SEX BIOLOGY AND PSYCHOLOGY 135 



state has been raised to a certain level. The result 

 of this is that special actions, associated generally 

 with bright colours or striking structures, with song 

 or with scent, come into being. 



The exact mechanism of the appearance of these 

 courtship-displays is a much-vexed point ; but it is 

 undoubted that they only occur in animals with 

 congress of the sexes and with minds above a certain 

 level of complexity, and that they are employed 

 in ceremonies between the two sexes at mating-time. 

 There can subsist no reasonable doubt that there exists 

 some causal connection between the associated facts. 



An important point, which has been commonly 

 overlooked, is that such characters and actions may 

 be either developed in one sex only, or in both. In 

 a large number of birds, such as egrets, grebes, cranes, 

 and many others, the courtship-displays are mutual, 

 and the characters used in them developed to a similar 

 extent in both sexes. Such characters are therefore 

 often not secondary sexual dijfferences, and we had 

 best use Poulton's term epigamic for them, 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 characters 

 of both kinds. Some, like voice and moustache, 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. 



1 See Huxley, '23. 



