Is it possible to say which of the wild horses of prehistoric times contributed to the making of the Shetland pony? To be in a position to hazard an answer to this question it will be necessary to refer to the more important links which are believed to connect modern breeds with Hyracotherium, generally regarded as the remote polydactylous ancestor of the horse family.
Hyracotherium lived in the south-eastern part of England—his remains occur in the London Clay near Herne Bay and in the Red Crag of Suffolk—i.e., in deposits formed two or three or it may be six million years ago. This ancient fossil horse—though not unlike a long-faced fox terrier—Owen regarded as a relation of the Hyrax or Coney of Palestine, hence the name Hyracotherium. Evolved in Western Europe—perhaps in England—this primitive small-brained terrier-on-hoofs not only wandered across Europe and Asia, but actually crossed into America—then freely connected with north-eastern Asia—and ranged at least as far south as New Mexico. So conservative was this small forerunner of the great horse family that the American representative from the Wasatch deposits only differs from the parent form in having slightly more complex teeth. The fore and hind foot of the American variety of Hyracotherium—generally known as Eohippus—is represented in fig. 10, and fig. 12 gives a restoration of Eohippus.
Though at the beginning of the Eocene period the milk-givers were backward and small (Eohippus was probably only 12 inches at the withers), the struggle for existence was probably keen enough. At any rate, the European varieties of Hyracotherium either died off without leaving descendants or gave rise to odd-toed ungulates which took no part in forming the modern Equidæ. But for the American branch of the Hyracotherium family there would have been no horses. In course of time Eohippus was supplanted by Protorohippus (a 14-inch horse longer in the limbs and with more complex teeth), which towards the close of the Eocene period gave place to the still more specialised Orohippus. It may be mentioned that Epihippus, a slightly modified descendant of Orohippus, may have lived side by side with the remote ancestor of the camels—a quaint even-toed ungulate about the size of a “jack-rabbit.”
The limbs of Orohippus[F10] are represented in fig. 11, and a restoration is given in fig. 13.
[F10] In the Yale collection there are five species of Orohippus from New Mexico and Wyoming.
[F11] R. S. Lull, ‘American Journ. of Science.’ 1907.
North America during Eocene times “was clad with forests in which grew both evergreen and deciduous trees distinctly modern in character. The moist climate gave rise to many streams and lakes, along the shores of which grew sedgy meadows that in turn gave rise to grassy plains.”[F11] In the following (Oligocene) period similar conditions for a time prevailed, but later, owing to the increasing aridity, broad meadow-lands and prairies made their appearance. The new environment produced larger and more active flesh-eaters, fleeter and more intelligent horses. One of the new and improved species is Mesohippus bairdi, an 18-inch horse with only a splint-like metacarpal representing the outer or fifth digit—a digit complete in all the Eocene horses. In this as in all the other Oligocene horses three of the four premolars, as in the recent Equidæ, resembled molars. Small and slender-limbed, Mesohippus bairdi was adapted for living in the open, but a larger species (Mesohippus intermedius) might be described as a “forest” horse,—though only 24 inches high, this forest-dwelling form had as long cannon-bones as a 33-inch Shetland pony of the “forest” or cart-horse type. Another American Oligocene type (Miohippus) from Oregon deserves mention, not so much because it was more specialised, but because it had a representative (probably a descendant) in Europe known as Anchitherium, which in Miocene times ranged from France to Bavaria. In this European species the last vestiges of the first and fifth digits had apparently disappeared. The fore limb of Mesohippus is represented in fig. 18, and a restoration is given in fig. 14.
It is impossible to say how many thousands of years are represented by the Eocene and Oligocene deposits, but an idea of the time that has elapsed since the beginning of the Tertiary period will be gained if it is mentioned that “when the fox-like Hyracotherium was wandering on the marshes of Kent not only was the Himalaya non-existent, but that along the line of its very heart—where the kiang now lives at an elevation of from thirteen thousand to sixteen thousand feet—extended an arm of the sea of no inconsiderable depth.”[F12]
[F12] Lydekker, ‘The Horse and its Relations,’ p. 242.