FERTILITY 629 



Ruminants breed readily in climates widely different from their 

 own. Carnivorous animals breed somewbat less freely in confine- 

 ment, and show considerable variation in different places. The 

 Canidre tend to be more fertile than the Felidse, 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 confinement only rarely 

 have young ones. Many of these animals, however, although failing 

 to conceive, are known to copulate freely. This is especially the 

 case with captive bears and monkeys, in which the typical phenomena 

 of prooestrum and oestrus occur. It would seem probable that the 

 sterility under these circumstances 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. Gallinaceous' 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 gulls 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 factors which control the mammary 

 metabolism. Moreover, in animals which are characterised by a 

 periodic growth of the secondary sexual characters, these sometimes 

 fail to make their appearance. The male linnet in captivity does 

 not assume its characteristic crimson breast, or the male bunting 

 (Eriiberizu passerina) the black colour on its head. Other birds, such 

 as a pyrrhula and an oriole, may acquire the appearance of the 

 hen, while a falcon (Falco albidus) has been observed to lose its adult 

 plumage. 1 These facts seem to show that the generative metabolism 

 may be so altered by changed conditions of existence as to induce 

 not merely a state of sterility, but also an interference with the 

 secretory activity of the essential organs of reproduction. 2 



Darwin says : " We feel at first naturally inclined to attribute 

 [such results] to loss of health, or at least to loss of vigour ; but this 



1 Darwin, loc. cit. 



2 The relation between the gonads and the secondary sexual characters, and 

 the apparent dependence of the latter upon the secretory activity of the former, 

 are discussed in Chapter IX. 



