144 WORLD-POWER AND EVOLUTION 



which promote glaciation. First, the precipitation of snow during 

 the winter becomes more abundant ; second, the lower temperature 

 at all times of the year lengthens the season when snow falls and 

 also prevents the snow from melting in the summer; and third, 

 the greater cloudiness also prevents the snow from melting. Thus 

 the area where snow lies throughout the year gradually increases. 

 After such an area is once established it becomes a region of high 

 atmospheric pressure. It then resembles Greenland and Ant- 

 arctica. In those great regions of ice-sheets the general move- 

 ment of the air is downward in the centers and outward on the 

 borders. Some of the world's most violent winds blow out from 

 the great ice-caps. Since the air in the centers of such regions 

 descends from far aloft, it is cold and thus preserves the ice and 

 snow. Only rarely do storms penetrate far into the high pressure 

 areas. They skirt the edges, however, in large numbers, and their 

 violence increases in proportion to the size and intensity of the 

 high pressure area caused by the ice-sheet. 



During the Glacial Period ice covered all of Scandinavia, and 

 much of Great Britain, northern Germany, and the Baltic region. 

 The Alps and to a less degree the Pyrenees were the seat of large, 

 permanent masses of ice which spread out far below the base of 

 the mountains. In winter a continuous cover of snow must have 

 connected these various ice-sheets. Thus all Europe north of 

 the Alps and the P^^renees was probably a high pressure area for 

 at least six months of each year. Asia north of the central 

 mountains was also a high pressure area. Under such circum- 

 stances practically all the storms must have been forced to follow 

 a route along the Mediterranean and across Syria, Mesopotamia, 

 and Persia to northern India. Moreover, even in summer many 

 storms must have been forced to follow this course. From all this 

 it appears that when man was making his most rapid mental 

 evolution he lived in a climate where severe storms were of great 

 frequency. Remember that at that time the direct rays of the 

 sun were probably even warmer than now. Thus when there were 



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