e Aceratherium is supposed to have migrated in early Miocene
times, when a mild climate and luxuriant vegetation prevailed far within
the arctic circle, it gave rise to the Ceratorhinus and the various
horned rhinoceroses of late Tertiary times and of those now living. In
America a number of large hornless rhinoceroses were developed--they
are found in the Upper Miocene, Pliocene, and Post-Pliocene
formations--and then became extinct. The true rhinoceroses have three
toes on all the feet.[185]

_The Pedigree of the Horse Tribe._

Yet more remarkable is the evidence afforded by the ancestral forms of
the horse tribe which have been discovered in the American tertiaries.
The family Equidae, comprising the living horse, asses, and zebras,
differ widely from all other mammals in the peculiar structure of the
feet, all of which terminate in a single large toe forming the hoof.
They have forty teeth, the molars being formed of hard and soft material
in crescentic folds, so as to be a powerful agent in grinding up hard
grasses and other vegetable food. The former peculiarities depend upon
modifications of the skeleton, which have been thus described by
Professor Huxley:--

"Let us turn in the first place to the fore-limb. In most
quadrupeds, as in ourselves, the fore-arm contains distinct
bones, called the radius and the ulna. The corresponding region
in the horse seems at first to possess but one bone. Careful
observation, however, enables us to distinguish in this bone a
part which clearly answers to the upper end of the ulna. This is
closely united with the chief mass of the bone which represents
the radius, and runs out into a slender shaft, which may be
traced for some distance downwards upon the back of the radius,
and then in most cases thins out and vanishes. It takes still
more trouble to make sure of what is nevertheless the fact, that
a small part of the lower end of the bone of a horse's fore-arm,
which is only distinct in a very young foal, is really the lower
extremity of the ulna.

"What is commonly called the knee of a horse is its wrist. The
'cannon bone' answers to the middle bone of the five metacarpal
bones which support the palm of the hand in ourselves. The
pastern, coronary, and coffin bones of veterinarians answer to
the joints of our middle fingers, while the hoof is simply a
greatly enlarged and thickened nail. Bu