THE ARRIVAL OF MAN IN EUROPE 



there has been any other such period since the 

 beginning of Tertiary times. How, then, shall 

 we explain the occurrence of two periods of 

 high eccentricity, one lasting 200,000 and the 

 other 1 80,000 years, without an accompanying 

 glaciation of the northern hemisphere ? 



This difficulty has been sometimes cited as 

 fatal to Mr. Croll's theory ; but when we fully 

 consider all the conditions of the case, we shall 

 see that it is not so. For we must remember that 

 it is not simply the cold, but it is the snow of the 

 glacial winter, that chills the summers and ren- 

 ders possible the accumulation of ice. To pro- 

 duce a glacial epoch, according to Mr. Croll's 

 theory, it is not enough that the mean winter 

 temperature of the northern hemisphere should 

 be lowered 35 F., unless there is enough con- 

 densation of moisture going on to produce an 

 abundant snowfall. Under such geographical 

 conditions as exist to-day, and as existed during 

 the Pleistocene period, there would be such a 

 condensation and such a snowfall ; but in the 

 Eocene and Miocene periods it was probably 

 otherwise. The explanation is not difficult. 



The most efficient promoters of condensation 

 are mountains, which, thrusting their cold sum- 

 mits high into the air, precipitate the surround- 

 ing moisture. It is a familiar fact that mountain- 

 ous districts are apt to be rainy, and that very 

 high mountains are usually covered with snow 

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