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 residt. 



Here the problem was left by Mr. Dar^vin ; but we have 

 shown that its solution may be carried a step further. If we 

 accept the association of some degi^ee of infertility, however 

 slight, as a not unfrequent accompaniment of the external 

 differences Avhich always arise in a state of nature between 

 varieties and incipient species, it has been shown that natural 

 selection luis 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. 



