398 THE FRESH-WATER LOCHS OF SCOTLAND 
much less than in the temperate zone, where it reaches from below zero 
to about 30° C. In no other part of the world, except perhaps the 
arctic, will we find lakes with such small annual temperature variations 
as in the tropics. Owing to the high temperatures, the evaporation 
from the surface and the drying up of the rivers in the dry season will 
produce great concentration of the water and great variation in its 
level. In the dry season large water-basins will be dried up, and in 
the rainy season again filled with water. The heavy rain in the rainy 
season will, owing to the enormous amount of decaying organic 
material, carry vast quantities of suspended material into the lakes, 
which, combined with the great quantities of organic material held in 
solution in the water owing to the high temperature, will produce the 
dark-coloured water which characterises so many of the tropical fresh- 
water lakes. 
We have now endeavoured to obtain, as far as possible, an insight 
into the variations in fresh-water lakes from the pole to the equator. 
I wish further merely to call attention here to a physical phenomenon 
of the fresh-water lakes which has only come to light during the last 
few years. It is a well-known fact that the specific gravity and the 
viscosity of the water vary in accordance with the temperature. At 
25° C. the viscosity of pure water is just half as great as at 0° ; the 
specific gravity varies in accordance with the viscosity, but in a much 
less degree. Quite provisionally we may suppose that the viscosity of 
the fresh water is reduced concomitantly with the rising temperature in 
the direction from north to south, and undergoes the greatest regular 
annual variations in that zone, i.e. the temperate zone, where the 
annual variations in temperature are greatest. It is evident that the 
variations in viscosit)^ and specific gravity, since the supporting power 
of the fresh water, and therewith the conditions for buoyancy, are 
dependent on these variations, are of the greatest importance to 
plankton organisms. 
In accordance with the plan laid down in the beginning, we shall 
now consider the communities of plants and animals in fresh-water 
lakes and their variations in the higher latitudes. In fresh-water 
lakes we may distinguish between three great communities : the 
littoral, the abyssal, and the plankton communities. Of the abyssal 
communities in the fresh water we know very little ; they very prob- 
ably contain many cosmopolitan species, but Loven's explorations in 
the Swedish lakes, the Russian investigations in the Baikal Lake, and 
Moore's in the African lakes have shown that the abyssal fauna of the 
fresh water contains many very peculiar organisms, more definite know- 
ledge of which will have to be obtained from future investigations. 
With the littoral communities we are best acquainted. Hitherto 
