ON THE CHARACTERISTICS OF THE NORTH AMERICAN FLORA. 563 



that there are good species which I had reckoned as synonyms, and some 

 that may rise to arboreal height which I had counted as shrubs. But, on 

 the other hand, and for the present purpose, it may be rejoined that the 

 list contained several trees, of as many genera, which were probably 

 carried from Asia into Europe by the hand of man. On Nyman's authority 

 I may put into this category cercis siliquastrum, ceratonia siliqua, 

 diospyros lotus, styrax officinalis, the olive, and even the walnut, the 

 chestnut, and the cypress. However this may be, it seems clear that the 

 native forest flora of Europe is exceptionally poor, and that it has lost 

 many species and types which once belonged to it. We must suppose 

 that the herbaceous flora has suffered in the same way. I have en- 

 deavoured 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 Carpathians 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 rehabi- 

 litation from that quarter until a very late period, was apparently prevented 

 by the prolongation of the Mediterranean to the Caspian, and probably 

 thence to the Siberian Ocean. If we accept the supposition of Norden- 

 skiold 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 escaped from and re-entered 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 han been open for their return. Europe fared much better, but has 

 suffered in its degree in a similar way.' 1 



Turning to this country for a contrast, we find the continent on the 

 eastern side unbroken and open from the arctic circle to the tropic, and 

 the mountains running north and south. The vegetation when pressed 

 on the north by on-coming refrigeration had only to move its southern 

 border southward to enjoy its normal climate over a favourable region of 

 great extent ; and, upon the recession of glaciation to the present limit, or 

 in the oscillations which intervened, there was no physical impediment 

 to the adjustment. Then, too, the more southern latitude of this country 

 gave great advantage over Europe. The line of terminal moraines, which 



1 Amer. Jonrn. Sci., III., 1. c, 194. 



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