226 VIGNETTES FROM NATURE. 



the North Pole, and were each pushed south- 

 ward one after another, as the cold grew 

 intenser, in concentric rings, so that the 

 arctic fauna in one age becomes the parent 

 of the North European fauna in the next age, 

 of the Mediterranean fauna in the succeeding 

 one, and of the Southern fauna in the epoch 

 after that again. So long as life remained 

 possible at the Pole at all, it was almost 

 absolutely true that the polar plants and 

 animals represented the most modern and 

 most highly evolved types at that moment 

 living upon earth. Spitzbergen, in fact, then 

 led the very van of progressive evolution. 



But since the planetary autumn has 

 thoroughly set in, this principle has ceased to 

 be quite true, and has been to some extent 

 reversed by the occurrence of that great pre- 

 mature cold spell, the glacial epoch. Towards 

 the close of the tertiary period, a combination 

 of astronomical and geological causes set up 

 a long interval of intense cold in the northern 

 hemisphere, which made all life impossible, 



