154 DEVELOPMENT AND HEREDITY. 



its efficiency be lost, thus causing a degeneration 

 of the limb in succeeding generations. 



The question, how does growth begin in a new 

 direction, and how do new parts originate, involves 

 the whole problem of the physiology of growth. The 

 human body may need four or five years to attain 

 the last twenty pounds of its weight, and yet if 

 twenty pounds' weight be lost by sickness, it may be 

 recovered in as many weeks. Why should this be so 

 if it be not that the nervous elements controlling 

 assimilation are more perfectly developed and more 

 efficient because of their continued exercise .^ Again, 

 why does a muscle grow larger and stronger by exer- 

 cise .'' The contraction of the muscle actually weak- 

 ens it, and also consumes its substance, so that each 

 contraction causes loss of both strength and material. 

 Therefore, after each contraction, there must be a 

 restoration of strength and substance ; and, as the 

 muscle grows stronger and larger by repetition of 

 the exercise, it follows that each restoration must 

 give the muscle more than the previous exercise 

 consumed, i.e. the restorative process becomes more 

 and more perfect by repetition. Thus we see that 

 the trophic nerves, like all others, gain facility by 

 practice. And we must remember also that the 

 trophic nerves, like all others, are only active as 

 the result of stimulation. 



