102 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 Origm 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 



' LJ/e and Letters, vol. ii. p 2.S * m,i 



