How the History of the Past Is Head 



In cases such as these, plants furnish reli- 

 able testimony as to climate, for they are less 

 adaptive than animals, or show their adaj^tation 

 much more plainly. When we go north we 

 find the trees growing smaller and smaller, 

 until finally they disappear; but the reindeer 

 and musk-ox are large animals, and the polar 

 bear and Greenland whale even larger than 

 their tropical relatives. 



And if fossils tell what the climate was 

 while the rocks in which they are contained 

 were forming, the rocks, on the other hand, 

 may show wliy the climate changed, and with 

 it the plant and animal life of that portion of 

 the globe. The upheaval of mountain ranges 

 has cut off warm and moisture-laden winds, 

 transforming verdure-clad plains into arid 

 wastes ; the slow rise of great masses of land 

 has cast a chill over vast areas, transfoiTaing 

 those species that — we know not why — can re- 

 spond to changed conditions, pushing south- 

 ward, or blotting out of existence those that 

 can not. The mountain range may countless 

 ages ago have been leveled to a plain, the con- 



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