ANIMAL LIFE OF POLAR REGIONS 513 



relatively short time. The only genuinely resident bird is the ptarmigan, 

 which extends northward of the 84th parallel. At 82° 30' N. latitude, 

 in Grinnell Land, only the snowy owl, snow bunting, and raven were 

 to be found nesting in addition to sea birds. In the Antarctic all the 

 birds depend on the sea and strictly speaking there are no land birds. 



The number of land mammals which survive the pessimum of 

 environmental conditions in the Arctic is very limited. The richest 

 fauna appears to be that of northeast Greenland, where there are 7 

 species in addition to the polar bear. They are hare and lemming, 

 musk ox and reindeer, ermine, wolf, and fox. In western and southern 

 Greenland there are scarcely more than 3 species. On the islands of 

 arctic Eurasia the number of species of mammals is very small, 5 on 

 Nova Zembla, 3 on Spitzbergen. The arctic fox is found on the northern- 

 most land reached by man. 20 It spreads from island to island by means 

 of ice floes, like the polar bear. 



The species in Greenland and the arctic islands of America differ 

 in part from those of the Old World. The caribou corresponds to the 

 reindeer, the hare is Lepus arcticus, distinct from the Eurasian 

 L. timidus, and the lemming Dicrostonyx hudsonius replaces D. tor- 

 quatus. The musk ox is now wholly confined to eastern Arctic America 

 and Greenland, though it ranged over all central Europe even in post- 

 glacial time, and is known to have been exterminated in Alaska within 

 the memory of man. The last stronghold of this species is in eastern 

 Greenland where some 10,000 were estimated to be still living in 1934. 21 



Arctic boundaries. — Opinions differ concerning the zoogeographic 

 delimitation of the arctic region. The Arctic Circle is an unsatisfactory 

 limit, for it cuts through very diverse regions. The tree limit offers a 

 fairly satisfactory line of demarcation, as in the high mountains. 

 Kiaer 22 adopts the annual isotherm of 0°, which extends the region 

 southward to the 51st parallel of north latitude in America. Friese, 23 

 however, suggests that this limit be set at —5° in North America on 

 account of the continental climate, with relatively high summer tem- 

 peratures. For our purposes, such an exact delimitation is not required. 

 All the islands of the Arctic Ocean, with the exception of Iceland 

 (which is forested), belong plainly to an arctic life zone. A mixture of 

 faunae takes place on the arctic borders of the continents, which 

 makes exact demarcation impossible and undesirable. The intimate 

 relation between faunal composition and climate makes it self-evident 

 that a sharply defined faunal boundary will be found only where 

 natural barriers, such as coast lines and mountain ranges, produce well- 

 defined climatic limits. 



