THE ARRIVAL OF MAN IN EUROPE 



not only obscures the sun's heat, but leads to 

 further heavy condensation in the shape of cold 

 rains. Now bear in mind that this state of 

 things goes on for at least half of the period 

 of 10,500 years, when the aphelion of the 

 northern hemisphere occurs between September 

 and March, and it is easy to see how the snow 

 and ice must so far gain the upper hand that 

 the intense summer heat cannot produce any 

 considerable impression on them, but the region 

 of " eternal snow," no longer confined to the 

 tops of lofty mountains, descends to the sea- 

 level throughout a large part of the northern 

 hemisphere. Thus we get far toward an expla- 

 nation of the causes of the Glacial epoch. But 

 still other causes have conspired with those here 

 pointed out to enhance the general effect. 



While the northern hemisphere was situated 

 as just described, the state of things in the south- 

 ern hemisphere must have been entirely differ- 

 ent. There the perihelion occurring in winter 

 and the aphelion in summer, with the same high 

 eccentricity, the summer would be twenty-six 

 days longer than the winter, and the climatic re- 

 sult would be perpetual spring. And this would 

 affect the flow of ocean-currents in such a way 

 as to deprive the northern hemisphere of its 

 only possible chance of escaping the glaciation 

 we have just depicted. Let us notice this point 

 carefully, for it is one of great importance. 

 53 



