Chap. XVIII. CHANGED CONDITIONS. 143 



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

 which must have undergone a great change in this respect, 

 >>reed 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 cade 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 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. Those animals 

 which usually breed freely under confinement, rarely breed, 

 as I was assured, in the Zoological Gardens, within a year or 

 two after their first importation. When an animal which is 

 generally sterile under confinement happens to breed, the 

 young apparently do not inherit this power: for had this 

 been the case, various quadrupeds and birds, which are 

 valuable for exhibition, would have become common. Dr. 

 Broca even affirms 66 that many animals in the Jardin des 

 Plantes, after having produced young for three or four suc- 

 cessive generations, become sterile; but this may be the 

 result of too close interbreeding. It is a remarkable circum- 

 stance that many mammals and birds have produced hybrids 

 under confinement cpiite as readily as, or even more readily 

 than, they have procreated their own kind. Of this fact many 

 instances have been given ; 67 and we are thus reminded of 

 those plants which when cultivated refuse to be fertilised by 



06 'Journal de Physiologic,' torn. subject, see F. Cuvier, in ; Anuales do 

 P- 347. Museum,' torn. xii. p. 119. 



* r For additional evidence on tnla 



