MATING CONTROLLED 177 



another — and, on the whole, there is a sameness 

 in the direction of flight taken by the same pairs 

 on different occasions. An interval of nearly 

 two months may thus elapse between mating 

 and nest-building, during which the sexes are 

 not only often apart but often separated by a 

 considerable distance. 



What does this species gain by the 

 individuals belonging to it mating so early in 

 the season ? If the appropriate condition which 

 leads the females to seek males were to arise 

 in each individual at a late date, the first stage 

 in the process — mating — would not be com- 

 pleted before the second — the discharge of the 

 sexual function — were due to begin. Thus, 

 instead of having ample time, the females 

 would have but a short period in which to 

 discover males ; and this in some cases might 

 lead to delay, in others to failure, and in 

 others again to needlessly severe competition, 

 entailing physical exhaustion at a critical 

 moment in their lives. Hence those females 

 in which the appropriate organic condition 

 developed early in the season would not only 

 bq more likely to find males, but would be 

 in a position to rear more broods than those 

 in which it developed late ; and they would 

 have a better chance of leaving ofiFspring, 

 which, in their turn, would reproduce the 

 peculiarities of their parents. Moreover, within 

 certain limitations, the more these successful 

 females varied in the date of their development, 

 the less, severe would be the competition, and 



