82 THE UNFINISHED BOOK. [1856. 



the more I think (and I cannot get the subject out of my 

 head), the more difficult I find it. If there were only some 

 half-dozen cases, I should not feel the least difficulty ; but 

 the generality of the facts of all islands' (except one or two) 

 having a considerable part of their productions in common 

 with one or more mainlands utterly staggers me. What a 

 wonderful case of the Epacridae ! It is most vexatious, also 

 humiliating, to me that I cannot follow and subscribe to the 

 way in which you strikingly put your view of the case. 

 I look at your facts (about Eucalyptus, &c.) as damning 

 against continental extension, and if you like also damning 

 against migration, or at least of enormous difficulty. I see 

 the ground of our difference (in a letter I must put myself 

 on an equality in arguing) lies, in my opinion, that scarcely 

 anything is known of means of distribution. I quite agree 

 with A. De Candolle's (and I dare say your) opinion that it 

 is poor work putting together the merely possible means of 

 distribution ; but I see no other way in which the subject can 

 be attacked, for I think that A. De Candolle's argument, 

 that no plants have been introduced into England except by 

 man's agency, [is] of no weight. I cannot but think that the 

 theory of continental extension does do some little harm 

 as stopping investigation of the means of dispersal, which, 

 whether negative or positive, seems to me of value ; when 

 negatived, then every one who believes in single centres will 

 have to admit continental extensions. 



... I see from your remarks that you do not understand 

 my notions (whether or no worth anything) about modifica- 

 tion ; I attribute very little to the direct action of climate, &c. 

 I suppose, in regard to specific centres, we are at cross 

 purposes ; I should call the kitchen garden in which the red 

 cabbage was produced, or the farm in which Bakewell made 

 the Shorthorn cattle, the specific centre of these species! 

 And surely this is centralisation enough ! 



I thank you most sincerely for all your assistance ; and 



