98 THE DISPERSAL OF SHELLS. 



haps insufficient to account for the difference in specific 

 distribution which the terrestrial and non-marine 

 aquatic groups of molkisca exhibit. The problem 

 presented is of importance, and a solution of it, which 

 seems likely to be the true one, has occurred to Mr. Belt, 

 who has pointed out that the variation of fresh-w^ater 

 species of animals and plants has probably been con- 

 stantly checked by the want of continuity of lakes and 

 rivers in time and space : — 



" In the great oscillations of the surface of the 

 earth, of which geologists find so many proofs, every 

 fresh- water area has again and again been de- 

 stroyed. It is not so with the ocean — it is continuous 

 — and as one part was elevated and laid dry, the 

 species could retreat to another. On the great 

 continents the land has probably never been totally 

 submerged at any one time ; it also is continuous 

 over great areas, and as one part became uninhabit- 

 able, the land species could in most cases retreat to 

 another. But for the inhabitants of lakes and rivers 

 there was no retreat, and whenever the sea overflowed 

 the land, vast numbers of fresh-water species must 

 have been destroyed. A fresh-water fauna gave place 

 to a marine one, and the former was annihilated so far 

 as that area was concerned. When the land again rose 

 from below the sea, the marine fauna was not destroyed 

 — it simply retired farther back. There is every reason 

 to believe that the production of species is a slow 

 process, and if fresh-water areas have not continued as 

 a rule through long geological periods, we can see how 



