It was once
thought that the earth was really shaken at times by vast and
sudden cataclysms, which destroyed its entire living population,
so that new kingdoms of plants and animals had to be created. But
we have interpreted the word revolution in a very different
sense. The series of changes and disturbances to which we give
the name extended over a period of hundreds of thousands of
years, and they were themselves, in some sense, the creators of
new types of organisms. Yet they are periods that stand out
peculiarly in the comparatively even chronicle of the earth. The
Permian period transformed the face of the earth; it lifted the
low-lying land into a massive relief, drew mantles of ice over
millions of miles of its surface, set volcanoes belching out fire
and fumes in many parts, stripped it of its great forests, and
slew the overwhelming majority of its animals. On the scale of
geological time it may be called a revolution.
It must be confessed that the series of disturbances which close
the Secondary and inaugurate the Tertiary Era cannot so
conveniently be summed up in a single formula. They begin long
before the end of the Mesozoic, and they continue far into the
Tertiary, with intervals of ease and tranquillity.
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