Second Principle of Evolution
And the second guiding principle I wish to lay down in advance is
that these great changes in the face of the earth, which explain
the progress of organisms, may very largely be reduced to one
simple agency--the battle of the land and the sea. When you gaze
at some line of cliffs that is being eaten away by the waves, or
reflect on the material carried out to sea by the flooded river,
you are--paradoxical as it may seem--beholding a material process
that has had a profound influence on the development of life. The
Archaean continent that we described was being reduced constantly
by the wash of rain, the scouring of rivers, and the fretting of
the waves on the coast. It is generally thought that these
wearing agencies were more violent in early times, but that is
disputed, and we will not build on it. In any case, in the course
of time millions of tons of matter were scraped off the Archaean
continent and laid on the floor of the sea by its rivers. This
meant a very serious alteration of pressure or weight on the
surface of the globe, and was bound to entail a reaction or
restoration of the balance.
The rise of the land and formation of mountains used to be
ascribed mainly to the cooling and shrinking of the globe of the
earth. The skin (crust), it was thought, would become too large
for the globe as it shrank, and would wrinkle outwards, or pucker
up into mountain-chains. The position of our greater
mountain-chains sprawling across half the earth (the Pyrenees to
the Himalaya, and the Rocky Mountains to the Andes), seems to
confirm this, but the question of the interior of the earth is
obscure and disputed, and geologists generally conceive the rise
of land and formation of mountains in a different way. They are
due probably to the alteration of pressure on the crust in
combination with the instability of the interior. The floors of
the seas would sink still lower under their colossal burdens, and
this would cause some draining of the land-surface. At the same
time the heavy pressure below the seas and the lessening of
pressure over the land would provoke a reaction. Enormous masses
of rock would be forced toward and underneath the land-surface,
bending, crumpling, and upheaving it as if its crust were but a
leather coat. As a result, masses of land would slowly rise above
the plain, to be shaped into hills and valleys by the hand of
later time, and fresh surfaces would be dragged out of the deep,
enlarging the fringes of the primitive continents, to be warped
and crumpled in their turn at the next era of pressure.
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.
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