THE FUNCTIONS OF THE SKIN 491 



more than one of colour, for a white rabbit, or an Arctic hare in 

 its winter coat of white, is immune to an intravenous injection of 

 nucleo-protein, which produces in the pigmented animal coagulation 

 of the blood. 



Pigment is used for purposes of offence as well as of defence. 

 It is particularly marked in reptiles, amphibia, and fishes. Pigmen- 

 tation also plays a marked part in the sexual relationships of some 

 animals, particularly of birds. The male bird is decked in vivid 

 colours, especially in the springtime of active courtship. True 

 albinos are devoid of pigment, and their irises are pink owing to 

 the reflexion of light from the blood circulating therein. 



