THE INHERITANCE OF FAMILY TRAITS 129 



Even though both partners hear, if they belong to the 

 same strain with a tendency to deafness the hability 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 obstacle 

 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: 



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 families many of whose members become 

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

 quently attributed to climatic causes and this belief is in- 

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

 But it will be found on inquiry that the afifected persons 

 are relatives and that their unrelated neighboi-s are not 

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

 severe climate merely brings out the latent weakness of the 



