To what extent the prolificness of these two species will go is yet to be determined, and there is an unexplored field still open among the other species of this genus; it is highly probable that a gradation may be established from sterility, up to perfect prolificacy. Not only do the female ass and the male onager breed together, but a male offspring of this cross, with a mare, produces an animal more docile than either parent, and combining the best physical qualities, such as strength, speed, &c.; whence the ancients preferred the onager to the ass, for the production of mules.[205] Mr. Gliddon, who lived upwards of twenty years in Egypt and other eastern countries, informs me this opinion is still prevalent in Egypt, and is acted upon more particularly in Arabia, Persia, &c., where the _gour_, or wild ass, still roams the desert. The zebra has also been several times crossed with the horse. 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.