84 THE LAW OF BIRTHS AND DEATHS 



pick up the rest for themselves, they will have a large 

 amount of exercise with moderate feeding. Compare 

 this with the case of the race-horse in training. The latter 

 obtains plenty of exercise, yet is rendered less fertile. 

 But the degree of fertility will be decided not by the 

 amount of the exercise but by the effect of the exercise 

 on the nervous system. The exercise of th., race-horse 

 is designed, in combination with a diet rich in proteids, 

 to stimulate the production of nervous energy. The 

 animal will expend much energy but will produce still 

 more, and the degree of fertility in either case will 

 be decided by the relation between production and 

 expenditure. 



Darwin provides a considerable quantity of evidence 

 showing the effect of confinement upon the fertility of 

 wild animals. He goes on to say : " Sufficient evidence 

 has now been advanced to show that animals when first 

 confined are liable to suffer in their reproductive systems. 

 We feel at first naturally inclined to attribute the result 

 to loss of health, or at least to loss of vigour ; but this 

 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, 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 of 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 beasts, except indeed 



