THE FALL OF THE YEAR. 227 



not only at the Pole itself, but even as far 

 south as London, Berlin, New York, and 

 Chicago. During that interval, all living 

 things were necessarily driven towards the 

 sub-tropical and equatorial regions ; and the 

 middle of the earth thus became for a while 

 the seat of the richest and most advanced 

 fauna and flora. Some seventy or eighty 

 thousand years since, if we may trust Dr. 

 Croll, backed by the almost unanimous 

 opinion of the greatest scientific authorities, 

 these unfavourable astronomical conditions 

 ended, and the vast glacial sheet cleared away 

 from the face of the northern hemisphere, at 

 least below the latitude of Greenland. But, 

 as Mr. Wallace believes, the geological con- 

 ditions remained unaltered ; and so, instead 

 of the Pole becoming once more habitable, it 

 still continues to be enveloped in perpetual 

 snow. From that time forward, the exiled 

 plants and animals, which had been driven 

 south by the advancing cold, have begun to 

 migrate northward once more, and to re- 

 Q 2 



