RETROSPECT.

N the preceding pages we have endeavoured to limn, but in faint and sketchy outlines, the biography of our planet. We now

propose still more briefly to connect the scattered ideas into a short summary. Perhaps the most difficult thing a person experiences when he comes into contact with geological teaching for the first time, is the great demand made upon his imagination for the article of Time, in which to account for geological phenomena.

It bewilders one to contemplate such a practical eternity, and we ask-" can all this be true? Many cannot accept the doctrine, but turn away sceptically discontented, thinking they are doing heaven service, by adhering to the older idea that the world is only some six thousand years old, as if the Deity were complimented by supposing His attributes were more honoured by limiting their display to six thousand years, than they are if extended into the past, and made eternal. The more we study the phenomena of geology, however,

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and the more knowledge of natural science we bring with us to the task, the more profoundly impressed do we become with the vast antiquity of our planet.

Many men fall into the error of supposing that in discovering new laws in the, universe, we are exiling the Deity, and giving to the operations of these laws the power that is really His. Even scientific men sometimes speak of the laws of nature as if they were entities, forgetting they use the term simply as a figure of speech. For, as Dugald Stewart has shown, the term "law" can only be applied in its correct sense to conscious agents, capable of understanding the rule of conduct laid down, of obeying it or disobeying it. When we apply it to such a system as that which guides the planets, which arranges the animal and vegetable kingdoms, and which directs the operations of physical geography, we are speaking of unconscious objects, which cannot obey law, inasmuch as they are not conscious of it. To their relationships, therefore, the word "law" is used as a figure of speech, and limits itself to the mode by which an active Providence is operating on matter.

Though it may seem strange indeed to hear that the world has been in existence millions of years, and that its surface has been covered by numberless creations of animals and plants, yet the true naturalist sees in these extinct faunas and floras, conjoined with the present, only one great and harmonious scheme! In the fossils of the rocks we have a graduated scale of animal and vegetable life, and we have

seen how, in spite of the imperfection of the geological record, it is possible to link object by object together, so that, when extended from the remote. past to the present, they form a connected chain.

Our planet's earliest existence seems to have been that of a cosmical mass-a sort of world-fog or vapour-something like those revealed by the telescope as being still in existence. Some of the best astronomers have shown that the probable origin of the entire Solar system has been a condensation of this cosmical vapour into planets, satellites, and planetary rings. Whether this was the case or not, it is certain that there is much in the shape and physical constitution of the planets to lend support to the idea. But, with the oblate shape of our globe, and its probable evolution from a cosmical mass, the geologist has little or nothing to do. But he knows, from the fact of igneous rocks having repeatedly been injected into the stratified rocks, so as to bind them together, as mortar does the bricks of a wall, that the interior of the globe still contains molten matter.

The first time the geologist can lay his hand on a formation distinct in its character from the primitive igneous rocks, it is when he comes to the Laurentian system. They are thirty thousand feet in thickness, and so contorted and changed by the pressure, heat, and mechanical forces to which they have been subjected since the infancy of the world, that all original characters have been obliterated. But, by

the aid of the microscope, the explorer is yet able to discern that the ancient sea along whose floors these mica-schists, gneisses, and quartzites were deposited as muds and sands was not a lifeless area, but was tenanted by lowly creatures after their kind. The only solitary known fossil from the altered limestones of the Laurentian formation-the Eozoon-is sufficient to prove this. And from the occurrence of this lowly-organized creature up to the present, we never afterwards lose sight of the graduated life-scheme recorded in the rocks! There is many a difficult chapter to spell out, many a leaf missing, but there is still sufficient left to interpret the stony scroll.

Above the Laurentian system lies the Cambrian. • But we should remember that this classification of the rocks into formations and systems is, at the best, but a harsh and forced one-a remnant of the time, not long ago, when men believed there were distinct creations and destructions of separate faunas and floras. Geological and, in fact, all natural history classification is but an arbitrary arrangement to enable the human mind, in its faintness, to grasp and arrange the multitudinous facts presented to it. In reality there is no separation of geological systems, but more or less of a graduation of one into another. The world's biography is like a man's, not like a butterfly's, consisting of metamorphosed states, each unlike the other, and definitely separated from it. In the Cambrian formation, we find that life, which had begun, as it were, from a point, was radiating like

the rays of light from a focus. Here we find the lowest order of shell-fish (brachiopods), worms, and, towards the later period, crustacea.

But it is in the Silurian system that we find the stream of life broadening out. The seas are full of coral-reefs, bivalve and univalve shells, huge crustacea, tolerably highly-endowed Trilobites, &c. At the close of the formation, we came on placoid fishes, the first vertebral types. Thus we find a lateral development of species, in size, and a vertical one in organization. Then comes the Devonian, or Old Red Sandstone epoch, whose seas abounded in strangely-clad and gigantic ganoid fishes, and whose deeper waters were busy with the manifold complexities of marine life. The dry land was scantily covered with a thin vegetation of a cryptogamous type, or of the lowliest of the exogens. Great freshwater lakes existed, set in beautiful frameworks of tree-fern and huge club-moss. But it is when the Carboniferous era commences that we find abundant evidence of a dense flora, although one of a very lowly kind. Every foot of dry land, where the circumstances were favourable, seems to have been densely covered with forests, the trees of which now find their nearest allies in our 66 Horse-Tails" and club-mosses. Enormous Sigillaria, Lepidodendra, and tree-ferns constituted this vegetation, whilst there was no lack of species of Conifera. In the Carboniferous limestone period, which immediately preceded that of the coal measures, we have ample