The relative or absolute sterility characterizing first crosses and hybrids depends upon a biological law which might be called the “Law of Similarity.” The degree of sterility, in either case,[1640] runs, at least to a certain extent, parallel with the general affinity of the forms that are united. Thus, most animal hybrids are produced by individuals belonging to the same genus, whilst species belonging to distinct genera can rarely, and those belonging to distinct families perhaps never, be crossed.[1641] The parallelism, however, is not complete, for a multitude of closely allied species will not unite, or unite only with great difficulty, though other species, widely different from each other, can be crossed with facility. Hence Mr. Darwin infers that the difficulty or facility in crossing “apparently depends exclusively on the sexual constitution of the species which are crossed, or on their sexual elective affinity, _i.e._, the ‘Wahlverwandtschaft’ of Gärtner.” But as species rarely, or never, become modified in one character, without being at the same time modified in many, and as systematic affinity includes all visible resemblances and dissimilarities, any difference in sexual constitution between two species would naturally stand in more or less close relation with their systematic position.[1642]
With regard to the instinct in question, man follows the general rule in the animal kingdom. Our notions of morality are closely connected with the instinctive feelings engraved in our nature; and bestiality is commonly looked upon as one of the most heinous crimes of which man can make himself guilty. Several passages both in ancient[1643] and modern writers[1644] prove the occasional occurrence of this crime, but always under circumstances analogous to those under which single birds sometimes form connections against nature,[1645] _i.e._, either because of isolation, or on account of vitiated instincts.[1646]
Supporters of the hypothesis that the several races of man are distinct species of the genus Homo, assert that an instinctive aversion similar to that which keeps different animal species from intermingling, exists also between the various human races.[1647] It may be noted by the way that, even if this were true, the idea that mankind consists of various species might be controverted; for certain races of domestic or semi-domesticated animals seem to prefer breeding with their own kind and refuse to mingle with others. Thus Mr. Bennett states that the dark and pale coloured herds of fallow deer, which have long been kept together in the Forest of Dean and two other places, have never been known to mingle. On one of the Faroe Islands, the half-wild native black sheep are said not to have readily mixed with the imported white sheep. And in Circassia, where six sub-races of the horse are known and have received distinct names, horses of three of these races, whilst living a free life, almost always refuse to mingle and cross, and will even attack each other.[1648] As for man, there are many races who dislike marrying persons of another race, but the motives are various. The different ideas of beauty no doubt play an important part. Mr. Winwoode Reade does not think it probable that negroes would prefer even the most beautiful European woman, on the mere grounds of physical admiration, to a good-looking negress.[1649] A civilized race does not readily intermingle with one less advanced in civilization, from the same motives as those which prevent a lord from marrying a peasant girl. And more than anything else, I think, the enmity, or at least, want of sympathy, due to difference of interests, ideas, and habits, which so often exist between distinct peoples or tribes, helps to keep races separate. But such reasons as these have nothing in common with the instinctive feeling which deters animals of distinct species from pairing with each other. Hence, when two races come into very close mutual contact, especially if they are at about the same stage of civilization, their dislike to intermarriage commonly disappears.