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Ch. XVIII.) LAND AND FRESH-WATER MOLLUSKS.

333

difference in the distribution of the land and freshwater species. According to Mr. Lovell Reeve, who has specially studied this question, out of many hundreds of land mollusks inhabiting the Caucasian province at its centre in Hungary and Austria, only ninety extend to the British Isles, and of these thirty-five do not reach Scotland. Upwards of two hundred species of Clausilia are to be found in the centre of the province, and of these only four reach England, and only one Scotland. Out of five hundred and sixty species of Helix inhabiting the Caucasian province, there are but twenty-four in Britain.

Whilst the distribution of the terrestrial mollusks of Europe is thus restricted in range, though the species are numerous, the fresh-water shells are few in species, but of wide distribution. Quoting again from Mr. Reeve:Of the Lymnæacea “ there are not six species, it may be safely stated, in all Europe, more than there are in Britain. They have no particular centre of creation. There is no evidence to show whether the alleged progenitors of our British species were created in Siberia, Hungary, or Thibet. There is scarcely any variation either in the form or number of the species in those remote localities. Of Planorbis scarcely more than fifteen species inhabit the whole Caucasian province, and we have eleven of them in Britain.”—“Of Physa and Lymnæa, it is extremely doubtful whether there are any species throughout the province more than we have in Britain. 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.” * * Lovell Reeve, “British Land and Fresh-Water Mollusks," p. 225.

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

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

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.

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 would be colonised by forms 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 productions are few and world-wide, whilst the sea has mollusks innumerable, and the land great variety and wealth of species. This 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 physical 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 still preserved in a few of the large rivers of the world.

To illustrate more clearly the theory I have advanced, I will take a supposititious case. In the southern states of America there is reason to suppose that since the glacial period there has been a great variation in the species of the fresh-water mollusk genus Melania, and in different rivers there are distinct groups of species. Now let us suppose that the glacial period were to return, and that the icy covering, gradually thickening in the north, should push down southward as it did once before. The great lakes of North America would be again filled with ice, and their inhabitants destroyed. As the ice advanced southward, the inhabitants of one river-system after another would be annihilated, and many groups of Melania entirely destroyed. On the retreat of the ice again the rivers and lakes would reappear, but the varieties of animals that had been developed in them would not, and their places would be taken by aquatic forms from other areas, so that the number of species would be thereby greatly reduced, and wide-spreading forms would be freed from the competition of many improved varieties.

Viewed in this light, the similarity of fresh-water productions all over the world, instead of being a difficulty in the way of the acceptance of the theory of natural selection, becomes a strong argument in favour of its truth; for we perceive that the number of marine, terrestrial, and fresh-water animals is in proportion to the more or less continuous development that was possible under the different conditions under which they lived.

Ch. XVIII.] CONTINUITY OF LAND AND AIR.

337

The same line of argument might be used to explain the much greater variety in some classes of terrestrial animals than in others. The land has often been submerged in geological history, and the classes that were best fitted to escape the impending catastrophes would be most likely to preserve the varieties that had been developed. The atmosphere has always been continuous, and the animals that could use it as a highway had great advantages over those that could not, and so we find the slow-moving terrestrial mollusks few in number compared with the multitudinous hosts of strong-flying insects ; similarly, the mammals are far outnumbered by the birds of the air, that can pass from island to island, and from country to country, unstopped by mighty rivers or wide arms of the sea.

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