This corresponds roughly to the Pleistocene
epoch of geology; it is included along with the much shorter time
during which civilization has existed, in the latest and shortest of
the geological periods, the Quaternary. It was the age of the mammoth
and the mastodon, the megatherium and Irish deer and of other
quadrupeds large and small which are now extinct; but most of its
animals were the same species as now exist. It was marked by the great
episode of the Ice Age, when considerable parts of the earth's surface
were buried under immense accumulations of ice, remnants of which are
still with us in the icy covering of Greenland and Antarctica.
_The Age of Mammals._ Before this period was a very much longer
one--at least thirty times as long--during which modern quadrupeds
were slowly evolving from small and primitive ancestors into their
present variety of form and size. This is the Tertiary Period or Age
of Mammals. Through this long period we can trace step by step the
successive stages through which the ancestors of horses, camels,
elephants, rhinoceroses, etc., were gradually converted into their
present form in adaptation to their various habits and environment.
And with them were slowly evolved various kinds of quadrupeds whose
descendants do not now exist, the Titanotheres, Elotheres, Oreodonts,
etc., extinct races which have not survived to our time. Man, as such,
had not yet come into existence, nor are we able to trace any direct
and complete line of ancestry among the fossil species known to us;
but his collateral ancestors were represented by the fossil species
of monkeys and lemurs of the Tertiary period.
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