438 ALPINE AND ARCTIC PLANTS 



passage over the intervening lowlands, because the extreme 

 heat of summer and cold of winter would be fatal to them. 

 We must suppose, therefore, that originally they extended their 

 range in the same way as the plants now inhabiting Arctic and 

 Antarctic lands disseminate themselves. The innumerable 

 islands in the polar seas are tenanted by the same species of 

 plants, some of which are conveyed as seeds by animals over 

 the ice, when the sea is frozen in winter, or by birds ; while a 

 still larger number are transported by floating icebergs and field 

 ice, on which soil containing the seeds of plants may be carried 

 in a single year for hundreds of miles. A great body of geo- 

 logical evidence has now been brought together to show that 

 this machinery for scattering plants, as well as for carrying 

 erratic blocks southward, and polishing and grooving the floor 

 of the ancient ocean, extended in the western hemisphere to 

 lower latitudes than that of the White Mountains. When these 

 last still constituted islands, in a sea chilled by the melting of 

 floating ice, we may assume that they were covered entirely by 

 a flora like that now confined to the uppermost or treeless 

 region of the mountains, except in such portions of the period 

 as were sufficiently cold to clothe their summits permanently in 

 snow. As the continent grew by the slow upheaval of the land, 

 and the islands gained in height, and the climate around these 

 hills grew milder, the Arctic plants would retreat to higher 

 zones, and finally occupy an elevated area, which probably had 

 been, at first, or in the Glacial period, always covered with per- 

 petual snow. Meanwhile the newly formed plains around the 

 base of the mountain, to which northern species of plants could 

 not spread, would be occupied by others migrating from the 

 south, and perhaps by many trees, shrubs, and plants, then first 

 created, and remaining to this day peculiar to North America." 

 The time to which the above views of Sir. C. Lyell would 

 refer the migration of the White Mountain flora, is historically, 

 very remote. The changes of level which have submerged the 



