148 THE AMERICAN NATURALIST. [Vor. XXXIV. 
are deaf; and when one parent is adventitiously deaf and the other 
hearing the percentage is 2.244. Here appears again the surpris- 
ingly large number of deaf children of hearing parents. But it is 
very evident that adventitious deafness is transmitted much less 
readily than congenital deafness. Indeed, it cannot be proven by 
these statistics that the former is ever inherited, because of the 
unknown error in the classification of the two varieties of deafness. 
The statistics show that the presence of deafness among the 
relatives of the parents increases very largely the chances that deaf 
children will be produced, and this is, of course, what would be 
expected. Taking the cases where both parents have deaf relatives 
(not including descendants), the percentage of deaf children where 
both parents are congenitally deaf is raised to 30.303; where both 
parents are deaf but only one congenitally deaf the percentage is 
10.903; where one parent was congenitally deaf and the other 
hearing there is again a large percentage, 24.286; and where 
both partners are adventitiously deaf there are 9.649 per cent of 
deaf children. Now, taking the cases where only one parent had 
deaf relatives, the percentages are 20.0, 5.536, 11.864, and 2.801, 
respectively. Finally, where neither partner had deaf relatives the 
figures are 4.167, 1.515, 15.789, and 0.364. In this last series the 
first and third percentages represent one child and three children, 
respectively, and the third one would be much reduced if one doubt: 
ful case were discarded. 
Over 45 per cent of the hearing parents whose family history is 
recorded had deaf relatives, while this is true of only about 32 per 
cent of the adventitiously deaf. This fact, taken in connection with 
the evidence as to the effect of history of deafness in the family of the 
parents in increasing the chances of deafness in the children, seems 
to explain to a great extent the large proportion of deaf children of | 
hearing parents compared with those of the adventitiously deaf which 
appears in the general statistics. But even when the factor of family 
history is taken into account, as in the tables summarized in the pre- 
ceding paragraph, there is still to be found a remarkably large pro- 
portion of deaf children of hearing parents. The author attempts 
to explain this on the ground of consanguinity. When both part- 
ners are deaf the largest percentage of deaf children is obtained 
from consanguineous marriages, 32.258 per cent; and in consan- 
guineous marriages where one partner is deaf and the other hear- 
ing the percentage is nearly as large, 29.851. In cases of deafness, 
then, the peculiarity of the parents seems to be more strongly 
