MEANS OF DISPERSAL. 1 19 



Heilprin) that the exact mode in which the diffusion 

 of the creatures has been effected is not known/ It 

 seems probable, however, from many considerations, 

 that they are carried from place to place by accidental 

 or occasional means of various kinds similar in many 

 respects to those which we have seen have almost 

 certainly operated for the dispersal of the fresh-water 

 groups, yet the very general absence of evidence on the 

 point is certainly surprising : hitherto, as Sir C. Lyell 

 remarked in the *' Principles," " the naturalist has not 

 witnessed the arrival of a new continental Helix on any 

 remote oceanic island, except by the aid of man," ' and, 

 indeed, it may be said that we have little or no actual 

 evidence of precise modes of dispersal even for short 

 distances on land. We shall probably find reason to 

 believe, however, that transportal through wide spaces 

 is by no means so rare as might at first sight have been 

 supposed, but the ultimate establishment of a colony in 

 a new home, which will always depend on many 

 complex contingencies and must often be altogether 

 impossible, is quite another matter ; and it should be 

 clearly understood that the colonization of a species on 

 an oceanic island, or at any other distant point, as the 

 result of the trans-oceanic dispersal of a few individuals, 

 must certainly be extremely rare and exceptional, and 

 is hardly likely to happen more than once, perhaps, 



^ " Geographical Distribution," ii. p. 525 ; Heilprin, " Geo- 

 graphical and Geological Distribution of Animals," 1887, p. 



53. 

 - " Principles," ii. p. 434. 



