In opposition to the view that these opinions are the results of experience, it may be urged that any infraction of the customs or laws of ancestors is commonly thought to call down divine vengeance. Father Veniaminof tells us that, among the early Aleuts, incest, which was considered the gravest crime, was believed to be always followed by the birth of monsters with walrus-tusks, beard, and other disfiguration;[2051] and among the Kafirs, according to Mr. Fynn, it is a general belief that the offspring of an incestuous union will be a monster—“a punishment inflicted by the ancestral spirit.”[2052] But whatever may be said of the other cases referred to, no such explanation can possibly hold good for the Arabs. Among them, marriage with a near relation involved no infringement of their marriage regulations. On the contrary, in spite of the opinions in favour of exogamy, the preference for marriage with a cousin was dominant among them, and a man had even a right to the hand of his “bint ‘amm,” the daughter of a paternal uncle.[2053] Taking all these facts into consideration, I cannot but believe that consanguineous marriages, in some way or other, are more or less detrimental to the species. And here, I think, we may find a quite sufficient explanation of the horror of incest; not because man at an early stage recognized the injurious influence of close intermarriage, but because the law of natural selection must inevitably have operated. Among the ancestors of man, as among other animals, there was no doubt a time when blood-relationship was no bar to sexual intercourse. But variations, here as elsewhere, would naturally present themselves; and those of our ancestors who avoided in-and-in breeding would survive, while the others would gradually decay and ultimately perish. Thus an instinct would be developed which would be powerful enough, as a rule, to prevent injurious unions. Of course it would display itself simply as an aversion on the part of individuals to union with others with whom they lived; but these, as a matter of fact, would be blood-relations, so that the result would be the survival of the fittest.