CHANGE IN EXPRESSION OF DISEASE 293 



of disease " seem to point to another consideration — that what 

 is inherited is sometimes a very general pecuHarity, which finds 

 this or that expression in relation to the conditions of the body, 

 which is a very variable soil, according to the liberating 

 stimuli which are available, such as the diet, climate, and 

 other conditions of life. 



It is well known in medicine that a predisposition or diathesis 

 may express itself in half a dozen different ways — being poly- 

 morphic, as it is said — though there may be one way or two 

 ways which, being most frequent, may be called " diagnostic " or 

 " distinctive." Thus, the tubercular tendency has several differ- 

 ent ways of expressing itself, probably depending mainly on the 

 nature of the nutritive and other environmental influences. 

 The parallel to this in natural conditions is seen when the same 

 kind of animal develops or grows differently in different sur- 

 roundings. 



But if the same disease may find different expression in, let 

 us say, three brothers, it is not surprising that the disease of 

 a parent may take a different, though analogous, form in the 

 offspring, and perhaps a third form in the grandchildren. It 

 may be intensified, or weakened, or directed on new lines, the 

 change depending, so far as we can see, partly on the amphimixis 

 or duality of the inheritance, and partly on the external con- 

 ditions. Thus, if both parents have a markedly phthisical 

 tendency, the probability is that there will be in the offspring 

 a more pronounced similar predisposition than if one of the 

 parents had belonged to an untainted stock ; or, again, apart 

 from amphimixis, a thorough change in habits and surroundings 

 may at least greatly inhibit the phthisical outcrop in the off- 

 spring. 



There is probably a very simple reason why a hereditary ten- 

 dency to nervous disease should have different expressions in 

 successive generations, and it is this : that many if not most 

 abnormal neuroses — e.g. epilepsy and insanity — emerge during 



