214 VIGNETTES FROM NATURE. 



by the elevation of great mountain ranges 

 about the polar regions, which became 

 nurseries for immense glaciers, and so sup- 

 plemented the natural chilling due to the 

 cosmical cycles. At any rate, whatever 

 theory we may adopt for its explanation, the 

 fact itself remains certain that from the eocene 

 age up to the glacial epoch the climate of the 

 earth grew steadily colder, the change being 

 of course most marked at either Pole, and 

 least noticeable in the equatorial district. 



Concomitantly with the steady decrease 

 of temperature thus forced upon the earth, a 

 new forest vegetation developed itself in 

 adaptation to the altered circumstances. 

 This modern cold-weather flora of course first 

 showed its face in the polar regions, or, to 

 speak more correctly, about the North Pole. 

 Here the fresh conditions first made them- 

 selves felt, and here all the familiar trees of 

 modern English woodlands had their generic 

 origin. In the eocene days the arctic flora 

 was still of a temperate or even sub-tropical 



