THE TUNDRA AND ITS FAUNA 31 
starfish, which prey upon them, are also abundant, and 
reach a large size. Where seaweed is abundant, sea- 
urchins occur ; according to Nordenskidld, west of Nova 
Zembla they are so numerous as almost to cover the 
bottom. The mud holds a considerable number of 
marine worms, and other less striking invertebrates 
also occur in numbers. Here, however, we must note 
that if the sea contributes a wealth of food to land 
animals, yet the land also gives something to the sea. 
The icebergs which detach themselves from the glaciers 
carry much rock-waste to the sea, and this forms a fine 
mud very favourable to animal life, for very many 
shore animals are mud-eaters. The ice-foot and drift- 
ice also contribute to the rock débris spread over the 
bottom, and the proximity of the glaciers to the shore 
in many places must mean that the rock-waste is 
carried by the turbid streams of summer direct to the 
ocean, instead of being spread over the land, as is for 
instance so much of the glacial mud of the Alps. In 
other words, the mud which in Central Europe forms 
such vast fertile areas as the Plain of Lombardy, is in 
Arctic regions carried seawards to help to feed the 
swarming life of the sea. It is this fact which explains 
the wealth of the Arctic in marine birds and mammals, 
and its poverty in land forms of either. The poverty 
of the land is made up for by the wealth of the sea. 
The characteristic marine mammals of the extreme 
north are the walrus and the earless seals, all of which 
spend a considerable part of their life on shore, espe- 
cially at the breeding season, and the whales, toothed 
and whalebone, which never voluntarily come on shore 
at all. All have, of course, had ‘terrestrial ancestors, 
and we may see one justification of the evolution of 
their adaptations to a marine habitat in the fact just 
