The existence of non-significant colors is, nevertheless, most important, for they form the material out of which natural or sexual selection can create significant colors. Thus the color of blood may be made use of for "complexion," while fat may be employed to produce white markings, as in certain insect larvae. The yellow, brown, and red fatty matters of the connective tissue are accumulated beneath the skin in patches, so as to produce patterns.
All animal color must have been originally non-significant, for, although selective agencies have found manifold uses for color, this fact can never have accounted for its first appearance. It has, however, been shown that this first appearance presents no difficulty, for color is always liable to occur as an incidental result. This is even true of the various substances which seem to be specially set apart for the production of color in animals; for pigments occur abundantly in the internal organs and tissues of many forms. The brilliant colors of some of the lower organisms are probably also non-significant. In all higher animals, however, the colors on the surface of the body have been significant for a vast period of time, so that their amount, their arrangement in patterns, their varying tints, and their relation to the different parts of the body, have all been determined by natural selection through innumerable generations. Because the origin of all pigments is to be found in the incidental result of the chemical and physical nature of organic compounds, it by no means follows that incidental or non-significant colors would have appeared at all on the surface of most animals. And we find as a matter of fact that such colors tend to disappear altogether, directly they cease to be useful, as in cave-dwelling animals. On the other hand, the non-significant color of blood or of fat would persist undiminished in such forms.
Just as natural selection may develop an appearance which harmonizes with the surroundings, out of the material provided by non-significant color, the same agency may lead to the disappearance of the latter when it impedes the success of an animal in the struggle for existence. Thus the red color of blood has disappeared in certain transparent fishes, which are thereby concealed from their enemies. Among the manifold possible variations of nature is that of a fish with colorless blood, which can, nevertheless, efficiently perform all the duties of this fluid. While such a variation would be no advantage to the great majority of vertebrates, it would be very beneficial to a fish which was already difficult to detect on the surface of the ocean on account of its transparency.
Colors may be useful in many ways, and are therefore always liable to be turned to account in one direction or another. They
