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. _Divorce._--In cases in which one partner to a marriage suffers from disease, divorce should be rendered as easy as possible. The interests of the children are often put forward as reasons against this course. There is no doubt that many married couples to-day refrain from separation or divorce solely because they regard it as to their children’s interest that they should continue to live together. But it is precisely on the ground of the children’s interest that such marriages ought to be dissolved, and that the child or children should remain with the healthy parent. The opposite course would only destroy the happiness of the healthy parent, without doing the other parent any good.