628 THE PHYSIOLOGY OF REPRODUCTION 



and produce larger litters of young than wild animals belonging to, 

 the same species ; and Darwin, who made the same observation, 

 attributed the increased fertility of the former to a long habituation 

 to a regular and copious food supply without the labour of seeking 

 for it. " It is notorious how frequently cats and dogs breed, and • 

 how many young they produce at birth. The wild rabbit is said to 

 breed four times yearly, and to produce each time at most six young ; 

 the tame rabbit breeds six- or seven times yearly, producing each 

 time from four to eleven young. . . . The ferret, though so closely 

 confined, is more prolific than its supposed wild prototype [the 

 polecat]. The wild sow is remarkably prolific; she often breeds 

 twice in the year, and bears from four to eight, and sometimes even 

 twelve, young ; but the domestic sow regularly breeds twice a year, 

 and would breed oftener if permitted ; and a sow that produces less 

 than eight at birth ' is worth little, and the sooner she is fattened for 

 the butcher the better.' The amount of food affects the fertility of 

 the same individual ; thus sheep which on mountains never produce 

 more than one lamb at birth, when brought down to lowland 

 pastures frequently bear twins. The difference apparently is not 

 due to the cold of the higher land, for sheep and other domestic 

 animals are said to be extremely prolific in Laj)land." ^ 



Darwin remarks that birds afford still better evidence of increased 

 fertility resulting from domestication. Thus, in its natural state 

 the female of Gallus bankiva, the wild representative of the common 

 fowl, lays only from six to ten eggs ; the wild duck lays from five 

 to ten eggs, as compared with eighty or a hundred produced by 

 the domestic dack in the course of the year. Similarly, the turkey, 

 the goose, and the pigeon are more , fertile in the domestic state, 

 though this is not the case with the pea-fowl. Among plants also 

 there are countless instances of increased fertility as a consequence 

 of cultivation.^ 



On the other hand, it is well known that wild animkls, when 

 removed from their natural conditions and brought into captivity, 

 often become partly or completely sterile. Darwin discusses this 

 phenomenon at some length, and cites numerous cases from different 

 groups of animals and birds. 



The Indian elephant, for example, seldom breeds in captivity, 

 although kept in a perfectly healthy condition and in its native 

 country. On the other hand, most members of the Suidte are 

 known to breed in menageries and zoological gardens, while many 



' Darwin, The Variation of Animals and Plants under Domestication, Popular 

 Edition, vol. ii., London, 1905. 



^ Cf. also Spencer (loo. dt.), who discusses this question at some length in 

 connection with his generalisation that Individuation and Genesis vary inversely. 

 See above, p. 624. 



