, the horse, and the rhinoceros, the
three forms of the Perissodactyl. The second of these types is the
Hyracotherium. It has no distinct equine features, and is known only
from the skull, but the authorities regard it as the progenitor (or
representative of the progenitors) of the horse-types. In size it must
have been something like the rabbit or the hyrax. Still early in the
Eocene, however, we find the remains of a small animal (Eohippus), about
the size of a fox, which is described as "undoubtedly horse-like." It
had only three toes on its hind feet, and four on its front feet; though
it had also a splint-bone, representing the shrunken and discarded fifth
toe, on its fore feet. Another form of the same period (Protorohippus)
shows the central of the three toes on the hind foot much enlarged, and
the lateral toes shrinking. The teeth, and the bones and joints of the
limbs, are also developing in the direction of the horse.

In the succeeding geological period, the Oligocene, we find several
horse-types in which the adaptation of the limbs to running on the firm
grassy plains and of the teeth to eating the grass continues. Mesohippus
has lost the fourth toe of the fore foot, which is now reduced to a
splintbone, and the lateral toes of its hind foot are shrinking. In the
Miocene period there is a great development of the horse-like mammals.
We have the remains of more than forty species, some continuing the main
line of development on the firm and growing prairies of the Miocene,
some branching into the softer meadows or the forests, and giving rise
to types which will not outlive the Tertiary. They have three toes on
each foot, and have generally lost even the rudimentary trace of the
fourth toe. In most of them, moreover, the lateral toes--except in the
marsh-dwelling species, with spreading feet--scarcely touch the ground,
while the central toe is developing a strong hoof. The leg-bones are
longer, and have a new type of joint; the muscles are concentrated near
the body. The front teeth are now chopping incisors, and the grinding
teeth approach those of the modern horse in the distribution of the
enamel, dentine, and cement. They are now about the size of a donkey,
and must have had a distinctly horsy appearance, with their long necks
and heads and tapering limbs. One of them, Merychippus, was probably in
the direct line of the evolution of the horse. From Hipparion some
of the authorities believe that the zebras