XII] Nervous Disease 229 



forms of paralysis, of deaf-mutism, and of mental disease. 

 From the analysis of considerable collections of such evidence 

 it is clear that in the present state of knowledge any 

 reduction of the phenomena into a simple scheme is im- 

 possible. The first difficulty is that the pathology of these 

 diseases is obscure and their diagnosis often imperfect, 

 various dissimilar affections passing by one name. 



Deaf-mutism, for instance, though at first sight a very 

 definite phenomenon, may obviously be due to congenital 

 deafness arising from many distinct causes, and hence the 

 genealogies of deaf-mutism cannot be treated as though 

 they all related to a single physiological condition^. 



Next in dealing with diseases of the nervous system it 

 must be remembered that many of them depend for their 

 appearance on the presence of external stimuli. Forms of 

 insanity which appear when the individual is subjected to 

 various strains and excitements may not appear at all if 

 these causes be absent. The element transmitted is evi- 

 dently the liability, not necessarily the developed condition. 

 The descent of such peculiarities is therefore beyond the 

 range of our analysis. 



The peculiar form of insanity known as Hereditary 

 Chorea is exceptional, in that it very clearly follows the 

 course of an ordinary dominant with few complications. 

 Adding all the families apparently from heterozygous {DR) 

 parents recorded as mating with unaffected persons I get 

 the totals 117 affected, 99 unaffected, as nearly approaching 

 the normal equality as we can expect when the nature of 

 the evidence is remembered. 



Note on Collecting Evidence as to Human Descent. 



As some persons may read this chapter who have not 

 leisure for study of the more elaborate phenomena of 

 Mendelism a few words as to the collection of evidence may 

 usefully be introduced here. The one essential point in 

 such collections is that the normal members of families 



* Dr C. J. Bond, Brit, Med. Jour., Oct. 28, 1905, calls attention to 

 evidence suggesting sex-limitation in certain deaf-mute families, sometimes 

 the male, sometimes the female being the affected sex. 



