32 THE PHYSIOLOGY OF REPRODUCTION 



climatic and environmental changes, and even by stimuli of a 

 more particular nature (cf. frogs, p. 20). But this power, which 

 all animals in some degree possess, of responding to altered con- 

 ditions, may none the less have arisen primarily to meet the re- 

 quirements of the next generation ; or, to speak more accurately, 

 that those animals which breed at a certain particular season 

 (or in response to certain conditions which prevail at that 

 season) have the advantage in being able to produce a new 

 generation to which this capacity to respond similarly will be 

 transmitted. In other words, the restriction of the breeding 

 habit to certain seasons may have been brought about under 

 the influence of natural selection to meet the necessities of the 

 offspring. 



Heape, however, has raised the objection l that this view is 

 inapplicable to the Mammalia, in which there is a period of 

 gestation of greatly varying length in the different species. If 

 the theory were correct, why, he asks, do some bats experience 

 a breeding season in the autumn, and not produce young until 

 the following June, although only two months are required for 

 the development of the embryo in 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, 2 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 favour- 

 able 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 con- 

 ditions, may sometimes have been provided for by modifications 



1 Heape, '* The Sexual Season of Mammals," Quar. Jour. Micr. Science, 

 vol. xliv., 1900. 



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

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

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

 influences." 



