Fossil Vegetation of America. 315 
species of plants which formed the coal, coupled with the law 
of Professor Pictet, we also establish a gradual diminution of 
the extent of this climate at the more recent date of the pre- 
valence of animal life, this gradual diminution continuing, at 
still more recent periods, until the present epoch ; —a fact of 
great value in comparing the ancient and the present condi- 
tions of the crust of the globe. Some difficulty has been felt 
with regard to the idea of an extensive area of uniform climate, 
from the supposition that the short period of light of an arctic 
tegion would not be sufficient for the growth of plants of so 
much luxuriance as those of the Coal formation. But it ap- 
pears to me that this difficulty vanishes under the following 
considerations. 
Plants have their times of alternate growth and hybernation, 
or rest; the latter might take place during the period of dark- 
hess, and their growth during the period of light. This, in an 
atmosphere of carbonic acid, with great heat and humidity, 
might be very rapid. The time of hybernation might be 
thought long ; but be it remembered that a plant is but a 
modification of a seed, and the period of rest of a seed, until 
I$ vegetation is called forth by favorable circumstances, is by 
. To means definite. Fern seeds have been known to vegetate 
after having been for many years at rest in the herbarium ; 
and although there are no experiments on record contrasting 
the length of the vitality of seeds with the length of rest which 
“€ same plant will endure, yet enough is known to prevent 
this length of hybernation from being an insuperable objection. 
l have a bulb, Stenomesson, which was two years in the 
Pocket of one of the officers of the Exploring Expedition ; it 
Stew and has flowered several times. To this may be added 
that the darkness near the poles is never very intense; and 
this might not have been without influence on the vegetation, 
Particularly of the lycopodiaceous and fern tribes, many of 
which naturally flourish best, now, when constantly in the 
densest shade, 
~ Hence, the importance of working up carefully the details 
