302 THE ORIGIN OF THE FLORA 



thousands of years may have elapsed between the 

 period when the temperature first began to fall at the 

 Pole and the glaciation of the British Isles. 



There was also probably more than one period of 

 maximum low temperature. Cycles of severe cold, 

 each of considerable length, seem to have alternated 

 with more genial, Interglacial Periods, when the ice 

 retreated for a time from Britain and the plains of 

 Central Europe. Finally, the climate became per- 

 manently temperate, the ice retreated to the polar 

 circle, and to the higher peaks of the European 

 and American mountains, and thus was ushered in 

 the era of to-day. 



The Glacial Period then was a long-continued 

 epoch, the changes of temperature being extremely 

 slow and gradual. Periods of cold alternated with 

 more genial epochs. As the tides ebb and flow, 

 so the wide-spread glaciation appears to have now 

 advanced, now retreated. 



We will not attempt to discuss here the arguments 

 which have been put forward to account for the 

 occurrence of such glacial periods. These remain 

 the province of the mathematician, physicist, and 

 astronomer. We may, however, add that the late 

 Tertiary Glacial Period is not the only ice age of 

 which geological evidence exists. Nor will we 

 concern ourselves with the proofs of the general 

 glaciation of Europe and North America. As 

 Darwin, in the " Origin of Species" (p. 310), remarks : 

 " The ruins of a house burnt by fire do not tell their 



