J890 PHYSIOLOGICAL SELECTION 173 



distinguished from linear transmutation, impossible. 

 This last difficulty can only be met by assuming that 

 the same beneficial variation arises in a number of 

 individuals simultaneously, for which assumption 

 our present knowledge furnishes no warrant. If 

 natural selection is brought forward as the sole factor 

 in the guidance of organic evolution, then he con- 

 sidered that these difficulties remain insurmount- 

 able ; if, however, it is regarded as a factor, even the 

 chief factor, then these difficulties vanish, it being 

 consistent, in the latter case, to hold the other 

 factor, or factors, responsible for an explanation of 

 the difficulties 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 supplementary to it, and was at the same time 

 capable of explaining the facts, of the inutility of 

 many specific characters, the cross-infertility of allied 

 species, and the non-occurrence of free intercrossing. 

 Very briefly indicated, Mr. Romanes' line 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 with 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 



