100 THE COLOUES OF ANIMALS 



duction of heat depends, just as it would tend to 

 diminish rather than promote the growth of hair. 

 This direct effect is obvious in animals which are un- 

 injured by variations of temperature. * The body of 

 a cold-blooded animal behaves in this respect like a 

 mixture of dead substances in a chemist's retort : 

 heat promotes and cold retards chemical action in 

 both cases.' But the higher vertebrates are warm- 

 blooded (homothermic) , and such direct effects of cold 

 would be fatal. ' In these animals there is obviously 

 a mechanism of some kind counteracting, and indeed 

 overcoming, those more direct effects which alone 

 obtain in cold-blooded animals.' The influence of 

 cold upon the nerves of the skin constitutes a stimulus 

 to that part of the central nervous system which 

 regulates the production of heat : thus cold indirectly 

 increases the amount of heat, and the temperature 

 of the body remains constant. I may mention that 

 the amount of heat produced in the body at any one 

 time may be gauged by the amount of oxygen ab- 

 sorbed in respiration.^ 



It is in every way probable that such changes in 

 colour as that of Sir J. Eoss' Lemming and the 

 American Hare are also indirectly caused by the cold, 

 which we may suppose acts as a stimulus to that part 



* For a further account of the regulation of temperature see 

 Professor Michael Foster's Physiology, from which the quoted 

 sentences are taken. I owe the correct understanding of the physical 

 cause of the change of colour to a conversation with Professor 

 Foster. 



