genus Eohippus, a little creature
not much larger than a cat, which has a short neck, relatively short
limbs, and in particular, short feet, with four functional digits and
a splint-like rudiment in the fore-foot, three functional digits and
a rudiment in the hind-foot. The forearm bones (ulna and radius) are
complete and separate, as are also the bones of the lower leg (fibula
and tibia). The skull has a short face, with the orbit, or eye-socket,
incompletely enclosed with bone, and the brain-case is slender and
of small capacity. The teeth are short-crowned, the incisors without
"mark," or enamel pit, on the cutting edge; the premolars are all
smaller and simpler than the molars. The pattern of the upper molars is
so entirely different from that seen in the modern horses that, without
the intermediate connecting steps, no one would have ventured to derive
the later from the earlier plan. This pattern is quadritubercular, with
four principal, conical cusps arranged in two transverse pairs, forming
a square, and two minute cuspules between each transverse pair, a tooth
which is much more pig-like than horse-like. In the lower molars the
cusps have already united to form two crescents, one behind the other,
forming a pattern which is extremely common in the early representatives
of many different families, both of the Perissodactyla and the
Artiodactyla. In spite of the manifold differences in all parts of the
skeleton between Eohippus and the recent horses, the former has stamped
upon it an equine character which is unmistakable, though it can hardly
be expressed in words.

Each one of the different Eocene and Oligocene horizons has its
characteristic genus of horses, showing a slow, steady progress in a
definite direction, all parts of the structure participating in the
advance. It is not necessary to follow each of these successive steps
of change, but it should be emphasised that the changes are gradual and
uninterrupted. The genus Mesohippus, of the middle Oligocene, may be
selected as a kind of half-way stage in the long progression. Comparing
Mesohippus with Eohippus, we observe that the former is much larger,
some species attaining the size of a sheep, and has a relatively longer
neck, longer limbs and much more elongate feet, which are tridactyl, and
the middle toe is so enlarged that it bears most of the weight, while
the lateral digits are very much more slender. The fore-arm bones have
begun to co-ossify and