294 HEREDITY AND DISEASE 



the period of development, and are due to defects or arrests in 

 the development, ultimately traceable to deficient nutrition 

 of the tissues, or to a lack of vigour in the germinal material 

 to begin with. What is inherited is this general tendency to 

 debility, and it is for the environmental influences to determine 

 the precise lines of least resistance. 



3. Some Predispositions to Disease are much more herit- 

 able than others. — Statistics seem to prove what a general 

 outlook suggests, that some predispositions to disease are much 

 more likely to have hereditary re-expression than others. But 

 the cautious student will bear in mind two saving clauses : 

 (i) that non-expression does not necessarily imply non-inherit- 

 ance, for a morbid character often skips a generation, or more 

 than one ; and (2) that recurrence does not necessarily imply 

 inheritance, for a particular predisposition may crop up de novo, 

 or, in other words, the fountain of variation may repeat itself. 

 No one will go the length of supposing that the rheumatic 

 tendency has not originated afresh over and over again, or 

 of tracing the whole burden of rheumatic disease back to man's 

 pre-human ancestry because rheumatism occurs in monkeys. As 

 already noted, acute rheumatism is probably microbic. 



A few illustrations of the variable probabilities of transmission 

 must suffice. In one long family history, gout is said to have 

 persisted for four centuries. Out of 523 gouty subjects, 309 

 had a family taint (about 60 %) ; out of 156 cases, 140 had a 

 family taint (about 90 %) ; various sets of cases show percentages 

 varying from 50 to 100. Out of 104 cases of diabetes mellitus, 

 22 had a family taint (about 20 %). In one long family history, 

 dealing with about 400 members, there were 26 cases of haemo- 

 philia ; in another, dealing with 100 members, there were 17 

 '' bleeders." 



Out of 901 admissions to an asylum, 477 had insane relatives ; 

 out of 321 cases of epilepsy, 105 had a family taint (about 35 %) ; 

 out of 208 cases of hysteria, 165 had a family taint (about 80 %). 



