58 THE LAW OF BIRTHS AND DEATHS 



to bodily exercise should then become more capable of 

 reinforcing voluntary stimuli." l 



From this passage it would appear that the question 

 as to whether the amount of nervous energy produced 

 can be increased by exercise is regarded by physiologists 

 as quite a debatable one. Yet that such is the case 

 can be shown by a few simple considerations and a few 

 simple facts. In the first place the power to regulate 

 the supply to suit the demand is a necessary condition 

 of the capacity of the organism for direct adaptation to 

 its environment. For if the amount of energy produced 

 by the system were a fixed quantity under all conditions, 

 then no man, however much he was trained or practised, 

 would be able to perform work which required more than 

 a moderate expenditure of nervous energy, or if able to 

 perform such work, when not doing so his system would 

 be irritable with energy which would allow him no rest. 



This is illustrated by a passage from the same writer : 

 " When a vigorous horse has been kept in the stable 

 for a long time, we see it when first brought out make 

 leaps and turns, and show by its lively paces a great 

 desire for movement. We then say that the animal is 

 frisky. This great vivacity has not, however, as its sole 

 object the manifestation of the joy it feels at being again 

 at liberty : it expresses the need for exercise which the 

 animal feels. Similarly, it is under the influence of the 

 need for exercise that the wild beasts in a menagerie 

 constantly prowl about their cages ; that children coming 

 out of the classroom leap and run about the recreation 

 ground, and that dogs run after each other in the streets." 2 



The obvious explanation of this need for exercise is 

 not that the animals referred to have an intelligent 



1 Physiology of Bodily Exercise, Fernand Lagrange, part i, chap. i. 

 * Ibid., part v, chap. i. 



