THE BREEDING SEASON 5 



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

 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 first 

 confined are eminently liable to suffer in their reproductive systems. 

 We feel at first naturally inclined 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 under 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." 1 



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. 2 



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 preliminary in character, I shall content 

 myself with stating a few general facts about the breeding season, 

 giving illustrations, taken from various groups of Vertebrates and 

 Invertebrates, of its seasonal recurrence, and the manner in which 

 this varies under altered conditions of life. 



PROTOZOA 



Among the Protozoa the organisms pass through successive phases 

 of vitality, which are comparable to the different age-periods of the 

 Metazoa. In such simple forms of life, fission or division into two 

 parts is the usual method of reproduction, 3 and the frequency of 

 its occurrence appears to depend more upon the phase which has 



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

 London, 1905. 



2 See especially page 18, where Bles's observations on the breeding habits 

 of Amphibia are referred to. 



3 In this process no material is lost, and two simple nucleated organisms 

 result. During the period of maturity referred to in the text, multiplication 

 is often preceded by union (either temporary or complete) of two individuals, 

 and this process is called conjugation (see p. 220, Chapter VI.). 



