4 Darwin, and after Darwin. 



Nevertheless he may have been right — and, as I shall 

 presently show, I believe he was right — in his funda- 

 mental premiss, that in the presence of free inter- 

 crossing natural selection would be powerless to effect 

 divergent evolution. Where he went wrong was in 

 not perceiving that geographical isolation is not the 

 only form of isolation. Had it occurred to him that 

 there may be other forms quite as effectual for the 

 prevention of free intercrossing, his essay could hardly 

 have failed to mark an epoch in the history of Dar- 

 winism. But, on account of this oversight, he really 

 weakened his main contention, namely, that in the 

 presence of free intercrossing natural selection must 

 be powerless to effect divergent evolution. This main 

 contention I am now about to re-argue. At present, 

 therefore, we have only to observe that Wagner did it 

 much move harm than good by neglecting to perceive 

 that free intercrossing may be prevented in many other 

 ways besides by migration, and by the intervention of 

 geographical barriers. 



In order that we may set out with clearer views 

 upon this matter, I will make one or two preliminary 

 remarks on the more general facts of isolation as these 

 are found to occur in nature. 



In the first place, it is obvious that isolation 

 admits of degrees : it may be either total or partial ; 

 and, if partial, may occur in numberless grades of 

 efficiency. This is so manifest that I need not wait 

 to give illustrations. But now, in the second place, 

 there is another general fact appertaining to isolation 

 which is not so manifest, and a clear appreciation 

 of which is so essential to any adequate considera- 

 tion of the subject, that I believe the reason why 



