(A) REMOTE CAUSES.—Among these are to be mentioned principally natural conditions, unfavorable social and hygi enic conditions, heredity, consanguinity and a few others of minor importance.
Natural Canditions.—In considering the unequal distribution of deaf-mutism, we are involuntarily led to the supposi tion that this phenomenon may be caused by varying natural conditions, among which soil and elevation seem to play an important part.
To H. Schmaltz is due the honor of having investigated the question of the importance of geological conditions and elevation in Saxony so thoroughly that his results are entirely to be relied on. In these investigations, which have em braced the minutest details which could possibly be of importance concerning the appearance of deaf-mutism, the author has weighed each separate point care fully. His conclusions are as follow: There is nothing to be said in favor of the hypothesis that soil, climate, or other territorial conditions influence the deaf mute rate, neither can the composition of the water be proved to affect it in any way, but it is the social and hygienic conditions which are decisive. Lemcke, in Mecklenburg-Schwerin, and 1Tcher mann, in Norway, were also unable to prove that geological conditions are a cause of deaf-mutism.
Unfavorable Social and Hygienic Con ditions.—Almost all authors who have considered the question of the connec tion between deaf-mutism and unfavor able social and hygienic conditions, ag,ree in ascribing to them great importance as causes of deaf-mutism. The statistical proofs in support of this hypothesis are not, however, on the whole, very satis factory. The best statistics are furnished by FI. Schmaltz, who has come to the following conclusions: "The industrial population, and especially that part of it which is worst off pecuniarily,—in fact, all who are in danger of degenerat ing both morally and physically on ac count of insufficient means, or poverty, and who consequently are unable or un willing to take the necessary care of their children,—all such persons exhibit the highest percentage of deaf-mutes among their descendants. Finally, when, in ad dition to all these unfavorable condi tions under which children are born, they are brought up by a family which, from various reasons, is, perhaps, more or less degenerated, and have to undergo all sorts of diseases in infancy without hav ing sufficient power of resistance, thus deaf-mutism is an only too common re sult." On the other hand, rchermann
states that in Norway unfavorable social and hygienic conditions are far from in creasing the deaf-mute rate, it being higher among the better-situated classes.
Heredity.— Opinions have differed greatly as to the heredity of deaf-mutism, the reason being that not only are the laws which govern the hereditability of pathological changes and diseases sub ject to different interpretations, and that the statistics employed have given dif ferent results, but also that the term "heredity" is used in different ways.
The term "heredity" is used by many authors to express the frequent appear ance of the same pathological condition in two consecutive generations, other in fluences having, of course, been excluded. The statistics which have been employed in attempts to solve the question of the frequency with which deaf-mutism ap pears in two consecutive generations have been based on two different meth ods: the one calculating how often deaf and-dumb persons had deaf-and-dumb parents, the other how frequently unions where the one or both parties were deaf and dumb resulted in deaf-and-dumb offspring.
The first mode of ascertaining the fre quency with which deaf-mutism appears in two generations, consisting in discov ering how often deaf-and-dumb subjects belonging to large groups of deaf-mutes are descended from deaf-and-dumb par ents, everywhere gives the result that deaf-mutes very seldom have deaf-and dumb parents. This is even the case when only congenitally deaf have been the objects of investigation, Uchermann, for instance, finding in Norway among 921 deaf-mutes with congenital deafness only 2 with deaf-and-dumb parents. This seems to prove that deaf-mutism is rarely inherited in the strictest significance of the term, or, as it might also be ex pressed, inherited directly. It must, how ever, be borne in mind that marriages contracted by deaf-mutes are, and es pecially have been, comparatively rare in Europe, and also that their fertility is smaller than that of other marriages; there can certainly be no doubt that the direct hereditability of deaf-mutism is certainly of much greater importance than might be supposed from the above mentioned statistics.