NATURE AND ORIGIN OF FRESH-WATER ORGANISMS 359 
find a seal, a herring, certain Cumacea and Schizopoda, the mollusc 
Cardium edule^ a Polychaete worm, and two Foraminifera of marine 
type. Lake Baikal, in Eastern Siberia, which is one of the largest 
fresh-water lakes in the world, is similarly inhabited by a seal, also 
by certain Harpacticoid Copepods and a Polycha?te worm. 
It is no wonder, then, that cases such as these, in which sea-organisms 
are living in fresh-water basins, have aroused wide interest. An 
inquiry into the past history of these inland seas affords some clue 
(particularly in the case of the Caspian) as to the meaning of the 
anomalies. During the early part of the Tertiary period, the 
Caspian appears to have belonged to a great sea which then covered 
the southern part of Russia, and was in direct communication with 
the ocean. Only since then has it become gradually cut off* from the 
sea and gradually freshened. If this is indeed the case, it is not 
difficult to believe that the marine forms which have been mentioned 
are forms which have persisted in the lake since it was actually a 
portion of the ocean. 
Inland basins which seem to be the modified remainders of isolated 
portions of the ocean are sometimes spoken of as relict seas 
{Reliktenseen)^ and the Caspian is manifestly an example of such. 
The case of Lake Baikal is by no means so satisfactorily proved from a 
geological point of view ; but however that may be, it is clear that in 
certain instances, at any rate, the existence of what we have called 
marine animal types in fresh water is merely an indication of the 
origin of that fresh- water basin, and not of a lack of distinctness 
between the two great groups of aquatic animals. 
If, then, certain apparent exceptions do not really invalidate our 
conception of a difference between marine and fresh-water organisms, 
we of necessity ask the question : Why are certain forms present in 
one case and not in the other P This at once takes us to the root of 
matters, for it not only involves a study of organisms in relation to 
their environment, but suggests the additional question : How did 
fresh- water life originate ? 
At the present day, the most varied forms of life, both animal 
and vegetable, are found in fresh water. Representatives of most of 
the principal groups are known, from the Protozoa up to the mammals 
themselves, and from the lowest Algae to the flowering plants. Yet 
there seems no escape from the conclusion that life had its origin in 
the ocean, and that all the fresh-water organisms with which we are 
acquainted must have been derived either directly or indirectly from 
that source. 
Our study of the different groups concerned has shown that, w^hile 
certain cases exist in which the forms all inhabit one medium or the 
other, in the greater number of cases some types are capable of exist- 
