discovery made a great sensation
among scientists. This little animal, the skeleton of which is no larger
than that of a fox, had four perfect toes, and a fifth splint on the
forefoot, and three toes on the hind foot. The teeth are herbivorous.

_Orohippus_, with a larger skeleton, was found in the middle Eocene
strata of Wyoming. Its feet are like those of its predecessor, except
that the splint is gone. The teeth as well as the feet are more like
those of the modern horse.

_Mesohippus_, the three-toed horse, found in the Miocene, shows the
fourth toe reduced to splints, and the skeleton as big as that of a
sheep. In this the horse family becomes fairly established.

_Hypohippus_, the three-toed forest horse, found in the middle Miocene
strata of Colorado, is a related species, but not a direct ancestor of
the modern horse.

_Neohipparion_, the three-toed desert horse, from the upper Miocene
strata, shows the three toes still present. But the Pliocene rocks
contain fossils showing gradual reduction of the two side toes,
modification of the teeth, and increase in size of the skeleton.

_Protohippus_ and _Pliohippus_, the one-toed species from the Pliocene
strata, illustrate these changes. They were about the size of small
ponies.

_Equus_, the modern horse, was represented in the Pliocene strata by a
species, now extinct, called _Equus Scotti_. This we may regard as the
true wild horse of America, for it was as large as the domesticated
horse, and much like it, though more like a zebra in some respects. No
one can tell why these animals, once abundant in this country, became
extinct at the end of the Tertiary Period. But this is undoubtedly true.

The types described form a series showing how the ancestors of the
modern horse, grazing on the marshy borders of ancient ponds, lived and
died, generation after generation, through a period covering thousands,
possibly millions, of years. Along the sides of the crumbling buttes
these ancient burying-grounds are being uncovered. Within a dozen years
several expeditions, fitted out by the American Museum of Natural
History, have searched the out-cropping strata in Dakota and Wyoming for
bones of mammals known to have lived at the time the strata were forming
in the muddy shallows along the margins of lake and marsh. Duplicate
skeletons of the primitive horse types above have been found, and vast
numbers of their scattered bones. Each summer geological excursions will