THE INHERITANCE OF FAMILY TRAITS 129 



Even though both partners hear, if they belong to the 

 same strain with a tendency to deafness the UabiUty to deaf 

 offspring is so high as to warrant warning strongly against 

 such a marriage (Fig. 99). 



Finally if one or both partners are adventitiously deaf 

 and have no deaf relatives then there is no eugenic obstaicle 

 to marriage, for such marriages result in a negligible propor- 

 tion of deaf offspring — ^in Fay's statistics only 2 out of 552. 



b. Otosclerosis. — This disease consists of a progressive 

 rigidity of the mucous coat of the tympanic membrane; 



•]• 



iUiiiiio 



Fig. 100. — ^Pedigree of "fistula auris congenita." Both of the original 

 pair were affected with a congenital aural fistula, with a fistulous canal anterior 

 and close to the ear; all persons represented by black symbols had a similar 

 fistula. Hartman, p. 56. 



usually associated with adhesions in the inner ear and altera- 

 tions of the windows (fenestra). It shows itself in an ever 

 increasing difficulty in hearing conversation. 



The inheritance of otosclerosis is a familiar fact. Most 

 persons know famiUes many of whose members become 

 "hard of hearing" as they grow older. The deafness is fre- 

 quently attributed to climatic causes and this beUef is in- 

 creased by the presence of many cases in the same locaUty. 

 But it will be found on inquiry that the affected persons 

 are relatives and that their unrelated neighbors are not 

 affected by the same climate. This makes it clear that a 

 severe climate merely brings out the latent weakness of the 



