232 EVOLUTION OF PLANTS 
pecially noticeable upon lofty mountains in the trppics. 
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 
