THE BREEDING SEASON 5 



in the higher animals is less liable to modification than is the 

 case with certain of the lower forms of life, there is abundant 

 evidence that among the former no less than among insects 

 the sexual functions are affected by external conditions and 

 food supply. 



Darwin remarks that any sort of change in the habits of hfe 

 of an animal, provided it be great enough, tends in some way to 

 affect the powers of reproduction. " The result depends more 

 on the constitution of the species than on the nature of the 

 change ; for certain whole groups are affected more than others ; 

 but exceptions always occur, for some species in the most fertile 

 groups refuse to breed, and some in the most sterile groups 

 breed freely." " Sufficient evidence has now been advanced 

 to prove that animals when fiist confined are eminently 

 liable to suffer in their reproductive systems. We feel at first 

 naturally incKned to attribute the result to loss of health, or at 

 least to loss of vigour ; but this view can hardly be admitted 

 when we reflect how healthy, long-lived, and vigorous many 

 animals are xmder captivity, such as parrots and hawks when 

 used for hunting, chetahs when used for hunting, and elephants. 

 The reproductive organs themselves are not diseased ; and the 

 diseases, from which animals in menageries usually perish, are 

 not those which in any way affect their fertility." ^ 



It would seem probable that failure to breed among animals 

 in a strange environment is due not, as has been suggested, to 

 any toxic influence on the organs of generation, but to the 

 same causes as those which restrict breeding in a state of nature 

 to certain particular seasons, and that the sexual instinct can 

 only be called into play in response to definite stimuli, the 

 existence of which depends to a large extent upon appropriate 

 seasonal and climatic changes.^ 



There are at present no sufficient data for a comparative 

 account of the physiology of breeding among the lower animals ; 

 and in the present chapter, which is prehminary in character, 

 I shall content myself with stating a few general facts about 



' Darwin, Variation of Animals and Plants, Popular Edition, vol. ii., 

 London, 1905. 



^ See especially page 20, where Bles's observations on the breeding habits 

 of Amphibia are referred to. 



