The idea that either infertility or geographical isolation is absolutely
essential to the formation of new species, in order to prevent the
swamping effects of intercrossing, has been shown to be unsound, because
the varieties or incipient species will, in most cases, be sufficiently
isolated by having adopted different habits or by frequenting different
stations; while selective association, which is known to be general
among distinct varieties or breeds of the same species, will produce an
effective isolation even when the two forms occupy the same area.
From the various considerations now adverted to, Mr. Darwin arrived at
the conclusion that the sterility or infertility of species with each
other, whether manifested in the difficulty of obtaining first crosses
between them or in the sterility of the hybrids thus obtained, is not a
constant or necessary result of specific difference, but is incidental
on unknown peculiarities of the reproductive system. These peculiarities
constantly tend to arise under changed conditions owing to the extreme
susceptibility of that system, and they are usually correlated with
variations of form or of colour.
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