ORIGIN OF COLOR IN ANIMALS. 97 



are generally susceptible of special explanations, and their contradic- 

 tion 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 re- 

 solved 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 de- 

 colorizing action of some secretions. Light is the principal excitant 

 capable of provoking the development of coloring-matter. Very sig- 

 nificant on this point is M. Paul Bert's account of his experiments 

 with the larvae 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 ques- 

 tion, 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 inter- 

 rupted 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, 



VOL. XXYIII. — Y 



