We must not expect to find records of its changing seasons in human history, still less to observe them personally.
[Illustration: Fig.

4 .-- Relative Length of Prehistoric and Historic Time.] There are indeed minor cycles of climate within this great cycle.

The great Ice Age through which the earth has so recently passed was marked by alternations of severity and mildness of climate, of advance and recession of the glaciers, and within these smaller cycles are minor alternations whose effect upon the course of human history has been shown recently by Professor Huntington ("The Pulse of Asia").

But the great cycles of the geologic periods are of a scope far too vast for their changes to be perceptible to us except through their influence upon the course of evolution.
_The Later Cycles of Geologic Time._ The Reptilian Era opens with a period of extreme elevation, which rivalled that of the Glacial Epoch and was similarly accompanied by extensive glaciation of which some traces are preserved to our day in characteristic glacial boulders, ice scratches, and till, imbedded or inter-stratified in the strata of the Permian age.

Between these two extremes of continental emergence, the Permian and the Pleistocene, we can trace six cycles of alternate submergence and elevation, as shown in the diagram (Fig.