16 THE COLOURS OF ANIMALS 



which resemble plants in possessing chlorophyll. Dr. 

 Hickson, however, believes that among corals ' the most 

 widely distributed colours will eventually be proved to 

 be allied to chlorophyll . . . and perform a very simi- 

 lar if not precisely identical physiological function.' 

 It is much to be desired that this interesting sugges- 

 tion, which Dr. Hickson supports by many arguments, 

 may be thoroughly tested as soon as possible. 1 



In the very common association of coloured sub- 

 stances with the important function of respiration, it 

 is clear that the colour is not more than incidental ; 

 while the fish with transparent blood shows that 

 colour is not indispensable for the due performance of 

 the function. Pigment is, however, of direct import- 

 ance 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 

 (retinal purple). 



The difference between the physiological importance 

 of colour in animals and plants is well shown by the 

 fact that a true albino variety (not merely a varie- 

 gated example) of a green plant could not live for any 

 length of iime. 



There are, however, certain cases among animals 

 in which it is extremely probable that colour is of 

 direct physiological value. It is well known that 

 dark colours readily absorb radiant heat, while light 



1 A Naturalist in North Cekbes (Hiokson, 1889), pp. 149-51. 



