Some Salient Points in the Science of the Earth

by Sir John William Dawson

If, for convenience of reference, we divide the whole history of the earth, from the time when a solid crust first formed on its surface and began to be ridged up into islands or mountains in the primeval ocean, into four great periods, we shall find that each can be characterized by some features in relation to the world of plants.

That Archean age, in which the oldest known beds of rocks were produced—rocks now greatly crumpled by the first movements of the thin crust, and hardened and altered by heat and pressure has, it is true, little to tell us. But, as elsewhere stated, even it has beds of Carbon in the form of Graphite—veritable altered coal seams—which the analogy of later formations would lead us to believe must have been accumulated by the growth of plants. This growth is indeed the only known cause capable of producing such effects. If we should ever be fortunate enough to find beds of the Laurentian series in an unaltered state, we may hope to know something of this old flora. Nor need we be surprised if it should prove of higher grade and more noble development than we should at first sight anticipate. If there ever was a time when vegetation alone possessed the earth, and when there were no animals to devour or destroy it, we might expect to find it in its first and best estate, perhaps not comparable in variety and complexity of parts with the flora of the modern world, but grand in its luxuriance and majesty. Of such discoveries, however, we have no certain indication at present.

If such a primeval flora as that above indicated ever existed, it must have perished utterly before the incoming of the next great age of the world—that known as the Palæozoic, whose rocks are surpassingly rich in the remains of animals, especially those of the lower or invertebrate classes and those that inhabit the waters.

In the oldest Palæozoic rocks we find no plants certainly terrestrial, but abundance of Algæ or seaweeds, and some gigantic members of the vegetable kingdom which seem to have been trees, with structures more akin to those of aquatic than to those of land plants.[82] At a somewhat early stage, however, in the rocks of this period, we discover a few undoubted land plants.[83] These seem to be allied to the modern Club mosses and to their humble relations, the pillworts[84] and other small plants of similar structure found in ponds and swamps. Some of them, indeed, appear to be intermediate between these groups. All these plants are Cryptogams, or destitute of true flowers, but do not belong to the lowest forms of that type. Thus, so far as we know, plant life on the land began possibly with certain large trees of algoid structures, and more certainly with the club mosses and pillworts and their allies, and these last in the form of species not tree-like in dimensions, but of very moderate size. The structures of these plants are already sufficiently well known to inform us that the plan and functions of the root, stem and leaf, and of spores and spore case were set up; and that the structures and functions of vegetable cells, fibres and some kinds of vessels were perfected, and all the apparatus introduced necessary for the fertilization and reproduction of plants of some degree of complexity. At the same time, the peculiar structures of the higher Algæ were brought to a pitch of perfection not surpassed if equalled in modern times, and which may have enabled plants so constructed to exist even on the land.

[82] Nematophyton, etc. See "Geological History of Plants."

[83] Psilophyton, Protannularia, etc.

[84] Rhizocarpeæ.

From these beginnings in the early Palæozoic, the progress of the vegetable kingdom went on, until, in the later parts of that great period, the Devonian and Carboniferous eras, it culminated in those magnificent forests which have left so many interesting remains, and which accumulated the materials of our great beds of coal. In these the families of the Club mosses, the Ferns and the Mare's-tails attained to a perfection in structure and size altogether unexampled in the modern world, and may be said to have overspread the earth almost to the exclusion of other trees. Here, however, two new families come in of higher grade, and leading the way to the flowering plants. These are the Pines and their allies and the Cycads, and certain intermediate forms, neither Pines nor Cycads, but allied to both.[85] This wonderful flora, which we have now the materials to reproduce in imagination almost in its entirety, decays and passes away in the Permian system, the last portion of the Palæozoic, and in entering into the third great period of the earth's history—the Mesozoic, we again find an almost entire change of vegetation. Here, however, we are able to understand something of the reasons of this. The Palæozoic floras seem to have originated in the North, and propagated themselves southward till they replenished the earth, and they were favoured by the existence at that time of vast swampy flats extending over great areas of the yet imperfectly elaborated continents. The Mesozoic floras, on the other hand, seem to have been of Southern or equatorial origin, and to have followed up the older vegetation as it decayed and disappeared, or retreated in its old age to its northern home. There is, of course, much in all this that we do not understand, but the general fact seems certain.