a. In the early
Eocene we find the Condylarthra (such as Phenacodus) with flat five-toed
feet, and such a mixed combination of characters that they "might serve
very well for the ancestors of all the later Ungulata" (Woodward).
We then presently find this generalised Ungulate branching into three
types, one of which seems to be a patriarchal tapir, the second
is regarded as a very remote ancestor of the horse, and the third
foreshadows the rhinoceros. The feet have now only three or four toes;
one or two of the side-toes have disappeared. This evolution, however,
follows two distinct lines. In one group of these primitive Ungulates
the main axis of the limb, or the stress of the weight, passes through
the middle toe. This group becomes the Perissodactyla ("odd-toed"
Ungulates) of the zoologist, throwing out side-branches in the tapir
and the rhinoceros, and culminating in the one-toed horse. In the other
line, the Artiodactyla (the "even-toed" or cloven-hoofed Ungulates), the
main axis or stress passes between the third and fourth toes, and the
group branches into our deer, oxen, sheep, pigs, camels, giraffes, and
hippopotamuses. The elephant has developed along a separate and very
distinctive line, as we shall see, and the hyrax is a primitive survivor
of the ancestral group.

Thus the evolutionist is able to trace a very natural order in the
immense variety of our Ungulates. He can follow them in theory as they
slowly evolve from their primitive Eocene ancestor according to their
various habits and environments; he has a very rich collection of fossil
remains illustrating the stages of their development; and in the hyrax
(or "coney") he has one more of those living fossils, or primitive
survivors, which still fairly preserve the ancestral form. The hyrax has
four toes on the front foot and three on the hind foot, and the feet are
flat. Its front teeth resemble those of a rodent, and its molars
those of the rhinoceros. In many respects it is a most primitive and
generalised little animal, preserving the ancestral form more or less
faithfully since Tertiary days in the shelter of the African Continent.

The rest of the Ungulates continued to develop through the Tertiary, and
fortunately we are enabled to follow the development of two of the most
interesting of them, the horse and the elephant, in considerable detail.
As I said above, the primitive Ungulate soon branches into three types
which dimly foreshadow the tapir