304 THE ORIGIN OF THE FLORA 



temperate Miocene flora of North America was 

 driven, in its turn, towards the south, and there being 

 no mountain ranges stretched across the continent 

 to bar its way, it found a haven in what are now the 

 tropics, and in some cases, it is believed, even migrated 

 into the Southern hemisphere. In Europe, however, 

 the retreat of the Miocene flora was barred by 

 the Alps and other links in the incomplete chain of 

 mountains, which stretches from the west to the east 

 across the continent. The flora, brought up sharp 

 against the highly-glaciated Alps, became almost 

 entirely extinct. A few survivals, however, preserved 

 by some good fortune, still lingered. 



"With the advancing wave of cold, the Alps and 

 the Eockies became gradually more and more glaciated, 

 and the ancient Alpines were driven downwards to 

 the Lowlands, and there mingled with the ancient 

 Arctic plants from the Far North. 



At the periods of maximum intensity of cold, the 

 plains of Europe and North America were populated 

 by Arctic and Alpine species. 



When, however, the ice again retreated northward, 

 many of the ancient Arctic plants returned to the 

 polar region and the old temperate, Miocene types 

 to North America, and there mingled with some 

 Arctic species which finally made their homes in the 

 Lowlands. 



In Europe, as we have seen, the Miocene flora 

 had been practically exterminated, and the present 

 Western European flora would seem to have originated 



