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sembled that of a sea-coast, with the waves rolling in upon it, and to the south-east the water extended to the horizon. Along the shore were strewn shells thrown up by the surf; and on examining them, I found them all to belong to well-known old-world genera— Unio, Planorbis, Ancylus, and Ampullari.
On this journey, all the beach was, as I have said, covered with water, and I saw no shells; but in the pools on the road were water-beetles swimming about, and these showed a surprising resemblance to the waterbeetles of Europe. Gyrinidæ swam round and round in mazy circles; Dytiscidæ came up to the surface for a moment, and dived down again to the depths below with a globule of air glistening like a diamond. Amongst the vegetation at the bottom and sides of the pools Hydrophilidæ crawled about, just as in ponds in England. Not only were those familiars there, but they were represented by species belonging to the typical genera -Gyrinus, Colymbetes, and Hydrophilus. Over these pools flew dragon-flies, whose larval stages are passed in the water, closely resembling others all over the world. All the land fauna was strikingly different from that of other regions; but the water fauna was as strikingly similar.
The sameness of fresh-water productions all over the globe is not confined to animal life, but extends to plants also. Alph. de Candolle has remarked that in large groups of plants which have many terrestrial and only a few aquatic species the latter have a far wider distribution than the former. It is well known to botanists that many fresh-water and marsh plants have an immense range over continents, extending even to the most remote islands.* The close affinities of fresh-water animals and plants have been noticed by many naturalists. Darwin saw with surprise, in Brazil, the similarity of the fresh-water insects, shells, &c., and the dissimilarity of the surrounding terrestrial beings compared with those of Britain.t Dr. D. Sharp informs me that water-beetles undoubtedly present the same types all over the world. He believes there is no family of Coleoptera in which tropical or extra-tropical species so closely resemble one another as in the Dytiscidce. Cybister is found in Europe, Asia, Africa, Australia, New Zealand, Brazil, and North America ; and the species have a very wide range. Dr. Sharp remarks that this wide distribution and great similarity of the Dytiscido is of special interest when we recollect that they are nothing but Carabidæ fitted for swimming, and yet that the Carabido are one of the groups in which the tropical members differ widely from the temperate ones.
For following up this branch of inquiry the study of the distribution of the mollusca offers special advantages. There are numerous marine, fresh-water, and terrestrial species and genera. They are slow moving; they have not the means of transporting themselves great distances, like insects, for example, that may easily and often pass over arms of the sea, or fly from one country to another. Their shells are the commonest of fossils; and in islands such as Madeira and St. Helena, where we have abundant remains of extinct land shells, there are few, if any, of extinct animals of other classes or of plants.
Taking the shells of Europe, we find a remarkable * Darwin, “ Origin of Species,” p. 417. ^ Ibid., p. 414.
Ch. XVIII.) LAND AND FRESH-WATER MOLLUSKS.
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 bave 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