THE FOSSIL EQUIDÆ.
The living members of the family of Horses are, as we have seen, restricted to the region of the Old World, and were unknown in the Americas and in Australia, when those countries were first discovered. From an examination, however, of their fossil remains, it is evident that in the Pliocene and Pleistocene times Horses were widely distributed in both North and South America. The bones and teeth in caves and river deposits of Europe also show that Wild Horses were very numerous in Europe in the latter age. We are even able to form an accurate idea of the European Wild Horse from the engravings which the ancient hunters of Reindeer have left behind in caves of Auvergne, Switzerland, and Derbyshire. The outline engraved on a bit of bone or a fragment of antler shows us an animal with a large head, thick neck, and big mane, coarse and clumsy in its points, as might be expected from an aboriginal wild breed not subject to the care and selection of man. The Horse, like the Bison and the Reindeer, formed a large part of the food of these ancient men of the caves, and was not domesticated. The true Horses begin to appear in Europe in the later Pliocene strata.
In the early Pliocene and late Miocene ages the family of Horses is represented by the HIPPARION, a small, slender, graceful animal, possessed of three well-defined toes, bearing hoofs, on each limb: one strong and large in the middle, while the two lateral toes are so small that they do not extend beyond the fetlock. They may be compared to dew-claws. The teeth are like those of the Horse, but shorter, and the pattern of the enamel on the grinding surface is more complicated. In the early Miocene and late Eocene the ANCHITHERIUMappears. Its orbit is not so completely encircled with bone as in the Horses and Hipparion.
“The shaft of the ulna,” writes Professor Huxley, “is stouter than in Hipparion, and is less closely united with the radius. The fibula appears—at any rate, in some cases—to have been a complete though slender bone, the distal end of which is still closely united with the tibia, though much more distinct than in the Hipparions and Horses. In some specimens, however, the middle of the shaft seems to have been incompletely ossified. Not only are there three toes in each foot, as in Hipparion, but the inner and the outer toes are so large that they must have rested upon the ground. Thus, so far as the limbs are concerned, the Anchitherium is just such a step beyond the Hipparion as the Hipparion is beyond the Horse, in the direction of a less specialised quadruped. The teeth are still more divergent from the Equine type. The incisors are smaller in proportion, and their crowns lack the peculiar pit which characterises those of Equus and Hipparion. The first grinder is proportionally much larger, especially in the upper jaw, and, like the other six, has a short crown and no thick coat of cement. The pattern of their crowns is wonderfully simplified. The fore and hind-ridges run with but a short obliquity across the crown, and the pillars are little more than enlargements of the ridges, while in the lower jaw these pillars have almost entirely disappeared. But the foremost of the six principal grinders is still somewhat larger than the rest, and the posterior lobe of the last lower molar is small, as in the other Equidæ.”
In all those respects in which Anchitherium departs from the modern Equine type it approaches that of the extinctPalæotheria; and this is so much the case that Cuvier considered the remains of the Anchitherium, with which he was acquainted, to be those of a species of Palæotherium. From these considerations it may be concluded that the highly specialised Horse has obtained its characteristics by descent from the Hipparion, and that again from the Anchitherium. In some cases on record there is a reversion towards the ancestral type, Horses having been born with tridactyle feet, similar in every respect to those of the Hipparion.
The lineage of the Horse is traceable yet further back by the discoveries of Marsh and Cope in New Mexico, Wyoming, and Utah, in North America, up to the Eohippus of the Lower Eocene, a small animal not larger than a Fox, and with three toes on the hind foot and four and a rudiment of a fifth on the fore foot. It must further be noted that the fossil Horses increased in size as they lost their toes, and that the living Horse is the biggest of the family.