f the same
species which when crossed are infertile or even sterile.

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. Hence, as fixed differences of form and
colour, slowly gained by natural selection in adaptation to changed
conditions, are what essentially characterise distinct species, some
amount of infertility between species is the usual result.

Here the problem was left by Mr. Darwin; but we have shown that its
solution may be carried a step further. If we accept the association of
some degree of infertility, however slight, as a not unfrequent
accompaniment of the external differences which always arise in a state
of nature between varieties and incipient species, it has been shown
that natural selection _has_ power to increase that infertility just as
it has power to increase other favourable variations. Such an increase
of infertility will be beneficial, whenever new species arise in the
same area with the parent form; and we thus see how, out of the
fluctuating and very unequal amounts of infertility correlated with
physical variations, there may have arisen that larger and more constant
amount which appears usually to characterise well-marked species.

The great body of facts of which a condensed account has been given in
the present chapter, although from an experimental point of view very
insufficient, all p