[91] Kirby and Spence, loc. cit. ante, pp. 582, 583.
[92] Darwin, Descent of Man; Romanes, Animal Intelligence, Mental Evolution in Animals, Mental Evolution in Man; Lubbock, Senses, Instincts, and Intelligence of Animals, and Ants, Bees, and Wasps; Hartmann, Anthropoid Apes; Büchner, Geistesleben der Thiere; Huber, Natural History of Ants, etc.
CHAPTER VIII
AUXILIARY SENSES
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,[93] 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.[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 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 Kölliker, of Würtzburg, 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 Kölliker, 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]
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 rôles 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.