THE INHERITANCE OF FAMILY TRAITS 93 



defect in the germ plasm, such that when the factor that 

 stimulates to complete mental development is absent from 

 the gexm plasm of both parents it will be absent from all 

 their offspring. Varied as are the mental conditions of the 

 persons in a family containing feeble mindedness the chil- 

 dren do not ordinarily surpass in mental development the 

 better developed parent. 



In considering heredity of mental disease we must not 

 forget that what is inherited is not, as in imbeciUty, a 

 tendency to incomplete mental development, but rather 

 a tendency such that a completely developed and apparently 

 normal mentality is liable under ordinary, or still more 

 under extraordinary, conditions to show disturbance of a 

 temporary or permanent nature. The more intimate nature 

 of this inherited tendency is probably varied. In some cases 

 there is doubtless an idiosyncrasy in the neurones, in other 

 cases there is a lack of resistance to infection or specific 

 poisons, again the trouble may be outside the neurones in 

 the supporting tissue or even in the blood vessels whose 

 walls may be peculiarly liable to weaken and burst; to waste 

 away; to thicken, occluding the lumen and shutting off 

 nutrition to a part of the brain. 



Before considering the inheritance of specific nervous 

 diseases it may be pointed out that what is inherited is often 

 a general nervous weakness — a neuropathic taint — showing 

 itself now in one form of psychosis and now in another. 

 Especially the lower types of mental defect may be carried 

 in the higher, i. e., departing least from the normal. 



b. The Neuropathic Makeup. — We have seen (page 77) 

 that imbecility, epilepsy and many forms of insanity are 

 due merely to the absence of some factor. It remains to 

 be considered how they behave amongst each other in 

 heredity. A pedigree worked out by Barr (1907) gives the 

 desired information (Fig. 56). 



