opinions on Isolation. 121 



that this character should undergo either increase or 

 decrease of its average size, form, colour, &c., there 

 will always be, in each succeeding generation, a suffi- 

 cient number of individuals — i. e. half of the whole — 

 which present variations in the required direction, 

 and which will therefore furnish natural selection 

 with abundant material for its action, without the 

 need of any other form of isolation. It is to be 

 regretted, however, that while thus so clearly pre- 

 senting the fact that free intercrossing is the very 

 means whereby natural selection is enabled to effect 

 monotypic evolution, he fails to perceive that such 

 intercrossing must always and necessarily render it 

 impossible for natural selection to effect polytypic 

 evolution. A little thought might have shown him 

 that the very proof which he gives of the necessity 

 of intercrossing where the transmutation of species 

 is concerned, furnishes, measure for measure, as good 

 a proof of the necessity of its absence where the multi- 

 plication of species is concerned. In justice to him, 

 however, it may be added, that this distinction be- 

 tween evolution as monotypic and polytypic (with 

 the important consequence just mentioned) still con- 

 tinues to be ignored also by other well-known evo- 

 lutionists of the " ultra-Darwinian " school. Professor 

 Meldola, for example, has more recently said that in 

 his opinion the "difficulty from intercrossing " has been 

 in large part — if not altogether — removed by Mr. 

 Wallace's proof that natural selection alone is capable 

 of effecting [monotypic] evolution ; while he regards 

 the distinction between monotypic and polytypic 

 evolution as mere " verbiage ^" 



^ Nature, vol. xliii. p. 410, and vol. xliv. p. 39. 



