THE FALL OF THE YEAR. 229 



the submergence which turned them into 

 islands, the comparative poverty of Hfe is 

 very noticeable. Nevertheless, the temperate 

 types have everywhere driven out the polar 

 species which preceded them, except on the 

 very summits of the principal mountains. 

 Near the top of Mount Washington, the 

 highest peak in the Eastern States, there 

 linger even now some few butterflies of a 

 species which is not found again till we reach 

 the arctic regions. Their ancestors were 

 stranded there by the receding glaciers, when 

 all the rest of their kind were driven north- 

 wards ; and there alone they have been able 

 to hold their own against the better adapted 

 southern kinds which came to occupy the 

 surrounding country. 



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