126 The Third and Fourth Generation 



eye, ultimately rendering the person blind. 

 In the particular form of. the disease here con- 

 sidered it does not develop until middle life. 

 Clarence Loeb in a study of hereditary blindness 

 published in 1909 tabulated the results of a 

 study of 304 families in which such blindness 

 occurs. There were 1,012 children, of whom 

 58 per cent were afflicted, which is about the 

 percentage expected when hybrid defectives 

 mate with normal individuals and the defect 

 is a "dominant character. Similar extensive 

 studies of congenital deafness and deaf-mutism 

 show that these are similarly heritable, though 

 just how the character behaves is not yet known, 

 for undoubtedly under "deafness" are included 

 a variety of diseased conditions that must be 

 studied separately before we shall know how 

 each is inherited. Care must be taken, too, to 

 distinguish between congenital deafness and 

 blindness — that which inheres in the germ 

 plasm — and those forms, due to accident or 

 contagious disease, which are acquired modifica- 

 tions and so not heritable. Thus measles often 

 produces deafness as one of its after effects. 



