230 ESSAYS. 



the vegetation which in our clay borders the ice, or by ice it- 

 self, it is difficult to form more than general conjectures — so 

 different and conflicting are the views of geologists upon the 

 Glacial period. But upon any, or almost any, of these views, 

 it is safe to conclude that temperate vegetation, such as pre- 

 ceded the refrigeration and has now again succeeded it, was 

 either thrust out of northern Europe and the northern At- 

 lantic States, or was reduced to precarious existence and di- 

 minished forms. It also appears that, on our own continent 

 at least, a milder climate than the present, and a considerable 

 submergence of land, transiently supervened at the north, to 

 which the vegetation must have sensibly responded by a north- 

 ward movement, from which it afterward receded. 



All these vicissitudes must have left their impress upon the 

 actual vegetation, and particularly upon the trees. They fur- 

 nish probable reason for the loss of American types sustained 

 by Europe. 



I conceive that three things have conspired to this loss. 

 First, Europe, hardly extending south of latitude 40°, is all 

 within the limits generally assigned to severe glacial action. 

 Second, its mountains trend east and west, from the Pyrenees 

 to the Carpathians 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. At- 

 tacked both on front and rear, much of the forest must have 

 perished then and there. Third, across the line of retreat of 

 those which may have flanked the mountain-ranges, or were 

 stationed south of them, stretched the Mediterranean, an im- 

 passable 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 re- 

 habilitation from that quarter until a very late period, was 

 apparently prevented by the prolongation of the Mediterra- 

 nean to the Caspian, and thence to the Siberian ocean. If we 



