3. A considerable part of this deafness is preventable under enlightened action. Medical science is principally in control of the situation, but there is also much that can be done in general measures for the protection of the health. In attacking the problem, the most immediate practical program lies in the arrest of those diseases, especially infantile and infectious diseases, that cause deafness.
4. Our evidence is incomplete to determine definitely whether adventitious deafness is increasing or decreasing relatively among the population; but it is hardly other than likely that it is decreasing. Although certain diseases producing deafness fail to show any extensive signs of abatement, there are other diseases from which there can be little doubt that deafness is decreasing.
5. In the outlook there is, on the whole, promise, both in respect to the treatment of deafness itself and of the diseases that lead to deafness, though it cannot be said in any sense that any large or general relief is at present in sight.
6. Of congenital deafness nearly half occurs in families often without any positively known strain to indicate a predisposition to deafness. Though concerning this deafness little in the present state of our knowledge can be predicated, it is likely that with measures to secure a race sound in all particulars there will be a reduction to a greater or less extent of such deafness.
7. Consanguineous marriages do not take place, so far as deafness as an effect is concerned, to any great extent; though where they do the consequences are very marked. Their relation to deafness consists apparently for the greatest part in the fact that the chances of its transmission are thereby intensified, there being also a very strong connection with the question of deaf relatives in general.
8. There are a certain number of families in society deeply tainted with deafness, in evidence both lineally and collaterally, and this deafness may be transmitted from parent to offspring.
9. Children of deaf parents are far more likely to be deaf than children of hearing parents.
10. The great majority of the children of deaf parents, however, are able to hear, the proportion of those who are not being small.
11. The likelihood of deaf offspring is not necessarily greater when both parents are deaf than when one is deaf and the other hearing.
12. The liability to deaf offspring depends in the greatest degree upon the presence or absence in the parents, deaf or hearing, of deaf relatives, and, to a less extent, upon whether or not the existing deafness is congenital—being especially great under a combination of these two conditions.