We are
now at the beginning of a long period of slow erosion and subsidence
which, if this interpretation of the geologic record be correct, will
in the course of time reduce the mountains to plains and submerge
great parts of the lowlands beneath the ocean. As compensation for the
lesser extent of dry land we may look forward to a more genial and
favorable climate in the reduced areas that remain above water.
[Illustration: Fig. 3.--Relative Length of Ages of Reptiles,
Mammals and Man.]
_Length of Geologic Cycles._ But these vast cycles of geographic and
climatic change will take millions of years to accomplish their
course. The brief span of human life, or even the few centuries of
recorded civilization are far too short to show any perceptible change
in climate due to this cause. The utmost stretch of a man's life will
cover perhaps one-two hundred thousandth part of a geologic period.
The time elapsed since the dawn of civilization is less than a
three-thousandth part. Of the days and hours of this geologic year,
our historic records cover but two or three minutes, our individual
lives but a fraction of a second. 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").
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