174 GEOEGE JOHN EOMANES issi- 



into new species. This supposition is borne out by 

 the nature of the flora and fauna of oceanic islands, 

 which are particularly rich in peculiar species, and 

 where intercrossing was, of course, prevented with 

 the original parent forms by the action of the geo- 

 graphical boundaries. 



However, closely allied species are not always, or 

 even generally, separated by geographical boundaries, 

 and the cross-infertility remains to be explained. 

 The cardinal feature of Mr. Romanes' theory is that 

 the initial step in the origin of species is the arising 

 of this infertility as an independent variation, by 

 which free intercrossing with the parent form on a 

 common area is prevented, and specific differentiation 

 rendered possible. Innumerable varieties are known 

 to occur which do not pass into distinct species, 

 the reason being that this initial variation, that is, 

 incipient infertility whereby the swamping effects of 

 intercrossing might be obviated, was lacking, and the 

 variations became re-absorbed. That is, given any 

 degree of sterility towards the parental form which 

 does not extend to the varietal form, then a new 

 species must take its origin. Without the bar of 

 sterility, in Mr. Romanes' opinion, free intercrossing 

 must render the formation of species impossible. 

 Mutual sterility is thus the cause, not the result, of 

 specific differentiation. As regards the occurrence of 

 this initial variation, the reproductive system is known 

 to be highly variable, its variability taking the form 

 either of increased fertility, or of sterility in all degrees, 

 and depending on either extrinsic causes (changes of 

 food, climate, &c.), or on an intrinsic cause arising in 

 the system itself. 



From the nature of this additional factor at work 



