THE BREEDING SEASON 29 



this is very various, and, above all, bears no proportion to the size 

 attained. . . . This length of time, which we may generally designate 

 as the period of individual growth, is not alike even for all the 

 individuals of the same species ; on the contrary, it depends on the 

 co-operation of so many different factors that it must necessarily 

 vary considerably. Now, if from any cause the period of individual 

 growth, say of the salmon, became changed in consequence of the 

 slower development of the embryo in the egg or of the young larvse, 

 most or all the young salmon thus affected would die in our climate, 

 because the greater heat of spring is injurious to them at that stage." 

 In a similar way it may be argued that the periodicity of the breeding 

 season, no less than the rate of growth, is governed by the necessities 

 of the young. No doubt this is true to a large extent, yet at the 

 same time it is equally evident, as has been shown above in 

 numerous instances, that this periodicity is greatly affected by 

 climatic and environmental changes, and even by stimuli of a more 

 particular nature (cf. frogs, p. 19). But this power, which all 

 animals in some degree possess, of responding to altered conditions, 

 may none the less have arisen primarily to meet the requirements 

 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. 1 



Heape, however, has raised the objection 2 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 



1 Westermarck (The History of Human Marriage, 5th Edition, London, 1921) 

 says it is " obvious that the sexual functions are, at least to some extent, affected 

 by different conditions in different species. This is shown by the fact that 

 every month or season of the year is the pairing time of one or another species 

 of Mammals." He goes on to cite examples. Moreover, he points out that 

 while the Adelie penguin rears its young in the warmest and lightest months, 

 the giant Emperor penguin does this in the dark season, so that they may be 

 fostered by their parents until the warm weather, and have the whole summer 

 in which to change their plumage (Levick, Antarctic Penguins, London, 1914). 

 Westermarck points out also that where the conditions amid which certain 

 animals live are fairly uniform throughout the year there is no sexual season. 

 He instances the whale, the elephant, and the birds of the Galapagos Islands 

 which are situated very near the equator. (Cf. p. 13.) 



2 Heape, "The Sexual Season of Mammals," Quar. Jour. Micr. Science, 

 vol. xliv., 1900. 



