A powerful instinct keeps animals from pairing with individuals belonging to another species than their own. “L’animal,” says M. Duvernoy, “a l’instinct de se rapprocher de son espèce et de s’éloigner des autres, comme il a celui de choisir ses aliments et d’éviter les poisons.”[1629] Among Birds, there are found a small number of wild hybrids, nearly all of which are in the order of Gallinae, and most of which belong to the genus Tetrao.[1630] But among Insects, Fishes, and Mammals, living in a state of nature, hybridism is unknown or almost so.[1631] And, even among domesticated mammals, some tricks are often required to deceive the male, and so to conquer its aversion to a female of a different species. The stallion, for instance, who is to cover a she-ass, is frequently first excited by the presence of a mare, for which, at the proper moment, the she-ass is substituted.[1632]
We may be sure that, were it not for this instinctive feeling, many more animal hybrids would be naturally produced than is the case. In the vegetable kingdom, where the play of instincts is altogether out of the question, bastards occur much more frequently;[1633] and in captivity a considerable number of animal hybrid forms are produced that are never met with in a state of nature.[1634] Yet, according to Mr. Darwin, there are good grounds for the doctrine of Pallas, that the conditions to which domesticated animals and cultivated plants have been subjected, generally eliminate the tendency towards mutual sterility, so that the domesticated descendants of species which in their natural state would have been in some degree sterile when crossed, become perfectly fertile.[1635]
The origin of this instinct, which helps to keep even closely allied species in a state of nature distinct, seems to be sufficiently clear. The number of species which have proved fertile together are very limited, and the fertility of the hybrid offspring is almost constantly diminished, often even to a very great extent. Of course, no one now talks of the sterility of hybrids as a moral necessity—hybrids being animalia adulterina,—or as the result of a special divine decree, that new species should not be multiplied indefinitely.[1636] M. Isidore Geoffroy has shown not only that hybrids may be fertile, but that “infertile” hybrids are, properly speaking, merely the hybrids which are most rarely fertile, their sterility never being absolute.[1637] Moreover, as has been pointed out by Mr. Wallace, in almost all the experiments that have hitherto been made in crossing distinct species, no care has been taken to avoid close interbreeding; hence these experiments cannot be held to prove that hybrids are in all cases infertile inter se.[1638] But looking to all the ascertained facts on the intercrossing of plants and animals, we may with Mr. Darwin conclude that some degree of sterility in hybrids is an extremely general result.[1639] This being the case with the hybrids of our domesticated animals, it must be so all the more with animals in a state of nature, which generally live under conditions less favourable to mutual fertility. It is easy to understand, then, that instincts leading to intercrossing of different species, even if appearing occasionally, never could be long-lived, as only those animals which preferred pairing with individuals of their own species, gave birth to an offspring endowed with a normal power of reproduction, and thus became the founders of numerous generations that inherited their instincts.
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.
Mongrels form, indeed, a large proportion of the inhabitants of the world. It is doubtful whether there are any pure races in Europe; not even the Basques can pretend to purity of blood.[1650] M. Broca found, when investigating the subject of stature, that nineteen-twentieths of the whole population of France presented, in various degrees, the characters of mixed races.[1651] In North America, different races intermingle more and more every day. In Greenland, according to Dr. Nansen, in the course of a century and a half there has been such an intermixture of races that it would now be extremely difficult, if not impossible, to find a true Eskimo throughout the whole of the west coast; and the Europeans, far from being disliked by the native women have succeeded in inspiring them with so much respect that the “simplest European sailor is preferred to the best Eskimo seal catcher.”[1652] In Mexico, the Spanish mixed breeds constitute two-thirds or three-fourths of the whole population;[1653] and South America, to quote a French writer, is “le grand laboratoire des nations hybrides ou métisses modernes.”[1654] Of twelve millions of mongrels, which is the estimated number of mongrels on the face of the globe, no fewer than eleven millions are found there.[1655] Even in remote Tierra del Fuego, according to Mr. Bridges, some mongrels of European fathers and indigenous mothers have appeared during the last few years.
In Asia there are numberless instances of intermixture of breed between the Tartars, Mongols, and Tunguses, and the Russians and Chinese, &c.[1656] In India there are many Eurasians; in the Indian Archipelago Chinese and Malays intermarry;[1657] and, in the Islands of the South Sea, the mongrels of European fathers amount to a considerable number. In Africa, the eastern Soudan is a great centre of mixed breeds between races much removed from one another. And, in Southern Africa, the Griquas—the offspring of Dutch colonists and Hottentot women—form a very distinct race.
As far as we know, there are no human races who, when intermingled, are entirely sterile. But as regards the degree of fertility of first crosses and of mongrels, the opinions of different anthropologists vary considerably. Those who do not believe in the unity of the human race have been especially solicitous to prove that crosses are almost inevitably followed by bad results in that respect. Thus Dr. Knox thinks that the half-breeds, if they were abandoned to themselves and no longer had access to pure races, would rapidly disappear, the “hybrid” being rejected by nature as a degradation of humanity.[1658] Dr. Nott asserts that, when two proximate species of mankind, two races bearing a general resemblance to each other in type, are bred together, they produce offspring perfectly prolific; but that, when species the most widely separated, such as the Anglo-Saxon and the negro, are crossed, the mulatto offspring are but partially prolific, and acquire an inherent tendency to run out, and become eventually extinct, when kept apart from the parent stocks.[1659] The same opinion is entertained by M. Broca, and by M. Pouchet, who thinks that the crossed race will exist only if it continues to be supported by the two creating types remaining in the midst of it.[1660]