NATURE AND NURTURE 6y 



It has seemed to some that the pathological hypertrophy of 

 certain muscles under abnormal conditions is evidence of an 

 inherent or innate capacity for adaptive response. For example, 

 pathological conditions which throw extra work upon the heart 

 are often followed by the hypertrophy of the heart itself ; and, as 

 these conditions are abnormal or exceptional, it is said that the 

 capacity of the heart for responding to them cannot be due to the 

 survival, in past generations, of those ancestors whose hearts thus 

 responded ; but a moment's thought will show that the survival of 

 ever}'^ mammal does depend upon the power of its heart to re- 

 spond to increasing demands by increasing efficiency. If the work- 

 ing capacity of the heart did not keep pace with the growth of the 

 "body, no mammal could grow up, but growth is a normal process, 

 common to all. No mammal could survive the great changes which 

 take place in the circulation before and at the time of birth, if the 

 capacity of its heart for doing work did not keep pace with the 

 normal changes in the amount of work which is required. As we 

 have already seen, page ii, that the responsive activities of liv- 

 ing things may be called out by either the normal stimulus or any 

 other which acts in the same way, the pathological hypertrophy 

 of the heart is no more than the past history of mammals would 

 lead us to expect. 



Improvement of our muscles under exercise is the outcome of 

 structural adjustments for bringing this useful end about — it is 

 an adaptation ; and the heart is as obviously fitted for improve- 

 ment by use as it is for propelling blood. Exercise fits a muscle 

 for its normal work only so far as structural adjustments for 

 bringing this about already exist, in the brain, and in the nervous 

 system, and in the muscle itself ; and the real problem, the origin 

 of the adaptation, is in no way different from that presented by 

 any other structural adjustment. 



This is still further illustrated by the fact that while many 

 organs are improved by normal or natural use, abnormal or un- 

 natural use is well called abuse. When our bodies are used in 

 what is popularly called the way they were intended to be used, 

 use is beneficial; but injudicious or excessive training may be as 

 pernicious as neglect. 



If we acquire no nurture except that which our nature pro- 



