THE SIBERIAN MIGRATION. 241 



have been discovered, lie either within or close to 

 the limits of the maximum extension of the supposed 

 northern ice-sheet, or within those of the former 

 Alpine glaciers. Whether we look upon the boulder- 

 clay as a marine or a terrestrial product, it is quite 

 conceivable that, in many instances, the remains of 

 the Arctic plants may have been carried by ice to 

 great distances from where they grew. The prob- 

 ability, however, is in favour of most of them 

 having lived where their remains are now found. 

 Now, it is a remarkable fact, that the single instance 

 in Europe of a deposit of Arctic plants having been 

 found far removed from the maximum extension 

 of the northern ice-sheet is the one quoted above, 

 viz., at Bovey Tracey, in Devonshire. Even up 

 to recent times Arctic plants may have persisted at 

 Bovey Tracey just as they do in Gal way under the 

 influence of a mild coast climate. Similar circum- 

 stances may have led to their survival along the 

 shores of the sea which deposited the North 

 European boulder-clay, while they moved north- 

 ward from the Alps along with the glaciers, which 

 always supplied them with an abundance of moisture. 

 Alpine plants probably became exterminated in the 

 plain of Central Europe at a much earlier period. 



