There has been a gradation in time for the vegetable as well as the animal world. With the marine population of the more ancient geological ages we find nothing but sea-weeds,—of great variety, it is true, and, as it would seem, from some remains of the marine Cryptogams in early times, of immense size, as compared with modern sea-weeds. But in the Carboniferous period, the plants, though still requiring a soaked and marshy soil, were aërial or atmospheric plants: they were covered with leaves; they breathed; their fructification was like that which now characterizes the ferns, the club-mosses, and the so-called "horse-tail plants," (Equisetaceae,) those grasses of low, damp grounds remarkable for the strongly marked articulations of the stem.
These were the lords of the forests all over the world in the Carboniferous period. Wherever the Carboniferous deposits have been traced, in the United States, in Canada, in England, France, Belgium, Germany, in New Holland, at the Cape of Good Hope, and in South America, the general aspect of the vegetation has been found to be the same, though characterized in the different localities by specific differences of the same nature as those by which the various floras are distinguished now in different parts of the same zone. For instance, the Temperate Zone throughout the world is characterized by certain families of trees: by Oaks, Maples, Beeches, Birches, Pines, etc.; but the Oaks, Maples, Beeches, Birches, and the like, of the American flora in that latitude differ in species from the corresponding European flora. So in the Carboniferous period, when more uniform climatic conditions prevailed throughout the world, the character of the vegetation showed a general unity of structure everywhere; but it was nevertheless broken up into distinct botanical provinces by specific differences of the same kind as those which now give such diversity of appearance to the vegetation of the Temperate Zone in Europe as compared with that of America, or to the forests of South America as compared with those of Africa.
There can be no doubt as to the true nature of the Carboniferous forests; for the structural character of the trees is as strongly marked in their fossil remains as in any living plants of the same character. We distinguish the Ferns not only by the peculiar form of their leaves, often perfectly preserved, but also by the fructification on the lower surface of the leaves, and by the distinct marks made on the stem at their point of juncture with it. The leaf of the Fern, when falling, leaves a scar on the stem varying in shape and size according to the kind of Fern, so that the botanist readily distinguishes any particular species of Fern by this means,—a birth-mark, as it were, by which he detects the parentage of the individual. Another indication, equally significant, is found in the tubular structure of the wood in Ferns. On a vertical section of any well-preserved Fern-trunk from the old forests the little tubes may be seen very distinctly running up its length; or, if it be cut through transversely, they may be traced by the little pores like dots on the surface. Trees of this description are found in the Carboniferous marshes, standing erect and perfectly preserved, with trunks a foot and a half in diameter, rising to a height of many feet. Plants so strongly bituminous as the Ferns, when they equalled in size many of our present forest-trees, naturally made coal deposits of the most combustible quality. It is true that we find the anthracite coal of the same period with comparatively little bituminous matter; but this is where the bitumen has been destroyed by the action of the internal heat of the earth.
Next to the Ferns, the Club-Mosses (Lycopodiacae) seem to have contributed most largely to the marsh-forests. They were characterized, then, as now, by the small size of the leaves growing close against the stem, so that the stem itself, though covered with leaves, looks almost naked, like the stem of the Cactus. Beside these, there are the tree-like Equiseta, in which we find the articulations on the trunk corresponding exactly to those now so characteristic of those marsh-grasses which are the modern representatives of this family of plants, with cone-like fructifications on the summit of the stem.
I would merely touch here upon a subject which does not belong to my own branch of Natural History, but is of the greatest interest in botanical research, namely, the gradation of plants in the geological ages, and the combination of characters in some of the earlier vegetable forms, corresponding to that already noticed in the ancient animal types. For instance, in the Carboniferous period we have only Cryptogams, Ferns, Lycopodiacae, and Equisetaceae. In the middle geological ages, Conifers are introduced, the first flowering plant known on earth, but in which the flower is very imperfect as compared with those of the higher groups. The Coniferae were chiefly represented in the middle periods by the Cycadae, that peculiar group of Coniferae, resembling Pines in their structure, but recalling the Ferns by their external appearance. The stem is round and short, its surface being covered with scars similar to those of the Ferns; while on the summit are ten or more leaves, fan-like and spreading when their growth is complete, but rolled up at first, like Fern-leaves before they expand. Their fruit resembles somewhat the Pine-Apple.
The mode of growth of the Coniferae recalls a feature of the Equisetaceae also, in the tufts of little leaves which appear in whorls at regular intervals along the length of the stem in proportion as it elongates, reminding one of the articulations on the stem of the Equisetaceae. The first cone also appears on the summit of the stem, like the terminal cone in the Equisetaceae and the Club-Mosses. Thus in certain types of the vegetable, as well as the animal creation of earlier times, there was a continuation of features, afterwards divided and presented in separate groups. In the present times, no one of these families of plants overlaps the others, but each has a distinct individual character of its own.
At the close of the middle geological ages and the opening of the Tertiary periods, the Monocotyledons become abundant, the first plants with flower and inclosed seed, though with no true floral envelope: but not until the two last epochs of the Tertiary age do we find in any number the Dicotyledonous plants, in which flower and fruit rise to their highest perfection. Thus there has been a procession of plants from their earliest introduction to the present day, corresponding to their botanical rank as they now exist, so that the series of gradation in the Vegetable Kingdom, as well as the Animal Kingdom, is the same, whether founded upon succession in time or upon comparative structural rank.
Some attempt has been made to reproduce under an artistic form the aspect of the world in the different geological ages, and to present in single connected pictures the animal and vegetable world of each period. Professor F. Unger, of Vienna, has prepared a collection of fourteen such sketches, entitled, "Tableaux Physionomiques de la Végétation des Diverses Périodes du Monde Primitif."
First, we have the Devonian shores, with spreading fields of sea-weed and numbers of the club-shaped Algae of gigantic size. He has ventured, also, to represent a few trees, with scanty foliage; but I believe their existence at so early a period to be very problematical.
Next comes the Carboniferous forest, with still pools of water lying between the Fern-trees, which, much as they affect damp, swampy grounds, seem scarcely able to find foothold on the dripping earth. Their trunks, as well as those of the Club-Moss trees which make the foreground of the picture, stand up free from any branches for many feet above the ground, giving one a glimpse between them into the dim recesses of this quiet, watery wood, where the silence was unbroken by the song of birds or the hum of insects. We shall find, it is true, when we give a glance at the animals of this time, that certain insects made their appearance with the first terrestrial vegetation; but they were few in number and of a peculiar kind, such as thrive now in low, wet lands.