COLORATION, Protective. The color of plants and animals is primarily due to the reflection of light, and is dependent in some cases on the pigment in the tissues of flowers, and in the skin, scales, hair or feathers of animals, and in others to structural surfaces which by diffraction give color. Plants are colored green by chlorophyll, so are cater pillars from eating plants. The ((chro matic function) is that adaptation of color of the skin of the frog, chameleon, squid, fish or crustacean, which rapidly changes so as to simulate the tints of the objects on which they rest. In most animals the coloration is due to pigment-cells (see CiutostKroPtioaes), which are more or less ramified, and which, under the stimulus of the prevailing color of the habitat, cause the animal to simulate in hue the ground, or tree, or sea-bottom. In most animals the coloration is permanent, in a few it changes with the change of color of the immediate surroundings of the animal, as in the chameleon. In insects, fishes and birds, where the variety of colors is almost endless, it has been found that even the most striking colors are aids to concealment of the identity of the wearer and thus are its protection.
Pigment in Animala.—That this is pri marily due to the action of light is proved by the fact that cave animals, or those living in darkness, are white or bleached out, so that the pigment cells become degenerate, the pig ment losing its more or less dark color, while in insects the colors are either optical or natural; the pigrnents, when present, are formed in the cellular layer of the skin (hypodermis). These colors tend to fade after death, but when en closed and preserved in air-tight sacs, such as the scales and hairs of butterflies, and the wing covers of beetles, they remain bright for a longer time, though eventually fading when dried specimens are exposed to the light. It has been shown that red, yellow, brown and black colors in the scales of certain butterflies are always due to pigments, while in a few cases greens, blues, violets, purples and whites are due to pigments in the scales themselves. Mayer finds that the pigments of the American silkworm moth (telea polyphemus) are de rived from the blood of the chrysalis. He has artificially produced several kinds of pigments from the blood, which are similar in color to various markings on the wings of the moth, and has found that chemical .reagents have the same effects on their manufactured products as on similar pigments in the wings of the living moth.
Origin of Spots, Bands or Stripes.—The markings of caterpillars, the stripes of the zebra, of many African antelopes and of the tiger are supposed by some writers to be due to the direct effects of light and shade, to shadows cast in jungle-grass or in forests. As proofs of this they instance the experiments of Steinach, who glued strips of black paper to the skins of frogs which were kept in the dark; when they were exposed to the light, only the uncovered parts of their skins retunied to a lighter hue,. while the covered parts remained
dark. The reflected green light in the case of frogs turning green when among leaves is by these writers believed to act directly upon the pigment cells. INhen the bottom of the vessel is covered with felt or with a wire net, the frogs become black, but recover their green color when a green branch is introduced into the vessel containing them. Again those cater pillars living among pine needles are green, striped with white, yellow and red, exactly harmonizing with the red and yellow or white portions of each cluster of needles; this is ascribed to the direct reflection of different shades of light on the moist skin of the cater pillar. In other caterpillars the longitudinal lines are broken up into spots, and if the process is carried on farther the spotting be comes transverse. Eyelike spots on certain caterpillars have been observed to be gradually formed at successive molts from what were originally continuous lines or stripes. It thus appears that the wonderful variety of colors and marlcings in animals is primarily due to the direct result of the environment, bringing about different results in animals of different groups and exposed to different environments. An alternative theory which has received wide spread acceptance is that built up by Abbott H. Thayer upon his observations covering many years, namely, that color in animals is based upon its ability to conceal them in their usual haunts. Obviously this theory works in ex cellently with the Darwinian theory of Natural Selection. The individuals whose accidental or inherited colorings most effectively concealed them would be those most certain to escape attacic and destruction, and survive as parents to reproduce in their progeny the peculiar colors which proved most protective. In this way, it is argued, the coloring of wild animals has eventually come to reflect the colors pre vailing in their native environments. The Thayer theory develops upon a basis of the obliteration of relief by the shading of the color from darkest on the upper side, which is best lighted from the sky, to lightest on the under side, which is naturally darkened by the shadow. The result is a flat tint conveying no suggestion of solidity. Upon this primary basis, obliteration of outline is accomplished by (1) a color system which is practically a picturing of the salient features of the background against which the animal is most likely to be seen; and (2) a color scheme which, though strilong.in tone, by its very boldness baffles recognition of the true form. (The theory is set forth in great detail in the book by G. H. Thayer, noted below). (For coloration due to changes of temperature see Seasonal Dimor phism under Distoantissi).