5 34 THE POPULAR SCIENCE MONTHLY. 



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 diffi- 

 culty, for color is always liable to occur as an incidental result. 

 This is even true of the various substances which seem to be spe- 

 cially set apart for the production of color in animals ; for pig- 

 ments 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 pig- 

 ments 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 disap- 

 pearance of the latter when it impedes the success of an animal in 

 the struggle for existence. Thus the red color of blood has disap- 

 peared 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, neverthe- 

 less, efficiently perform all the duties of this fluid. While such a 

 variation would be no advantage to the great majority of verte- 

 brates, 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 



