CLIMATE OF PLEISTOCENE PERIOD. 35 



fidence is in nearly the same latitude as Bodo in Norway, where 

 the temperature of July is + 54 0- 5, and that of January + 26° - 6, 

 a difference of only 27 c, 9. This greater range of temperature 

 in Northern Asia and North America naturally affects to a very 

 considerable degree the vegetable and animal products. Thus, 

 as every one knows, there are vast tracts in those regions which 

 are subject to a climate that forbids the growth of trees. In 

 these wide " barren grounds " mosses and lichens form the 

 prevailing vegetation, and next to these come grasses, sedges, 

 and rushes, and dwarf willows. This treeless zone presents a 

 very irregular margin towards the south. Thus in North 

 America it descends to Labrador in latitude 57°, from which, 

 as we follow it across that continent, it gradually rises to 

 higher and higher latitudes until it reaches the delta of the 

 Mackenzie Eiver in 69° N. lat. After leaving the Mackenzie, 

 it trends more towards the south, and terminates on the shores 

 of Behring Strait in 65° N. lat. On the opposite or Asiatic 

 coast, the boundary line between the tundras or barrens and the 

 region of trees begins in 63° N. lat., and sweeps away in a north- 

 westerly direction till it reaches the Lena in 71° N. lat., after 

 which it again extends more to the south and crosses the Obi a 

 little beyond the Arctic Circle. It now sweeps farther and 

 farther to the north as it traverses Europe, so that only a 

 narrow fringe of " treeless ground " appears in Lapland and the 

 north of Bussia. 



Immediately south of the " barrens " of North America and 

 the " tundras " of Siberia comes the belt of Arctic forests, which 

 are composed almost exclusively of coniferous trees. These 

 forests cover an immense territory, and extend with hardly any 

 interruption across the three northern continents, forming a zone 

 which is 15° to 20° in breadth. The limits of arboreal vegeta- 

 tion are of course determined by climatic conditions, the area to 

 the north being swept in winter by cold winds coming from the 

 ice -laden Arctic seas, before the breath of which every green 

 thing shrivels up. Vast areas of the barren grounds during 

 that season are covered with thick snow — lake and river and 



