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 Nordenskiold, 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 froin 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 debris 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 Ufe 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 



