MEANS OF DISPERSAL. 1 39 



and ova must often be carried down stream, sometimes 

 to great distances, along with floating timber, vegetable 

 debris of many kinds, pumice, &c._, which must fre- 

 quently be stranded on the low and shelving banks of 

 river-margins and on islands in mid-stream, but I do 

 not know that we have any actual evidence of dispersal 

 in this way. The landing of a tree in a condition 

 similar to that of the pollard-elm above mentioned, 

 however, would certainly be sufficient to account for 

 the presence say of Helix lapicida — quite unknown 

 perhaps in the surrounding neighbourhood — on some 

 such island or river-bank, for a whole colony of snails, 

 possibly from a locality several miles distant, might 

 thus be safely landed in a new home. Dr. R. Brown 

 (or some author edited by him),^ mentions having seen, 

 on his way up some American river, the huge trunk of 

 a tree " floating leisurely down stream with plants 

 blooming on its decayed surface,^' and weeks afterwards, 

 on coming down the river, he has noticed, as he assures 

 us, the same tree " left high and dry on the banks, with 

 the same plants, still in flower or in seed, growing upon 

 it." The pumice-stones floated down by the Amazons, 

 as Mr. Bates states, are sometimes stranded on the 

 banks in different parts of the river.- Many of the 

 smaller snails may frequently be floated in the hollow 

 stems, or kexes, of large umbelliferous plants, for these, 

 often lying loosely upon the ground, are liable to be 

 swept away by floods, and molluscs are known to hide 



^ See " Our Earth and its Story,'^ (Cassell : no date), p. 306. 

 - " Naturalist on the River Amazons," ed. 5, 1884, p. 248. 



