LAND SHELLS : THEIR TENACITY OF LIFE. 97 



carried there from Porto Santo, but they have not 

 colonized the island ; on the other hand^ European 

 land-shells, believed to have been introduced by man, 

 are said to flourish both in Madeira and Porto Santo.^ 



It must be admitted that neither fresh-water nor land- 

 shells are really well furnished with means for dispersal ; 

 the transportal of a species of either group over a large 

 expanse of ocean, or to great distances on land, with 

 subsequent establishment, must be an extremely rare 

 and exceptional occurrence, and one which happens, 

 perhaps, only once or twice in many hundreds of years. 

 It can hardly be argued, therefore, that fresh-water 

 species have been kept uniform by constant or frequent 

 transportal. Numerous facts, it is true, apparently 

 indicating means of dispersal for fresh-water kinds 

 have been referred to, and these are doubtless of value 

 as helping us to understand how it is that many have 

 been able to wander so far from their several birth-places, 

 but it cannot be said that they explain the wide ranges 

 and consequent uniformity which obtain in fresh-water 

 as against more restricted ranges and greater variety on 

 land. 



It seems evident, as Mr. Belt has remarked, that on 

 land there has been more variation or that the varieties 

 which have arisen in fresh-water have less frequently 

 been preserved. Mr. Darwin has shown that in fresh- 

 water competition will have been less severe than on land, 

 and consequently new forms will have been more slowly 

 produced ; - but this consideration, taken alone, is per- 

 ' " Origin," p. 357. 2 .. Origin," p. 83. 



H 



