28o HEREDITY AND DISEASE 



" blot on the brain " was transmitted from generation to 

 generation ; and (3) that there seems to be httle warrant for 

 beheving in the transmission of a nervous disorder of exogenous 

 origin. 



(i) In most cases the facts seem to suggest that what is 

 inherited and transmitted is a general predisposition to some 

 dislocation or derangement of the nervous system. If such 

 a dislocation or derangement occur in a case where we can 

 exclude the probability of its being due to any infection, intoxi- 

 cation, or lesion of external origin, we must refer it to some 

 initial defect or disturbance in the organisation of the germ. 

 As such, it is likely enough to be transmitted, whether it be 

 hysteria or epilepsy, melancholia or idiocy ; but it does not by 

 any means follow that it must be transmitted, or that, if trans- 

 mitted, it will have in the offspring the form it took in the 

 parent. In fact, the frequency with which the expression 

 changes almost forces us to conclude that what is inherited is 

 something general, not specific. Another reason for this con- 

 clusion is to be found in the fact that the nervous disorder is 

 so often associated with some more general constitutional dis- 

 turbance. Thus the association of hysteria, epilepsy, chorea, 

 etc., with rheumatism is well known. In such cases it is probably 

 more accurate to speak of the inheritance of a constitutional 

 vice, a derangement of metabolism, and to avoid expressions 

 which suggest that there is, to begin with, anything definitely 

 wrong with the cerebral machinery. In the third place, it is 

 instructive to note that the cerebral equipment may work weU 

 for years of ordinary life, and yet break down hopelessly in 

 face of some extraordinary excitement or some constitutional 

 crisis (puberty, parturition, menopause, etc.), which again 

 suggests the inheritance of general weakness rather than the 

 inheritance of specific disease. 



The fact that predispositions to nervous diseases so often 

 change in particular expression from generation to generation 



