ds, for here we are to deal
with some of the great questions of heredity--though hereditary deafness
and congenital deafness are not altogether one and the same thing.[41]
For the purposes of our inquiry, let us think of the congenitally deaf
as divided into three great classes in respect to their family
relations: 1. the offspring of parents who were cousins; 2. the
offspring of parents who were themselves deaf or members of families in
which there are other deaf relatives; and 3. the product of families
without either consanguinity or antecedent deafness. Of these three
classes the first two only will engage our attention. Of the last,
comprising, according to the census, nine-twentieths, or 44.4 per cent,
of the congenitally deaf, there is not much that we can say. For a great
part of it there no doubt exists in the parent, or perhaps in a more
remote ancestor, some abnormal strain, physical or mental, in the nature
of disease or other defect. But in respect to such deafness we have too
little in the way of statistical data to help us arrive at any real
determination; and for it as a whole we shall have to wait till we have
greater knowledge of eugenics and the laws of heredity.[42]

THE OFFSPRING OF CONSANGUINEOUS MARRIAGES

Not all the deaf born of consanguineous marriages are congenitally deaf,
but as the majority are so, and as the fact of the parents being blood
relatives is assumed to have at least a contributing influence in the
result, we may consider the matter in this place. It is in fact closely
connected with the question of deaf relatives in general.

In the census investigations,[43] of the number who answered on this
point, 2,525, or 7.4 per cent, have parents who were cousins. Of these
cases, deafness occurred in 87 per cent before the fifth year of age,
and in 60 per cent at birth. Of all the deaf born without hearing, 13.5
per cent are the offspring of consanguineous marriages. The proportion
of those born deaf is thus nearly twice as great when the parents are
cousins as it is among the whole class of the congenitally deaf; and the
proportion is also nearly twice as great of the offspring of
consanguineous marriages among the congenitally deaf as the proportion
of the deaf from such marriages among the total number of the deaf.
Moreover, 55.0 per cent of the offspring of cousin-marriages have deaf
relatives of some kind, and of the congenitally deaf from
cousin-marriages, 65.6 per cent have dea