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.