S8B M. Brongniart on the Vegetation of 



rally appear to belong tx> genera still existing ; and the specific 

 differences, although almost always perceptible when the fossil 

 plants are carefully compared with the living species of the same 

 genera, are often very slight. 



From the manner in which we have defined what we name a 

 period of vegetation, it may therefore be said that the vegeta- 

 tion which covered the earth, during the deposition of the ter- 

 tiary formations, and that which at present grows upon its sur- 

 face, belong to the same period. 



It will be seen that the manner in which we have above con- 

 sidered the essential characters of the vegetation, at the different 

 epochs of formation of the earth's crust, is almost entirely re- 

 moved from the influence of the errors of detail, which it is im- 

 possible to avoid in a science still in its infancy. Should more 

 precise observations or new discoveries make known in the old 

 formations some plants of more than one of the classes which 

 we have admitted, or even some species of one of the classes 

 which have appeared to us to be wanting at this epoch, still the 

 essential relations of these classes to each other would be but 

 slightly modified. Thus, it might be proved that certain yet 

 little known genera .of the coal formation are true dicotyledo- 

 nous plants ; yet it would not be the less certain that the vascu- 

 lar cryptogamic plants, were by much the most numerous vege- 

 tables during the first period of vegetation. Some leaves of 

 really dicotyledonous plants might be discovered In the Lias, or 

 Jura limestone ; yet these species being necessarily very rare, 

 would not change the essential relations between the number of 

 species belonging to the other classes, and the gymnospermous 

 phanerogamic plants ; the Cycadeae, in particular, would not the 

 less be the characteristic plants of this epoch. Thus, whatever 

 further discoveries may be made, it may be established with all 

 the certainty which the sciences of observation present, that the 

 essential characters of the four periods which we have above in- 

 dicated, can be but slightly modified, and that these periods 

 themselves will always remain perfectly distinct. 



The vast numerical predominance of the vascular cryptogam 

 mic plants, that is to say, of the Filices, Equisetaceae, and Ly- 

 copodiaceae, and the great development of these plants are the 

 essential characters of the first period. 



