440 THE UNFINISHED BOOK. [1856. 



P. S. — After giving much condensed, my argument versus 

 continental extensions, I shall append some such sentence, 

 as that two better judges than myself have considered these 

 arguments, and attach no weight to them. 



C. Darwin to J. D. Hooker. 



Down, August 5th [1856]. 

 ... I quite agree about Ly ell's letters to me, which, 

 though to me interesting, have afforded me no new light. 

 Your letters, under the geological point of view, have been 

 more valuable to me. You cannot imagine how earnestly I 

 wish I could swallow continental extension, but I cannot ; 

 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 Epacridse ! 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 posssible 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, 

 of no weight. I cannot but think that the theory of conti- 

 nental extension does do some little harm as stopping investi- 

 gation of the means of dispersal, which, whether negative or 

 positive, seems to me of value ; when negatived, then every 



