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When we come to examine the methods by which, or through which, many of the lower animals protect themselves from their enemies, we soon discover that some of these means are very wonderful indeed. It is not my purpose to discuss instinctive protective habits in this chapter; I wish rather to call attention to two senses, which are to be observed in certain of the lower animals, and which man and some of the higher animals have lost in the process of evolution. I refer to tinctumutation, the “color-changing” sense, and the sense of direction, or, as it is commonly and erroneously termed, the “homing instinct.” Neither of these faculties is instinctive, but they are, on the contrary, true senses, just as hearing, or taste, or smell is a sense. Careful dissections and repeated experiments have shown me, beyond peradventure, that these two psychical habitudes have their centres in the brains (ganglia) of animals which possess them.

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 chromatophores. If the web of a frog’s foot be placed on the stage of a microscope and examined with an achromatic lens, the chromatophores 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 chromatophores, when shrunk, become brown, and the contracted yellow ones, when dilated, become greenish yellow. When all the chromatophores 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 chromatophore, which was filled with iridescent crystals. They were only visible, as spots of metallic lustre, when the cells were in a state of contraction. He observed these latter chromatophores in a fish belonging to Gobius, the classical name of which is Gobius ruthensparri. 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 chromatophores 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 anæsthetic, 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 chromatophores 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 Koelliker, of Wuertzburg, 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 Koelliker, 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.

There is a marked difference between the functions of the chlorophyll bodies found in plants and the chromatophores 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 chromatophores, 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 chromatophores 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 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.

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 shell and membranes, yet pigment-cells develop and disappear again.

A butterfly emerges from the cocoon 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 chromatophores. 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 chromatophores 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 chromatophores. As soon as the optic nerve was severed, the chromatophores 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 chromatophores 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.

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 chromatophores 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, and thus incapable of assuming the color of the objects around it, the eyes being unable to act as a medium of communication between them and the chromatophores of the skin." 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.

Pouchet cut the spinal cord close to the brain, yet the chromatophores 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 chromatophores 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.

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 assert, 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 chromatophores 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 lustrous 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 butterflies also possess the chromatophoric function. The chromatophores 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.

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 shells 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 circumscribed and well-marked accumulations or corpuscles of nerve-cells and nerve-filaments.

This sense centre can easily be destroyed without inflicting injury on the circumjacent 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.

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.

The agile flea is another “homesteader,” and if marked, its favorite resting-place on a dog or cat can easily be determined. After feeding, it will invariably return to a certain spot in order to enjoy its nap in peace; for, strange as it may seem, fleas are sound sleepers, and, what is more, seem to require a great deal of sleep.

Ants are, of the entire insect world, probably the most gifted home-finders. Time and again have I tested them in this, sometimes taking them what must have been, to these little creatures, enormous distances from their nests before freeing them. Of course the ants experimented with were marked, otherwise I could not have watched them successfully. When an ant is taken into new surroundings and set free, it at first runs here and there and everywhere. As soon, however, as it regains its equanimity and recovers from its fright, it turns toward home. At first it proceeds slowly, every now and then climbing tall blades of grass, and from these high places viewing the surrounding country in search of landmarks. As soon as it arrives among scenes partially familiar to it, it ceases to climb grass-blades or weeds, and accelerates its pace. When it arrives among well-known and accustomed surroundings it runs along at its utmost speed, and fairly races into its nest.

The burying beetle has a regular abode, to which it invariably returns after performing the offices of mortician to some defunct bird, beast, or reptile. This insect grave-digger, by the way, is remarkably expert at its business, and will bury a frog or a bird in a very short time. As soon as it has buried the dead animal and deposited its eggs, it returns to its domicile beneath some log or stone.

Some snakes likewise are exceedingly domestic, and have their regular dens, to which they resort on occasions. The homing sense seems to be rather highly developed in them, for they can find their way back to their dens from great distances. I have had under observation for the past three years a garden snake, locally known as a “spreading viper”; this snake was brought to me by a friend when it was only a foot long, so I have known her (for it is a female) ever since her infancy. Owing to some antenatal accident, this reptile has a malformed head, so that I can readily recognize her at a distance of fifteen, twenty, or even thirty feet. Last year she reared her first brood of young, which I was fortunate enough to see with her on several occasions. Her den is on my lawn; and in the autumn of last year she conducted her brood to it, where they hibernated until spring. If I remember correctly, on the 29th of March she came out of her den accompanied by a dozen of her progeny, all but four (two pairs) of which I killed. Snakes subserve a very useful purpose in the economy of nature, but it is well to keep them in limits, for, when very numerous, they become dangerous to young birds, especially after they have passed the second year.

With the exception of the anthropoid apes all mammals possess the homing sense in a higher or lower degree; this is true also of birds. Experiments with the nesting robin show conclusively that this bird can find its way back to its nest when carried fifty miles from its home and then set free among wholly unknown surroundings. The well-known exploits of the carrier-pigeon are so familiar that they scarcely need comment. On May 3, 1898, two carrier-pigeons, en route for Louisville, rested for a time at Owensboro, Kentucky; these birds had been set free at New Orleans, Louisiana. The duck and the goose sometimes have this sense very highly developed. I once knew a goose to travel back home after having been carried in a covered basket for the distance of eighteen miles. A drake and duck have been known to return to their home after being carried a distance of nine miles by railway. Instances of home-returning by dogs, cats, horses, etc., are of such common occurrence that I hardly need call attention to them; the following instance is so unique, however, that I will present it:

In the fall of 1861, a gentleman of Vincennes, Indiana, visited his father at Lebanon, Kentucky; when this gentleman started to return home, his father gave him a yoke of young steers, which he drove, via Louisville, Kentucky, to Vincennes.

Shortly after his arrival at this last-mentioned town, the steers made their escape, swam the river at Owensboro, Kentucky, 160 miles below Louisville, Kentucky, and, in a week or so, were found one morning at the gate of their old home at Lebanon. Directed by their homing sense alone, these animals had made a journey of several hundred miles over a route they had never seen!

Fishermen are aware that certain fish choose localities for lurking-places, which they will share with no other fish. The black bass, and brook trout, and sturgeon, and goggle-eye are familiar examples of fish which have this habit.

On one occasion, I performed the following experiment: I took a black bass from its home near a sunken stump, and, after passing a short piece of thread through the web of its tail and knotting it, replaced it in the river, two miles below its lurking-place. The next day I saw it in its old home, clearly recognizable by the bit of thread which waved to and fro in the clear water as the fish gently moved its tail!

In an examination of phenomena such as have been discussed in this chapter, ay, throughout this book, we must lay aside the dogmatic assertions of our superstitious ancestors, who, to paraphrase Roscoe, “when awed by superstition, and subdued by hereditary prejudices, could not only assent to the most incredible proposition, but could act in consequence of these convictions, with as much energy and perseverance as if they were the clearest deductions of reason, or the most evident dictates of truth."

It will take the human race many, many years to unlearn, and to recover from the effects of the superstitious cult of the shaman, who exists, not only among savages, but also in the most highly civilized races of the world! Superstition is the antithesis of knowledge; in fact, it is but another name for ignorance.

There is yet another exceedingly interesting psychical trait to be noticed in the lower animals, especially in insects; I refer to the instinctive habit, letisimulation (letum, death, and simulare, to feign). The word “instinctive” must not be used, however, when this stratagem is to be observed in the higher animals other than the opossum; for many of these animals sometimes make an occasional and a rational use of it, as I will endeavor to show in the next chapter.