Opinions on Isolation. 107 



the important words. " I almost wish I could believe 

 in its importance to the same extent with you ; for 

 you well show, in a manner which never occurred to 

 me, that it removes many difficulties and objections." 

 These words are important, because they show that 

 Darwin had come to feel the force of the " difficulties 

 and objections" with regard to divergent evolution 

 being possible by means of natural selection alone, 

 and how readily they could be removed by assuming 

 the assistance of isolation. Hence, it is much to be 

 deplored that Wagner presented a single kind of 

 isolation (geographical) as equivalent to the principle 

 of isolation in general. For he thus failed to present 

 the complete and, therefore, the true philosophy 

 of the subject to Darwin's mind ; and in this, as 

 in certain other respects which I shall notice later 

 on, served rather to confuse than to elucidate the 

 matter as a whole. 



To sum up. Although in his later years, as shown 

 by his correspondence, Darwin came to recognize 

 more fully the swamping effects of free intercrossing, 

 and the consequent importance of " separation " for 

 the prevention of these effects, and although in this 

 connexion he likewise came more clearly to dis- 

 tinguish between the " two cases " of monotypic 

 and poly ty pic evolution, it is evident that he never 

 worked out any of these matters " thinking it pru- 

 dent," as he wrote with reference to them in 1878, 

 ' now I am growing old, to work at easier subjects 1 ." 

 Therefore he never clearly saw, on the one hand, 

 that free intercrossing, far from constituting a " diffi- 

 culty " to monotypic evolution by natural selection, 

 1 Life and Letters, vol. iii. p. 161. 



