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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 aU 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
;