SEX BIOLOGY AND SEX PSYCHOLOGY 135 



to unite as best they may. Congress of the sexes 

 eliminates the major part of this waste, and is uni- 

 versal above a certain level. This is in itself the 

 basis for other changes. As the mind, or shall we 

 say the psycho-neural organization, becomes more 

 complex, the sexual instinct becomes more inter- 

 woven with the general emotional state; and a large 

 number of animals appear not to mate unless their 

 emotional 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 con- 

 gress 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 simi- 

 lar extent in both sexes. Such characters are there- 

 fore often not secondary sexual differences, and we 

 had best use Poulton's term epigamic for them. 



