CHAP. XXII.] MOLLUSCA. 525 



peculiar species of Achatinella. Another striking feature of the 

 distribution of land molluscs, is the richness of islands as com- 

 pared with continents. The Philippines contain more species 

 than all India ; and those of the Antilles according to Mr. Bland 

 almost exactly equal the numbers found in the entire American 

 continent from Greenland to I'atagonia. Taking the whole world, 

 it appears that many more species of land-shells are found in the 

 islands than on the continents of the globe, a peculiarity that 

 obtains in no other extensive group of animals. 



Looking at these facts it seems probable, that the air-breathing 

 molluscs have been chiefly distributed by air- or water-carriage, 

 rather than by voluntary dispersal on the land. Even seas and 

 oceans have not formed impassal)le barriers to their diffusion ; 

 whereas they only spread on dry land with excessive slowness and 

 difficulty. The exact mode in which their diffusion is effected is not 

 known, and it may depend on rare and exceptional circumstances ; 

 but it seems likely to occur in two ways. Snails frequently 

 conceal themselves in crevices of trees or under bark, or attach 

 themselves to stems or foliage, and either by their operculum or 

 mucous diaphragm, are able to protect themselves from the in- 

 jurious effects of salt water for long periods. They might there- 

 fore, under favourable conditions, be drifted across arms of the 

 sea or from island to island ; while wherever there are large 

 rivers and occasional floods, they would by similar means be 

 Avidely scattered over land areas. Another possible mode of dis- 

 tribution is by means of storms and hurricaues, which would 

 carry the smaller species for long distances, and might occasionally 

 transport the eggs of the larger forms. Aquatic birds might 

 occasionally get both shells and eggs attached to their feet or 

 their plumage, and convey them across a wide extent of sea. 

 But whether these, or some other unknown agency has acted, the 

 facts of distribution clearly imply that some means of transport 

 over water is, and has been, the chief agent in the distribution of 

 these animals ; but that its action is very rare or intermittent, so 

 that its effects are hardly perceptible in the distribution of single 

 species. 



Another important factor in enabling us to account for the 



