382 THE FRESH- WATER LOCHS OF SCOTLAND 
Baltic lakes (G. West, 1905, p. 968). Here the Characew go down to 
20-25 feet, and Fontinalis antipyretica even to 40 feet (G. West, 1905, 
pp. 982-983). 
The conditions offered to organisms by the real lakes and by the 
small ponds and pools differ greatly ; in the latter especially, the 
variations in temperature are very great, principally in spring. There 
is therefore a very considerable difference between the famia and 
flora of the pools and of the large lakes, especially in the southern 
parts of the zone. There is, however, but little information on these 
matters as yet. 
The Baltic Fresh- water Lakes 
A great many researches enable us to judge of the conditions of 
life in the Baltic lakes ; the most important will be mentioned in 
the sequel. 
Many countries bordering upon the Baltic are very rich in lakes, 
especially Finland, Ponierania, and Prussia, to a somewhat less degree 
South Sweden, and Denmark least of all. The great majority of all 
these lakes are in some way indebted to the Glacial Age for their 
origin. Their number was formerly much greater, and the area of 
the present lakes also much larger. From a series of valuable papers 
we can judge of the origin of the North German lakes, their topo- 
graphy and geography (see especially Geinitz, 1886, p. 1 ; Wahnschaffe, 
1891, p. 1 ; Bludau, 1894, p. 1 ; Steusloff, 1907, p. 427 ; Halbfass, 1901, 
p. 1, 1903a, pp. 592 and 706 ; Braun, 1903, p. 1, 1907, p. 8 ; 
Seligo, 1900, p. 1, 1905a, p. 1, 1907, p. 1 ; Ule, 1894, p. 1, 1898, 
p. 25; Penck, 1894, vol. ii. p. 266; Keilhack, 1887, p. 161). 
From Sweden we have principally Trybom's investigations of the 
lakes in Jonkoping and Malmohusliin (1893, 1895, 1896, 1899, 1901). 
The Swedish explorations give us information regarding the glacial 
lakes of former times and the kind of soil left by them. It has been 
shown how lakes have been dammed up by the masses of ice of the 
Glacial Age, and how, on the retreat of the ice, the water has 
hollowed out enormous valleys by erosion and left a drained lake-bottom 
consisting of clay and sand. In the case of several lakes it appears that 
the ice has kept the height of the water far above that of the present 
time, so that a number of the present small separate lakes were formerly 
only one large lake. On all this see especially Gavelin (1907, p. 1), 
Munthe (1907, p. 1), Westergard (1906, p. 408), Bobeck (1906, 
p. 481). 
For Danish lakes we have no corresponding literature ; here only 
Madsen (1903, p. 1) can be cited. 
It is common to the lakes recorded here that they hardly deserve 
this name. All come under ForeFs definition of pond-lakes, and 
