Ch. XVIII.] EXPLANATION OF FRESH-WATER FORMS. 335 



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 variation has been constantly checked by 

 the destruction, first in one part, then in another, of all 

 the fresh-water species ; and on these places being again 

 occupied by fresh water they were colonised by species 

 from other parts of the world. Thus species of restricted 

 range were always exposed to destruction because their 

 habitat was temporary and their retreat impossible, and 

 only families of wide distribution could be preserved. 

 Hence I believe it is that the types of fresh-water pro- 

 ductions are few and world-wide, whilst that the sea has 

 mollusks innumerable, and the land great variety and 

 wealth of species. Their variety is in the ratio of the 

 continuity of their habitats in time and space. 



It follows also, from the same reasoning, that old and 

 wide-spread types are more likely to be preserved in 

 fresh-water areas than on land or in the sea, for the 

 destruction of wide-ranging species is effected more 

 by the competition of improved varieties than by phy- 

 sical causes ; so that when variation is most checked 

 old forms will longest survive. Therefore I think it 

 is that, amongst fishes we find some old geological types 



