334: THE NATURALIST IN NICARAGUA. [Ch. XVIII. 



The wide distribution of species inhabiting fresh, water 

 compared with those living on land has not escaped the 

 comprehensive mind of Darwin, and in explanation of the 

 fact, he has shown how fresh- water shells may be carried 

 from pool to pool, or from one river or lake to others 

 many miles distant, sticking to the feet of water-fowl, or 

 to the elytra of water-beetles. Whilst the distribution of 

 water mollusks may be thus accounted for, it does not 

 explain the greater variety and more restricted range of 

 the land species. They have at least equal means of 

 dispersion, compared with the sluggish, mud-loving water- 

 shells of our ponds and ditches. Why should the one 

 have varied so much and the other so little ? We might 

 at first sight have expected the very reverse, on the 

 theory of natural selection. In large lakes and in river 

 systems isolated from others, we might look for the con- 

 ditions most favourable for the variation of species, and 

 for the preservation of the improved varieties. 



It is evident that tjiere must have been less variation, 

 or that the varieties that arose have not been preserved. 

 I think it probable that the variation of fresh-water 

 species of animals and plants has been constantly checked 

 by the want of continuity of lakes and rivers in time and 

 space. In the great oscillation of the surface of the 

 earth, of which geologists find so many proofs, every 

 fresh-water area has again and again been destroyed. 

 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 pro- 

 bably never been totally submerged at any one time ; it 

 also is continuous over great areas, and as one part 

 became uninhabitable, the land species could in most 



