328 Different Floras of the Rock- Formations. 
Does this vegetation, thus reduced to the forms which we 
are led to consider as the most simple and least perfect, 
owe this special nature to a first phase of development in the 
organization of the vegetable kingdom, which had. not, yet 
attained the perfection which it afterwards reached ; or. is 
it owing to the influence of the physical conditions under 
which the surface of the earth was then placed? These are 
questions we cannot answer. 
I shall merely remind the reader that I have already 
pointed out the analogy which this predominance of Acro- 
genous Cryptogams establishes between the vegetation of 
this first period, and that of the small islands of the equato- 
rial and southern temperate zone, in which the maritime cli- 
mate reaches its highest degree. 
However, this predominance is not such as to entail, as 
during the carboniferous period, the exclusion of phanero- 
gamous vegetables, and this complete exclusion would seem 
more favourable to the idea of a gradual development. of the 
vegetable kingdom. 
Lastly, we are not sufficiently acquainted stith the influence 
of the nature of the atmosphere on the life of vegetables, 
when it is prolonged during their whole existence, to know 
whether considerable differences. in the composition of this 
atmosphere, and in particular the very probable presence of 
a much greater proportion of carbonic acid, might not favour 
the existence of certain classes of the vegetable kingdom, 
and be opposed to that of other groups. 
I shall terminate this glance at the vegetation of the car- 
boniferous period, by remarking, that the coal-formation, 
which is almost the only one containing these remains, is -- 
evidently a terrestrial formation and from fresh water ; that > 
the beds of coal which it contains are the result of the accu- a 
mulation on the place of the remains of vegetables which . 
covered the surface after the manner of layers of turf onthe» j 
soil of great forests; that it is only in exceptional circum=: a 
stances that these beds alternate with beds containing the» 4 
remains of marine animals, and may be considered as-result=" . 
ing from the transportation into the sea of the terrestrial, 
vegetables which are found there. ) 
