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 polytypic evolution, it is evident that he never 

 worked out any of these matters — " thinking it pru- 

 dent," as he wrote with I'eference to them in 1878, 

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

 Therefore he never clearly saw, on the one hand, 

 that free intercrossing, far from constituting a " diffi- 

 culty '' to monotypic evolution by natural selection, 

 ' Life and Letters, vol. iii. p. i6i. 



