256 HEREDITY AND DISEASE 



itself, for instance, in " vulnerability of the protective epithelia," 

 — in fact, in a deteriorated power of resistance to the tubercle 

 bacillus. 



In the same way, to take a case provisionally non-bacterial, 

 it seems probable that gout is not, as such, transmissible, but 

 that what is inherited is a constitutional peculiarity (arising 

 originally as a germinal variation), which expresses itself in an 

 altered mode of eliminating nitrogenous waste — a constitutional 

 vice which may be exacerbated by excess of food and alcohol. 



4. Acquired and Innate Abnormal Conditions should be 

 distinguished. — Closely similar abnormal states of the body 

 may arise in two different ways, and their heritability will 

 differ with the mode of origin. If the abnormal condition is 

 inborn in the strict sense — i.e. if it is the expression of a con- 

 stitutional peculiarity arising originally as a germinal variation — 

 the probability of transmission is often great. But if the ab- 

 normal condition has been induced adventitiously by external 

 influences (including food, drink, poisons, etc.), then the proba- 

 bility of transmission is slight. The distinction is a real one, 

 but it is not always readily drawn in actual practice. 



Thus the difficulty of distinguishing inborn deafness from 

 exogenous or adventitious deafness — the result, for instance, of 

 various infectious diseases, — may, perhaps, explain a curious pecu- 

 liarity in E. A. Fay's statistics (3,078 marriages, 6,782 children). 

 The percentage of deaf children in families where both parents 

 were deaf was 8-458, while in families where only one parent was deaf 

 the percentage was larger — namely, 9-856. There seems something 

 wrong here, and the explanation may be that there are two quite 

 different phenomena slumped under the title deafness — viz. innate 

 or idiopathic deafness, and acquired or exogenous deafness. 



As the case appears instructive, let us pursue it further. Where 

 both parents were believed to be congenitaliy deaf the percentage 

 of deaf children was 25-931 ; where one parent was deaf congenitaliy 

 and the other adventitiously, it was 6-538 ; where both parents were 

 adventitiously deaf, it was only 2-326. Where one parent was 

 congenitaliy deaf and the other normal, 11*932 per cent, of the 



