232 EVOLUTION OF PLANTS 



pecially noticeable upon lofty mountains in the tropics. 

 Thus in Jamaica the writer has collected upon the 

 highest peaks of the island such northern plants as 

 strawberries and brambles, buttercups and northern 

 species of club-mosses, none of which occur elsewhere 

 in the island nor on the adjacent mainland. A similar 

 occurrence of northern plants upon high tropical moun- 

 tains has been repeatedly observed. The presence of 

 these northern plants on the summit of tropical moun- 

 tains has been explained by the supposition that their 

 ancestors were driven south by the advance of the gla- 

 cial ice-sheet, and with the retreat of the latter, and the 

 corresponding increase in the temperature in the low- 

 lands, they retreated up to the cooler regions of the 

 mountain summits, not being able to live in the hot 

 lowlands. 



Where an extensive chain of mountains occurs, run- 

 ning north and south, it is possible to see how the 

 northern plants follow them, ascending higher and 

 higher as they proceed southward, seeking in this way 

 the same climatic conditions they have left behind them. 

 In the United States the Appalachian Mountains, the 

 Rockies, and the ranges of the Pacific slope, are all 

 beautiful illustrations of this method of distribution of 

 northern plants. 



Comparing the north temperate regions of the eastern 

 and western hemispheres, we find that eastern North 

 America much more nearly resembles eastern Asia 

 than it does the much nearer regions of western Europe. 

 The latter region lies, for the most part, much further 

 north than any part of the United States, and being 

 cut off from the south by high mountains, its whole 



