86 THE LAW OF BIRTHS AND DEATHS 



of health merely. They will produce more energy, but 

 they will also expend more. There will be occasional 

 spells of fasting or insufficient feeding which, together 

 with the restless activity which results, will reduce the 

 nervous charge very low. But the " fat, well-fed " 

 animals will at all times have a nervous system highly 

 charged with energy which they have no opportunity 

 to work off. 



Again : " The carnivora in the Zoological Gardens 

 were formerly less freely exposed to the air and cold than 

 at present, and this change of treatment, as I was informed 

 by the former superintendent, Mr. Millar, greatly increased 

 their fertility." * 



This is much the same story. Exposure to the air and 

 cold would lead to greater physical activity, and probably 

 the cold would have a more direct effect by diverting 

 some of the energy to the purpose of maintaining bodily 

 heat. To anyone watching an animal in confinement 

 prowling restlessly about its cage or flinging itself against 

 the bars it is clear that the animal's system is highly 

 charged with energy which can find no outlet. This 

 feeling has been experienced by most people when confined 

 to the house by wet weather or similar causes. The 

 effect of low temperature in this connection is well stated 

 by Lagrange : * ' The need for exercise is developed with 

 more intensity in proportion to the lowness of the tem- 

 perature ; by severe and piercing cold we are much more 

 driven to action than by great heat. In this case the 

 need for exercise is derived from the instinct which leads 

 us to produce heat by movement." 2 



Darwin mentions the great longevity of parrots and 

 the fact that even in England they are believed to have 



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

 * Physiology of Bodily Exercise, Feruand Lagrange, part v, chap. i. 



