MB. G. J. ROMANES ON PHYSIOLOGICAL SELECTION. 379 



geograpMcal barriers or by migration, it is more easy to prove 

 the evolution of new species as a consequence than it is when in- 

 tercrossing has been prevented by physiological barriers ; for in 

 the latter case the older and the newer forms will probably continue 

 to occupy the same area, and thus there will be no independent 

 evidence to show that the severance between them was due to the 

 prevention of intercrossing. Nevertheless, all the evidence which 

 I have of the large part that geographical barriers and migration 

 have played in the evolution of species by the prevention of inter- 

 crossing -with parent forms, goes to show the probable importance 

 of physiological barriers when acting in the same way. Hence it 

 will be better to postpone this line of argument till the appearance 

 of my next paper, where I shall hope to show, from evidence fur- 

 nished by the geographical distribution of species, how predomi- 

 nant a part the prevention of intercrossing has played in the 

 evolution of species. Here, therefore, it will be enough to offer 

 a few general remarks. 



In the first place, the theory of physiological selection has this 

 great advantage over the theory of natural selection, namely, that 

 the swamping effects of free intercrossing on the new variety — 

 or on the incipient species — are supposed to be from the first 

 excluded by the very fact of the variation itself. This is so 

 obvious an advantage that it appears needless to dwell upon its 

 consideration. 



But, in the next place, I- may observe that, in so many cases as 

 species do originate by physiological selection, the subsequent 

 influence of natural selection admits of being considerably en- 

 hanced. 'For when once this physiological separation between a 

 variety and its parent-stock has been effected, there will be less 

 likelihood than before of any useful variations which may subse- 

 quently arise in the former being again obliterated by intercrossing. 

 This is evident, because the possibilities of intercrossing would 

 now be restricted to a much smaller number of individuals, and 

 therefore the influence of intercrossing would not be so detrimental 

 to the continuance of any beneficial variation. In other words, the 

 primary variation of the reproductive system would serve to pro- 

 tect any secondary variations of a useful kind which might after- 

 wards arise elsewhere ; just as happens in the analogous case 

 where intercrossing is prevented by geographical barriers, or by 

 migration in different directions of varying descendants from a 

 common centre. 



