172 THE DISPERSAL OF SHELLS. 



It cannot be supposed that slugs often cross the 

 sea, and trans-oceanic migration for long distances ^ 

 if ever occurring, is likely to be extremely rare. Dr. 

 Scharff has stated, indeed, that the sea, '^ which 

 is the principal means of communication for other 

 animals, and plants, between mainland and island, forms 

 an almost impassable barrier for slugs, sea water being 

 deadly both to their eggs and themselves ; " I do not 

 think it necessarily safe, however, to assume, as that 

 author does,, that *' if we find the slugs of mainland and 

 island agreeing in anatomical characters, we may 

 generally conclude that the island must have had a land 

 connection with the mainland at some time or other," ^ 

 for this seems to imply that the creatures are in- 

 capable of crossing the ocean, and it is a fact, as I 

 hear from Dr. Scharff, that their chances of surviving 

 a sea passage are regarded by him as practically nil. 

 But what do we actually know of the ways in which the 

 various organisms are or have been distributed over the 

 globe ? Unthought-of means, even for the dispersal of 

 slugs, may be in operation, or may have operated in 

 former times. I have not ascertained whether any of 

 the true oceanic islands, independently of man's dispersal, 

 possessed slugs belonging to continental species or 

 genera, but such I presume was the case ; some kinds 

 certainly seem to enjoy fairly wide ranges, but in many 

 cases I suppose it would be difficult to ascertain to what 

 extent man has affected their distribution. Few land 



' R. F. Scharff, " Sci. Trans. Royal Dublin Soc," (2), iv. (1S91), 



