( 320.) 
On the Chronological Exposition of the Periods of Vegetation, 
and the different Floras which have succeeded each other on 
the Earth’s surface. According to the views of M. BRone- 
NIART. 
If, after having studied fossil vegetables, with respect to 
their organization, and the manner of determining their rela- 
tions, with vegetables actually existing, without attending to 
the geological position which they occupy, we compare the 
different forms which have inhabited the surface of the earth 
at the different epochs of its formation, we shall find that 
great differences are observable in the nature of the vege- 
tables which have been successively developed, and which 
replaced those which had been destroyed by the revolutions 
of the globe, and changes in the physical state of its surface. 
These are not merely specific differences, or slight modi- 
fications of the same types, but often of a much more im- 
portant character,—such as new genera or families replacing 
genera and families destroyed and completely distinct ; or 
rather, a numerous and varied family is reduced to a few 
species, while another, which was faintly marked by a few 
rare individuals, becomes all of a sudden numerous and pre- 
dominating. 
This is what is most commonly remarked in passing from 
one geological formation to another ; but, when considering 
these transformations as a whole, a more general and im- 
portant result presents itself in an unquestionable manner, 
namely, the predominance, in the most ancient periods, of 
acrogenous-cryptogamous vegetables (Ferns and Lycopodia- 
cee); later, the predominance of gymnospermous dicotyle- 
dons (Cycadee and Coniferce), without any mixture hitherto 
of angiospermous dicotyledons ; and, in the last place, during 
the chalk formation, the appearance and speedy predomi- 
nance of angiospermous vegetables, both dicotyledons and 
monocotyledons. ‘These differences, so remarkable in the 
composition of the vegetation of the earth, which I have long 
since pointed out, and which all sound observations, of recent 
date, appear to me to confirm, shew that we may divide the 
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