186 
DARWINISM 
CHAP. VII 
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 in¬ 
crease 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 infer¬ 
tility 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 experi¬ 
mental point of view very insufficient, all point to the general 
conclusion we have now reached, and afford us a not unsatis¬ 
factory 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. 
