The human species

by A. de Quatrefages

This proportion—three-eighths to five-eighths—appears to be very favourable to the maintenance of hybrid races; it is the proportion which characterises the famous leporides, the result of the crossing of the hare and the rabbit. But can these hybrids, of which so much has been said, maintain themselves without reverting to the parental types? M. Roux evidently believed it, and it is still asserted by M. Gayot. But the testimony of those who have established and impugned their assertions leaves scarcely any room for doubt. Isidore Geoffroy, who had at first believed in their fixity, and had spoken of it as a conquest, did not hesitate afterwards to admit the reversion. The fact has been established in the Jardin d’Acclimatation, and M. Roux himself, upon the assertion of M. Faivre, appears to have abandoned his previous assertions. The observations and experiments made by the Agricultural Society of Paris clearly show that the leporides, sent or presented by the breeders themselves, had entirely reverted to the rabbit type. Lastly, M. Sanson, discussing the anatomical side of the question, has arrived at the same conclusions. Moreover, whoever will credit the observations made by M. Naudin upon the Linariæ, will easily recognise the reversion and the disordered variation exhibited by the leporides of the Abbé Cagliari, who was the first to obtain a fertile crossing between the hare and the rabbit.

These phenomena appear in an equally well marked manner in the result of the cross between the silkmoth (Bombyx cynthia) and the castor-oil silkmoth (Bombyx arrindia), obtained by M. Guérin Méneville. The hybrids of the first generation were almost exactly intermediate between the two species, and resembled each other. In the second this uniformity disappeared, in the third the dissimilarity had increased, some of the insects having reassumed all the characters of the paternal or maternal types. In the seventh generation this curious experiment was destroyed by ichneumons. But, as M. Valée, their intelligent breeder, told me, nearly all the moths had returned to the type of the Bombyx arrindia. The resemblance to what took place in the case of M. Naudin’s Linariæ is here complete.

V. The phenomenon of the reversion of the descendants of a hybrid to the paternal or maternal type, or disordered variation, has given rise to some interpretations which it will be well to rectify, and has also raised important questions.

The attempt has been made to assimilate the latter to the oscillations presented by mongrels for some generations. But daily experience should suffice to refute this opinion. Breeders are crossing races every day for some purpose or other, and they would never do so if the crossing were to result in the production of a disorder which would exhibit the smallest resemblance to that displayed by the Linariæ of M. Naudin, and the silkmoths of Guérin Méneville. They expect, however, a few irregularities more or less marked, in the first generations, but they know that the race will soon settle while the disorder would only increase if the crossing had taken place between species.

Again, an attempt has been made to consider the facts of atavism and reversion as identical. There is, however, a fundamental difference between them, for the mongrel which by atavism reassumes the characters of one of its paternal ancestors, for example, still preserves its mixed nature. This is proved by the possibility of its offspring of the first or second generation reproducing, on the contrary, the essential traits of its own maternal ancestors. Darwin gives many examples of facts of this nature from the agricultural history of his country. One of the best to quote is that furnished by the genealogy of a family of dogs observed by Girou de Buzareingues. These animals were crosses between the setter and spaniel. Now one male, a setter to all appearances, united with a female of pure setter breed, produced spaniels, which makes it evident that the latter blood was by no means annihilated, and that the return to the setter type was only apparent.

It is different in the cases of reversion displayed by hybrids, for one of the two bloods is irrevocably expelled. We are justified in making this assertion in the case of mammalia, by experience extending as far back as the Roman period, or at least as far as the seventeenth century. Titires and musmons have never since those times had offspring affected by atavism. A ram and sheep have never been known to produce a kid, nor a male and female goat to produce a lamb. It is the same with plants, according to statements with which M. Naudin has kindly furnished me.

Far from being similar, the phenomena of atavism and reversion are absolutely different and characteristic, the one of crossing between races, the other of crossing between species. The first proclaims the persistency of the physiological connections between all the representatives, more or less modified, of one species; the second proves the complete rupture of the same connections between the descendants of two species accidentally brought into contact by the promoter of the hybridism.

VI. In none of the preceding cases has hybridism, no matter in what degree, given rise to a series of individuals descended the one from the other; and preserving the same characters. An exception is, however, known to this general fact. It is unique, and is produced in the vegetable kingdom from the crossing of wheat with Ægilops ovata.

The hybrid of the first generation from these two species is sometimes produced naturally, and was regarded by Requien as a species. Fabre, who frequently met with it in the fields, considered it to be the commencement of the transmutation of the Ægilops into wheat. Afterwards a quadroon hybrid, accidentally obtained and cultivated during several years, gave him descendants resembling the beardless wheat of the South. It was the result of reversion. Fabre, however, who did not recognise the hybrid, thought it was a transmutation, and flattered himself that he had discovered wild wheat in the Ægilops.