LAKES AND TARNS 135 



of fishes, often with several incarnations between. It 

 is only thus that the world keeps agoing. The multi- 

 plication of the unicellular plants on the surface of 

 the lake is sometimes so prolific that the water comes 

 to look like living green soup. In some cases, where 

 certain kinds of Oscillarias are abundant, the well- 

 known phenomenon of "red water" occurs. There is 

 often a congestion of life in this "water-flowering," 

 but it passes quickly and a balance is restored. 



We have discussed the freshwater fauna in another 

 book, the " Wonder of Life," so that we must keep 

 here to a very short statement. The kinds of animals 

 that are found in a lake must depend mainly on the 

 chances that there have been of peopling the water 

 and on the physical and chemical conditions. The 

 chemical composition of the water depends on the 

 nature of the basin and on what the streams bring 

 down. The depth is important in affecting tempera- 

 ture, illumination, and pressure; it also influences the 

 changes of level, for wide, shallow lakes show the 

 loss from evaporation in dry seasons more markedly 

 than compact, deep lakes. Lakes often differ in colour, 

 and this affects the penetrability of the water by light. 

 The colour of the water depends on various factors 

 e.g. t the dust in suspension in the water, the number 

 of unicellular Algae, and the amount of organic matter 

 brought in from the peat. 



By the shore of the lake there are nests of water 

 birds, such as Moorhen and Coot. The shallows are 

 inhabited by a few fishes e.g., the Miller's Thumb 

 (Coitus gobio), by Water Snails and freshwater bi- 

 valves, by many insect larvae and countless small 

 Crustaceans, by simple worms and two or three fresh- 



