entirely, reflex in character, we at once see the psychological importance of this discovery. This fact makes the phenomenon of tinctumutation an involuntary act on the part of the animal possessing the chromatic function, and thus keeps inviolate the fundamental laws of evolution, which, were the facts otherwise, would be broken.[101]

[101] This simple fact of involuntary action renders the sensual nature of the function all the more apparent.--W.

By a series of experiments on frogs I have confirmed the conclusion of Pouchet _in toto_, and have even solved, so I believe and unhesitatingly a.s.sert, the puzzling problem of the physiological _modus operandi_ of the wonderful phenomenon of tinctumutation.

For a very long time I believed that this function was a distinct sense, and, five years ago, I set to work in search of the sense's centre.

After many dissections I found it (in the frog) lying immediately below the optic centres and closely connected with them. Nerve-fibres of the sympathetic can easily be traced and can be seen to penetrate this centre. When this centre is artificially stimulated either with the point of a needle or with a mild electric current, tinctumutation can be incited at will.

Again, when this centre is destroyed (which can be done without injury to the optic centres), the chromatophoric function ceases--the phenomenon of tinctumutation is no longer observable.

That the sympathetic nerves are the carriers of the messages from the optic nerve and the color-changing centre, can be demonstrated by other means than by excision of the nerve. Atropine, to a certain extent, paralyzes the sympathetic when given in sufficiently large doses, and injections of this drug beneath the skin of a frog render the division of the sympathetic unnecessary. The chromatoph.o.r.es will not respond to light impressions if the animal be placed thoroughly under the influence of atropine.

A large number of the lower animals possess the chromatophoric function.

Several years ago, I placed in a large cistern several specimens of gilt catfish. This is a pond fish and is quite abundant throughout the middle United States. It is of a beautiful golden yellow color on the belly and sides, shading into a l.u.s.trous greenish yellow on the back and head.

Several months after these fish had been placed in the cistern, it became necessary to clean the latter, and the fish were taken out. They were of a dusky drab color when first taken out, but soon regained their vivid tints when placed in a white vessel containing clear water. They had evidently changed color in order to harmonize with the black walls and bottom of the cistern.

Certain katydids are marked tinctumutants. I took one from the dark foliage of an elm and placed her on the lighter-colored leaves of a locust. She could be easily seen when first placed on the locust; in a few moments, however, she had faded to such an extent that she was barely noticeable.

The larvae of certain moths, beetles, and b.u.t.terflies also possess the chromatophoric function. The chromatoph.o.r.es in the larva of _Vanessa_ are very numerous, and this grub is a remarkably successful tinctumutant; the same can be said of the larvae of certain varieties of _Pieris_.

The power of changing color so as to resemble, in coloring, surrounding objects is evidently one of Nature's weapons of defence. In some animals it is developed in a wonderful manner. Wherever it is found it becomes to the animal possessing it a powerful means of defence by rendering it inconspicuous, and in some instances wholly unnoticeable.

After nine years of careful, systematic, and painstaking investigation, I am prepared to affirm that, besides the senses, sight, smell, taste, touch, hearing, and tinctumutation, certain animals have yet another sense, the sense of locality, or of direction, commonly called the "homing instinct." This remarkable function of the mind is not an instinct any more than the sense of sight or smell is an instinct, but is, on the contrary, a true sense; for I have demonstrated by actual experiment that it has a centre in the brains (ganglia) of some of the animals possessing it, just as the other senses have their centres. And, since this centre has been found in certain species, and that, too, in creatures very low in the scale of animal life, it is reasonable to infer that it is present in the brains (ganglia) of all those animals which evince the so-called "homing instinct."

