THE FAUNA OF BRITAIN. 131 



others again hold that a portion is pre-glacial and the rest 

 glacial and post-glacial. Those who have studied the subject 

 most closely feel convinced that the south-western or Lusitanian 

 fauna, and also the flora, must have arrived before the Glacial 

 period and survived the latter in these Islands. It seems 

 reasonable to suppose, therefore, that the climate cannot 

 have been very severe during the so-called Ice-Age. This 

 Lusitanian fauna must be looked upon as the oldest portion of the 

 British fauna. The Alpine and Oriental migrations arrived next. 

 After these came the Arctic, and finally the Eastern or Siberian. 

 As the fossil evidence is most complete with regard to the last, 

 we are able to determine with precision not only the direc- 

 tion whence this migration came, but approximately its geo- 

 logical age. It arrived in Germany from the east after the 

 deposition of the lower boulder-clay. Since the boulder-clay 

 is looked upon as a glacial deposit, the Siberian migration 

 reached Central Europe after the first portion of the Glacial 

 period had passed. In England it makes its first appearance 

 in the Forest-Bed, which would therefore correspond to the 

 " Loess " formation of Central Europe. All the other migrations 

 are older than the Siberian. They must therefore have come 

 to Great Britain during the earlier part of the Glacial period 

 or before it. 



The chapter concludes with a short statement on the physical 

 geography of the British Islands during the time when these 

 migrations entered them. That there existed a continuous 

 coast-line between France and Ireland is proved by the 

 occurrence of a considerable number of identical shore 

 species, whilst the former existence of a freshwater lake on 

 the site of the present Irish Sea is indicated by the dis- 

 tribution of some freshwater fishes. 



