In point of geological fact, the story of the earth has been one
prolonged series of changes in the level of land and water, and
in their respective limits. These changes have usually been very
gradual, but they have always entailed changes (in climate, etc.
) of the greatest significance in the evolution of life. What was
the swampy soil of England in the Carboniferous period is now
sometimes thousands of feet beneath us; and what was the floor of
a deep ocean over much of Europe and Asia at another time is now
to be found on the slopes of lofty Alps, or 20,000 feet above the
sea-level in Thibet. Our story of terrestrial life will be, to a
great extent, the story of how animals and plants changed their
structure in the long series of changes which this endless battle
of land and sea brought over the face of the earth.
As we have no recognisable remains of the animals and plants of
the earliest age, we will not linger over the Archaean rocks.
Starting from deep and obscure masses of volcanic matter, the
geologist, as he travels up the series of Archaean rocks, can
trace only a dim and most unsatisfactory picture of those remote
times.
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