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. 1 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 alga?; 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 
berls 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 atmo- 
1 Sketch of Palasobotany in Fifth Annual Report of U. S. Geological Survey, 
1883-84, pp. 363-452, with diagrams. Sir J. William Dawson, speaking of 
the value of leaves for the determination of fossil plants, says : “ In my own 
experience I have often found determinations of the leaves of trees confirmed 
by the discovery of their fruits or of the structure of their stems. Thus, in 
the rich cretaceous plant-beds of the Dunvegan series, we have beech-nuts 
associated in the same bed with leaves referred to Fagus. In the Laramie 
beds I determined many years ago nuts of the Trapa or water-chestnut, and 
subsequently Lesquereux found in beds in the United States leaves which he 
referred to the same genus. Later, I found in collections made on the Red Deer 
River of Canada my fruits and Lesquereux’s leaves on the same slab. The 
presence of trees of the genera Carya and Juglans in the same formation was 
inferred from their leaves, and specimens have since been obtained of silicified 
wood with the microscopic structure of the modern butternut. Still we are 
willing to admit that determinations from leaves alone are liable to doubt.”-— 
The Geological History of Plants, p. 196. 
