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They have yielded fossils in an excellent state of conservation and in unexampled number and variety. The researches of Leidy and others have shown that forms allied to the Hipparion and the Anchitherium are to be found among these remains.
I use the word "type" because it is highly probable that many forms of Anchitherium-like and Hipparion-like animals existed in the Miocene and Pliocene epochs, just as many species of the horse tribe exist now; and it is highly improbable that the particular species of Anchitherium or Hipparion, which happen to have been discovered, should be precisely those which have formed part of the direct line of the horse's pedigree.
From Hipparion some of the authorities believe that the zebras may have been developed. Miohippus, Protohippus, and Hypohippus, varying in size from that of a sheep to that of a donkey, are other branches of this spreading family. In the Pliocene period the evolution of the main stem culminates in the appearance of the horse, and the collateral branches are destroyed.
As to the mere action of surrounding conditions, the same Professor remarks: "Any modification affecting the density of the soil might so far relate to the changes of limb-structure, as that a foot with a pair of small hoofs dangling by the sides of the large one, like those behind the cloven hoof of the ox, would cause the foot of Hipparion, e.g., and a fortiori the broader based three-hoofed foot of the Palæothere, to sink less deeply into swampy soil, and be more easily withdrawn than the more concentratively simplified and specialized foot of the horse.
That the Anchitherium type had become metamorphosed into the Hipparion type, and the latter into the Equine type, in the course of that period of time which is represented by the latter half of the Tertiary deposits, seemed to me to be the only explanation of the facts for which there was even a shadow of probability.
For instance, he supposes that the differences between the extinct three-toed Hipparion and the horse arose suddenly. He thinks it difficult to believe that the wing of a bird "was developed in any other way than by a comparatively sudden modification of a marked and important kind;" and apparently he would extend the same view to the wings of bats and pterodactyles.
Mivart, such as the sudden development of the wings of birds or bats, or the sudden conversion of a Hipparion into a horse, hardly any light is thrown by the belief in abrupt modifications on the deficiency of connecting links in our geological formations. But against the belief in such abrupt changes, embryology enters a strong protest.
The genus Equus is represented as far back as the latter part of the Miocene epoch; but in deposits belonging to the middle of that epoch its place is taken by two other genera, Hipparion and Anchitherium; and, in the lowest Miocene and upper Eocene, only the last genus occurs. A species of Anchitherium was referred by Cuvier to the Paloeotheria under the name of P. aurelianense.
But it is more valuable than the European Hipparion for the reason that it is devoid of some of the peculiarities of that form peculiarities which tend to show that the European Hipparion is rather a member of a collateral branch, than a form in the direct line of succession. Next, in the backward order in time, is the Miohippus, which corresponds pretty nearly with the Anchitherium of Europe.
The hind toes are three, like those of the fore leg; and the middle metatarsal bone is much less compressed from side to side than that of the horse. In the Hipparion the teeth nearly resemble those of the Horses, though the crowns of the grinders are not so long; like those of the Horses, they are abundantly coated with cement.
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