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of non-scientific writers that Darwinism is "dead" or superseded.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.
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