Much ingenuity has been expended in attempts to account for the origin of the horror of incest.  The main reason why it has so far remained more or less of a mystery, is that each writer advanced a single cause, which he pressed into service to explain all the facts, the result being confusion and contradiction.  In my opinion different agencies must be assumed in different cases.  When we find among Australians, American Indians (and even the Chinese), customs, enforced by the strongest feelings, forbidding a man to marry a woman belonging to the same clan or having the same surname, though not at all related, while allowing a marriage with a sister or other near blood relative, we are obviously not dealing with a question of incest at all, but with some of the foolish taboos prevalent among these races, the origin of which they themselves have forgotten.  Mr. Andrew Lang probably hit the nail on the head when he said (258) in regard to the rule which compels savages to marry only outside of the tribe, that these prohibitions “must have arisen in a stage of culture when ideas of kindred were confused, included kinship with animals and plants, and were to us almost, if not quite, unintelligible.”  To speak of instinct and natural selection teaching the Veddahs to abhor marriage with an elder sister while making union with a younger sister the proper marriage (Westermarck, 292) is surely to assume that instinct and natural selection act in an asinine way, which they never do—­except in asses.

In a second class of cases, where lower races have ideas similar to ours, I believe that the origin of domestic chastity must be sought in utilitarian practices.  In the earlier stages of marriage, girls are usually bought of their parents, who profit by the sale or barter.  Now when a man marries a girl to be his wife and maid of all work, he does not want to take her to his home hampered by a bevy of young children.  Fathers guilty of incestuous practices would therefore be unable to dispose of their daughters to advantage, and thus a prejudice in favor of domestic purity would gradually arise which a shrewd medicine man would some day raise to the rank of a religious or social taboo.

As regards modern society, Darwin, Brinton, Hellwald, Bentham, and others have advocated or endorsed the view that the reason why such a horror of incestuous unions prevails, is that novelty is the chief stimulus to the sexual feelings, and that the familiarity of the same household breeds indifference.  I do not understand how any thinker can have held such a view for one moment.  When Bentham wrote (Theory of Legislation, pt. iii., chap.  V.) that “individuals accustomed to see each other from an age which is capable neither of conceiving desire nor of inspiring it, will see each other with the same eyes to the end of life,” he showed infinitely less knowledge of human nature than the author of Paul and Virginia, who makes a boy and a girl