Some years ago it was estimated, by Brown, that of the one thousand species of plants found in the rocks of the Primary Age, especially of the Carboniferous period, not less than eight hundred and seventy-two were Fern-like, the remaining species including about seventy-seven Coniferæ and Cycadæ, forty Thallophytes, mostly Algæ, and about twenty undetermined plants. We see from this estimate that the Fern-like plants were the characteristic feature of the Carboniferous period, and must have flourished in a much greater profusion than at the present day, the Tree-ferns of tropical climates, even, giving one no idea of the luxuriance of their growth in those ancient days. Indeed, whole orders have passed away: the Calamites and Asterophyllites, resembling the Horse-tails, having no living representatives, while the Sigillariæ and Lepidodendrons have degenerated into the Club-moss of our forests. As commonly known, the Lycopod of the woods is a delicate moss-like plant: that of the Sunda Islands is often twenty-five feet high. The Lepidodendrons of the Carboniferous period, closely allied to living Club-moss, attained, however, a height of from forty to sixty feet, while their diameter at the roots was as much as twelve to fifteen feet. The Sigillariæ are similar in many respects to the Lepidodendrons, often as high, though more slender. The general aspect of the Carboniferous period was that of a great Fern forest and a jungle of gigantic Club-mosses, with some Coniferæ and Cycadæ; these, however, but rarely seen, comparatively speaking. The gradual decomposition of these plants resulted in the formation of the vast coal-fields so characteristic of this period. In the marshes of these forests first appeared the Batrachia (Frogs, etc.), together with the Centipedes, the May-fly, Locust, and Beetle orders among Insects. We see, therefore, that the tree of the development of life, as proposed in the chapters on Botany and Zoology, is in perfect
