MATING CONTROLLED 171 



varying conditions of existence. But before 

 we proceed to examine the particular ways 

 in which it has been modified to suit the needs 

 of particular classes of species, and the reason 

 for such modifications, we must inquire whether 

 there is not some way in which it has been 

 serviceable alike to every species, or at least 

 to a large majority of them. 



Success in the attainment of reproduction 

 depends upon the successful discharge of the 

 sexual function ; and the discharge of the sexual 

 function depends primarily upon an individual 

 of one sex coming into contact with one of the 

 opposite sex at the appropriate season and 

 when its appropriate organic condition arises. 

 Now the power of locomotion is so highly 

 developed in birds that it may seem unreason- 

 able to suppose that males and females would 

 have any difficulty in meeting when their 

 inherited nature required that they should do 

 so, still less reasonable to suggest that this 

 power might even act as a hindrance to 

 successful mating. Nevertheless, if we try to 

 picture to ourselves the conditions which would 

 obtain if the movements of both sexes were in 

 no wise controlled, and mating were solely 

 dependent upon fortuitous gatherings, we shall 

 come, 1 fancy, to no other conclusion than that 

 much loss of valuable time and needless waste 

 of energy would often be incurred in the search, 

 and that many an individual would fail to 

 breed just because its wanderings took it into 

 districts in which, at the time, there happened 



