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 point to the general conclusion we have now reached, and afford us a not unsatisfactory solution of the great problem of hybridism in relation to the origin of species by means of natural selection.

Further experimental research is needed in order to complete the elucidation of the subject; but until these additional facts are forthcoming no new theory seems required for the explanation of the phenomena.
FOOTNOTES: [Footnote 51: Darwin's _Animals and Plants under Domestication_, vol.
ii.pp.

163-170.] [Footnote 52: For a full account of these interesting facts and of the various problems to which they give rise, the reader must consult Darwin's volume on _The Different Forms of Flowers in Plants of the same Species_, chaps, i.-iv.] [Footnote 53: See _Nature_, vol.xxi.p.