Radiolaria, Sponges, Corals, Sharks, Mudfishes,
Duckbills, etc., do not change (except within the limits of their
family) during millions of years, because they keep to an
environment to which they are fitted. On the other hand, certain
fishes, reptiles, etc., remain in a changing environment, and
they must change with it. The process has its obscurities, but we
make them darker, it seems to me, with these semi-metaphysical
phrases.
It has seemed advisable to take this further glance at the
general principles and current theories of evolution before we
extend our own procedure into the Tertiary Era. The highest types
of animals and plants are now about to appear on the stage of the
earth; the theatre itself is about to take on a modern
complexion. The Middle Ages are over; the new age is breaking
upon the planet. We will, as before, first survey the Tertiary
Era as a whole, with the momentous changes it introduces, and
then examine, in separate chapters, the more important phases of
its life.
It opens, like the preceding and the following era, with "the
area of land large and its relief pronounced." This is the
outcome of the Cretaceous revolution.
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