THE FAUNA OF BRITAIN. 93 



marks, and of these a few have migrated southward 

 along the coasts of the great continents. Thus we 

 meet with various species of Coitus as far south as 

 California and Japan, on the American and Asiatic 

 coasts of the Pacific respectively. In Europe, two 

 species, viz., C. scorpio and C. bubalis, range as far 

 south as the French coast. Our freshwater Cottns, 

 the Miller's thumb (Cottus gobio), has migrated to 

 us from the north with the Arctic species. All the 

 freshwater forms, indeed, of this genus are typically 

 Arctic. 



A large number of land and freshwater invertebrates 

 too have no doubt reached us from the north. Some 

 of them may have originated in Scandinavia or within 

 the Arctic Circle, but others probably came still 

 farther, either from America or even from Asia, and 

 used the Arctic land-connection via Greenland in their 

 migration to Europe. As I shall give a number of 

 additional instances of such migrants in the succeed- 

 ing chapters, I need not, perhaps, dwell upon them 

 now any longer, except to mention a few of the 

 more typical ones. Vertigo alpestris, a minute snail 

 with an amber-coloured shell, and our freshwater 

 pearl-mussel, Unto (Margaritana) margaritifer y belong 

 to this migration. Then among butterflies we may 

 cite the Marsh-ringlet (Coenonympha t)phon\ and 

 among beetles, Pelophila borealis and BletJiisa multi- 

 punctata. There are a number of northern spiders, 

 among which a few certainly indicate an Arctic 

 origin, or at any rate, that they have wandered to 