In the process of civilization certain of the five senses in man become dull and blunted; thus, the sense of smell in the Tagals of the Philippine Islands is much more acute than it is in the civilized European, and what is true of the sense of smell is also true of the other senses, save that of touch, in all primitive peoples. This last sense seems to be much more acute in civilized man than it is in savages. This, for certain psychical reasons, unnecessary to detail here, is a necessary result of evolutionary growth and development.[102]

[102] Compare Tyler, _Anthropology_; De Quatref.a.ges, _The Human Species_; Peschel, _The Races of Man_; Lombroso, _L'Uomo Delinquente_; Ellis, _The Criminal_; the writer, "Criminal Anthropology," _N. Y.

Medical Record_, January 13, 1894.

As far as I have been able to learn, after much research in natural history, the anthropoid apes do not show that they possess the sense of direction in a marked degree; thus we see that the immediate ancestors of pithecoid man had already begun to lose this sense, which in man is entirely wanting, and the absence of which should not be a matter of surprise in the slightest degree, but rather a result that should be expected.

Evidences of this sense are to be observed in animals of exceedingly low organization. On one occasion, while studying a water-louse, as I have already described elsewhere in this book, I saw the little creature swim to a hydra, pluck off one of its buds, then swim a short distance away and take shelter behind a small bit of mud, where it proceeded to devour its tender morsel. In a short while, much to my surprise, the louse again swam to the hydra, again procured a bud, and again swam back to its hiding-place. This occurred three times during the hour I had it under observation. The louse probably discovered the hydra the first time by accident; but when it swam back to the source of its food-supply the second time and then returned again to its sheltering bit of mud, it clearly evinced conscious memory of route and a sense of direction.

The common garden-snail is a homing animal, and it will always return to a particular spot after it has made an excursion in search of food. In front of my dwelling there is a brick wall capped by a stone coping; the overhanging edge of this coping forms a moist, cool home in summer for hundreds of snails. Last summer I took six of these creatures, and, after marking their sh.e.l.ls with a paint of gum arabic and zinc oxide, I set them free on the lawn some distance away from the wall. In course of time, four of them returned to their homes beneath the stone coping; the other two were probably killed and eaten by blackbirds, numbers of which I noticed during the day feeding on the sward.

The centre of the sense of direction in snails is located at the base of the cephalic ganglion (brain); this ganglion lies immediately between and below the "horns" (eye-stalks), and is composed of several circ.u.mscribed and well-marked acc.u.mulations or corpuscles of nerve-cells and nerve-filaments.

This sense centre can easily be destroyed without inflicting injury on the circ.u.mjacent sense centres. Whenever this is done, the snail loses its sense of direction and locality, and cannot find its way back to its home when it is carried thence, and deposited amid new surroundings. It is not killed by the mutilation, for I have seen marked snails in which this sense centre had been destroyed, alive and apparently in good health, several weeks after having undergone this operation; they found temporary homes wherever they chanced to be.

The limpet is likewise a homing animal, and invariably returns to its home after journeys in search of food. Lieutenant L----, an officer in the British navy, once told me that he had repeatedly had specimens of this animal under observation for months at a time, and that they always had particular spots, generally depressions in rocks, which they regarded as homes, to which they would always return after excursions in search of sustenance. Romanes makes a similar statement.[103]

[103] _Animal Intelligence_, pp. 28, 29.

Some beetles have their homing sense highly developed; thus, in Mammoth Cave, the blind beetle (_Adelops_) has its particular home, and will always return to it even when it is set free at a considerable distance.

Notwithstanding the fact these insects are blind, and that darkness reigns in this immense cavern, they have periods of rest corresponding with the diurnal rest-periods of kindred species living in daylight; hence, it is easy to study their habits at home and abroad.

I have frequently marked these beetles and then set them free some distance away from their domiciles; they would hide themselves at once beneath stones or clods of earth, but as soon as they had recovered from their fright they would turn towards home, and would not stop, if left unmolested, until they arrived at their particular and individual homing places. Truly a most wonderful exhibition of the homing sense!

At first, these beetles are, probably, directed and governed by their sense of direction alone, but as soon as they arrive among familiar surroundings, memory comes to their aid.