76 
ISLAND LIFE. 
[part I. 
restriction to limited areas; and it is only by bearing these 
considerations in mind that we can find a satisfactory ex- 
planation of the many anomalies we meet with in studying 
their distribution. 
The Dispersal of Land Mollusca . — The only other group 
of animals we need now refer to is that of the air-breathing 
mollusca, commonly called land-shells. These are almost as 
ubiquitous as insects, though far less numerous ; and their wide 
distribution is by no means so easy to explain. The genera 
have usually a very wide, and often a cosmopolitan, range, while 
the species are rather restricted, and sometimes wonderfully so. 
Not only do single islands, however small, often possess peculiar 
species of land-shells, but sometimes single mountains or valleys, 
or even a particular mountain side, possess species or varieties 
found nowhere else upon the globe. It is pretty certain that 
they have no means of passing over the sea but such as are very 
rare and exceptional. Some which possess an operculum, or 
which close the mouth of the shell with a diaphragm of secreted 
mucus, may float across narrow arms of the sea, especially 
when protected in the crevices of logs of timber ; while in the 
young state when attached to leaves or twigs they may be 
carried long distances by hurricanes . 1 Owing to their exceedingly 
slow motion, their powers of voluntary dispersal, even on land, 
are very limited, and this will explain the extreme restriction of 
their range in many cases. 
Great Antiquity of Land-Shells . — The clue to the almost 
universal distribution of the several families and of many genera, 
is to be found, however, in their immense antiquity. In the 
Pliocene and Miocene formations most of the land-shells are 
either identical with living species or closely allied to them, 
1 Mr. Darwin found that the large Helix pomatia lived after immersion 
in sea-water for twenty days. It is hardly likely that this is the extreme 
limit of their powers of endurance, but even this would allow of their being 
floated many hundred miles at a stretch, and if we suppose the shell to be 
partially protected in the crevice of a log of wood, and to be thus out of 
water in calm weather, the distance might extend to a thousand miles or 
more. The eggs of fresh-water mollusca are known to attach themselves 
to the feet of aquatic birds, and this is supposed to account for their very 
wide diffusion. 
