120 THE CAUSATION OF DISEASE. 



normally evolved tissues — that is to say, although reversions 

 to remotely ancestral pathological states are improbable, yet 

 reversions to physiological states of a bygone era may lead 

 to pathological states under our present E. We must, how- 

 ever, allow that reversions to true-pathological states of a few 

 generations back are quite possible ; and this fact alone should 

 guard us against asserting that a disease has no element of 

 heredity in it when we can get no history of its heredity, for, as 

 Sir James Paget observes, it is impossible, in the most fortu- 

 nate cases, to trace the family history beyond five generations. 



The tendency to reversion when two animals are crossed has, 

 I believe, an interesting application in pathology. We may, I 

 think, safely infer from this principle that the more unlike any 

 two individuals are, the greater will be the tendency towards a 

 reversion in the offspring of their union ; nay, I believe we may 

 go further than this, and expect that, if the parents show a 

 great unlikeness in any one particular, there will be a special 

 tendency to a reversion as regards that particular. 



Now, if by thus uniting individuals showing great unlike- 

 ness in one particular we can effect a reversion to a status quo 

 ante, might we not hope, by judiciously mating an individual 

 showing a strong tendency to a particular disease, to cause 

 by reversion a complete disappearance of this in the offspring ? 

 Not that I am sanguine enough to suppose that men and women 

 will ever trouble themselves very much about such very un- 

 romantic considerations, but it occasionally falls to the lot of 

 the physician to advise as to the desirability of a particular 

 marriage, and it behoves him, therefore, to consider all the aspects 

 of the case. All will admit that nothing could justify assent 

 to marriage between individuals showing a tendency to a 

 serious disease, say insanity; but should we be altogether 

 justified in withholding our sanction if one of the two showed 

 a clean ancestral chart as regards that disease ? I know many 

 physicians regard the probability of the disorder appearing 

 in some of the offspring of such a couple as very strong, and 

 they are ready to cite several cases in which insanity was 

 inherited from one side only. But, it may be retorted : What 

 proof is there that the other ancestral line is perfectly free from 

 insanity, or some allied neurosis ? Of how few families could this 



