ve and breed the next generation. If the change in the environment
(in the food or climate, for instance) is prolonged and increased for
hundreds of thousands of years, we shall expect to find a corresponding
change in the animals and plants.

We shall find such changes occurring throughout the story of the earth.
At one important point in the story we shall find so grave a revolution
in the face of nature that twenty-nine out of every thirty species of
animals and plants on the earth are annihilated. Less destructive and
extreme changes have been taking place during nearly the whole of the
period we have to cover, entailing a more gradual alteration of the
structure of animals and plants; but we shall repeatedly find them
culminating in very great changes of climate, or of the distribution of
land and water, which have subjected the living population of the earth
to the most searching tests and promoted every variation toward a more
effective organisation. [*]

* This is a very simple expression of "Darwinism," and will
be enlarged later. The reader should ignore the occasional
statement of non-scientific writers that Darwinism is "dead"
or superseded. The questions which are actually in dispute
relate to the causes of the variation of the young from
their parents, the magnitude of these variations' and the
transmission of changes acquired by an animal during its own
life. We shall see this more fully at a later stage. The
importance of the environment as I have described it, is
admitted by all schools.

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 wer