14 VERTEBRATE REPRODUCTIVE CYCLES 



his study of the wild rabbit by stating that it differs 

 from the tame animal in the sharp delimitation of its 

 breeding season, in the intensity of its breeding during 

 this season, and in its power of maintaining pregnancy 

 during lactation. 



Besides the domestic animals, those other mammals 

 that have come uninvited to live with man in his houses 

 also tend to breed throughout the year. This is true of 

 the house mouse^^* and of the brown rat, although in the 

 latter a greater proportion of the females are pregnant 

 from March to June than at other times of the year.^^^ 



So all the evidence points to an increase in fertility 

 among those mammals that live in the comparative 

 comfort of men's houses. However, a suggestion of 

 which notice must be taken is that all these species were 

 uniquely constituted in this respect before they were 

 tamed, and in support of this it has been pointed out 

 that most animals when forced into captivity in zoological 

 gardens have tended rather towards sterility.202 In view 

 of the extreme rarity of continuous breeding among wild 

 animals in temperate regions, this suggestion does not 

 carry great weight, and the sterility of animals in zoos 

 may well be due to the depressing effects of an abnormal 

 life (see also p. 41), effects which the domestic animals 

 overcame long ago if indeed they ever felt them. 



It is unfortunate that so few data on this subject are 

 provided by the lower vertebrates. Hens and ducks, 

 being highly bred for egg production, are outside the 

 present argument, while reptiles, amphibians, and fishes 

 are not normally kept indoors. 



5. OESTROUS CYCLES 



So far in this chapter only yearly reproductive cycles 

 have been considered, and for the male this completes 

 the story. In some species the shedding of the sperm 

 occupies only a few hours or days while in others the 

 supply is continuous over a period of many months, but 



