30 THE PHYSIOLOGY OF REPRODUCTION 



these animals ; why do roe-deer in Germany breed in autumn, while 

 the embryo does not develop beyond the segmentation stage until 

 the following spring ; and why does the seal take eleven or twelve 

 months for gestation when a large dog requires only nine weeks ? 

 Heape believes that the recurrence of the breeding season is governed 

 directly by climatic, individual, and maternal influences, 1 and that 

 " variation in the rate of development of the embryo, in the length 

 of gestation, and in the powers of nursing, are quite sufficient to 

 provide for the launching of the young at a favourable time of the 

 year." 



I cannot altogether concur in Heape's view of this question. 

 For it seems to me by no means improbable that whereas the 

 necessities of the offspring, under changed environmental conditions, 

 may sometimes have been provided for by modifications 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 breediinr. 

 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 environ- 

 mental 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 

 cestrous 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 



1 Under the heading of "individual influences" Heape includes special 

 nervous, vascular, and secretory peculiarities of the individual and its haliits 

 of life. The length of the gestation and lactation periods he calls " maternal 

 influences." 



