The peculiar character of the vegetable tissue in the trees of the Carboniferous period, containing, as it did, a large supply of resin drawn from the surrounding elements, confirms the view of the atmospheric conditions above stated; and this fact, as well as the damp, soggy soil in which the first forests must have grown, accounts for the formation of coal in greater quantity and more combustible in quality than is found in the more recent deposits. But stately as were those fern forests, where plants which creep low at our feet to-day, or are known to us chiefly as underbrush, or as rushes and grasses in swampy grounds, grew to the height of lofty trees, yet the vegetation was of an inferior kind. 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.