The study of these extinct forms has afforded valuable indications as to the physical state of the earth, and as to its climate at different epochs. This study requires the conjunct labours of the Zoologist, the Botanist, and the Petralogist. The vegetation of the globe, during the different stages of its formation, has undergone very evident changes. At the same time there is no reason to doubt that the plants may all be referred to the great classes distinguished at the present day--namely, Thallogens, including such plants as Lichens, Algæ, and Fungi; Acrogens, such as Ferns and Lycopods; Gymnosperms, such as Cone-bearing plants and Cycads; Endogens, such as Palms, Lilies, and Grasses; and Exogens, such as the common trees of Britain (excluding the Fir), and the great mass of ordinary flowering plants. The relative proportion of these classes, however, has been different, and the predominance of certain forms has given a character to the vegetation of different epochs. The farther we recede in geological history from the present day, the greater is the difference between the fossil plants and those which now occupy the surface. At the time when the coal-beds were formed, the plants covering the earth belonged to genera and species not existing at the present day. As we ascend higher, the similarity between the ancient and the modern flora increases, and in the latest stratified rocks we have in certain instances an identity in species and a considerable number of existing genera. At early epochs the flora appears to have been uniform, to have presented less diversity of forms than at present, and to have been similar in the different quarters of the globe. The vegetation also indicates that the nature of the climate was different from that which characterises the countries in which these early fossil plants are now found. DETERMINATION OF FOSSIL PLANTS. [Illustration: Fig. 1.] [Sidenote: Fig. 1. Section of _Peuce Withami_, after Lindley and Hutton, a fossil Conifer of the coal epoch. Punctated woody tissue seen.]