18431882] LAND MOLLUSCS 441 



following passage occurs : " I shall have to discuss and think more Letter 336 

 about your difficulty of the temperate and sub-arctic forms in the S. 

 hemisphere than I have yet done. But I am inclined to think that I 

 am right (if my general principles are right), that there would be little 

 tendency to the formation of a new species during the period of migra- 

 tion, whether shorter or longer, though considerable variability may have 

 supervened. 



To J. D. Hooker. Letter 337 



Down, Dec. loth [1856]. 



It is a most tiresome drawback to my satisfaction in 

 writing that, though I leave out a good deal and try to 

 condense, every chapter runs to such an inordinate length. 

 My present chapter on the causes of fertility and sterility 

 and on natural crossing has actually run out to 100 pages 

 MS., and yet I do not think I have put in anything 

 superfluous. . . . 



I have for the last fifteen months been tormented and 

 haunted by land-mollusca, which occur on every oceanic 

 island ; and I thought that the double creationists or con- 

 tinental extensionists had here a complete victory. The 

 few eggs which I have tried both sink and are killed. No 

 one doubts that salt water would be eminently destructive to 

 them ; and I was really in despair, when I thought I would 

 try them when torpid ; and this day I have taken a lot out of 

 the sea-water, after exactly seven days' immersion. 1 Some 

 sink and some swim ; and in both cases I have had (as yet) 

 one come to life again, which has quite astonished and 

 delighted me. I feel as if a thousand-pound weight was 

 taken off my back. Adios, my dear, kind friend. 



I must tell you another of my profound experiments ! 

 [Frank] said to me : " Why should not a bird be killed (by 



1 This method of dispersal is not given in the Origin ; it seems, 

 therefore, probable that further experiments upset the conclusion drawn 

 in 1856. This would account for the satisfaction expressed in the 

 following year at the discovery of another method, on which Darwin 

 wrote to Sir J. D. Hooker: "The distribution of fresh- water molluscs 

 has been a horrid incubus to me, but I think I know my way now. 

 When first hatched they are very active, and I have had thirty or forty 

 crawl on a dead duck's foot ; and they cannot be jerked off, and will live 

 fifteen or even twenty-four hours out of water" (Life and Letters, 11^ 

 p. 93). The published account of these experiments is in the Origin, 

 Ed. I., p. 385. 



