(c) What has been said regarding alcoholism applies also to some extent to morphinism.
(d) Tuberculosis is not directly transmitted by inheritance. But the predisposition to tuberculosis is so transmitted; that is to say, it is regarded as unquestionable that an inferior power of resistance to the virus of tubercle passes by inheritance from parent to offspring. Those predisposed to tuberculosis tend to have a vigorous sexual impulse, and exhibit a high degree of fertility.
(e) Gonorrhœa lessens or destroys fertility to such an extent that, in from 40 to 50 per cent. of childless marriages, the sterility is referable to gonorrhœal infection. Gonorrhœa, i.e. specific gonorrhœal urethritis or vaginitis, is not itself transmissible by inheritance; but gonorrhœa in the parent may lead to certain diseases in the offspring, and the most important of these is ophthalmia of the new-born—the commonest cause of blindness.
(f) Syphilis is in most cases transmissible by inheritance. The children of syphilitic parents are commonly feeble-minded or idiotic; or bodily, mentally, or morally degenerate; and they possess an inferior power of resistance to diseases.
The Age of the Parents.—The age of the parents at the time of procreation has a marked influence upon the health of the offspring. The parents should not be too young. It is not well that the mother should be less than twenty, or the father less than twenty-four years of age. Many of the children of such extremely youthful parents are weakly, and have poor health. On the other hand, the parents should not be very old. When the mother is over forty and the father over fifty years of age, the children are apt to be weakly; also they are apt to be left orphaned at a comparatively early age. It is also undesirable that there should be a great difference between the ages of the parents. Thus the parents should be young, but not too young. The younger they are, the more children can they procreate—first of all, because their fertility is greater in youth, and, secondly, because their married life lasts longer. Qualitatively, also, the children of such marriages are usually healthier. As a matter of fact, the age indicated is the normal age for marriage—the age at which the great majority enter upon marriage.
The Marriage of Near Kin.—We still lack precise information to enable us to decide whether the marriage of near kin is injurious to the offspring of such marriages. According to certain (unofficial) statistics, of 1000 marriages, from 7 to 11 are those of persons near akin. Naturally, in certain remote and inaccessible districts, the proportion of such marriages is greater than this. Two very different views have been put forward by scientific authorities. According to one school, the marriages of persons near akin either prove completely sterile, or else produce offspring defective in body or in mind. According to the other school, blood relationship has per se no influence upon the offspring of the unions of nearly-related persons; where the parents (in such marriages) are themselves free from defects transmissible by inheritance, there is no reason to anticipate that the offspring will be in any way defective, unless other noxious influences (in addition to the kinship of the parents), such as disease, debility, exhaustion from previous excesses, &c., have been at work. But if such noxious influences have affected both parents, the danger of some congenital defect appearing in the children is very great. We certainly see many cases in which the marriage of near kin results in the procreation of large and thoroughly healthy families; but, in contrast with these, we also see many cases in which the offspring of such marriages have proved non-viable or degenerate (especially, blind, deaf-mute, idiotic, insane, polydactylous, or affected with congenital developmental defects).
Marriages between persons whose qualities are extremely divergent, and marriages between persons whose qualities are too closely similar, are alike undesirable. Neither the offspring of marriages between those belonging to different races nor the offspring of marriages between those too closely related appear to be a gain for the race. It is the degree of divergence or of resemblance which here plays the decisive part.
It remains a subject of controversy whether the institution of exogamy originated from the fact that it was regarded as desirable to refresh the blood of the tribe by cross-fertilisation with the blood of another tribe. But it is incontestably established that since historic times began there has existed in the human race a natural antipathy to incestuous sexual relations, and more especially to incestuous marriages. The mental abnormalities seen in the offspring of incestuous marriages are to a large extent explicable from the fact that the incestuous relationship was itself the outcome of mental abnormality in those who entered upon that relationship.
The possibility of the hereditary transmission of disease is greater in the case of the marriage of near kin than it is in ordinary marriages. When both parents suffer from the same transmissible defect, the likelihood that that defect will manifest itself in the offspring in a graver form is greatly increased. Thus, in the case of the offspring of nearly related persons, the probability of the appearance of certain transmissible defects is considerable. For perfectly healthy parents are rarities, and when husband and wife are closely related the probability that both will suffer from the same transmissible defect or disorder is relatively high.
Disease in the Parents from the Legal Standpoint.—In relation to marriage, disease has a twofold significance: on the one hand, it may be a factor leading to the dissolution of the marriage (owing to divorce, nullity of marriage, venereal infection); on the other hand, the existence of disease may prevent marriage. Very naturally, the latter factor is of preponderant importance, because we lay the chief stress upon prevention. It is for this reason that so little attention is paid to the legal significance of the former factor.