FRESH-WATER SHELLS. 5 



allied forms, which " must have proceeded from a single 

 source," prevail throughout the world/ 



Much has to be explained, also, even in local distri- 

 bution. Every naturalist remembers having seen shells 

 in puddles formed after periods of excessive rain, in 

 pools in quarries and pits, in drinking-troughs and 

 water-butts, in tanks on the roofs of buildings, in newly- 

 formed reservoirs and artificial lakes, in ponds on open 

 pastures far away from the nearest streams, &c. As 

 the Rev. James Dalton long ago remarked, the creatures 

 seem to possess a " mysterious faculty " of finding their 

 way to the most unlikely habitations.^ Dispersal by 

 some means must be constantly going on. 



That almost every isolated cattle-pond which a farmer 

 digs perhaps near the middle of an upland pasture should 

 come to possess a molluscan fauna within a few years of 

 its formation, though a matter of common observation, 

 is certainly a surprising fact, and it is remarkable that 

 writers of local lists when recording the existence of 

 shells in such places have so generally passed over the 

 circumstance without comment. If multiple birthplaces 

 were possible it is evident, of course, that the spreading 

 of forms from their several centres would have to be 

 explained, for no believer in multiple centres, I imagine, 

 would go so far as to suppose that cattle-ponds are 



' " Origin," p. 344 ; but see Semper, " Animal Life," p. 298. 



' J. Dalton, "Zoologist," xix. (1861), 7318-9. Early in 1890 

 Mr. W. H, Heathcote saw a specimen of Limns&a trwicatula 

 crawling on the top of the tower of St. James' Church, Preston, 

 ninety feet or more from the ground ! 



