But in the
case of an island, or of a country partly surrounded by barriers, into
which new and better adapted forms could not freely enter, we should then
have places in the economy of nature which would assuredly be better filled
up if some of the original inhabitants were in some manner modified; for,
had the area been open to immigration, these same places would have been
seized on by intruders. In such cases, slight modifications, which in any
way favoured the individuals of any species, by better adapting them to
their altered conditions, would tend to be preserved; and natural selection
would have free scope for the work of improvement.
We have good reason to believe, as shown in the first chapter, that changes
in the conditions of life give a tendency to increased variability; and in
the foregoing cases the conditions the changed, and this would manifestly
be favourable to natural selection, by affording a better chance of the
occurrence of profitable variations. Unless such occur, natural selection
can do nothing. Under the term of "variations," it must never be forgotten
that mere individual differences are included. As man can produce a great
result with his domestic animals and plants by adding up in any given
direction individual differences, so could natural selection, but far more
easily from having incomparably longer time for action. Nor do I believe
that any great physical change, as of climate, or any unusual degree of
isolation, to check immigration, is necessary in order that new and
unoccupied places should be left for natural selection to fill up by
improving some of the varying inhabitants.
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