72 THE CAUSATION OF DISEASE. 



necessary to the medical man — we must ever keep clearly in 

 mind this doctrine of sexual heredity. This principle shows us 

 that many of the differences between the sexes depend upon 

 use and disuse, and are not necessary differences. Hence it 

 would be possible to cancel, to a greater or less extent, many 

 of the sexual differences ; the exceedingly irritable nervous 

 system of woman, for instance, might be rendered more stable 

 by judicious bringing-up during many successive generations. 



George Eliot recognized the principle of sexual heredity, — 

 the fact that a peculiarity passes potentially through the sex in 

 which it does not appear ; for in " Middlemarch" she makes 

 Mr. Brooke say : " I had it myself — that love of knowledge .... 

 though that sort of thing does not often run in the female line ; 

 or it runs underground, like the rivers in Greece — it comes out 

 in the sons." 



Although the characters acquired by one sex tend in the 

 offspring to appear more especially in the same sex, we should 

 err in concluding that the other sex is wholly unaffected. 

 Were such the case, the daughters of a man who had per- 

 sistently, from youth upward, abused his health, would in no 

 wise suffer from the father's transgressions. Whether, how- 

 ever, in such cases of parental transgression the male offspring 

 necessarily suffer most, I cannot say, but the question is cer- 

 tainly worthy of investigation. 



Are there any special conditions favourable to the strict 

 limitation to one sex of an inherited structural peculiarity ? 

 We have already seen that one necessary condition is the 

 acquisition of the peculiarity during the period of sexual life. 

 A further favourable circumstance is the operation during many 

 generations upon one sex only of the E tending to produce the 

 peculiarity. Finally, I think it probable that when the tissue 

 specifically acted upon is more or less limited, the resulting 

 change is more apt to be confined to the sex in which it is 

 acquired — one would expect, namely, that a strictly local modi- 

 fication of tissue (although this must to some extent modify 

 other tissues by altering the E) would be more apt to be 

 sexually limited than one more general. 



This view may be rendered clearer by stating the converse case. 

 When the entire organism is affected for good or evil, one would 

 expect the good or evil effect to be more impartially inherited 



