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 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, 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 variation 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 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.