are generally susceptible of special explanations, and their contradiction of the other facts is only apparent.
In consideration of the influence of external agencies on coloration, we distinguish between two classes: those forces which can be resolved into a rapid vibration—light, heat, and electricity—the action of which is very marked; and other more complex agencies, among which we include food, captivity, moisture, and the colorizing and decolorizing action of some secretions. Light is the principal excitant capable of provoking the development of coloring-matter. Very significant on this point is M. Paul Bert's account of his experiments with the larvæ of the axolotl: "Pale on issuing from the egg, they become colored by the deposition of pigment under the influence of light. In the dark, or in red light, the pigment is not developed." From this we learn that the less refrangible rays have no influence on the production of pigment; it is therefore by the rapidity, and not by the amplitude of its vibrations, that light acts upon the formation of coloring-matter. An analogous example is furnished by the Proteus, which, having been drawn out from its dark hole, becomes gradually colored by light. We may compare with these observations that the negro baby is, when first born, of only slightly different color from the white; and the fact that certain parts of his body may already show the negro tinge does not contradict our theory of the dependence of the color on the action of light, but is only the mark of a hereditary tendency to become black. I do not intend to assert that light is the sole cause of pigment-coloration, for that would be contrary to the facts; but it is generally the exciting and sometimes the necessary means for the development of the coloring-matter. It plays a part like that of the spark in combustion, which has no effect upon an incombustible body, in the same way that light produces no colorizing effect upon an albino. There is, then, an aptitude to become colored, which varies according to races, and may not always exist. The question, however, of the ultimate cause of coloration is not solved, but only pushed back; for we are ignorant of the cause of this aptitude, and are obliged, to explain it, to have recourse to the laws of heredity and natural selection.
The rich coloration of deep-sea animals apparently contradicts the facts we have cited, but does not really do so. For it is principally the red, or less refrangible, neutral rays, the passage of which is interrupted by the water, while the blue, violet, and ultra-violet rays, which are the active ones in coloration, pass through it to a considerable depth. Furthermore, we know that the molecules composing the tissues of these animals are subject to vibratory movements analogous to those of light, which are represented to us by phosphorescence; and we may conceive those vibrations to be intense enough to produce a coloration like that which is the effect of sunlight.
As a rule, the parts of animals most exposed to rays of light are,
