CHAP, ii.l LAND-SHELLS AND INSECTS. 31 



clue 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. Althougli 

 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. It is 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- 



