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 Patagonia. 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 impassable 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 
widely scattered over land areas. Another possible mode of dis- 
tribution is by means of storms and hurricanes, 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 
