67. The origin of exogamy.
Again, Dr. Westermarck shows that the practice of exogamy or the avoidance of intermarriage did not at first arise between persons recognised as blood relations, but between those who lived together. “Facts show that the extent to which relatives are not allowed to intermarry is nearly connected with their close living together. Generally speaking the prohibited degrees are extended much further among savage and barbarous peoples than in civilised societies. As a rule the former, if they have not remained in the most primitive social condition of man, live not in separate families but in large households or communities, all the members of which dwell in very close contact with each other.”[158] And later, after adducing the evil results of self-fertilisation in plants and close interbreeding in animals, Dr. Westermarck continues: “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 recognised 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.”