Page:Darwinism by Alfred Wallace 1889.djvu/422

{{RunningHeader|398|DARWINISM|CHAP. plants; and although there are still indications of vast gaps in our knowledge, due, no doubt, to the very exceptional conditions required for the preservation of plant remains, we now possess evidence of a more continuous development of the various types of vegetation. According to Mr. Lester F. Ward, between 8000 and 9000 species of fossil plants have been described or indicated; and, owing to the careful study of the nervation of leaves, a large number of these are referable to their proper orders or genera, and therefore give us some notion—which, though very imperfect, is probably accurate in its main outlines—of the progressive development of vegetation on the earth. The following is a summary of the facts as given by Mr. Ward:—

The lowest forms of vegetable life—the cellular plants—have been found in Lower Silurian deposits in the form of three species of marine algae; and in the whole Silurian formation fifty species have been recognised. We cannot for a moment suppose, however, that this indicates the first appearance of vegetable life upon the earth, for in these same Lower Silurian beds the more highly organised vascular cryptogams appear in the form of rhizocarps—plants allied to Marsilea and Azolla,—and a very little higher, ferns, lycopods, and even conifers appear. We have indications, however, of a still more ancient vegetation, in the carbonaceous shales and thick beds of graphite far down in the Middle Laurentian, since there is no other known agency than the vegetable cell by means of which carbon can be extracted from the {{hws|atmo|atmosphere}}