THE CAUSATION OF DISEASE. 297 



tude of functional troubles which would be entailed upon the 

 other, compelled, as he would be, to sit for hours together 

 over a book ! If he were a man of nervous temperament, 

 he would probably go mad. Yet had these two individuals 

 from the first followed each the other's occupation, it is 

 very possible that each would have become adapted to it ; 

 and this leads to the distinction between actual and potential 

 normality of S in relation to a particular E. By an " actual 

 normality " of S, I mean existing normality of S, to a particu- 

 lar E — admitting, that is, of harmonious inter-action with it ; 

 by a "potential normality," a capacity to become normal to it. 

 The process by which such normality is attained is none other 

 than personal adaptation (= direct equilibration), so that, from 

 the medical point of view, we may define personal adapta- 

 tion or direct equilibration as that process of structural alteration 

 by ivhich an organism becomes capable of living healthily in a 

 particular E. Perfect adaptation of S to E constitutes normality 

 for both, and there is not normality of either until this occurs. 

 During life there is a continual struggle towards a perfect 

 adaptation of the S to the E by which it is surrounded, that 

 is, the S is continually striving to become normal as regards 

 its E, and this adaptation occurring during the brief span of 

 human life is but a part of that grand scheme of adaptation 

 which underlies the whole process of organic evolution. 



Thus it is that a man who works chiefly with his brain, 

 who uses his muscles but little, becomes gradually adapted 

 to his mode of life — the muscular system, including the heart, 

 sinks to the level of his inactive life, and the nutritive, 

 nervous, and muscular systems all act in harmony. The 

 change is a work of time. Few individuals take naturally to 

 long hours of reading and muscular inactivity, and, indeed, all 

 school work is for the most part forced ; but by degrees the 

 power of attention is acquired, till what was once effort 

 becomes ease — nay, till what was at first, perhaps, actually 

 injurious, becomes beneficial — till books and thoughts are as 

 so much food for which the mind craves, and without which 

 life were a misery. The individual, in short, grows into 

 harmony with his surroundings : the S and E, at first abnormal, 

 become normal. So again with the muscle-worker : his tissues, 



