464 DISTRIBUTION OF FOSSIL FERNS. 
remains of Ferns decrease continually in num- 
ber, as we ascend from the most ancient to the 
most recent strata, he founds upon this fact an 
important conjecture, with respect to the succes- 
sive diminutions of temperature, and changes of 
cUmate, which the earth has undergone. Thus 
in the great Coal formation there are about 120 
known species of Ferns, forming almost one half 
of the entire known Flora of this formation ; 
these species represent but a small number of 
the forms which occur among living Ferns, and 
nearly all belong to the Tribe of Polypodiacese, 
in which Tribe we find the greater number of 
existing arborescent species.* Fragments of the 
stems of arborescent Ferns occur occasionally in 
the same formation. M. Brongniart considers 
these circumstances as indicating a vegetation, 
analogous to that of the Islands in the equinoctial 
regions of the present Earth ; and infers that 
the same conditions of Heat and Humidity which 
favour the existing vegetation of these islands, 
prevailed in still greater degree during the for- 
* In plate 1, figs. 7, and 37, represent two of the graceful 
forms of arborescent Ferns which adorn our modern tropics, where 
they attain the height of forty and fifty feet. 
An arborescent Fern forty-five feet high (Alsophila bruno- 
niana), from Silhet in Bengal, may be seen in the staircase of the 
British Museum. The stems of these Ferns are distinguished 
from those of all arborescent Monocotyledonous plants, by the 
peculiar form and disposition of the scars, from which the Petioles 
