1. The vegetation of the globe has varied at different epochs of the earth's history.
2. 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.
3. All fossil plants may be referred to the great classes of plants of the present day, Acotyledons, Monocotyledons, and Dicotyledons.
4. The fossil species are different from those of the present flora, and it is only when we reach the Tertiary periods that we meet with some genera which are without doubt identical.
5. Fossil plants are preserved in various conditions, according to the nature of their structure, and the mode in which they have been acted upon. Sometimes mere casts of the plants are found, at other times they are carbonised and converted into coal, while at other times, besides being carbonised, they are infiltrated with calcareous or siliceous matter, and finally, they may be petrified.
6. Cellular plants, and the cellular portions of vascular plants, have rarely been preserved, while woody species, and especially Ferns, which are very indestructible, have retained their forms in many instances.
7. In some cases, especially when silicified or charred, the structure of the woody stems can be easily seen in thin sections under the microscope.
8. The determination of fossil plants is a matter of great difficulty, and requires a thorough knowledge of structure, and of the markings on stems, roots, etc.
9. The rocks containing organic remains are called fossiliferous, and are divided into Primary, Secondary, and Tertiary, or into Palæozoic, Mesozoic, and Cainozoic, each of these series being characterised by a peculiar facies of vegetable life.
10. The mere absence of organic remains will not always be a correct guide as to the state of the globe.