Il8 THE DISPERSAL OF SHELLS. 



and no doubt, as Mr. Wallace remarks, the general 

 stability of continents and oceans has been accompanied 

 by constant changes of form.^ The building up of great 

 continental extensions for the purpose of explaining 

 facts in distribution, as Mr. Darwin more than once 

 observed, has doubtless tended to check the investiga- 

 tion and study of means of dispersal, and has been an 

 ill-service to science.'^ 



It seems to have been supposed by some writers that 

 voluntary migration is the only means of dispersal 

 possessed by these creatures, but in view of the facts 

 connected with their distribution referred to in the last 

 chapter we are bound to admit that they have been 

 carried, occasionally at least, from one place to another, 

 even over considerable expanses of ocean, unless indeed, 

 we fall back upon imaginary extensions of land, or 

 take for granted — as Reeve did — '^ **the doctrine of a 

 plurality of progenitors for each species.^' We are con- 

 fronted at once, however, with a statement by Mr. 

 Wallace (repeated as recently as 1887 by Professor 



^ " Origin,"' p. 323 ; " Island Life/' p. 502, ed. 2, p. 534. 



' See letters to Lyell and Hooker, 1856, " Life and Letters," 

 ii.(i888), pp. 78-82; and see also Wallace," Island Life," p. 10, ed. 2, 

 same page : " If we once admit that continents and oceans may 

 have changed places over and over again (as many writers main- 

 tain), we lose all power of reasoning on the migrations of ancestral 

 forms of life, and are at the mercy of every wild theorist who 

 chooses to imagine the former existence of a now-submerged 

 continent to explain the existing distribution of a group of frogs 

 or a genus of beetles." 



2 '' Land and fresh-water Mollusks," 1863, P- -54- 



