3. That in which animals of unquestionably distinct species produce a progeny which are prolific _inter se_, but have a tendency to run out. 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."