The Quagga was more striped than is sometimes represented in illustrations. According to Dr. Noack, from whose paper[[153]] upon the animal I quote here, the transverse stripes reached back as far as the buttocks; they were, however, completely absent from the legs. The animal is, as every one knows, probably completely extinct. In the year 1836 it was still abundant; in 1864 the last specimen ever exhibited was received by the Zoological Society. Mr. W. L. Sclater thinks that it may have survived in the Orange River Colony as late as 1878, but admits that any certainty is difficult, as it was frequently confounded by the Boers with Burchell's Zebra. Its rarity is emphasised by the fact that it is not mentioned in the recent work of that most skilful of hunters, Mr. F. Selous. Gaudry places the Quagga nearest of all living Equidae to the Hipparion gracile of Pikermi.
Fossil Equidae.—The existing Equidae all belong to the genus Equus, though there are some who would (quite unnecessarily) divide off the Zebras as a genus Hippotigris. The genus Equus itself goes back in time to the Pliocene, during which epoch there lived in India E. sivalensis, the same species according to some with the E. stenonis of Europe. None of these species, Old World or New, are easily to be separated from E. caballus. But many names have been given to them. It is of course perfectly conceivable that they may have differed among themselves as much as do the existing Zebras and Asses, the separation of which would be hardly possible did we know their bones only. There are, however, extinct genera, undoubtedly related so closely to Equus as to be placed in the same family, though clearly separable as genera. Hipparion is one of these genera; its remains are known from Europe, Asia, and North Africa, from beds of Miocene and Pliocene times. A large number of different species have been described. It was a beast of about the size of a Zebra. The principal characters are that each foot has three toes, of which, however, the two side ones are smaller than the central toe. There is a marked round fossa on the maxillary bone, a feature shared by the South American Onohippidium.[[154]] The pattern of
the molar teeth is, too, a little different from that of Equus. Protohippus of the North American Pliocene is also three-toed, but the two additionally-developed toes are smaller than in Hipparion. Other forms are dealt with below in connexion with the ancestry of Perissodactyles. It is a curious fact about Hipparion, which is not now regarded as on the direct line of equine descent, that the edges of the enamel plates of the molars may show a complicated folding very like that presented by that clearly terminal form of Perissodactyle life, the gigantic Elasmotherium. This is indicative of high specialisation, which ended in extinction.
Ancestry of the Horses.—The Lophiodontidae and the Palaeotheriidae are two of the most interesting extinct families of Perissodactyles; for among them we find what would appear to be the ancestral forms of both the existing Tapirs and Horses. The Rhinoceroses also would seem to be derivable from the Palaeotheriidae. The very vagueness of the characters of these creatures, considered from a classificatory point of view, has led to much diversity in their placing. This though gratifying to the evolutionist is tiresome to the writer who wishes to give a methodical account of their various characters. It will be best perhaps not to attempt an accurate placing or to reconcile conflicting opinion, but to give some salient features of osteology which lead to the belief in their relationship to existing groups of Perissodactyles. A book upon the history of mammals would be incomplete without some account of that well-ascertained series of forms which seem to connect these primitive Perissodactyles with the modern Horse. Equus, in fact, is not only the "show horse" of the doctrine of evolution, but also the "stalking horse."
In the Eocene of both Europe and America are met with a number of forms from which we may start. Hyracotherium, regarded on the one hand as the type of a sub-family of the Equidae themselves, and on the other as a member of the family Lophiodontidae, was a small-sized animal, three feet or so in length; it possesses the complete Eutherian dentition with a slight diastema. The orbits are not separated from the temporal fossa; the fore-limbs were four-toed, the hind three-toed, with moderately long metapodia, especially on the hind-feet. The shoulder blade
has a well-marked coracoid process. The radius and ulna are separate; so too are the tibia and fibula. Eohippus, belonging to the same sub-family, is slightly more primitive; for the hind-feet have a rudiment of digit I. Orohippus is a little nearer to the Horses in that the molar teeth have acquired a little further advance towards the equine type. Instead of the tubercles of the teeth remaining for the most part separate, they have fused into a set of ridges, of which, however, the pattern is less complex than in the modern Horses. In other respects Orohippus is much like Hyracotherium. Pachynolophus seems to be but a synonym.
The next stage is shown by Mesohippus, a Lower Miocene form, usually referred to the neighbourhood of Palaeotherium. It has nearly lost one of the toes of the fore-foot, a rudiment only remaining; the metapodials, at any rate of the fore-feet, seem to be slightly increased in length. The orbit is not encircled by bone, but there is a strong process from the frontal, which nearly meets the zygomatic arch.
Anchitherium, from the Upper Miocene, is not far removed in structure from the last-mentioned form; it is a trifle nearer the existing Horse in several points. The ulna is further reduced and fused with the radius below: the rudiment of digit V is still more rudimentary; the two lateral digits are smaller in proportion to the central one than they are in Mesohippus; the fibula is fused below with the tibia. From this form to Equus is a small series of steps, characterised by the still further reduction of all the digits except III, by the still further reduction of the already rudimentary ulna and fibula, and by the increasing depth of the molar teeth, which are of course, in Equus, hypselodont.
Another interesting conclusion may seem to follow when we consider the geographical range of the ancestral Horses. Hyracotherium and Pachynolophus occurred both in the Old and New World. From them may have arisen the Horses of both hemispheres. After that point there is a division. Mesohippus is American, and we get at Equus in that continent through Desmatippus and Protohippus. On the other hand there are no remains known of Mesohippus in Europe; and unless subsequent researches prove the existence of Mesohippus, we have to rely upon forms which are placed with Anchitherium and Hipparion.
It seems that in America the next genus in the direct line of equine descent to Mesohippus is Miohippus. It is smaller in