Then it is highly probable, that the great continents were not yet formed, but that a series of islands, barrier reefs, and inland seas, prevailed generally over the earth’s surface, being still chiefly oceanic. Consequently no great rivers could, in such circumstances, be in existence, rolling down like the Ganges, Nile, and Mississippi more stony detritus and mud than arborescent matter, and all to be mixed and confounded in one indiscriminate mass. Atmospheric influences, too, must have been widely different from what they now are; for all the cast-off apparel of a summer’s luxuriance is, we see year after year, speedily dissipated by the droughts, or absorbed back as _humus_ into the earth, and when spring returns the ground is parched and bare. A difference of temperature must also be taken into the list of modifying causes; for the plants, during the coal era, are nearly of a class—a few great types with little variety of structure—one and the same in every region—and approaching the characters, most of them, of the existing tropical flora. The climate, according to Mr. Bunbury, was characterized by excessive moisture, by a mild and steady temperature, and the entire absence of frost; and it has been established by Mr. Darwin’s interesting observations on Chiloe and other islands of the southern temperate zone, that extreme heat is not necessary to the existence of a very luxuriant and quasi-tropical vegetation. Mr. Austen, on the other hand, thinks that the temperature of Great Britain has not much changed since the coal period, because few of the fossil-ferns, found in the coal-measures, present any fructification, while those in more southern latitudes possess it; and, by experiments made by himself, it appears that the existing ferns of tropical climates would not fructify at a low temperature. Still, the great general fact remains unquestioned, that tree-ferns during the carboniferous age grew gigantically and in vast forests, where they do not grow at present over all the zones of the earth; and where now growing, in three out of the four zones, that the whole family are reduced to the size of small herbaceous plants.