NORTH AMERICAN FLORA. 2S 
way. I have endeavored to show how this has naturally come 
about. I cannot state it more concisely than in the terms 
which I used six years ago. 
‘IT conceive that three things have conspired to this loss of 
American, or as we might say, of normal types sustained by 
Europe. First, Europe, extending but little south of lat. 40°, 
is all within the limits of severe glacial action. Second, its 
mountains trend east and west, from the Pyrenees to the Car- 
pathians and the Caucasus beyond: they had glaciers of 
their own, which must have begun their work and poured 
down the northward flanks while the plains were still covered 
with forest on the retreat from the great ice forces coming 
from the north. Attacked both on front and rear, much of 
the forest must have perished then and there. 
“Third, across the line of retreat of whatever trees may 
have flanked the mountain ranges, or were stationed south of 
them, stretched the Mediterranean, an impassable barrier... . 
Escape by the east, and rehabilitation from that quarter until 
a very late period, was apparently prevented by the prolon- 
gation of the Mediterranean to the Caspian, and probably 
thence to the Siberian Ocean. If we accept the supposition 
of Nordenskjéld that, anterior to the Glacial period, Europe 
was ‘ bounded on the south by an ocean extending from the 
Atlantic over the present deserts of Sahara and Central Asia 
to the Pacific,’ all chance of these American types having es- 
eaped from and reéntered Europe from the south and east 
seems excluded. Europe may thus be conceived to have been 
for a time somewhat in the condition in which Greenland is 
now. . . . Greenland may be referred to as a country which, 
having undergone extreme glaciation, bears the marks of it in 
the extreme poverty of its flora, and in the absence of the 
plants to which its southern portion, extending six degrees 
below the arctic circle, might be entitled. It ought to have 
trees and it might support them. But since their destruction 
by glaciation no way has been open for their return. Europe 
fared much better, but has suffered in its degree in a similar 
way.” 
Turning to this country for a contrast, we find the conti- 
