BOTANY. 



ON THE SUCCESSIVE PERIODS OF VEGETATION OCCUPYING THE 



SURFACE OF THE EARTH. 



WHEN we compare the different forms of plants which have in- 

 habited the surface of the earth at different epochs of its formation, we 

 shall perceive that great differences present themselves in the na- 

 ture of the vegetables which have been successively developed, and 

 have replaced those destroyed by the revolutions and the changes in 

 the physical condition of its surface. These are not mere specific dif- 

 ferences, slight modifications of the same types ; but more frequently 

 they are profound differences, in such sort that new genera or families 

 take the place of genera and families destroyed and completely dis- 

 tinct ; or a numerous and varied family is reduced to a few species, 

 whilst another, poorly represented by a few individuals, becomes all 

 at once numerous and predominant. This is what strikes us most 

 commonly in passing from one geological formation to another ; but 

 in considering these transformations collectively, a more general and 

 more important result presents itself in an unmistakable manner, 

 namely, the predominance in the most ancient times of Acrogenous 

 Cryptogamic plants (Ferns and Lycopodiacea?) ; later, the predomi- 

 nance of Gymnospermous Dicotyledons (Cycadeas and Coniferae), 

 without the admixture yet of a single Angiospermous Dicotyledon ; 

 finally, during the cretaceous formation, the appearance, and soon the 

 predominance, of Angiospermous plants, both Dicotyledons and Mono- 

 cotyledons. These very remarkable differences in the composition of, 

 the vegetation of the earth show that we may divide the long series 

 of ages which have presided over this successive birth of the different 

 forms of the vegetable kingdom into three long periods, which I shall 

 denominate the reign of the Acrogens, that of the Gymnosperms, 

 and that of the Angiosperms. These expressions merely indicate the 

 successive predominance of each of these three great divisions of the 

 vegetable kingdom, and not the entire exclusion of the others ; thus 

 in the two first, the Acrogens and the Gymnosperms exist simultane- 

 ously, only the former prevail at first over the latter in number and in 



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