Brothers and sisters have more in common, mentally and physically, than any other members of the same race can possibly have with one another. But nobody falls in love with his sister. A profound instinct has taught even the lower races of men (for the most part) to avoid such union of the all-but-identical. In the higher races the idea never so much as occurs to us. Even cousins seldom fall in love--seldom, that is to say, in comparison with the frequent opportunities of intercourse they enjoy, relatively to the remainder of general society. When they do, and when they carry out their perilous choice effectively by marriage, natural selection soon avenges Nature upon the offspring by cutting off the idiots, the consumptives, the weaklings, and the cripples, who often result from such consanguineous marriages. In narrow communities, where breeding in-and-in becomes almost inevitable, natural selection has similarly to exert itself upon a crowd of _cretins_ and other hapless incapables. But in wide and open champaign countries, where individual choice has free room for exercise, men and women as a rule (if not constrained by parents and moralists) marry for love, and marry on the whole their natural complements. They prefer outsiders, fresh blood, somebody who comes from beyond the community, to the people of their own immediate surroundings. In many men the dislike to marrying among the folk with whom they have been brought up amounts almost to a positive instinct; they feel it as impossible to fall in love with a fellow-townswoman as to fall in love with their own first cousins. Among exogamous tribes such an instinct (aided, of course, by other extraneous causes) has hardened into custom; and there is reason to believe (from the universal traces among the higher civilisations of marriage by capture) that all the leading races of the world are ultimately derived from exogamous ancestors, possessing this healthy and excellent sentiment.