FLIGHT OF PLANTS AND ANIMALS. 433 



out of northern Europe and the northern Atlantic States or 

 was reduced to precarious existence and diminished forms. It 

 also appears that, on our own continent at least, a milder cli- 

 mate than the present, and a considerable submergence of land, 

 transiently supervened at the north, to which the vegetation 

 must have sensibly responded by a northward movement, from 

 which it afterward receded. 



All these vicissitudes must have left their impress upon 

 the actual vegetation, and particularly upon the trees. They 

 furnish probable reason for the loss of American types sus- 

 tained by Europe. 



I conceive that three things have conspired to this loss : 

 1. Europe, hardly extending south of latitude 40°, is all within 

 the limits generally assigned to severe glacial action. 2. Its 

 mountains trend east and west, from the Pyrenees to the Car- 

 pathians and the Caucasus beyond, near its southern border ; 

 and they had glaciers of their own, which must have begun 

 their operations, and poured down the northward flanks, while 

 the plains were still covered with forest on the retreat from 

 the great ice-wave coming from the north. Attacked both on 

 front and rear, much of the forest must have perished then 

 and there. 3. Across the line of retreat of those which may 

 have flanked the mountain-ranges, or were stationed south of 

 them, stretched the Mediterranean, an impassable barrier. 

 Some hardy trees may have eked out their existence on the 

 northern shore of the Mediterranean and the Atlantic coast. 

 But, we doubt not, taxodium and Sequoias, magnolias, and 

 Liquidambars, and even hickories and the like, were among the 

 missing. Escape by the east, and rehabilitation from that 

 quarter until a very late period, was apparently prevented by 

 the prolongation of the Mediterranean to the Caspian, and 

 thence to the Siberian Ocean. If we accept the supposition 

 of Nordenskiold, 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 or re-entered Europe from the south and east is 

 excluded. Europe may thus be conceived to have been for a 

 time somewhat in the condition in which Greenland is now, 



