no THE FIRST PRINCIPLES OF HEREDITY 



percentage of deafness among children in cases collected 

 by E. A. Fay was as follows : 



Percentage of 

 Deaf Children. 

 Both parents congenitally (inborn) deaf . . . . 25-9 



One parent congenitally deaf, the other accident- 

 ally deaf . . . . . . . . . . . 6*3 



One parent congenitally deaf, the other normal . . 1 1 '9 

 Both parents accidentally deaf . . . . . , 2*3 



One parent accidentally deaf, the other normal . . 2-2 



This shows definitely how great the inheritability of inborn 

 deafness is. Acquired deafness seems to make no differ- 

 ence to the children, seeing that we have only a small 

 percentage of deaf children (2-3 per cent.) from accidentally 

 deaf parents (not more than would be expected in an 

 average population) ; and, furthermore, where one parent 

 is congenitally deaf, the percentage of deaf children is 

 actually higher in those cases where the other parent 

 is normal than in those cases where he is accidentally 

 deaf. 



There are a number of diseases which can hardly be said 

 to be inheritable in the ordinary sense. Tuberculosis is 

 due to a bacillus ; none the less, it is now a generally 

 accepted fact that a predisposition, consisting of a certain 

 " vulnerability of the protective epithelium," is an essential 

 factor for its acquisition. Gout and rheumatism, too, are 

 such constitutional diatheses to which certain persons are 

 heir and which need only an additional external stimulus 

 the spark in the powder magazine in order to break out 

 in full force. Insanity and the various forms of neuroses 

 belong to the same class. Though we cannot define each 

 time in what this predisposition consists, we know that 

 under given conditions only certain people, evidently pre- 

 disposed, fall victims to these diseases. 



