298 THE CAUSATION OF DISEASE. 



by long exercise, are hardened and strengthened to work 

 against gravity ; the mind, never awakened into full activity, 

 soon becomes adapted to the narrow world in which it lives, 

 and is happy therein. What wonder, then, that if two such 

 individuals could exchange places, there should be terrible 

 commotion, and, it may be, actual disease ! 



In order to test the potential normality or adaptability of 

 a particular S to a particular E, the exposure to that E should 

 be as gradual as possible, to allow time for a gradual altera- 

 tion to the structural state normal to it. By suddenly altering 

 the E we do not test the potential normality, but the actual 

 normality to it at the time of change. The truth of this is 

 manifested in training for athletic feats. If a man who for 

 many months has been sitting at a desk, suddenly attempts to 

 climb a mountain 10,000 feet high, or to run 100 yards at full 

 speed, nothing but disaster can result. Think of the sudden 

 strain put upon the attenuated heart-walls and delicate valves ! 

 Put scientifically, the essence of training is the compelling 

 the heart, valves, arteries, and muscular system to gradually 

 undergo their maximum physiological hypertrophy. 



And, indeed, to bring out the total potentiality of adapta- 

 bility, much skill is required, and the physician is often called 

 upon to exercise it. He, too, has to train the body ; he has 

 to modify the delicate S, to convert potential normality into 

 actual, and thus, by a very careful process of education, to 

 adapt a weak tissue to its E. Thus, a nervous system here- 

 ditarily weak may, by skilful treatment, be so strengthened 

 during the period of development as to be rendered practically 

 normal as regards an average external-body -E. Such training, 

 however, requires the greatest skill, and we have probably not 

 yet acquired a tithe of what we may one day hope to possess. 



From the above considerations, it is clear that the whole 

 question of normality turns upon the healthy inter-working 

 of S and E — that health is the only real criterion of normality. 

 Is an individual healthy ? Then are his S and E both normal. 

 But there is no fixed, unalterable standard of normality for 

 either. Each can only be considered in relation to the other. 

 Is there, then, no proximate standard ? We may, in truth, 



