Even if it could be proved, however, that mixture of races produces lessened fertility of first crosses and of mongrels, this would not make it necessary for us to reject the doctrine of the unity of mankind. It is true that the domesticated varieties both of animals and of plants, when crossed, are as a general rule prolific, in some cases even more so than the purely bred parent varieties; whereas species, when crossed, and their hybrid offspring, are almost invariably in some degree sterile. But this rule is not altogether without exceptions. Even Agassiz condemned the employment of fertility of union as a limiting principle. He considered this a fallacy, “or at least a _petitio principii_, not admissible in a philosophical discussion of what truly constitutes the characteristics of species.”[1678] Thus the red and yellow varieties of maize are in some degree infertile when crossed, and the blue-and the red-flowered forms of the pimpernel, considered by most botanists to be the same species, as they present no differences of form or structure, are, according to Gärtner, mutually sterile. Moreover, Mr. Darwin’s investigations on dimorphic and trimorphic plants have shown that the physiological test of lessened fertility, both in first crosses and in hybrids, is no safe criterion of specific distinction.[1679] As for animals, Professor Vogt asserts that, in the opinion of experienced breeders, certain races can with difficulty be made to pair, and the fertility of the mongrels soon diminishes, whilst other races pair readily and are prolific.[1680] Sir J. Sebright says, “Although I believe the occasional intermixture of different families to be necessary, I do not, by any means, approve of mixing two distinct breeds, with the view of uniting the valuable properties of both: this experiment has been frequently tried by others as well as by myself, but has, I believe, never succeeded. The first cross frequently produces a tolerable animal, but it is a breed that cannot be continued.”[1681] CHAPTER XIV PROHIBITION OF MARRIAGE BETWEEN KINDRED The horror of incest is an almost universal characteristic of mankind, the cases which seem to indicate a perfect absence of this feeling being so exceedingly rare that they must be regarded merely as anomalous aberrations from a general rule.