We can not, however, attach any certain and definite value in years to geological time, but must content ourselves with the general statement that it has been vastly long in comparison to that covered by human history. Bearing in mind this great duration of geological time, and the fact that it probably extends from a period when the earth was intensely heated, its crust thin, and its continents as yet unformed, it will be evident that the conditions of life in the earlier geologic periods may have been very different from those which obtained later. When we further take into account the vicissitudes of land and water which have occurred, we shall see that such changes must have produced very great differences of climate. The warm equatorial waters have in all periods, as superficial oceanic currents, been main agents in the diffusion of heat over the surface of the earth, and their distribution to north and south must have been determined mainly by the extent and direction of land, though it may also have been modified by the changes in the astronomical relations and period of the earth, and the form of its orbit. We know by the evidence of fossil plants that changes of this kind have occurred so great as, on the one hand, to permit the plants of warm temperate regions to exist within the Arctic Circle; and, on the other, to drive these plants into the tropics and to replace them by Arctic forms. It is evident also that in those periods when the continental areas were largely submerged there might be an excessive amount of moisture in the atmosphere, greatly modifying the climate in so far as plants are concerned. Let us now consider the history of the vegetable kingdom as indicated in the few notes in the right-hand column of the table. The most general subdivision of plants is into the two great series of Cryptogams, or those which have no manifest flowers, and produce minute spores instead of seeds; and Phænogams, or those which possess flowers and produce seeds containing an embryo of the future plant. The Cryptogams may be subdivided into the following three groups: 1. _Thallogens_, cellular plants not distinctly distinguishable into stem and leaf. These are the Fungi, the Lichens, and the Algæ, or sea-weeds. 2. _Anogens_, having stem and foliage, but wholly cellular. These are the Mosses and Liverworts. 3. _Acrogens_, which have long tubular fibres as well as cells in their composition, and thus have the capacity of attaining a more considerable magnitude. These are the Ferns (_Filices_), the Mare’s-tails (_Equisetaceæ_), and the Club-mosses (_Lycopodiaceæ_), and a curious little group of aquatic plants called Rhizocarps (_Rhizocarpeæ_). The Phænogams are all vascular, but they differ much in the simplicity or complexity of their flowers or seeds. On this ground they admit of a twofold division: 1. _Gymnosperms_, or those which bear naked seeds not inclosed in fruits. They are the Pines and their allies, and the Cycads.