The far
greater complexity of the environment, together with the occurrence of
variations in constitution and habits, will often allow of effective
isolation, even here, producing all the results of actual physical
isolation. As we have already explained, one of the most frequent modes
in which natural selection acts is, by adapting some individuals of a
species to a somewhat different mode of life, whereby they are able to
seize upon unappropriated places in nature, and in so doing they become
practically isolated from their parent form. Let us suppose, for
example, that one portion of a species usually living in forests ranges
into the open plains, and finding abundance of food remains there
permanently. So long as the struggle for existence is not exceptionally
severe, these two portions of the species may remain almost unchanged;
but suppose some fresh enemies are attracted to the plains by the
presence of these new immigrants, then variation and natural selection
would lead to the preservation of those individuals best able to cope
with the difficulty, and thus the open country form would become
modified into a marked variety or into a distinct species; and there
would evidently be little chance of this modification being checked by
intercrossing with the parent form which remained in the forest.
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