4. That which takes place between closely proximate species; among mankind, for example, and among those domestic animals most essential to human wants and happiness; here the prolificacy is unlimited.
It seems to be a law that in those genera where several or many species exist, there is a certain gradation which is shown in degrees of hybridity; some having greater affinity than others. Experiments are still wanting to make our knowledge perfect, but we know enough to establish our points.
There are many points we have not space to dwell on, as the relative influence of the male and female on the offspring; the tendency of one species to predominate over another; the tendency of types to "crop out" after lying dormant for many generations; the fact that in certain species some of the progeny take after one parent and some after the other, while in other cases the offspring presents a medium type, &c.
The genus Equus (Horse) comprises six species, of which three belong to Asia, and three to Africa. The Asiatic species are the Equus Caballus (Horse), Equus Hemionus (Dzigguetai), and Equus Asinus (Ass). Those of Africa are the Equus Zebra (Zebra), Equus Montanus (Daw), and the Equus Quaccha (Quagga). The horse and ass alone have been submitted to domestication from time immemorial; the others have remained wild.
It is well known that the horse and ass produce together an unprolific mule, and as these two species are the furthest removed from each other in their physical structure, Dr. Morton long since suggested that intermediate species bred together would show a higher degree of prolificness, and this prediction has been vindicated by experiments recently made in the Garden of Plants at Paris, where the ass and dzigguetai have been bred together for the last ten years. "What is very remarkable, these hybrids differ considerably from each other; some resemble much more closely the dzigguetai, others the ass." In regard to the product of the male dzigguetai and the jenny, Mr. Geoffroy St. Hilaire says:[203]—
"Another fact, not less worthy of interest, is the fecundity, if not of all the mules, at least the firstborn among them; with regard to this, the fact is certain; he has produced several times with Jennies, and once with the female dzigguetai, the only one he has covered."[204]
At a meeting of the "Société Zoologique d'Acclimation,"
M. Richard (du Cantal) "parle des essais de croisements de l'hémione avec l'anesse, et dit qu'ils ont donnè un mulet beaucoup plus ardent que l'âne. Il asserte que les produits de l'hémione avec l'âne, sont féconds, et que le métis, nommé Polka, à déja produit."
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.