THE USES OF ANIMAL COLOR. 535 



may be of direct physiological value to the organism, or may 

 assist in the struggle for existence by deluding other species, or 

 by aiding the individuals of the same species, or they may be in- 

 timately connected with courtship. * 



The color of chlorophyl, which causes the green appearance 

 of vegetation, must be intimately connected with the important 

 changes which take place in this substance in the presence of 

 light. It is well known that under these circumstances carbon 

 dioxide (popularly called " carbonic acid ") can be split up, and its 

 carbon made to unite with the elements of water, forming organic 

 substance. Although this process has been much studied, it is still 

 very imperfectly understood. It is clear, however, that the color 

 of chlorophyl, involving the special absorption of certain light- 

 waves, has some direct bearing upon the changes which occur, 



No equally clear instance has been proved to occur in the ani- 

 mal kingdom, except in those few forms which resemble plants 

 in possessing chlorophyl. Dr. Hickson, however, believes that 

 among corals " the most widely distributed colors will eventually 

 be proved to be allied to chlorophyl, . . . and perform a very 

 similar if not precisely identical physiological function." It is 

 much to be desired that this interesting suggestion, which Dr. 

 Hickson supports by many arguments, may be thoroughly tested 

 as soon as possible.* 



In the very common association of colored substances with the 

 important function of respiration, it is clear that the color is not 

 more than incidental ; while the fish with transparent blood shows 

 that color is not indispensable for the due performance of the 

 function. Pigment is, however, of direct importance for vision ; 

 it is always present in the eyes of animals, except in the case of 

 albinos, and it is said that even they possess the essential visual 

 pigment associated with the termination of the optic nerve (reti- 

 nal purple). 



The difference between the physiological importance of color 

 in animals and plants is well shown by the fact that a true albino 

 variety (not merely a variegated example) of a green plant could 

 not live for any length of time. 



There are, however, certain cases among animals in which it is 

 extremely probable that color is of direct physiological value. It 

 is well known that dark colors readily absorb radiant heat, while 

 light colors do so with difficulty. For this reason black clothes 

 are most trying, and white most comfortable, in the hottest 

 weather. Conversely, a dark surface readily parts with heat by 

 radiation, while a white surface retains heat far more effectually. 



A few writers had suggested that these principles may explain 



* A Naturalist in North Celebes (Hickson, 1889), pp. 149-151. 



