Devonian.
Palæozoic
VI. Older Palæozoic.
Eozoic
VII. Archæan.
Now the actual length of these various periods was very different. The epoch of the Present Day is only in its commencement, and is like a thin line if compared with the broad bands of the past epochs. By far the greatest of the periods is the Archæan, and even the Older Palæozoic is probably longer than all the others taken together. It is, however, so remote, and the rocks which were formed in it retain so little plant structure that is decipherable, so few specimens which are more than mere fragments, that we know very little about it from the point of view of the plant life of the time. It includes the immense indefinite epochs when plants began to evolve, and the later ones when animals of many kinds flourished, and when plants, too, were of great size and importance, though we are ignorant of their structure. Of all the seven divisions of time, we can say least about the two earliest, simply for want of anything to say which is founded on fact rather than on theoretical conclusions.
Although these periods seem clearly marked off from one another when looked at from a great distance, they are, of course, but arbitrary divisions of one long, continuous series of slow changes. It is not in the way of nature to make an abrupt change and suddenly shut off one period—be it a day or an æon—from another, and just as the seasons glide almost imperceptibly into one another, so did the great periods of the past. Thus, though there is a strong and very evident contrast between the plants typical of the Carboniferous period and of the Mesozoic, those of the Permian are to some extent intermediate, and between the beginning of the Permian and the end of the Carboniferous—if judged by the flora—it is often hard to decide.
It must be realized that almost any given spot of land—the north of England, for example—has been beneath the sea, and again elevated into the air, at least more than once. That the hard rocks which make its present-day hills have been built up from the silt and débris under an ocean, and after being formed have seen daylight on a land surface long ago, and sunk again to be covered by newer deposits, perhaps even a second or a third time, before they rose for the time that is the present. Yet all these profound changes took place so slowly that had we been living then we could have felt no motion, just as we feel no motion to-day, though the land is continuing to change all around us. The great alternations between land and water over large areas mark out to some extent the main periods tabulated on [p. 34], for after each great submersion the rising land seems to have harboured plants and animals with somewhat different characters from those which inhabited it before. Similarly, when the next submersion laid down more rocks of limestone and sandstone, they enclosed the shells of some creatures different from those which had inhabited the seas of the region previously.
Through all the periods the actual rocks formed are very similar—shales, limestones, sandstones, clays. When any rocks happen to have preserved neither plant nor animal remains it is almost impossible to tell to which epoch they belong, except from a comparative study of their position as regards other rocks which do retain fossils. This depends on the fact that the physical processes of rock building have gone on throughout the history of the globe on very much the same lines as they are following at present. By the sifting power of water, fine mud, sand, pebbles, and other débris are separated from each other and collected in masses like to like. The fine mud will harden into shales, sandgrains massed together harden into sandstones, and so on, and when, after being raised once more to form dry land, they are broken up by wind and rain and brought down again to the sea, they settle out once again in a similar way and form new shales and sandstones; and so on indefinitely. But meantime the living things, both plant and animal, have been changing, growing, evolving, and the leafy twig brought down with the sandgrains in the flooded river of one epoch differs from that brought down by the river of a succeeding epoch—though it might chance that the sandgrains were the same identical ones. And hence it is by the remains of the plants and animals in a rock that we can tell to which epoch it belonged. Unless, of course, ready-formed fossils from an earlier epoch get mixed with it, coming as pebbles in the river in flood—but that is a subtle point of geological importance which we cannot consider here. Such cases are almost always recognizable, and do not affect the main proposition.
From the various epochs, the plants which have been preserved as fossils are in nearly all cases those which had lived on the land, or at least on swamps and marshes by the land. Of water plants in the wide sense, including both those growing in fresh water and those in the sea, we have comparatively few. This lack is particularly remarkable in the case of the seaweeds, because they were actually growing in the very medium in which the bulk of the rocks were formed, and which we know from recent experiments acts as a preservative for the tissues of land plants submerged in it. It must be remembered, however, that almost all the plants growing in water have very soft tissues, and are usually of small size and delicate structure as compared with land plants, and thus would stand less chance of being preserved, and would also stand less chance of being recognized to-day were they preserved. The mark on a stone of the impression of a soft film of a waterweed would be very slight as compared with that left by a leathery leaf or the woody twig of a land plant.