as proceeded more evenly.

The story of life on the earth since the Coal-forest period is
similarly quickened by three revolutions. The first, at the close of
the Carboniferous period, is the subject of this chapter. It is the most
drastic and devastating of the three, but its effect, at least on the
animal world, will be materially checked by a profound and protracted
reaction. 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. In some degree the cold penetrates
the whole earth. The rich forests shrink slowly into thin tracts of
scrubby, poverty-stricken vegetation. The loss of food and the bleak and
exacting conditions of the new earth annihilate thousands of species
of the older organisms, and the more progressive types are moulded into
fitness for the new environment. It is a colossal application of natural
selection, and amongst its results are some of great moment.

In various recent works one reads that earlier geologists, led astray by
the nebular theory of the earth's origin, probably erred very materially
in regard to the climate of primordial times, and that climate has
varied less than used to be supposed. It must not be thought that, in
speaking of a "Permian r