2.5, and when both are deaf, 9.2; while the
percentage of deaf children born of them when only one parent is deaf is
9.8, and when both are deaf, 8.4. This is apparently a very strange
result, though it probably may be accounted for in some part on the
theory that it is not so much deafness itself that is inherited, but
rather an abnormality of the auditory organs, or a tendency to disease,
of which deafness is a result or symptom, and that with different
pathological conditions in the parent there is less likelihood of
deafness resulting.

The most significant part of the results seems to be found, as before,
in respect to whether or not deaf parents are themselves congenitally
deaf or have deaf relatives. On the one hand, when one or both of the
parents are adventitiously deaf, the percentage of marriages resulting
in deaf children is 5.6, and the percentage of deaf children is 4.2;
when both parents are so, the percentages are lower: 3.5 and 2.3. The
percentages rise when one parent is adventitiously deaf, and the other
congenitally: 8.1 and 6.5. In respect to deaf relatives of parents, the
percentages are very low when neither has such relatives: 2.3 and 1.2.
The lowest percentages of all are in the case where both parents are
adventitiously deaf and neither has deaf relatives: 0.7 and 0.3.

On the other hand, we find the proportion of marriages resulting in deaf
offspring and the proportion of deaf children much greater when there is
congenital deafness in one or both parents, when one or both have deaf
relatives, and greatest of all when these influences are combined. When
one or both parents are congenitally deaf, the percentage of marriages
resulting in deaf offspring is 13.1, and the percentage of deaf children
is 12.1; when both parents are so, the percentages are doubled: 24.7 and
25.9. When one parent has deaf relatives and the other has not, the
percentages are 6.6 and 6.4; when both have, the percentages are nearly
four times as great: 23.5 and 20.9. When both parents are congenitally
deaf but neither has deaf relatives, the percentages are 7.1 and 4.1.
When both are adventitiously deaf and both have deaf relatives, the
percentages are 17.5 and 9.6. When both are congenitally deaf and one
has deaf relatives, the percentages are 16.3 and 20.0; and when both
have deaf relatives, the percentages are 28.4 and 30.3.

The evidence is very strong, then, with regard to the form of deafness
and the presence or ab