INTRODUCTORY NOTE. 



The distribution of fresh-water and land shells has been 

 looked upon as presenting special difficulties on the 

 theory of single birth-places or " centres of creation." 

 Mr. Darwin, in characteristic letters, in 1856-7, spoke of 

 being driven mad by land molluscs, and fresh-water 

 kinds, he said, had been a horrid incubus.^ In the 

 " Origin of Species/' however, he was able to suggest 

 several possible means of dispersal both for terrestrial 

 and aquatic groups, and speaking of the latter he re- 

 marked that many facts throwing light on the subject 

 would doubtless be discovered." Quite a number of 

 facts of the kind indicated have been recorded since the 

 publication of the " Origin,'"' and a collection of these 

 together with some hitherto unpublished items — both 

 as regards fresh-water and land shells — is now given. 

 In compiling this, and the essays on subsidiary subjects 

 also given, the writer has received constant help in the 

 way of notes, references, &c., from a number of friends 

 and correspondents whose courteous co-operation is 



' " Life and Letters," vol. ii. pp. 85, 93. 



"^ " Origin," 1859, p. 385, and see ed. 6, pp. 344, 353- 



