Land and Fresh-Water Mollusks 255 
Neither of Ancylus, which lives attached, limpet-like, to 
sticks and stones, and has very limited facilities of migration, 
are there any species throughout the province more than we 
have in Britain.” ? 
The wide distribution of species inhabiting fresh water 
compared with those living on land has not, as we have seen, 
escaped the comprehensive mind of Darwin, and in explana- 
tion 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, the greater 
variety and more restricted range of the land species is not 
explained. 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 conditions most favourable for the varia- 
tion of species, and for the preservation of the improved 
varieties. 
It is evident that there 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 oscillations 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 probably 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 
cases retreat to another. But for the inhabitants of lakes 
and rivers there was no retreat, and whenever the sea over- 
flowed 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 
1 Lovell Reeve, British Land and Fresh-Water Mollusks, p. 225. 
