136 ESSAYS OF A BIOLOGIST 



whether they are developed in one or in both sexes. ^ 

 The human species, in accordance with its com- 

 plexity and flexibility of brain, has epigamic char- 

 acters of both kinds. Some, like voice and mous- 

 tache, are different in the two sexes, others, such as 

 colour of eyes and lips, the hairlessness of the body 

 and grace of limbs and carriage, are common to both. 

 In the vertebrate stock, two main lines of evolu- 

 tion 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, persumably 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 over- 

 ride other components of the mind. As a result, gen- 

 eral emotional stimulus may play an important part 

 in inducing readiness to pair, and we accordingly 



3 See Huxley, '23. 



*See Howard, '20; Carr-Saunders, '22. 



