594 THE PHYSIOLOGY OF REPRODUCTION 



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 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 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 

 domesticated animals also under new conditions occasionally 

 show signs of lessened fertility, and that animals such as the 

 canary, which now breed readily in a state of captivity, were 

 formerly often sterile. 



Bles' observations, 1 to which reference has already been 

 made (p. 20), seem to have a bearing on this question. This 

 observer, who has kept various kinds of Amphibia in captivity, 

 has shown that axolotls can only be induced to breed under 

 certain special environmental conditions. By feeding them 

 copiously in summer and allowing them to hibernate in winter, 



1 Bles, "The Life-History of Xenopus Icevis" Trans. Roy. Soc. 

 Edinburgh, vol. xli., 1906. 



