But this question of
the swamping effects of intercrossing will be considered in another
chapter.
Mr. Darwin was of opinion that, on the whole, the largeness of the area
occupied by a species was of more importance than isolation, as a factor
in the production of new species, and in this I quite agree with him. It
must, too, be remembered, that isolation will often be produced in a
continuous area whenever a species becomes modified in accordance with
varied conditions or diverging habits. For example, a wide-ranging
species may in the northern and colder part of its area become modified
in one direction, and in the southern part in another direction; and
though for a long time an intermediate form may continue to exist in the
intervening area, this will be likely soon to die out, both because its
numbers will be small, and it will be more or less pressed upon in
varying seasons by the modified varieties, each better able to endure
extremes of climate. So, when one portion of a terrestrial species takes
to a more arboreal or to a more aquatic mode of life, the change of
habit itself leads to the isolation of each portion.
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