592 THE PHYSIOLOGY OF REPRODUCTION 



On the other hand, it is well known that wild animals, 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 Suidae are 

 known to breed in menageries and zoological gardens, while 

 many Ruminants breed readily in chmates widely different from 

 their own. Carnivorous animals breed somewhat less freely in 

 confinement, and show considerable variation in different places. 

 The Canidae tend to be more fertile than the Fehdse, while the 

 members of the bear group breed less easily. Rodents as a 

 general rule fail to breed after being brought into captivity, but 

 there are several exceptions. Monkeys also when kept in con- 

 finement only rarely have young ones. Many of these animals, 

 however, although faihng to conceive, are known to copulate 

 freely. This is especially the case with captive bears and monkeys, 

 in which the typical phenomena of procestrum and oestrus occur. 

 It would seem probable that the sterility under these circiun- 

 stances results from a failure to ovulate, due possibly to an 

 absence of ripe follicles in the ovaries. 



Among birds, members of the hawk group very seldom 

 breed in captivity. The graminivorous birds show considerable 

 variation, some, like the canary, breeding freely in aviaries 

 (although it was some time before it became fully fertile), while 

 others, like the finches, only occasionally reproduce their kind 

 when kept in confinement. Galhnaceous birds, on the other hand, 

 show an unusual capacity to breed in captivity, and the same 

 is the case with pigeons, ducks, and geese. Certain kinds of 

 guUs also are known to breed readily when kept in open spaces 

 in zoological gardens. 



As pointed out by Darwin, there is other evidence that 

 changed conditions of life may induce a disturbance of the 

 sexual functions. Thus when conception does occur under 

 confinement, the offspring are sometimes born dead or ill-formed, 

 or otherwise show signs of insufficiency of nourishment. The 

 mother's milk may fail, indicating an interference with those 



