386 THE FRESH-WATER LOCHS OF SCOTLAND 
for a long time, though it has been little studied. Outside this belt, 
again, it has sometimes, but not always, been possible to find a so-called 
" Grundalgenzone (Brand, 1896, p. 8). On the vegetation in the 
Baltic lakes see especially Warming (1897, p. 164), Ostenfeld (1905, 
p. 377), Baagoe and Kolpin Ravn (1896, p. 288), Carlsson (190J2), 
Brand (1896, p. 1), Klinge (1890, p. 264). In the httoral zone we 
further find an extremely rich animal life. Many species of Mollusca 
with an enormous number of individuals are to be found, and enormous 
quantities of insects are also hatched there. 
The lakes of this zone are specially characterised by the important 
part played by the organic life. It is not here, as in the other zones, 
the lakes which impose conditions to which the organisms must adapt 
themselves ; it is the organic life which has got the upper hand of the 
lakes, transforming them through fundamental changes in the shape 
of the lake basins and in the chemical and physical qualities of the 
water. 
Through the influence of the waves and the ice the material of 
the littoral zone which decays in autumn undergoes pulverisation. 
The detritus thus formed mixed with the large quantities of clay, 
lime, and organic material carried in by the rivers, together with the 
plankton, produce precipitation, filling up and transforming the 
original lake-basins. The lake-bottom is covered by enormous layers 
of material originally mainly organic, which through the action of 
the bottom fauna and bacterial flora is transformed into clay and cal- 
careous gytjes. The filling- up process always begins in the primary 
inlets of the lakes, so that the lakes are rounded and approximate 
to the circular shape. The result of the filling-up process is the 
rapid closure of the lake-basins : one lake after another becomes 
closed ; tracts of land rich in lakes become dry plains or give place 
to peat-bogs and meadows, a process which but rarely goes on so 
rapidly farther north. Owing to the large quantities of plankton, 
the nature of the bottom is to a great degree determined by the 
quality of the plankton (Diatom gytjes, Cyanophycea gytjes, Chitin 
gytjes; Wesenberg-Lund, 1901, p. 110), 
The rich life of the littoral region further influences to a high 
degree the thermic conditions. The fact that the temperature of 
our lakes in summer can rise so high is due, I believe, to a very great 
extent to the organic life in the littoral region. On warm summer 
days temperatures of 30-35° C. may occur in the dark, warmth- 
abstracting, and warmth-producing heaps of detritus. Similar tem- 
peratures are also found in the Sphagnum and Hypnum moss-beds often 
found at the upper ends of the creeks in our large lakes, and also in 
the dense vegetation of Myriophyllum^ Hydrocharis^ and other plants 
which lie much nearer to the free surface of the water. That these 
