THE CAUSATION OF DISEASE. l8l 



amplify the history of the causation of hereditary structure. 

 Such a proceeding, however, would have interfered with the 

 proper sequence of our argument ; and hence this subject will 

 be treated of in the second part of this work, for much that is 

 pathologically important is brought to light by examining 

 into the evolutionary history of man. 



The Principle, of Structural Mean. — Inasmuch as, in the 

 case of man, two individuals are engaged in the reproductive 

 process, the offspring tend towards a certain mean of the two 

 parental structures. A certain mean, be it noted, but not an 

 exact mean, for some one or other paternal or maternal charac- 

 ter may be prepotent. 



If the E were exactly the same for each and every child 

 born to the same parents, all the offspring would be exactly 

 alike, save only such differences as depend upon sex — the 

 male offspring on the one hand, the female on the other. 

 But, owing to the instability of the E, no two children 

 can be exactly the same. Under the principle of structural 

 mean, the blending in the offspring of tendencies derived from 

 the two parents has to be considered — notably, the blending of 

 physiological with pathological tendencies, and of pathological 

 with pathological, the latter constituting the so-called inter- 

 marriage of disease. 



When the pathological tendencies are alike in both 

 parents, the chances of the offspring inheriting the disease are 

 very strong. Nevertheless, even in this case, there is a 

 tendency to revert to the normal state, owing to the vis 

 medicatrix naturce. But when the morbid tendency comes 

 from the one parent only, there is less tendency to inherit- 

 ance. This may be partly explained by regarding the parents 

 as a cross in respect of this pathological particular, for, when 

 parents are very unlike in some one particular they may be 

 regarded as a cross in respect to it ; and when two individuals 

 are crossed, there is a tendency to reversion to the status quo 

 ante the development of that particular or those particulars 

 in virtue of which they become a cross. If one parent, for 

 instance, is highly neurotic, while the other possesses a strong 







