At the end of the Chalk period,
some millions of years later, there will be a second revolution,
and it will have a far more enduring and conspicuous result,
though it seem less drastic at the time. Yet there will be
something of a reaction after a time, and at length a third
revolution will inaugurate the age of man. If it is clearly
understood that instead of a century we are contemplating a
period of at least ten million years, and instead of a decade of
revolution we have a change spread over a hundred thousand years
or more, this analogy will serve to convey a most important
truth.
The revolutionary agency that broke into the comparatively even
chronicle of life near the close of the Carboniferous period,
dethroned its older types of organisms, and ushered new types to
the lordship of the earth, was cold. The reader will begin to
understand why I dwelt on the aspect of the Coal-forest and its
surrounding waters. There was, then, a warm, moist earth from
pole to pole, not even temporarily chilled and stiffened by a few
months of winter, and life spread luxuriantly in the perpetual
semi-tropical summer. Then a spell of cold so severe and
protracted grips the earth that glaciers glitter on the flanks of
Indian and Australian hills, and fields of ice spread over what
are now semitropical regions.
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