This opinion is corroborated by sta tistics founded on the second mode of estimating the frequency with which deaf-mutism appears in two consecutive generations, viz.: by calculating how fre quently unions where one or both parties are deaf and dumb result in deaf-mute offspring. The European statistics of this kind are but few and small, the reason being mentioned above, while the excellent American statistics collected by E. A. Fay are very comprehensive, mar riages contracted by deaf-mutes being so much more frequent in the United States. The principal results of Eu ropean statistics have been that a deaf and-dumb child was born in about every thirtieth or thirty-first union where one party was deaf and dumb, and that deaf mute offspring were much more fre quently the result of unions where both parties were deaf and dumb. The sta tistics published by Fay are based on vestigations of over 5000 marriages con tracted by deaf-mutes and have given the result that over 9 per cent. of these re sulted in "deaf offspring, and, curiously enough, the marriage where both parties were deaf did not result more frequently in deaf offspring than those where only the one party was deaf." Fay also found that marriages of congenital deaf persons and of deaf persons with deaf relatives gave a far greater liability to deaf off spring.
If, now, the term "heredity" is used to express the conspicuous frequency with which the same abnormality appears in the same family, the hereditability of deaf-mutism becomes still more evident. The frequency with which deaf-mutism appears among the parents of deaf-mutes has been mentioned above. Cases of deaf-mutism among the grandparents, great-grandparents, etc., of deaf-mutes, which should prove the direct heredity per saltum, as it is termed, must neces sarily be still less frequent, as marriages between deaf-mutes were very rare in the first half of this century. If we, how ever, look for cases of deaf-mutism in other branches of the deaf-mutes' family tree, we find in all statistics that--con sidering that deaf-mutism is a compara tively - rare pathological condition — a great number of deaf-mutes are to be found among the uncles, aunts, great uncles, great-aunts, cousins, and second cousins of deaf-and-dumb persons. Ac cording to European statistics, embrac ing a large number of deaf-mutes, about every sixteenth deaf-mute has one deaf and-dumb relative among the category above mentioned (parents, grandparents, brothers, and sisters excepted), the point where deaf-mutism most often appears corresponding to generations co-ordinate with the parents. These statistics have also shown that it is almost exclusively congenital deafness which plays a part in this respect. Deaf-mutism, finally, is to be met with more frequently among the brothers and sisters of deaf-mutes, and there are statistics as to congenital deaf-mutes according to which 50 per cent. of these had one or more deaf-and dumb brothers or sisters. The appear ance of deaf-mutism in two or more children of the same parents is very characteristic, and there are few patho logical conditions which show such a tendency to appear in the same branch of a family, there even being cases on record where ten deaf-and-dumb chil dren were born in the same marriage.
frequency among the relatives of deaf mutes and with about double the fre quency among the relatives of congenital deaf-mutes as among the relatives of deaf-mutes with acquired deafness.
Their appearance is particularly clearly demonstrated by several genealog ical tables published by Dahl and Ucher mann, of which the one depicted below is an interesting example.
[Albinism, retinitis pigmentosa, and malforniations are also frequently found among the relatives of deaf-mutes; these anomalies are probably to be considered as signs of degeneration, deaf-mutism it self being undoubtedly in several cases a degenerative phenomenon. These
anomalies, however, might also be con Finally, if by heredity we understand the frequent appearance in a family of not only one pathological condition, but of several others related to it anatomic ally or etiologically, we shall see that he redity is a most important factor in the etiology of deaf-mutism. It is, namely, proven by several comprehensive statis tics that partial or total deafness due to different ear diseases (which have not led to deaf-mutism, on account of the lesser degree of the loss of hearing or of its unilateral appearance, or of its later development in life), insanity, epilepsy, idiocy, stammering, and other defects of speech, hysteria, and several other nerv ous diseases appear with conspicuous The laws which may, then, be sup posed to regulate the heredity of deaf mutism are difficult of interpretation, and seem in many respects to differ from those which relate to other pathological conditions and diseases. This may be accounted for by supposing that, as the causes of deaf-mutism in general are numerous and varied, so are also the causes of each individual case. The cir cumstance that deaf-mutism, so far as its etiolog,y is concerned, must be divided into two distinct classes, the congenital and the acquired, the latter of necessity including numerous cases in which deaf ness is to be traced to accidental causes, is alone sufficient to render the interpre tation of the laws of heredity, by the help of investigations which embrace deaf-mutes in general, of the greatest difficulty. When we add to this that, although the importance of heredity in deaf-mutism is undoubted and consider able, there are other factors of at least equal importance, and that there is much which tends to neutralize the transmis sion of morbid tendencies (favorable so cial conditions, crossed marriages, etc.), it will be evident that there is much which renders a just explanation of the laws of heredity anything but an easy task. If we compare deaf-mutism with hmmophilia, which it resembles so far as heredity is concerned, we shall see how correct the above statements are. }Immo philia—which, like deaf-mutism, may pass over several generations and accu mulate in a single, being also most fre quent among males and in the children of fruitful marriages—is, etiologically, but little complicated, partly because it is not related to any other anomaly, and partly because heredity is the governing cause. With deaf-mutism it is very dif ferent. It, too, may accumulate in single generations, being most frequent in brothers and sisters and much less fre quent in the older generations. In these, however, there can be found a compara tively large number of cases of partial or total deafness, insanity, epilepsy, etc., which seems to indicate that deaf-mutism is, in many cases, a combined result of the transmission of various influences. These influences fall into two groups: those which originate in ear diseases, and those which originate in nervous disease in the family. Now, as the morbid anatomy of deaf-mutism proves that in the majority of cases the deafness caus ing deaf-mutism arises from abnormali ties of the nervous parts of the auditory organ,—the labyrinth,—there is reason to suppose that in many eases deaf-mut ism is caused by the transmission of the above dual influences through the par ents. Supposing this hypothesis to be correct, our knowledge of the, laws of heredity in deaf-mutism assumes at once a more distinct form, though we cannot ever expect it to be as clear as it is, for instance, in regard to the laws which govern lunmophilia, for, as above men tioned, the causes of deaf-mutism are too numerous and varied. Even twins, who would seem to be exposed to exactly the same influences during feetal life, are sometimes the one a deaf-mute, the other a normal subject.