126 HEREDITY IN RELATION TO EUGENICS 



of the parents the larger the proportion of deaf children as 

 the following table shows : — 



Per cent deaf offtpring 

 Partners "cousins," degree unreported 19.4 



" first or second cousins 34.6 



" nephew and aunt (1 family) 75.0 



The interpretation of this fact would seem to be that the 

 nearer the relationship the greater the chance that both 

 parents lack the same element and so all of their children 



I 



n 



D D D D HtW D D 



OriiiK:) 



m 



Fig. 95. — Pedigree of deaf mutes. Two deaf mute cousins each belonging 

 to fraternities having several deaf mutes marry one another. Both of their 

 children (II) are deaf. Each child marries a hearing wife and of 4 children 

 aU hear. Fay, 1898, No. 2621. 



tend to lack it. In Figs. 96 to 100 are given some pedigrees 

 of deaf mute families. They show that, under certain cir- 

 cumstances, probably identity of defect in parents, the 

 children will all be similarly defective. 



The studies of Bell (1906) based on the census returns of a 

 large proportion of the deaf population of the United States 

 show the importance of consanguineous marriages in favoring 

 the production of deaf mute offspring. He finds (p. 17) ''of 

 the 2,527 deaf whose parents were cousins, 632, or 25 per 

 cent, are congenitally deaf, of whom 350, or 55.4 also have 

 deaf relatives of the classes specified; while among the 

 53,980 whose parents were not so related the number of 

 congenitally deaf is 3,666 or but 6.8 per cent, of whom only 

 1,023 or 27.9 per cent have deaf relatives." 



