NORTH AMERICAN FLORA. 273 



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. 



" I 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 Nordenskjold 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- 

 caped from and reentered 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- 



