The genus canis contains a great many species, as domestic dogs, wolves, foxes, jackals, &c., and much discussion exists as to which are really species and which mere varieties. In this genus experiments in crossing have been carried a step further than in the Equidæ, but there is much yet to be done. All the species produce prolific offspring, but how far the prolificness might extend in each instance is not known; there is reason to believe that every grade would be found except that of absolute sterility which is seen in the offspring of the horse and ass.
The following facts are given by M. Flourens, and are the result of his own observations at the Jardin des Plantes.
"The hybrids of the dog and wolf are sterile after the third generation; those of the jackal and dog, are so after the fourth.
"Moreover, if one of these hybrids is bred with one of the primitive species, they soon return, completely and totally, to this species.
"My experiments on the crossing of species have given me opportunities of making a great many observations of this kind.
"The union of the dog and jackal produces a hybrid—a mixed animal, an animal partaking almost equally of the two, but in which, however, the type of the jackal predominates over that of the dog.
"I have remarked, in fact, in my experiments, that all types are not equally dominant and persistent. The type of the dog is more persistent than that of the wolf—that of the jackal more than that of the dog; that of the horse is less than that of the ass, &c. The hybrid of the dog and the wolf partakes more of the dog than the wolf; the hybrid of the jackal and dog, takes more after the jackal than dog; the hybrid of the horse and the ass partakes less of the horse than the ass; it has the ears, back, rump, voice of the ass; the horse neighs, the ass brays, and the mule brays like the ass, &c.
"The hybrid of the dog and jackal, then, partakes more of the jackal than dog—it has straight ears, hanging tail, does not bark, and is wild—it is more jackal than dog.
"So much for the first cross product of the dog with the jackal. I continue to unite, from generation to generation, the successive products with one of the two primitive stocks—with that of the dog, for example. The hybrid of the second generation does not yet bark, but has already the ears pendent at the ends, and is less savage. The hybrid of the third generation barks, has the ears pendent, the tail turned up, and is no longer wild. The hybrid of the fourth generation is entirely a dog.
"Four generations, then, have sufficed to re-establish one of the two primitive types—the type of the dog; and four generations suffice, also, to bring back the other type."[206]
From the foregoing facts, M. Flourens deduces, without assigning a reason, the following non sequitur:—
"Thus, then, either hybrids, born of the union of two distinct species, unite and soon become sterile, or they unite with one of the parent stocks, and soon return to this type—they in no case give what may be called a new species, that is to say, an intermediate durable species."[207]
The dog also produces hybrids with the fox and hyena, but to what extent has not yet been determined. The hybrid fox is certainly prolific for several generations.
There are also bovine, camelline, caprine, ovine, feline, deer with the ram, and endless other hybrids, running through the animal kingdom, but they are but repetitions of the above facts, and experiments are still far from being complete in establishing the degrees which attach to each two species. We have abundant proofs, however, of the three first degrees of hybridity. 1st. Where the hybrid is infertile. 2d. Where it produces with the parent stock. 3d. Where it is prolific for one, two, three, or four generations, and then becomes sterile. Up to this point there is no diversity of opinion. Let us now inquire what evidence there is of the existence of the 4th degree, in which hybrids may form a new and permanent race.
To show how slow has been our progress in this question, and what difficulties beset our path, we need only state that the facts respecting the dog, wolf, and jackal, quoted above from Flourens, have only been published within the last twelve months. The identity of the dog and wolf has heretofore been undetermined, and the degrees of hybridity of the dog with the wolf and jackal were before unknown. These experiments do not extend beyond one species of wolf.
M. Flourens says:—
"Les espèces ne s'altèrent point, ne changent point, ne passent point de l'une à l'autre; les espèces sont fixés."
"If species have a tendency to transformation, to pass one into another, why has not time, which, in everything, effects all that can happen, ended by disclosing, by betraying, by implying this tendency.
"But time, they may tell me, is wanting. It is not wanting. It is 2000 years since Aristotle wrote, and we recognize in our day all the animals which he describes; and we recognize them by the characters which he assigns.... Cuvier states that the history of the elephant is more exact in Aristotle than in Buffon. They bring us every day from Egypt, the remains of animals which lived there two or three thousand years ago—the ox, crocodiles, ibis, &c. &c., which are the same as those of the present day. We have under our eyes human mummies—the skeleton of that day is identical with that of the Egyptian of our day."