Not only is the absolute age of the woman of importance in deciding on the advisability of marriage, but the relative ages of the proposed husband and wife must also be taken into account, first of all in respect of the wife’s possible fertility, and secondly in respect of her general health. The most suitable arrangement is that in which there is no marked difference in age. The husband may be, and indeed in existing social circumstances almost necessarily is, somewhat older than his wife, as much perhaps as eight or ten years. But a very great disparity of age (in either direction) is a serious error. If a very young girl marries an elderly man, or a developed matron marries a young man, the true purpose of marriage is unfulfilled, the eternal laws of nature and all ethical principles are infringed. In the breeding of animals, the fundamental principle has long prevailed that the animals chosen for coupling should be well suited each to the other and should be in perfect physical condition; and breeders are also familiar both with the favorable influence of good nourishment and with the advantage of the opportune crossing of distinct varieties. The same principles are equally applicable to the human race, neglected as they commonly are in practice. With regard to the marriage of near kin, we can only remark that the marriage of those closely related by blood should as far as possible be avoided, and that such a marriage must be absolutely prohibited when in both families there is a history of tuberculosis, mental disorders, diabetes, and the like. When first cousins contemplate marriage, it is indispensable, not only that both individuals should be in perfect health, but also that on neither side there should be any serious family history of transmissible disease or transmissible morbid tendency; and, further, it is absolutely necessary that no such marriage of near kin should have taken place in the proximate ancestry of the cousins, _i. e._, their cousinship must not be a double one, derived both from the paternal side and the maternal. It is indeed to be recommended, with a view to the production of a healthy and powerful posterity, that marriage should bring about a crossing of healthy individuals proceeding from different families, different places, and different constitutional types. An instance of the advantage to be found in this practice is pointed out by _Ribbing_, who shows that the most powerful aristocracy in Europe, that of England, by the gradual creation of new peers, on the one hand, and by the gradual decline of younger sons and their descendants into the middle class, on the other, has undergone a continual crossing with less exalted but originally sounder stocks; in this way its vigor and fertility have been maintained, in contrast to the nobility of many continental states, which has so largely perished, in consequence of its exclusiveness in the matter of marriage.