Every geologist, indeed, admits "oscillations of
climate," as Professor Chamberlin puts it. But amidst all these
oscillations we trace a steady lowering of the temperature.
Unless we put a strained and somewhat arbitrary interpretation on
the facts of the geological record, earlier ages knew nothing of
our division of the year into pronounced seasons and of the globe
into very different climatic zones. It might plausibly be
suggested that we are still living in the last days of the
Ice-Age, and that the earth may be slowly returning to a warmer
condition. Shackleton, it might be observed, found that there has
been a considerable shrinkage of the south polar ice within the
period of exploration. But we shall find that a difference of
climate, as compared with earlier ages, was already evident in
the middle of the Tertiary Era, and it is far more noticeable
to-day.
We do not know the causes of this climatic evolution-- the point
will be considered more closely in connection with the last
Ice-Age--but we see that it throws a flood of light on the
evolution of organisms. It is one of the chief incarnations of
natural selection. Changes in the distribution of land and water
and in the nature of the land-surface, the coming of powerful
carnivores, and other agencies which we have seen, have had their
share in the onward impulsion of life, but the most drastic
agency seems to have been the supervention of cold.
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