omacodon) was much smaller than
the hog of to-day, and had strong canine teeth, but in the Oligocene
the family gave rise to a large and numerous race, the Elotheres. These
"giant-pigs," as they have been called, with two toes on each foot,
flourished vigorously for a time in Europe and America, but were
extinguished in the Miocene, when the true pigs made their appearance.
Another doomed race of the time is represented by the Hyopotamus,
an animal between the pig and the hippopotamus; and the Oreodontids,
between the hog and the deer, were another unsuccessful branch of the
early race. The hippopotamus itself was widespread in Europe, and
a familiar form in the rivers of Britain, in the latter part of the
Tertiary.

The camel seems to be traceable to a group of primitive North American
Ungulates (Paebrotherium, etc.) in the later Eocene period. The
Paebrotherium, a small animal about two feet long, is followed by
Pliauchenia, which points toward the llamas and vicunas, and Procamelus,
which clearly foreshadows the true camel. In the Pliocene the one branch
went southward, to develop into the llamas and vicunas, and the other
branch crossed to Asia, to develop into the camels. Since that time they
have had no descendants in North America.

The primitive giraffe appears suddenly in the later Tertiary deposits of
Europe and Asia. The evidence points to an invasion from Africa, and,
as the region of development is unknown and unexplored, the evolution of
the giraffe remains a matter of speculation. Chevrotains flourished in
Europe and North America in the Oligocene, and are still very primitive
in structure, combining features of the hog and the ruminants. Primitive
deer and oxen begin in the Miocene, and seem to have an earlier
representative in certain American animals (Protoceras), of which the
male has a pair of blunt outgrowths between the ears. The first true
deer are hornless (like the primitive muskdeer of Asia to-day), but by
the middle of the Miocene the males have small two-pronged antlers, and
as the period proceeds three or four more prongs are added. It is some
confirmation of the evolutionary embryonic law that we find the antlers
developing in this way in the individual stag to-day. A very
curious race of ruminants in the later Tertiary was a large antelope
(Sivatherium) with four horns. It had not only the dimensions, but
apparently some of the characters, of an elephant.

The elephant itself, the last