38 THE CAUSATION OF DISEASE. 



necessary to observe, however, that, even though they all 

 exactly corresponded to the structural mean,' they would still 

 be natural variations ; for, being a mean of the two parental 

 structures, they would differ from each. But, upon ultimate 

 analysis, it becomes evident that variations in E are the prime 

 .cause of natural variations, for, if the world were peopled 

 afresh from two individuals, identical in all respects save in 

 such structural peculiarities as depend upon sex, and if the E 

 of all their descendants were identical, all men on the one 

 hand, and all women on the other, would be exactly the same. 



Now, the E may give rise to variations both compatible and 

 incompatible with health. The former alone are usually 

 spoken of as natural variations, but let us bear the fact care- 

 fully in mind that a disease attended by structural change (and 

 most diseases are*) is quite as much a natural variation as an 

 ordinary physiological variation. Shakespeare was a physio- 

 logical variation. Pope was both a physiological and a patho- 

 logical variation : he was a genius, but he had a crooked back. 



Here let me remark that it is necessary to distinguish two 

 classes of pathological variations. We may speak of these as 

 the sub-pathological and the true-pathological. 



Sub-pathological variations are such as render the individual 

 prone to respond pathologically to specific forms of E, and it is 

 very necessary to bear in mind that an otherwise perfectly 

 healthy individual may vary in this way. This fact will be 

 insisted upon elsewhere. Meanwhile it is sufficient to point 

 out that an individual who responds pathologically to an E, 

 which to the community at large is healthy, is, in a manner of 

 speaking, a pathological variation, even although he be healthy 

 in all other respects ; and the same is true of an otherwise 

 healthy individual who succumbs to an ordinary zymotic virus ; 

 for any individual who is incapable of doing successful battle 

 against an average quantity of mal-environment, succumbs to 

 the law of natural selection — is, in fact, an unfit, and, therefore, 

 so to speak, a pathological, variation. 



An individual may be said to be a true-path ological variation 

 when his tissues are altered by actual disease. 



* The part which structural change plays in disease will be considered in 

 Part III. 



