356 POLAR PROBLEMS 



6000 feet, pour down the slopes under the pull of gravity. Conversely, 

 the still greater cold of favorably placed localities at sea level, such as 

 the Bay of Whales, from which Amundsen began his polar journey, 

 is usually accompanied by calm atmosphere. 



The Sub-Antarctic Zone 



Situated within or closely outside the pack-ice zone, or at least 

 in similar relation to the outer limit of icebergs, are the fifteen or more 

 islands and small archipelagoes which make up the sub-Antarctic 

 chain. With the exception of the Tristan da Cunha group and St. 

 Paul and Amsterdam, these are beyond the sinuate limit of tree growth. 

 Most of them lie within the belt of westerly winds, many are still 

 actively glaciated, all are characterized by cool summers and slight 

 yearly temperature amplitudes, and all consequently suffer from the 

 inhibiting effect of the controlling oceanic climate which results in a 

 paucity of land fauna and flora far more marked than anything to be 

 found in corresponding latitudes of the northern hemisphere. Few 

 of the typically sub- Antarctic members, such as South Georgia, 

 Kerguelen, and Macquarie, have a mean annual temperature which 

 rises much above freezing, but the extent of an ice-cap depends largely 

 upon the height of the land. Thus Macquarie is almost but not quite 

 glacial, while lofty South Georgia has a permanent neve and great 

 consolidation of ice in the valleys. Islands farther southward, such 

 as the South Shetlands and South Orkneys, are essentially Antarctic 

 in all their climatic and biotic features. 



The sub-Antarctic islands form a natural transition zone between 

 south temperate and polar life. Here we find, for example, the handful 

 of southernmost flowering plants (except for two species which barely 

 reach the Antarctic Continent), the southernmost insects save for 

 lowly organized Collembola and wingless flies, the southernmost 

 earthworms, land birds, and waterfowl of northern affinities (e. g. 

 ducks) , Here also a large proportion of such sea birds as albatrosses 

 and diversified types of petrels, which range widely in the southern 

 hemisphere, penetrating even as far southward as the pack ice, find 

 their sole nesting grounds. In like manner many of these islands pro- 

 vide the beaches of nativity for typically sub-Antarctic seals, such 

 as the sea elephant {Mirounga leonina) and the southern fur seals 

 (Arctocephalus) , species which like some of the birds are widely dis- 

 persed beyond the temperate seas and which also extend their ranges 

 far northward via the paths of cool currents. South of the surface 

 isotherm of 6° C, however, they are replaced by other species of 

 distinctly Antarctic type. 



In the present consideration of Antarctic zoogeography it seems 

 advisable to treat the Antarctic and sub-Antarctic zones as a unit, 



