Imagine some early continent with its population of animals and
plants. Each bay, estuary, river, and lake, each forest and marsh
and solid plain, has its distinctive inhabitants. Imagine this
continent slowly sinking into the sea, until the advancing arms
of the salt water meet across it, mingling their diverse
populations in a common world, making the fresh-water lake
brackish or salt, turning the dry land into swamp, and flooding
the forest. Or suppose, on the other hand, that the land rises,
the marsh is drained, the genial climate succeeded by an icy
cold, the luscious vegetation destroyed, the whole animal
population compelled to change its habits and its food. But this
is no imaginary picture. It is the actual story of the earth
during millions of years, and it is chiefly in the light of these
vast and exacting changes in the environment that we are going to
survey the panorama of the advance of terrestrial life.
For the moment it will be enough to state two leading principles.
The first is that there is no such thing as a "law of evolution"
in the sense in which many people understand that phrase. It is
now sufficiently well known that, when science speaks of a law,
it does not mean that there is some rule that things MUST act in
such and such a way.
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