But a deeper and still more interesting question remains: "How has this instinctive aversion to marriage between persons living closely together originated?" We cannot help feeling that through his masterly solution of this difficult problem Westermarck has at last brought us very near to the truth. He finds the key to it in the biological law of similarity.[394] It is demonstrated that a "certain degree of similarity as regards the reproductive system of two individuals is required to make their union fertile and the progeny resulting from this union fully capable of propagation." But the similarity must not be _too_ close. A certain amount of differentiation is requisite; but the differentiation must not be too great.[395] There must be homogeneity combined with heterogeneity. Among domestic animals close interbreeding, it is well known, leads to infertility and degeneration; and Darwin's researches prove that self-fertilization in the vegetable kingdom produces the same results.[396] There is abundant evidence tending to show that what is true of plants and the lower animals is true also of man. "Taking all these facts into consideration," says Westermarck, in closing his argument, "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. Whether man inherited the feeling from the predecessors from whom he sprang, or whether it was developed after the evolution of distinctly human qualities, we do not know."[397] [394] WESTERMARCK, _op. cit._, chap, xiii, and compare chap, xv, 334 ff.