386 MK. G. J. ROMANES ON PHYSIOLOGICAL SELECTION. 



considered is here presented in a special and aggravated form ? 

 At all events, I know tbat, after having duly and impartially 

 considered the matter, to me it does appear that unless the 

 swamping effects of intercrossing with the parent form on an 

 overcrowded area is in some way prevented, to begin with, 

 natural selection could never have any material supplied by which 

 to go on with. Let it be observed that I regard Mr. Darwin's 

 argument as perfectly sound where it treats of the divergence of 

 species, and of their further divergence into genera ; for in these 

 cases the physiological barrier is known to be already present. 

 But in applying the argument to explain the divergence of indi- 

 viduals into varieties, it seems to me that here, more than any- 

 where else, Mr. Darwiu has strangely lost sight of the formidable 

 difficulty in question ; for in this particular case so formidable 

 does the difficulty seem to me, that I cannot believe that natural 

 selection alone could produce any divergence of specific cha- 

 racter, so long as all the individuals on an overcrowded area 

 occupy that area together. Tet, if any of them quit that area, and 

 so escape from the unifying influence of free intercrossing*, 

 these individuals also escape from the conditions which Mr. Dar- 

 win names as those that ,are needed by natural selection in order 

 to produce divergence. Therefore, it appears to me that, under 

 the circumstances supposed, natural selection alone could not 

 produce divergence ; the most it could do would be to change 

 the whole specific type in some one direction (the needful varia- 

 tions in that one direction being caused by some general change 

 of food, climate, habit, &c., afiecting a number of individuals 

 simultaneously), and thus induce transmutation of species in a 

 linear series, each succeeding member of which might supplant its 

 parent form. But in order to secure diversity, multiplication, or 

 ramification of species, it appears to me obvious that the primary 

 condition required is that of preventing intercrossing with parent 

 forms at the origin of each branch, whether the prevention, be from 

 the first absolute, or only partial. And, after all that has been 

 previously said, it is needless again to show that the principles of 

 physiological selection are at once the only principles which are 

 here likely to be efficient, and the principles which are fully 

 capable oi doing all that is required. For species, as they now 



* As Mr. Darwin elsewhere observes, " Intercrossing plays a very important 

 part in nature by keeping the individuals of the same species, or of the same 

 variety, true and uniform in character " (p. 81). 



