The beech is a tree essentially of the temperate zone, having
its northern limit considerably southward of the oak, fir, birch, or
aspen, and its entrance into Denmark was no doubt due to the
amelioration of the climate after the glacial epoch had entirely passed
away. We thus see how changes of climate, which are continually
occurring owing either to cosmical or geographical causes, may initiate
a struggle among plants which may continue for thousands of years, and
which must profoundly modify the relations of the animal world, since
the very existence of innumerable insects, and even of many birds and
mammals, is dependent more or less completely on certain species of
plants.
_The Struggle for Existence on the Pampas_.
Another illustration of the struggle for existence, in which both plants
and animals are implicated, is afforded by the pampas of the southern
part of South America. The absence of trees from these vast plains has
been imputed by Mr. Darwin to the supposed inability of the tropical and
sub-tropical forms of South America to thrive on them, and there being
no other source from which they could obtain a supply; and that
explanation was adopted by such eminent botanists as Mr.
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