PREHISTORIC MAN. 



597 



would be stopped, and the northern portion of our hemisphere would be, as 

 it undoubtedly was, frozen over and covered with snow. 



When we consider the millions of years since the earth is supposed to 

 have been launched into space, we can imagine that the Glacial periods would 

 occur frequently, and considering the very slow "precession" movement 

 there, and the alternating tropical climate with graduations of temperature 

 for thousands of years they would last long. The great Glacial period is 

 computed to have begun 240,000 years ago and lasted 160,000 years with 

 alternations of comparative summer; and so the years went on, season 

 succeeding season, altering the appearance of the earth, and causing suc- 

 cessive changes in the distribution of animal and vegetable life. Then the 

 great mammalia, the mammoth and hippopotamus, with the hyzena, lion, 

 and other felidae came, and went when Arctic animals usurped their places. 



Fig. 687. Mammoth and Irish Elk. 



At the later Glacial epoch man must have arrived in Britain, and " this 

 being so," says Professor Geikie, " it is startling to recall in imagination those 

 grand geological revolutions of which he must have been a witness ... He 

 entered Britain at a time when our country was joined to Europe across the 

 bed of the German Ocean ; at a time when the winters were still severe 

 enough to freeze over the rivers in the south of England ; at a time when 

 glaciers nestled in our upland and mountain valleys, and the Arctic 

 mammalia occupied the land. He lived here long enough to witness a 

 complete change of climate, to see the Arctic mammalia vanish from 

 England, and the .hippopotamus and its congeners take their places. At 

 a later date, and while a mild and genial climate still continued, he beheld 

 the sea slowly gain upon the land, until, little by little, step by step, a large 

 portion of our country was submerged a submergence which, as we know, 

 reached in Wales to the extent of 1,300 feet or thereabouts." 



We find that the land underwent many subsequent changes ; it rose 



