100 THE WONDER OF LIFE 



It is said that the freshwaters occupy about 1,800,000 

 square miles, but that is a small fraction of the total of 

 about 197,000,000 for the earth's surface. In some 

 countries, however, the freshwater area is very considerable; 

 thus in Finland it is estimated at about 13 per cent. The 

 relative smaEness of the freshwaters is made up for in a way 

 by the scattered distribution and the correlated great 

 diversity in character. How many difierent forms there 

 are, with no unity except in the word fresh — the large 

 deep lake with storms like those at sea, the mountain tarn 

 with its dark mysterious surface, the shallow pond with a 

 population in many respects difEerent from that of the 

 lake, the ephemeral pool, the permanent well, the swamp, 

 the ditch, the brook and the river. Nor do these exhaust 

 the list; thus in a detailed German classification we find 

 a special subdivision for water-pipe fauna. It is recorded 

 that before the improvement of the filtering in connexion 

 with the water-supply of a large town on the Continent, 

 no fewer than sixty-one animals were obtained from the 

 pipes — ^including eels, sticklebacks, water-snails, insect 

 larvae, worms, and the freshwater sponge. For practical 

 purposes, it may be noted, large intruders are often unim- 

 portant. The serious thing is when some fungus, hke 

 Crenothrix, takes up its abode in the pipes. 



Of the various forms which accumulations of fresh water 

 may assume, the lake or loch is most important. It is 

 distinguished from the pond not so much by its size as by 

 depth, which reaches a maximum in Lake Baikal, with its 

 760 fathoms. In typical lakes we can readily distinguish 

 (1) the relatively shallow shore-area, (2) the open water, 

 and (3) the dark quiet dreary plain at the foot of the steep 

 slope or talus which runs from the shore-shelf downwards. 



