1890 PHYSIOLOGICAL SELECTION 165 



factor, or factors, responsible for an explanation of 

 the difl&culties in question. It was the object of 

 this paper to suggest another factor in the formation 

 of species, which, although independent of natural 

 selection, was in no way opposed to it, and might be 

 called supplevientary to it, and was at the same time 

 capable of explaining the facts, of the inutility of 

 vianij specific characters, the cross-infertility of allied 

 species, and the non-occurrence of free intercrossing. 

 Very briefly indicated, Mr. Eomanes' hne of argument 

 is as follows : — Every generation of every species 

 presents an enormous number of variations, of which 

 only the ones that happen to be useful are preserved 

 by natural selection. The useless variations are 

 allowed to die out immediately by intercrossing. 



Consequently, if intercrossing be prevented, there 

 is no reason why unuseful variations should not be 

 perpetuated by heredity quite as much as useful ones 

 when under the nursing influence of natural selection. 

 Thus, if from any cause, a section of a species is 

 prevented from intercrossing Tsdth the rest of its 

 parent form, it is to be expected that new varieties — : 

 for the most part of a trivial and unuseful kind — 

 should arise within that section, and in time pass 

 into new species. This supposition is borne out by 

 the nature of the flora and fauna of oceanic islands, 

 which are particularly rich in pecuhar species, and 

 where intercrossing was, of course, prevented with 

 the original parent forms by the action of the 

 geographical 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. Eomanes' 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 



