The geographical distribution of animals, with a study of the relations ...

chap, ii.] LAND-SHELLS AND INSECTS. 31due to the fact, that ponds and marshes are constantly frequentedby wading and swimming birds which arepre-eminently wanderers,and which frequently carry away with them the seeds ofplants, and the eggs of molluscs and aquatic insects. Freshwater molluscs just hatched were found toa duck's foot suspended in an aquarium ;attach themselves toand they would thus beeasily carried from one lake or river to another, and by the helpof different species of aquatic birds, might soon spread all overthe globe. Even a water-beetle has been caught with a smallliving shell (Ancylus) attached to it ;and these fly long distancesand are liable to be blown out to sea, one having been caught onboard the Beagle when forty-five miles from land. Althoughfresh water molluscs and their eggs must frequently be carriedout to sea, yet this cannot lead to their dispersal, since saltwater is almost immediately fatal to them ;forced toand we are thereforeconclude that the apparently insignificant and uncertainmeans of dispersal above alluded to are really what haveled to their wide distribution.The true land-shells offer a stillmore difficult case, for they are exceedingly sensitive to theinfluence of salt water; they are not likely to be carried byaquatic birds, and yet they are more or less abundant all overthe globe, inhabiting the most remote oceanic islands. It hasbeen found, however, that land-shells have the power of lyingdormant a long time. Some have lived two years and a halfshut up in pill boxes ;and one Egyptian desert snail came to lifeafter having been glued down to a tablet in the British Museumfor four years !We are indebted to Mr. Darwin for experiments on the powerof land shells to resist seawater, and he found that when theyhad formed a membranous diaphragm over the mouth of theshell they survived many days'immersion (in one case fourteendays) ; and another experimenter, quoted by Mr. Darwin, found thatout of one hundred land shells immersed for a fortnight in the sea,twenty-seven recovered.It is therefore quite possible for them tobe carried in the chinks of drift wood for many hundred milesacross the sea, and this is probably one of the most effectualmodes of their dispersal. Very young shells would also some-