I02 Darwin, and after Darwin. 



beings appears to me to indicate is, that isolation is the chief 

 concomitant or cause of the appearance of new forms {I well 

 know there are some staring exceptions) \ 



And again : — 



With respect to original creation or production of new forms, 

 I have said that isolation appears the chief element ^ 



Next, in the earlier editions of the Origin of Species 

 this view is abandoned, and in its stead we meet 

 with the opinion that geographical isolation lends 

 a certain amount of assistance to natural selection, 

 by preventing free intercrossing. But here we must 

 note two things. First, the distinction between mono- 

 typic and polytypic evolution is not defined. Secondly, 

 the levelling effect of free intercrossing in nature, and 

 hence its antagonism to divergence of character by 

 natural selection; is not sufficiently recognized ; while, 

 on the other hand, and in consequence of this, the 

 importance of isolation as a factor of evolution is 

 underrated — not only in its geographical, but likewise 

 in all its other forms. 



Taking these two points separately, the only 

 passages in Darwin's writings, so far at least as I 

 can find, in which any distinction is drawn between 

 evolution as monotypic and polytypic, are those in 

 which he deals with a somewhat analogous distinction 

 between artificial selection as intentional and un- 

 conscious. He says, for example : — 



In the case of methodical selection, a breeder selects for some 

 definite object, and if the individuals be allowed freely to inter- 

 cross, his work will completely fail. But when many men, 

 without intending to alter the breed, have a nearly common 



' Life and Letters, vol. ii. p. 28, * Jbid. 



