THE INHERITANCE OF FAMILY TRAITS 125 



tries have forbidden the marriage of persons of this class. 

 Yet the inheritance of deaf mutism has been disputed and, 

 indeed, without careful consideration of the separate family 

 histories the method of inheritance seems truly obscure. 

 I The most extensive data on the marriage of deaf are those 

 collected by Fay (1898). He finds that, when both parents 

 are congenitally deaf (Figs. 93, 94), of the 335 matings 25 



Fig. 93 Fig. 94 



Fig. 93. — Pedigree of deaf mutism. Parents both deaf; the father at 3 

 years; the mother before birth. The first two children died shortly after birth; 

 the other two are deaf mutes — one born so; the other following a slight blow 

 on the head. Saint Hilaire, 1900, p. 31. 



Fig. 94. — Pedigree of deaf mutism. Father mother, and 3 children, 

 all deaf mutes from birth. Saint Hilaire, 1900, p. 31. 



per cent yield some deaf offspring; and of the total of 77D 

 offspring 26 per cent are deaf. It is clear that such marriages 

 are, in the long run, dangerous. That all children of such 

 marriages are not deaf is doubtless due to the fact that the 

 parents are not deaf in the same way and that one parent 

 brings into the combination what the other lacks. The 

 contrast between the result of marriages of two congenitally 

 deaf parents and two who are adventitiously deaf is shown 

 by the fact that the latter yield only 2.3 per cent deaf chil- 

 dren. 



If, on the other hand, the partners belong to the same 

 deaf mute strain, i. e., are related, the percentage of mar- 

 riages yielding some deaf mute offspring rises to 45, and the 

 proportion of deaf offspring to 30 per cent (Fig. 95). But 

 that is not the whole story, for the closer the relation.^hip 



