74 M, Brongniart on the different Floras which 



composition of the vegetation of the earth, to which I called at- 

 tention long ago, and which all recent observations, properly 

 appreciated, appear to me to confirm, 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 Acro- 

 gens, 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 simultaneously, only 

 the former prevail at first over the latter in number and in size, 

 while in the later period the converse holds. But during these 

 two reigns, Angiospermous plants appear to be wholly absent, or 

 are only announced by a few rare signs, doubtful and very dif- 

 ferent from existing forms, marking moreover the presence of a 

 few Monocotyledons rather than that of Angiospermous Dicoty- 

 ledons. 



Each of these three reigns thus characterized by the predo- 

 minance of one of the great divisions of the vegetable kingdom, 

 is commonly subdivided into several periods, during which forms 

 very analogous, belonging to the same families and often to the 

 same genera, are perpetuated; then these periods themselves 

 comprise several epochs, during which vegetation does not appear 

 to have undergone any notable changes. But in many cases we 

 are still without materials for establishing these last subdivisions 

 with precision, either from the fact that the exact geological 

 position of the strata which inclose the impressions of the plants 

 is not well determined, or that the mode of distribution of the 

 species of plants in the different layers of the same formation, 

 has not been carefully made out. I doubt not therefore that 

 these different epochs, during which the vegetation has preserved 

 its characters in an invariable manner, will be multiplied much 

 more considerably than can be done in the actual state of our 

 knowledge, when carefully collected materials have been brought 

 together in greater abundance. 



For the moment, I believe the following general division may 

 be admitted : — 



I. Beign of the Acrogens. 



1. Carboniferous Period. 



(Not divisible into distinct epochs in the existing state of 

 knowledge.) 



2. Perm^n Period. 



(Forming but one epoch.) 



