"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."