Geology and Natural History. 149 
also, at least as different as the present floras of Europe and China, 
or of California and Pennsylvania. 
How the adaptive diversification into these physiological groups 
came to pass, is a question of the same nature as that of the diver- 
com- 
pared with those of subsequent ages. They would seem to have 
and conifers correspond to those two categories. Among them 
there may have been species adapted to the long polar twilights, 
Pp : 
as our ferns often inhabit the forests, and as some cultivated 
conifers, such as ytomeria Japonica, prefer the shade. The 
sn have destroyed the primeval vegetation of the north, 
difficult to conceive what was passing during the immense period 
of the secondary formations, on account of the subsequent disper- 
sion of the land surfaces, the extent of the seas which covered 
Europe, and the fewness of the fossil plants which have thus far 
been studied. At the commencement of the Tertiary period, the 
megatherms occupied all the ex surface up to the 48th de- 
gree of north latitude, that is to say, all the present hot and tem- 
perate region. ‘The other classes became detached little by little 
from the vegetation that preceded, and localized toward the north 
and upon the mountains, in proportion as the increased cold drove 
out the prior possessors. Gradually the megatherms lost territory 
and the others acquired it. This is the simple expression of the 
ere hypothesis 
ow and by what means has this physiological grouping in 
flected? H 
uestion is the same as that which respects the 
was originally only one kind of plant at a time there was 
certainly only one climate. ity of climate carries with it 
physiological unity for the vegetation o the epoch, whatever it 
een. 
that the actual physiological groups 
to t his 
