898 DARWINISM chap. 



plants ; and although there are still indications of vast gaps in 

 our knowledge, due, no doul)t, 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 develoj^ment 

 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 

 sup2)ose, 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 

 be. Is 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- 



^ Sketch of Paloeohotany 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-nnts 

 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 Avas 

 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 doiibt." — 

 The GeUoykal History of Plants, p, 196, 



