Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the
organic and inorganic conditions of life will generally be in a great
degree uniform; so that natural selection will tend to modify all the
individuals of a varying species throughout the area in the same
manner in relation to the same conditions. Intercrosses, also, with
the individuals of the same species, which otherwise would have
inhabited the surrounding and differently circumstanced districts,
will be prevented. But isolation probably acts more efficiently in
checking the immigration of better adapted organisms, after any
physical change, such as of climate or elevation of the land, etc.;
and thus new places in the natural economy of the country are left
open for the old inhabitants to struggle for, and become adapted to,
through modifications in their structure and constitution. Lastly,
isolation, by checking immigration and consequently competition, will
give time for any new variety to be slowly improved; and this may
sometimes be of importance in the production of new species. If,
however, an isolated area be very small, either from being surrounded
by barriers, or from having very peculiar physical conditions, the
total number of the individuals supported on it will necessarily be
very small; and fewness of individuals will greatly retard the
production of new species through natural selection, by decreasing the
chance of the appearance of favourable variations.
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