structure of the foot. The
toes of the hind foot, which are made use of in progression, are even in
number in one sub-order and are odd-numbered in the other sub-order.

The sub-order of odd-toed ungulates, or _Perissodactyla_, includes in
our day only the horses, asses, zebras, and quaggas (united together in
the family _Equidae_); the tapirs, the rhinoceroses, and the little
hyrax--the coney of Scripture. In ancient times, however, this sub-order
was a very large one, but the great majority of the forms belonging to
it, which formerly lived, have now become extinct.

The sub-order of even-toed ungulates, or _Artiodactyla_, comprises all
oxen, sheep, goats, antelopes, giraffes, deer, chevrotains,[10] llamas,
and camels. All these, from their practice of "chewing the cud," are
called "ruminants," and they are multitudinous in kinds. The great
plains of Southern Africa are the special home of most kinds of
antelope, and the giraffe is exclusively African. Deer have their
head-quarters in Asia, though they exist in South America as well as
throughout the Northern Hemisphere.

Besides the ruminating artiodactyles there is also an extensive group of
non-ruminating artiodactyles, made up of all the various kinds of swine
(including the American peccaries), together with the hippopotamus, now
found nowhere but in Africa. Distinct as are the ruminating and
non-ruminating artiodactyles now, they were in ancient time connected by
a great number of intermediate forms which have utterly passed away.

The llamas of South America represent the camels of the Old World, where
the latter are to-day exclusively found. When South America was
discovered by the Spaniards, llamas were the only beasts of burthen
found there, and, indeed, the only cattle of any kind then and there
existing; although horses had formerly abounded and had become extinct
in South America at a long anterior period.

Somewhat allied to ungulates, but distinct from them, are the elephants,
which form an order (_Proboscidea_) by themselves--an order once rich in
many species widely distributed over the earth.

Hardly less familiar than our domestic animals, are our hares, rabbits,
mice, squirrels, and their allies, which together form an "order" called
_Rodentia_ from the gnawing habits of its members which nourish
themselves on vegetable substances. This order of rodents is very rich
in species, and consists of many genera grouped in several distinct
families-