A good case by which to exhibit further our conception of a species is that afforded by the species which are united in the genus Equus–the horse-genus. There are living at the present day several wild kinds of Equus–namely, the wild horse, or Tarpan, of the Gobi desert of Mongolia, called after the Russian explorer Przewalski; two kinds of Asiatic wild ass, called the Kiang and the Onegar; the African wild ass, and two or three kinds of zebra. There are, besides, many kinds of domesticated horses, ranging from the Shetland pony to the Flemish dray horse, and from the Shire horse to the Arab. Then there are many kinds of fossil extinct horses known, some of which clearly must be placed in the genus Equus with the living kinds; others which have to be separated into special genera (Hippidium, Onohippidium, etc.). Now, as to the living forms or form-kinds of the genus Equus–which are we to regard as true species, and which are only varieties and races of lower significance than species? The answer is clear enough in regard to several of them. The wild Mongolian horse and all the domesticated horses are varieties, races, or breeds of one species, judged not only by such marks as the possession of callosities on both the hind and the fore legs, but also by the test of breeding. They breed together and produce persisting races. But the asses and the zebras, though they will form mules with the horse, do not in a state of nature freely breed with it. When an ass or zebra is mated by man with the horse it will produce hybrids, called "Mules," but will not in "a state of nature" _establish_ a hybrid race. The asses and the zebras are distinct from the horse, not only in markings and certain details of shape and hair, but in the fact that they cannot be fused into one race with him. There are no sufficient experiments on the aloofness of zebras and asses from one another in regard to breeding, although it seems that they cannot establish a mixed race, and are, therefore, distinct species judged by that test as well as by their form and marking. It is not known whether the so-called species of wild ass–the Asiatic and the African–would prove to produce fertile or infertile mules if intercrossed, nor has the test been applied to the very differently-marked local races of the African zebras–Grevy's zebra, Burchell's zebra, and the mountain zebra. It is likely enough that the three or more "species" distinguished among zebras on account of their being differently striped, and existing in different localities, would be found to breed freely together, and prove themselves thus to be entitled to be regarded as local "varieties" or "races," but not as fully-separated true species.