136 ESSAYS OF A BIOLOGIST 



In the vertebrate stock, two main lines of evolution 

 as regards sexual relationships may be traced. The 

 first is predominant in mammals : here, in most 

 species, the female will not receive the male except 

 at fixed times, which are determined by a purely 

 physiological mechanism, the internal secretion of 

 the gonad (reproductive organ). Here we conse- 

 quently find that the rule is for the males to fight for 

 the possession of the females, not to display before 

 them. In the monkeys, presumably as a result of 

 a lessened dependence of mental upon physiological 

 processes, bright colours and special adornments of 

 various parts of the body are frequently developed. ^ 



In the birds, on the other hand, although here too 

 the internal secretion of the gonad delimits a period 

 in which alone congress of the sexes can occur, it 

 does not act for such a sharply-limited time as in 

 the mammal, nor is it so intense as completely to 

 override other components of the mind. As a 

 result, general emotional stimulus may play an 

 important part in inducing readiness to pair, and we 

 accordingly find display of some sort, either by the 

 male alone or by both sexes, present in the great 

 majority of species. It is at least partly in correlation 

 with this that beauty of voice and brilliant appearance 

 is far commoner in birds than in mammals 



The monkeys represent in some way a transitional 

 stage towards that seen in man, in whom the 



^ See Howard, '20 ,• Carr- Saunders, '22. 



