IXXX INTRODUCTION. [CH. 



seas^ or vice versa. The most striking diflPerence appears 

 to be with respect to those species called '^^ littoral/'' 

 which are more subject to climatal influence than the 

 inhabitants of deep "water. In the case of freshwater 

 shells^ the same rule and exceptions seem to prevail. 

 The common pond-snail {Limn(Ea peregra) is diffused 

 over the whole of Eui'ope, as well as over considerable 

 tracts of North America and Northern Asia ; and it is 

 only by calling them " representative " species and gi^'ing 

 them other names that any pretence can be made for 

 distinguishing certain British species of Limncea, Physttj 

 and Pisidium from those which are brought from very 

 distant parts of the world. This diffusion of freshwater 

 shells has been attributed to the chance transport by 

 birds ; but I am inclined to believe that it had a different 

 and very remote origin_, and that it took place long before 

 the present distribution of land and water. Land-shells 

 are much more restricted in their range ; and with the 

 exception of two minute species {Helios pulchella and 

 Cochlicopa lubrica), besides a few other snails which 

 have been introduced^ and as it were domesticated, by 

 Man, T am not aware of any kinds which are common 

 to both hemispheres. In Thibet and Cashmir, indeed, 

 many of the land-shells are said to belong to the same 

 species as inhabit Great Britain ; but these are probably 

 the descendants of ancient immigrants during the Gla- 

 cial epoch from more northern latitudes. The mode by 

 which the Mollusca have become distributed throughout 

 the different and remote areas in which they are now 

 found living or in a fossil state has in all probability 

 been the same from the time of their creation. Their 

 natural tendency is to disperse either in search of food 

 or from a migratory instinct ; and, although the pace 

 of a snail is proverbially slow, time and the action 



