CHARACTERISTICS AND DISTRIBUTION OF LAKES 519 
adjoining land is small in quantity. In salt lakes, again, there may 
be a chemical precipitation of salts on the floor of the lakes. 
In addition to a rise and fall of the surface of the lake due to Motions, 
the varying amount of rainfall in the region, there may be a rise at 
one end of a lake produced by the heaping up of water through 
strong winds and gales, and, in addition to the ordinary waves, 
standing waves, called seiches, have been detected in most lakes. At 
the boundary-line separating layers of water of different temperature 
and density, what are called " temperature seiches have been dis- 
covered in the Scottish and other lakes. 
Lakes are inhabited by a great variety of organisms, but both Organisms, 
species and genera are much less numerous than in the ocean, and 
some whole classes — the Echinoderms, for example — are unrepresented. 
The ocean was almost certainly the original home of living beings, 
and relatively few species have been able to establish themselves in 
the less congenial fresh water.^ In very deep lakes the bottom fauna 
is represented by only a few species, or life may be wholly absent, 
with the exception of bacteria. In temperate regions there appears 
to be active vertical circulation of the water, even in the deepest 
lakes, at least twice a year, when the maximum density point (39°'l F., 
4° C.) is reached at the surface. In tropical regions, however, it is 
probable that, owing to an absence of, or much less active, vertical 
circulation, there may be insufficient oxygen to support animal life 
at the bottom of very deep lakes. There is a well-marked cosmo- 
politanism in the plankton organisms of lakes. Indeed, the fresh- 
water plankton is regarded as the oldest community of organisms on 
the earth. 
Lakes may be compared to oceanic islands. Just as an oceanic Compared 
island presents many peculiarities in its rocks, soil, fauna, and flora, iSands?^'^^^^*^ 
due to its isolation from the masses of continental land, so does a 
lake present individuality and special peculiarities in its physical, 
chemical, and biological features, owing to its position with reference 
to the drainage from the surrounding land, and its separation from 
the mass of waters represented by the great oceans. 
The surface of the earth, with which we are daily in direct contact, 
is composed of lithosphere, hydrosphere, and atmosphere, and these Hydrosphere, 
all interpenetrate. Lakes, rivers, underground water, the water of 
hydration in the lithosphere, and the water- vapour of the atmosphere, 
must all be regarded as belonging to outlying portions of the 
hydrosphere, which consists mainly of the waters of the great ocean 
basins. 
Lakes may be classified in a great variety of ways, but no method 
1 R. Quinton, Ueau de m,er milieu organique, Paris, 1904. 
