154 DARWINJSM chap. 



and numbers of species of rodents, very rarely breed in 

 confinement ; while other species do so more or less freely. 

 Hawks, vultures, and owls hardly ever breed in confinement ; 

 neither did the falcons kept for hawking ever breed. Of the 

 numerous small seed- eating birds kept in aviaries, hardly 

 any breed, neither do parrots. Gallinaceous birds usually 

 breed freely in confinement, but , some do not ; and even 

 the guans and curassows, kept tame by the South American 

 Indians, never breed. This shows that change of climate has 

 nothing to do ^vith the phenomenon ; and, in fact, the same 

 species that refuse to breed in Europe do so, in almost every 

 case, when tamed or confined in their native countries. This 

 inability to reproduce is not due to ill -health, since many 

 of these creatures are perfectly vigorous and live very long. 



With oui' true domestic animals, on the other hand, 

 fertility is perfect, and is very little affected by changed 

 conditions. Thus, we see the common fowl, a native of 

 tropical India, living and multiplying in almost every part of 

 the world ; and the same is the case with our cattle, sheep, 

 and goats, our dogs and horses, and especially mth domestic 

 pigeons. It therefore seems probable, that this facility for 

 breeding under changed conditions was an original ])roperty 

 of the species which man has domesticated — a property 

 which, more than any other, enabled him to domesticate them. 

 Yet, even with these, there is evidence that great changes of 

 conditions affect the fertility. In the hot valleys of the 

 Andes sheep are less fertile ; while geese taken to the high 

 plateau of Bogota were at first almost sterile, but after some 

 generations recovered their fertility. These and many 

 other facts seem to show that, mth the majority of animals, 

 even a slight change of conditions may produce infertility or 

 sterility ; and also that after a time, when the animal has 

 become thoroughly acclimatised, as it were, to the new 

 conditions, the infertility is in some cases diminished or 

 altogether ceases. It is stated by Bechstein that the canary 

 was long infertile, and it is only of late years that good 

 breeding birds have become common ; but in this case no 

 doubt selection has aided the change. 



As showing that these phenomena depend on deep-seated 

 causes and are of a very general nature, it is interesting 



