The choicest production of nature for gardening purposes, is the simple natural loam of a fertile pasture, which has not been cultivated for many years. A thick old sod, with a bit of turfy loam, which has not been disturbed by the plow for a long time, contains the elements of a good soil, in a condition which can scarcely be imitated by the art of man. A grape border formed of such materials alone, finely broken up, is perhaps equal, at first, to any composition that can be made. Why is this? In what manner and by what agencies has nature elaborated in this natural soil the elements of plant-growth, the sources of such certain and perfect fertility? We have often heard these questions asked, but until recently have never heard or seen a philosophical solution of the problem. Gardeners generally look upon the thing as a great mystery, a secret past finding out. Geology, we think, penetrates the heart of this mystery, and reveals the secret in a most satisfactory manner.

Geology, investigating the nature of the various strata, rocks, and other component parts of the earth, has been compelled to examine carefully the processes by which soil is formed, and presents to us the astonishing but undeniable fact, that the most important agents in the formation of both rocks and soil have been insects, or animalculas, a vast proportion of them too small to be distinguished by the naked eye. Some of the limestone rocks are almost entirely composed of the remains of cretaceous or shellcovered animalcules, of a size so minute that millions are condensed within the limits of a cubic inch. The creation of immense rocky reefs, hundreds of miles in extent, by the coral insect, is another instance of the part performed by minute creatures in the formation of the earth. The true soil makers are also insects, and the chief of these is our old acquaintance, the common earth-worm, (Lumbricus terrestris,) or angle-worm, universally found in all good carbonaceous soils. This humble and seemingly useless creature is shown, by scientific research, to be the great natural mould-maker, or loam-former, of the entire earth.

It is this poor, senseless worm which composts the elements of virgin loam and rich turfy sod, so much prized by all gardeners, which prepares, by mastication and digestion, as it were, the mineral and vegetable parts of the soil, the rocks and leaf-mould which form the basis of all soils, but which, without the agency of this worm, would lie separate, unmixed, and unfertile. How this is done, let us consider.

The ant and the earth-worm are called by geologists the pioneers of cultivation. As the honey-bee precedes the march of civilization towards the great forests, so the ant and the earth-worm precede the gardener in the cultivation of the soil. On a newly-made gravel walk, which is a good representation of a barren, rocky soil, the first insect which makes its appearance is the industrious little ant. These insects burrow into the sand, work it over and over, drop their excrement, die, and mingle their bodies with the ant-heaps, attract other insects, collect vegetable and animal matter, and thus organize the first elements of soil. Now coarse, rank grass starts around and among these ant-hills, and these primitive forms of vegetable life dying, create the food of new and higher classes of plants, which, again dying, year after year, soon deposit a layer of vegetable matter or leaf-mould upon the surface of the sand and gravel. After this comes in the earth-worm, and the richer the layer of vegetable matter, the sooner does the great soil-maker appear upon the scene.

The earth-worm, it would seem, feeds not only upon vegetable matter, but upon sand and rotten rock, or at least in its travels up and down through the surface material of the earth it passes through its intestines not only vegetable fibre, but coarse particles of sand, and a great variety of rocky substances rich in mineral elements, which it mixes, ferments, and composts, and finally ejects, in the shape of excrementitious worm-casts, upon the surface of the earth, thus furnishing the materials of pure loam, in the most perfectly combined and most finely divided form; just that rich, but simple, pure, natural, mysterious "bit of virgin loam," which the experienced gardener, the world over, so ardently covets. Neither vegetable matter, nor mineral matter, unmixed, will create a fertile soil. Nor will merely mixing these substances together form a perfect loam. The rocks must be weather-worn, and decomposed by the action of air and water, by oxygen and carbonic acid. The vegetable fibre must decay and become converted into humus or pure mould.

The earth-worm must feed upon these changing substances, and thns hasten the chemical changes, by its simple digestion, mixing and combining all in its poor maw, and finally ejecting its manurial treasures upon the surface of the earth, rich, carefully prepared, perfect plant-food, containing all the elements of fertility, united with a degree of skill beyond the art of the chemist or the gardener.

Here, then, is the great vegetable mould-maker, the great natural compost-former, a common earth-worm. It is said by scientific observers, that the earth-worm will create, by its excrement ejected in the shape of worm-casts, an inch in depth of mixed or perfect soil over the entire surface of a rich field within the short period of three years. But the labors of the worm do not end with the first formation of the soil; it is constantly at work, opening the subsoil, and admitting air and water through its barrows, and bringing up the rich mineral elements from below, well digested and divided, to restore the waste of cultivation. And it is this poor worm which when we rest our exhausted soil, in a measure renews the fertility of our fields, by its never-ceasing chemical and mechanical labors in sand and leaf-mould. The presence of the earth-worm, and its little heaps of masticated soil, in large numbers, may thus be taken as the evidences of a fertile loam; for the little mounds which the worm throws up from its burrows is the great natural fertilizer of the earth, before which the special fertilizers of man sink into comparative insignificance, as crude and ignorant compounds.

In view of these facts, the gardener will no longer look upon the earthworm (as he has sometimes done) as a worthless and destructive creature, which eats up his good loam, but as his co-laborer in the cultivation of the earth, his superior as a compost-maker and sod-former, and the manufacturer of his much-prized virgin loam. Thus happily does science penetrate and explain the seeming mysteries of nature.

[A happy defence of one of the gardener's best friends. There is really nothing useless in the vast world of nature; nothing made in vain. In connection with this subject, we may mention that we have only to cover a piece of ground for a while with some boards to greatly increase its fertility: the reason of this is, that the boards afford a convenient shelter for the worms to work under, and the earth will be found full of their exuviae. For the same reason, the soil under surface rocks is always more fertile than that surrounding them; and many similar cases will be suggested by Mr. Bright's article. We hope he will continue a subject he has so well begun. - Ed].