THE BREEDING SEASON 33 



in the rate of development or length of gestation, yet in other 

 cases a similar result may have been effected by alterations in 

 the season of breeding. The mere fact that breeding in any 

 one species occurs, as a rule, periodically at a time which is on 

 the whole well suited to the requirements of perpetuating the 

 race, is itself presumptive evidence that the periodicity of the 

 breeding season is controlled (through natural selection) by the 

 needs of the next generation. Further, the breeding season 

 having been fixed at one period in the history of the species, 

 the same season would probably be retained (in the absence of 

 disturbing factors) by the descendants of that species under the 

 directive influence of heredity. This view is in no way opposed 

 to the doctrine that the sexual capacity is developed in the 

 individual in response to definite stimuli, which are largely 

 environmental and often seasonal. 



The occurrence of a succession of " heat " periods within the 

 limits of a single breeding season no doubt arose in consequence 

 of the increased opportunity afforded thereby for successful 

 copulation. The number and frequency of the " heat " periods 

 under these circumstances are affected by the conditions under 

 which an animal lives in just the same kind of way as the 

 periodicity of the breeding season is affected, as will be shown 

 in the succeeding chapter on the oestrous cycle in the Mammalia. 

 Concerning the immediate cause of " heat/' and the nature of 

 the mechanism by which it is brought about, something will 

 be said later (Chapter IX.). 



The origin of the breeding season is a wider question. For 

 its complete solution, as pointed out by Heape, a comparative 

 study of the sexual phenomena in the lower animals is essential, 

 while, as already remarked, sufficient data for a comprehensive 

 treatment of this subject do not at present exist. 



That the breeding season occurs in some animals " as the 

 result of a stimulus which may be effected through the ali- 

 mentary canal is demonstrated by the effect upon ewes of certain 

 stimulating foods." 



" That it is associated with a stimulus which is manifested 

 by exceptional vigour and exceptional bodily ' condition ' is 

 demonstrated by the pugnacity of the males at such times, by 

 the restless activity of the females, by the brilliant colouring of 



c 



