2 THE DISPERSAL OF SHELLS. 



ance to the investigation and study of means of dispersal. 

 Obviously, we are not now authorized in explaining all 

 the difficulties which geographical distribution presents 

 by suggesting multiple centres of creation or former 

 radical changes in the relative positions of land and sea. 

 The wide ranges and singular distributions now en- 

 joyed by many organisms seem, in these circumstances, 

 well-nigh inexplicable ; the ranges of some genera, as 

 everyone is aware, are almost world-wide, some species 

 extend over immense areas, and, moreover, there are 

 cases in which the same or closely- allied forms occur at 

 isolated points in remote parts of the world. Highly 

 effectual means of dispersal of some kind or other must 

 certainly have been in operation, for it is clear that the 

 distribution, at least of many groups, cannot have 

 resulted from gradual migration by ordinary modes of 

 progression, and this is the more apparent, of course, 

 with pre-eminently slow-moving inland animals like 

 fresh-water and land mollusca. To such creatures 

 evidently, as far as voluntary migration is concerned, 

 even small arms of the sea, arid deserts, and elevated 

 mountain-chains must be almost, cr, perhaps, absolutely, 

 impassable ; but it has been remarked that such ob- 

 stacles are not likely to have endured so long as the 

 oceans.^ It will not be forgotten, of course, that the 

 great changes of level known to have taken place 

 '* within the period of existing organisms^' certainly 

 remove many difficulties, and it should be borne in 



^ See "Origin," p. 317. 



