630 THE PHYSIOLOGY OF REPRODUCTION 



view can hardly be admitted when we reflect how healthy, long- 

 lived, and vigorous many animals are under captivity, such as 

 parrots, and hawks when used for hawking, chetahs when used for 

 hunting, and elephants. The reproductive organs themselves are 

 not diseased; 1 and the diseases from which animals in menageries 

 usually perish are not those which in any way affect their fertility. 

 The failure of animals to breed under confinement has been some- 

 times attributed exclusively to a failure in their sexual instincts. 

 Tins 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, indirectly through the 

 reproductive system itself being disturbed. Moreover, numerous 

 cases have been given of 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. . . . 

 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 sterile. Change of food cannot be the chief cause; for 

 ostriches, ducks, and many other animals, which must have under- 

 gone 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 food 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." Darwin shows 

 also that close confinement by itself does not necessarily cause 

 sterility, since such animals as the rabbit and ferret breed freely in 

 cramped hutches. The general conclusion reached is that "any 

 change in the habits of life, whatever these habits may be, if great 

 enough, tends to affect in an inexplicable manner the powers of 

 reproduction. The result depends more on 'the constitution of the 

 species than on the nature of the change ; for certain whole groups 

 are affected more than others ; but exceptions always occur, for some 

 species in the most fertile groups refuse to breed, and some in the 

 most sterile groups breed freely." 



In support of these conclusions Darwin shows further that 



1 Few observations have been made upon the condition of the gonads in 

 animals in captivity, but Branca ("Recherches sur le Testicule et les Voies 

 sprrmatiques dans L6muriens en captivite," Jour. d<> F An<tt. <( l I'/it/*., vol. xl., 

 1904) states that in captive lemurs he could find no spermatozoa in the testicles. 



