The heavy rains which have done so much harm to the standing corn have at least had the effect of making the country look greener and lovelier than I have seen it look for many seasons. There is now a fresh verdure about the upland pastures and pine woods which almost reminds one of the deep valleys of the Bernese Oberland in early spring. Last year’s continuous wet weather gave the trees and grass a miserable draggled appearance; but this summer’s rain, coming after a dry spring, has brought out all the foliage in unwonted luxuriance; and everybody (except the British farmer) agrees that we have never seen the country look more beautiful. Though the year is now so far advanced, the trees are still as green as in springtide; and the meadows, with their rich aftermath springing up apace, look almost as lush and fresh as they did in early June. Londoners who get away to the country or the seaside this month will enjoy an unexpected treat in seeing the fields as they ought to be seen a couple of months sooner in the season.
Here, on the edge of the down, where I have come up to get a good blowing from the clear south-west breeze, I have just sat down to rest myself awhile and to admire the view, and have reverted for a moment to my old habit of snail-hunting. Years ago, when evolution was an infant — an infant much troubled by the complaints inseparable from infancy, but still a sturdy and vigorous child, destined to outlive and outgrow its early attacks — I used to collect slugs and snails, from an evolutionist standpoint, and put their remains into a cabinet; and to this day I seldom go out for a walk without a few pill-boxes in my pocket, in case I should happen to hit upon any remarkable specimen. Now here in the tall moss which straggles over an old heap of stones I have this moment lighted upon a beautifully marked shell of our prettiest English snail. How beautiful it is I could hardly make you believe, unless I had you here and could show it to you; for most people only know the two or three ugly brown or banded snails that prey upon their cabbages and lettuces, and have no notion of the lovely shells to be found by hunting among English copses and under the dead leaves of Scotch hill-sides. This cyclostoma, however, — I must trouble you with a Latin name for once — is so remarkably pretty, with its graceful elongated spiral whorls, and its delicately chiselled fretwork tracery, that even naturalists (who have perhaps, on the whole, less sense of beauty than any class of men I know) have recognised its loveliness by giving it the specific epithet of elegans. It is big enough for anybody to notice it, being about the size of a periwinkle; and its exquisite stippled chasing is strongly marked enough to be perfectly visible to the naked eye. But besides its beauty, the cyclostoma has a strong claim upon our attention because of its curious history.
Long ago, in the infantile days of evolutionism, I often wondered why people made collections on such an irrational plan. They always try to get what they call the most typical specimens, and reject all those which are doubtful or intermediate. Hence the dogma of the fixity of species becomes all the more firmly settled in their minds, because they never attend to the existing links which still so largely bridge over the artificial gaps created by our nomenclature between kind and kind. I went to work on the opposite plan, collecting all those aberrant individuals which most diverged from the specific type. In this way I managed to make some series so continuous that one might pass over specimens of three or four different kinds, arranged in rows, without ever being able to say quite clearly, by the eye alone, where one group ended and the next group began. Among the snails such an arrangement is peculiarly easy; for some of the species are very indefinite, and the varieties are numerous under each species. Nothing can give one so good a notion of the plasticity of organic forms as such a method. The endless varieties and intermediate links which exist amongst dogs is the nearest example to it with which ordinary observers are familiar.
But the cyclostoma is a snail which introduces one to still deeper questions. It belongs in all our scientific classifications to the group of lung-breathing mollusks, like the common garden snail. Yet it has one remarkable peculiarity: it possesses an operculum, or door to its shell, like that of the periwinkle. This operculum represents among the univalves the under-shell of the oyster or other bivalves; but it has completely disappeared in most land and fresh-water snails, as well as among many marine species. The fact of its occurrence in the cyclostoma would thus be quite inexplicable if we were compelled to regard it as a descendant of the other lung-breathing mollusks. So far as I know, all naturalists have till lately always so regarded it; but there can be very little doubt, with the new light cast upon the question by Darwinism, that they are wrong. There exists in all our ponds and rivers another snail, not breathing by means of lungs, but provided with gills, known as paludina. This paludina has a door to its shell, like the cyclostoma; and so, indeed, have all its allies. Now, strange as it sounds to say so, it is pretty certain that we must really class this lung-breathing cyclostoma among the gill-breathers, because of its close resemblance to the paludina. It is, in fact, one of these gill-breathing pond-snails which has taken to living on dry land, and so has acquired the habit of producing lungs. All molluscan lungs are very simple: they consist merely of a small sac or hollow behind the head, lined with blood-vessels; and every now and then the snail opens this sac, allowing the air to get in and out by natural change, exactly as when we air a room by opening the windows. So primitive a mechanism as this could be easily acquired by any soft-bodied animal like a snail. Besides, we have many intermediate links between the pond-snails and my cyclostoma here. There are some species which live in moist moss, or the beds of trickling streams. There are others which go further from the water, and spend their days in damp grass. And there are yet others which have taken to a wholly terrestrial existence in woods or meadows and under heaps of stones. All of them agree with the pond-snails in having an operculum, and so differ from the ordinary land and river snails, the mouths of whose shells are quite unprotected. Thus land-nails have two separate origins — one large group (including the garden-snail) being derived from the common fresh-water mollusks, while another much smaller group (including the cyclostoma) is derived from the operculated pond-snails.
How is it, then, that naturalists had so long overlooked this distinction? Simply because their artificial classification is based entirely upon the nature of the breathing apparatus. But, as Mr. Wallace has well pointed out, obvious and important functional differences are of far less value in tracing relationship than insignificant and unimportant structural details. Any water-snail may have to take to a terrestrial life if the ponds in which it lives are liable to dry up during warm weather. Those individuals alone will then survive which display a tendency to oxygenise their blood by some rudimentary form of lung. Hence the possession of lungs is not the mark of a real genealogical class, but a mere necessary result of a terrestrial existence. On the other hand, the possession of an operculum, unimportant as it may be to the life of the animal, is a good test of relationship by descent. All snails which take to living on land, whatever their original form, will acquire lungs: but an operculated snail will retain its operculum, and so bear witness to its ancestry; while a snail which is not operculated will of course show no tendency to develop such a structure, and so will equally give a true testimony as to its origin. In short, the less functionally useful any organ is, the higher is its value as a gauge of its owner’s pedigree, like a Bourbon nose or an Austrian lip.