Isolation. 33 



is so assisted, natural selection may become and, 

 I believe, does become the most efficient of all 

 the forms of isolation in promoting divergence of 

 character. For, in the first place, of all the forms 

 of isolation natural selection is probably the most 

 energetic in promoting monotypic evolution; so that 

 under the influence of such isolation monotypic 

 evolution probably advances more rapidly than 

 it does under any other form of isolation. In the 

 second place, when polytypic evolution has been 

 begun by any of these other forms of isolation, and 

 natural selection then sets to work on each of the 

 resulting branches, although natural selection is thus 

 engaged in as many different acts of monotypic evolu- 

 tion as there are thus separate cases supplied to it by 

 these other forms of isolation, the joint result of all 

 these different acts is to hurry on the polytypic 

 evolution which was originally started by the other 

 forms of isolation. So to speak, natural selection is 

 the forcing heat, acting simultaneously on each of the 

 separate branches which has been induced to sprout 

 by other means ; and in thus rapidly advancing the 

 growth of all the branches, it is still entitled to be 

 regarded as the most important single cause of diver- 

 sification in organic nature, although we must hence- 

 forth cease to regard it as in any instance the 

 originating cause or even so much as the sustaining 

 cause. 



So much by way of summary and recapitulation. 

 I will now briefly consider the only objections 

 which, so far as I can see, admit of being brought 

 against the foregoing doctrine of Isolation as held 

 by Mr. Gulick and myself. These possible objections 



III D 



