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." 68.