RETROSPECT OF FOSSIL BOTANY.
RETROSPECT OF FOSSIL BOTANY.
If we pass from the consideration of details of structure, and of botanical affinities, to a general survey of the mineralized remains of the vegetable kingdom, we perceive that from the palæozoic deposits, to those which are contemporaneous with the human race,—from the coal-measures to the peat-bogs of modern times,—vast accumulations of vegetable matter, in various states of carbonization, have been produced from the imbedded relics of the terrestrial floras that flourished during the respective periods of their formation; petrifaction, or the transmutation of vegetable tissues into stone, from the infiltration of siliceous, calcareous, or metallic solutions, being an accidental process, dependent on the physical conditions under which the trees and plants were submerged, and entombed in the strata.
Although the entire system of vegetable life which prevailed during the earlier ages of the world is but partially revealed by the fossil remains which geological researches have brought under the examination of the naturalist,—for numerous tribes of plants may have existed of which no traces have been detected, while of species of delicate tissues all vestiges may have perished,—yet a review of the facts hitherto obtained, presents some highly important and unexpected results, as to the characters of the successive floras which prevailed during the palæozoic, secondary, and tertiary epochs. And though deductions of this nature must, in the present state of our knowledge, be regarded in the light of shifting hypotheses to be modified or abandoned with the progress of discovery,—yet the predominating types which characterize the flora of one system of formations, differ so essentially from those of another, that it may be reasonably inferred such apparent distinctions are the effect of organic laws, and not illusions arising from our misinterpretation of the natural records of former conditions of the vegetable world.
The absence in the most ancient deposits of the entire class of Angiosperms, or flowering plants, which constitutes the leading features of the floras with which we are familiar,—the abundance of unknown types of Cryptogamia, and the extinction or disappearance of those tribes in the succeeding formations, and the prevalence of new species and genera belonging to another class;—the predominance in one flora, both in number and variety, of certain tribes, and their decadence in the next period; while a family subordinate in the antecedent epoch, and known but by a small number of species, suddenly acquires a pre-eminence both in numbers and variety;—are phenomena, which the facts brought before us in the course of this argument, present in a striking point of view.
Assuming these data as the basis of a philosophical generalization, M. Brongniart arranges the known species of fossil plants into three grand systems, which correspond with the great geological periods, comprehended in the palæozoic, secondary, and tertiary formations.
The first or most ancient flora is characterized by the predominance of Cryptogamic Acrogens—the Ferns and Club-mosses; the second by the large development of the Dicotyledonous Gymnosperms—the Cycads and Conifers: the third by the appearance and prevalence of the Angiosperms, both dicotyledonous and monocotyledonous. The following table presents a concise view of the results of M. Brongniart's investigation.[184]
[184] For details, and a masterly review of the subject, the original Memoir must be consulted. See Tableau des Genres de Végétaux Fossiles, considérés sur le point de vue de leur Classification Botanique et de leur Distribution Géologique, par M. Adolphe Brongniart. Paris, 1849.
| CHARACTER OF THE FLORAS. | GEOLOGICAL EPOCHS. | |
| I. Règne des Acrogènes; the Flora of Vascular Cryptogamia. | } | The Devonian, Carboniferous, and Permian, Formations. |
| II. Règne des Gymnospermes; the Flora of Cycadaceæ and Coniferæ. | } | The Triassic, Jurassic (or Lias and Oolite), and Wealden, Formations. |
| III. Règne des Angiospermes. Flora of Dicotyledonous and Monocotyledonous flowering plants, or Angiosperms. | } | The Cretaceous, and Tertiary (Eocene, Miocene, and Pliocene), Formations. |
It must be observed that this table is only designed to indicate the successive predominance of each of the three classes of the vegetable kingdom, in the respective epochs, and not the entire exclusion of the others. Thus, in the two first, both Acrogens and Gymnosperms existed; but in the first period the former greatly exceeded the latter, both in number and magnitude; while in the next the Gymnosperms acquired the ascendancy; but in both these epochs, from the Devonian to the Wealden inclusive, very few if any Angiosperms, or flowering dicotyledons, existed. With the Cretaceous period the Angiosperms appear in great numbers, and in the Tertiary epochs acquire the importance they possess in the existing floras.