deafness in the family the percentage of
consanguinity is the highest. That is, of those who had both (a) and
(b) relatives ten per cent were the offspring of cousins, while of
those who had neither (a) nor (b) relatives only three per cent were
the offspring of cousins. It is natural to assume that as a rule where
the deaf have either (a) or (b) deaf relatives, deafness is
hereditary, for the probability of two cases of deafness occurring in
the same family, uninfluenced by heredity would be very small. It is
likely also that a great many of the deaf who stated that they had no
deaf relatives were mistaken, for few people are well enough informed
in regard to their ancestry to answer this question definitely. Not
one man in thousands can even name all of his great-grandparents, to
say nothing of describing their physical or mental traits. Others may
have understood the inquiry to refer only to living relatives and
therefore have omitted almost all reference to their ancestors. These
possible errors might easily explain all the excess of the percentage
of consanguinity among those reported as having no deaf relatives over
the probable percentage of consanguineous marriage in the general
population. But this very probability that comparatively few deaf
ancestors have been reported increases the probability that the
greater part of the (a) relatives were brothers and sisters rather
than ancestors. Now of the 26,221 deaf having deaf relatives, 17,345
have only (a) relatives, and if these are largely living brothers and
sisters the relationship would "work both ways," so that if there were
two deaf children in a family, each would have an (a) deaf relative.
In the Census of Ireland figures above quoted it will be remembered
that among families which were the offspring of cousins the proportion
having two or more deaf children was three times as great as among
those who were not the offspring of consanguineous unions. If this
follows in America, it largely accounts for the high percentage of the
congenitally deaf who are the offspring of cousin marriages, and
especially of those who have (a) deaf relatives.

TABLE XXX.
------------------------------------------------------------------------
| | Consanguinity |
| | of Parents. | Per Cent.
| |----------------------------------------------
Class of Deaf |