198 



INTEE-GLACIAL PERIODS. 



[Ch. X. 



skill to detect tlie evidence of tlie moraines and erratics of an 

 older time as in the case of a palimpsest to recover the work 

 of the original author, which had been purposely washed out 



room 



From 



the prevalence of a colder climate at the close of the 

 Tertiary, and in the early part of the Post-tertiary periods 

 has been derived from two perfectly independent sources 



of evidence ; the first of which may 



called inorganic, 



such as erratic blocks, moraines, and the polishing and 

 striation of rocks ; and second, the organic, such as the arctic 

 character of the shells found in the drift of temperate regions. 

 But another or third proof was also pointed out by the late 

 Edward Forbes, as derivable from the present geographical 

 distribution of animals and plants in mountainous regions, 

 especially in high latitudes, in Europe and North America. 

 After the refrigeration of the northern hemisphere had lasted 

 for thousands of years, an arctic fauna and flora must have 

 inhabited the lower- lands of temperate latitudes, at a time 



when the more elevated parts of 



permanent 



same country were 

 On the return of a 



warmer 



climate, when the excess of snow was gradually 

 reduced, the arctic species of plants, insects, birds, and mam- 

 malia, would ascend to the higher parts of each continent, 

 while the plains would be invaded by species migrating from 

 the south. Hence an arctic fauna and flora, which once 



om 



* 



tinuously over what are now the temperate regions of 

 America, Europe, and Asia, became restricted to the summits 

 of the highest chains, such as the Alps or the mountains of 

 Scotland, Scandinavia, and New Hampshire in the United 

 States. The identity of the species now found in isolated 

 patches at or near the tops of so many widely separated 

 mountains would have been inexplicable, had not the geolo- 

 gist discovered that about the close of the Tertiary era there 

 was a glacial epoch instead of that high temperature formerly 

 assigned to times preceding the historical.* 



f> 



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to 



* Edward Forbes' Memoirs of Geol. Survey, vol. i. p. 399. 1846 



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