88 THE LAW OF BIRTHS AND DEATHS 



sterility of the parrot. The effect, too, would be pro- 

 portionate to the change in the habits of the species from 

 an active to an inactive life, and also proportionate to 

 the change from a poor diet to a rich one. For if a species 

 lives under hard conditions the optimum point for fertility 

 will be adjusted to hard conditions. These considerations 

 throw considerable light on certain facts mentioned by 

 Darwin : 



" We have seen that carnivorous animals are more 

 fertile under confinement than most other mammals. 

 The reverse holds good with carnivorous birds." 1 The 

 change in the habits of the birds is much greater. The 

 animals are able to obtain a certain amount of exercise 

 in confinement. The birds, accustomed to traverse 

 vast distances when at liberty with meals uncertain and 

 often far between, can do little in captivity but mope 

 upon a perch all day. Add to this that they will be well 

 and regularly fed. The tiger is, weight for weight, far 

 stronger and more formidable than the lion. This points 

 to a higher development of nervous energy. Consequently 

 we need not be surprised when we are told that the tiger 

 is much less fertile in captivity than the lion. 



" The carnivora, with the exception of the Plantigrade 

 division, breed, though with conspicuous exceptions, 

 about half as freely as ruminants." 2 This represents 

 about what we should expect in view of the greater change 

 in the habits of the carnivora. These latter are more 

 conspicuous for nervous energy than the herbivora. 

 A lion or a tiger bears much the same relation to an antelope 

 or other herbivorous animal that a sprint runner bears 

 to a long-distance runner. The former are capable of 

 putting forth an enormous amount of nervous energy 



1 Variation of Animals and Plants under Domestication, chap, xviii. 

 8 Ibid. 



