HOLARCTIC SUB-REGIONS. 69 



also in Europe. On the other hand, the two regions exhibit a 

 marked inter-relationship by the possession of a number of identical 

 specific forms, as Accipenser sturio (sturgeon), Perca fluviatilis 

 (perch), Salmo salar (salmon), Esox lucius (pike), Lota vulgaris 

 (ling), &c. 



The correspondence existing between the vertebrate faunas of 

 the Old and the New World divisions of the Holarctic tract extends 

 also to the Invertebrata, and is especially marked in the case of the 

 beetles (Staphylinida?, Carabidae), butterflies, and the land and 

 fresh-water mollusks (Limna3a, Planorbis, Physa, Paludina, Val- 

 vata). The land- snails (Helicidae) and naiades (Unionidae) are very 

 largely developed, the latter more particularly in the American 

 streams, where distinctive types appear to be relegated to the dif- 

 ferent water- courses. The eastern melanians are wholly wanting 

 in America, where they are replaced (principally to the east of the 

 Mississippi Kiver) by the members of the allied family of the Stre- 

 pomatida3 (lo, Goniobasis, &c.). 



The Holarctic realm may be conveniently divided into the fol- 

 lowing sub-regions: 



1. The Boreal Sub-Region, which extends northward into the 

 Polar Sea, and whose southern confines are fixed approximately by 

 the northern limits of the cultivation of the cereals, and the southern 

 limits of the migration of the reindeer. In the Western Hemisphere 

 it comprises most of the region lying to the north of the United 

 States and Canada boundary-line, and in Eurasia the tract lying 

 north of a line starting from about the sixty-sixth parallel of latitude, 

 on the Norwegian coast, and passing southeastward to the East Asiat- 

 ic coast, in about latitude fifty degrees north. The fauna of this 

 region is a very homogeneous one, and, generally speaking, also a 

 limited one. Among the more distinctive mammalian forms, which 

 comprise almost exclusively only ruminants, carnivores, and rodents, 

 are the Arctic fox (Canis lagopus), polar-bear, glutton, ermine, mink, 

 sable, walrus, variable hare, lemming, and reindeer. The musk-ox, 

 which occurs fossil in the Quaternary deposits of Europe, is at the 

 present time found only in America. Among the more character- 

 istic birds are the snow-partridges (Lagopus), snowy-owl (Surnia 

 nivea), Iceland falcon (Falco candicans), eider-duck (Somateria mol- 

 lissima), and various alks (Alca), divers (Colymbus), and guillemots 

 (Una, Lomvia). Captain Markham observed the footprints of the 



