18 ‘THE TUNDRA AND ITS FAUNA 
drains away as the snow melts is favourable to plant 
growth, while level ground is very unfavourable. A 
sunny exposure also exerts much influence. The Sver- 
drup expedition noted that the Arctic hare is prac- 
tically limited to the regions where large valleys open 
out into fiords, for only here is vegetation abundant 
enough to supply its needs. 
As regards topography one must note that the tundra, 
like the semi-arid lands of other latitudes, is charac- 
terized by a very youthful condition of drainage. As 
already stated, the rivers are temporary, chemical 
erosion does not occur, and therefore the process of 
carving up the land into hill and valley goes on very 
slowly, and the land is imperfectly drained. The 
imperfect drainage is aided by the fact that the ex- 
treme coldness of the soil is unfavourable to bacterial 
life, and thus humus accumulates and favours the 
growth of bog mosses as contrasted with higher plants. 
These hold up the water like a sponge, and so hinder 
drainage. When Nansen and his party left the inland 
ice of Greenland they descended to a little tarn, whose 
water drained by a stream to the distant fiord. In 
their descent, as the leader notices, they found that the 
main streams had often transverse tributaries, which 
greatly hindered progress. The picture may be taken 
as characteristic of the tundra. Lakes fed by melting 
ice, streams showing a very undeveloped drainage 
system—these are characteristic features, which, aided 
by the climate, account for the very local development 
of a covering of vegetation, and therefore for the 
curiously local appearance of animals. 
Without discussing the plants of the tundra in detail 
we may note those points which are of special impor- 
tance for animal life. The first of these is the great 
