Chap. XVIII. CHANGED CONDITIONS. 159 



tivity, such as parrots, and hawks when used for hawking, 

 chetahs when used for hunting, and elephants. The reproduc- 

 tive organs themselves are not diseased ; and the diseases, from 

 which animals in menageries usually perish, are not those which 

 in any way affect their fertility. No domestic animal is more 

 subject to disease than the sheep, yet it is remarkably prolific. 

 The failure of animals to breed under confinement has been 

 sometimes attributed exclusively to a failure in their sexual 

 instincts: this may occasionally come into play, but there is 

 no obvious reason why this instinct should be especially liable 

 to be affected with perfectly tamed animals, except indeed in- 

 directly through the reproductive system itself being disturbed. 

 Moreover, numerous cases have been given of various animals 

 which couple freely under confinement, but never conceive ; or, 

 if they conceive and produce young, these are fewer in number 

 than is natural to the species. In the vegetable kingdom 

 instinct of course can play no part ; and we shall presently see 

 that plants when removed from their natural conditions are 

 affected in nearly the same manner as animals. Change of 

 climate cannot be the cause of the loss of fertility, for, whilst 

 many animals imported into Europe from extremely different 

 climates breed freely, many others when confined in their 

 native land are completely sterile. Change of food cannot be 

 the chief cause ; for ostriches, ducks, and many other animals, 

 which must have undergone a great change in this respect' 

 breed freely. Carnivorous birds when confined are extremely 

 sterile ; whilst most carnivorous mammals, except plantigrades, 

 are moderately fertile. Nor can the amount of food be the 

 cause; for a sufficient supply will certainly be given to valuable 

 animals ; and there is no reason to suppose that much more 

 iood would be given to them, than to our choice domestic 

 productions which retain their full fertility. Lastly, we may 

 infer from the case of the elephant, chetah, various hawks, and 

 of many animals which are allowed to lead an almost free life 

 in their native land, that want of exercise is not the sole 

 cause. 



It would appear that any change in the habits of life, what- 

 ever these habits may be, if great enough, tends to affect in 

 an inexplicable manner the powers of reproduction. The result 



