164 THE DISPERSAL OF SHEELS. 



several times come across castings of this kind, con- 

 taining soft seeds, live worms, etc. 



It must be borne in mind, of course, that the 

 transportal of shells by these agencies to distant 

 points over the ocean can only very rarely happen, but, 

 as Mr. Wallace has remarked, " nature can afford to wait," 

 and if but once in a thousand years, he adds, " a single 

 bird should convey two or three minute snails to a 

 distant island, this is all that is required for us to find 

 that island well stocked with a great and varied 

 population of land-shells." ^ We must remember, how- 

 ever, that resulting colonization is much more rare than 

 mere transportal, though, of course, snails carried by 

 birds into the interior of new countries or islands, or to 

 some little distance inland, are more likely to be able to 

 establish themselves than are those stranded upon the 

 shores with drift-timber, etc. The creatures are carried, 

 we may feel sure, for comparatively short distances 

 within given tracts of land much more frequently, and 

 many apparent anomalies in local distribution no doubt 

 have thus arisen, but even here the ultimate establish- 

 ment of a new colony seems likely to be a rare event. 

 Additional ways in which short migrations may possibly 

 be brought about might be referred to. Small shells, such 

 as those of the genus Vertigo, and eggs of various kinds 

 must occasionally happen to be adhering to twigs, etc., 

 carried off by birds when building, and these are some- 

 times collected at some little distance from the nest ; 

 every one, no doubt, remembers having seen rooks 

 * " Geographical Distribution," i. p. 32. 



