174 THE FOUNDATIONS OF ZOOLOGY 



genitally deaf, and husband deaf from unknown causes. None of 

 the 10 1 children of these 40 marriages are reported as deaf. 



In the second group, where 5 to 6 per cent of the children 

 are deaf, 87 are the children of 37 marriages where the hus- 

 band was congenitally deaf and wife adventitiously deaf ; and 

 139 are the children of 55 marriages where both husband and 

 wife were adventitiously deaf. We must bear in mind, while 

 considering this last case, that adventitious deafness may indicate 

 an hereditary predisposition ; for many of the pupils of the asylums 

 who lost their hearing after birth have deaf relatives, and thus show 

 that their deafness is not strictly adventitious, in the scientific sense, 

 but is due to a congenital predisposition to deafness. 



In the third class, where from 12 to 18 of the children are 

 congenitally deaf, 124 are the children of 51 marriages where 

 husband was adventitiously and wife congenitally deaf; 66 were 

 children of 16 marriages of hearing husband and congenitally 

 deaf wife ; 72 were children of 26 marriages where wife hears 

 and husband is congenitally deaf; and 71 of 29 marriages of con- 

 genitally deaf husband with wife deaf from unknown causes. In 

 all the families in this group one parent was congenitally deaf. 



In the fourth class, where 31.78 per cent of the children are 

 congenitally deaf, all the parents in the 52 marriages, with 151 

 children, are congenitally deaf. 



While too few to give quantitative results, these statistics prove 

 that it is the congenital and not the adventitious deafness which 

 descendants have to fear. 



Careful study of the history of these pupils of the asylums 

 shows that the relatives of deaf persons must also be taken into 

 consideration, and that statistical data which do not include this 

 factor are inadequate as a basis for generalization on inheritance. 

 Of the 26 families in which both parents are deaf and have con- 

 genitally deaf children, there are 5 families in which one of the 

 parents has a deaf parent, 17 families in which both parents have 

 deaf relatives of the same generation, 4 in which one parent has 

 deaf relatives of the same generation, and only 5 in which no deaf 

 relatives of the same generation are reported. 



Of the 26 families in which both parents are congenitally deaf 

 and have hearing children only, there is not one parent, so far as 



