Il6 THE DISPERSAL OF SHELLS. 



tion and dispersal. With the great age of the genera 

 and higher groups in view, however, and admitting that 

 the creatures are able to migrate on land, even with 

 extreme slowness, and that they possess some means, 

 however rarely occurring, of crossing the sea, we need 

 hardly feel surprise at their having spread into all lands 

 ever connected with continents, and even into the most 

 remote and isolated islands of the open ocean. ^ 



Something no doubt is to be attributed to unaided or 

 voluntary migration over land by gradual progression 

 generation after generation, for, as Mr. Wallace states, 

 there is a natural tendency among animals to roam in 

 every direction in search of fresh pastures ; " but snails 

 are proverbially slow, and their powers of voluntary 

 dispersal, therefore, must necessarily be very limited.^ 

 It appears, however, that they are in a better position 

 in this respect than fresh-water kinds, many of which, 

 more especially the bivalves, (as far as their own 

 powers are concerned) seem to be more or less com- 

 pletely restrained within the limits of the river-basin in 

 which they happen to live. Dr. Binney has remarked that 

 snails are not instinctively restricted to particular local 

 habitations, having no regular places of breeding or of 

 shelter, and this, he thought, would favour their diffusion, 

 but it seems, from observations which have been made 

 since he wrote, that certain kinds return from time to 



* See " Geographical Distribution," ii. p. 526, &c. ; " Island 

 Life," p. yy. 



^ "Geographical Distribution," i. p. 10, 

 3 " Island Life," p. 76, 



