CHAP. II. ] LAND-SHELLS AND INSECTS. 31 
due to the fact, that ponds and marshes are constantly frequented 
by wading and swimming birds which are pre-eminently wan- 
derers, and which frequently carry away with them the seeds of 
plants, and the eggs of molluscs and aquatic insects. Fresh 
water molluscs just hatched were found to attach themselves to 
a duck’s foot suspended in an aquarium ; and they would thus be 
easily carried from one lake or river to another, and by the help 
of different species of aquatic birds, might soon spread all over 
the globe. Even a water-beetle has been caught with a small 
living shell (Ancylus) attached to it; and these fly long distances 
and are liable to be blown out to sea, one having been caught on 
board the Beagle when forty-five miles from land. Although 
fresh water molluscs and their eggs must frequently be carried 
out to sea, yet this cannot lead to their dispersal, since salt 
water is almost immediately fatal to them ; and we are therefore 
forced to conclude that the apparently insignificant and uncer- 
tain means of dispersal above alluded to are really what have 
led to their wide distribution. The true land-shells offer a still 
more difficult case, for they are exceedingly sensitive to the 
influence of salt water; they are not likely to be carried by 
aquatic birds, and yet they are more or less abundant all over 
the globe, inhabiting the most remote oceanic islands, It has 
been found, however, that land-shells have the power of lying 
dormant a long time. Some have lived two years and a half 
shut up in pill boxes ; and one Egyptian desert snail came to life 
after having been glued down to a tablet in the British Museum 
for four years ! 
We are indebted to Mr. Darwin for experiments on the power 
of land shells to resist sea water, and he found that when they 
had formed a membranous diaphragm over the mouth of the 
shell they survived many days’ immersion (in one case fourteen 
days) ; and another experimenter, quoted by Mr. Darwin, found that 
out of one hundred land shells immersed for a fortnight in the sea, 
twenty-seven recovered. Itis therefore quite possible for them to 
be carried in the chinks of drift wood for many hundred miles 
across the sea, and this is probably one of the most effectual 
modes of their dispersal. Very young shells would also some- 
