72 CHANGE OF CLIMATE. [CHAP. V. 



thousands of them planted round the foot of Mount Washington, 

 they would never be able, in any number of years, to make their 

 way to its summit. We must suppose, therefore, that originally 

 they extended their range in the same way as the flowering 

 plants now inhabiting Arctic and Antarctic lands disseminate 

 themselves. The innumerable islands in the Polar seas are 

 tenanted by the sarme species of plants, some of which are con 

 veyed 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, on which soil containing the seeds of plants 

 may be carried in a single year for hundreds of miles. A great 

 body of geological evidence has now been brought together, to 

 some of which I have adverted in a former chapter ,* 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 the White Mountains. When these last still 

 constituted islands, in a sea jphilled by the melting of floating ice, 

 we may assume that they vfrere covered entirely by a flora like 

 that now confined to the Uppermost or treeless region of the 

 mountains. As the}contin*fet grew by the slow upheaval of the 

 land, and the islands gai$p in height, and the climate around 

 their base grew milder/pfe Arctic plants would retreat to higher 

 and higher zones,, .awd finally occupy an elevated area, which 

 probably had been at first, or in the glacial period, always covered 

 with perpetual 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 period when the White Mountains ceased to be a group 

 of islands, or when, by the emergence of the surrounding low 



* Ante, p. 17. 



t For speculations on analogous botanical and geographical changes in 

 Europe, the reader may refer with advantage to an excellent essay by 

 Professor Edward Forbes, on the Origin of the British Fauna and Flora, 

 Memoirs of Geol. Survey of Great Britain, vol. i. p. 336. 1846. 



