VII FAUNA OF THE CASPIAN SEA 2125 
Sea’ shows us in an interesting manner the effects of isolation 
and changes in salinity, etc., on the inhabitants of a basin .which 
once formed part of the ocean. The waters of the Caspian Sea 
are not fresh, but they are on the average about one-third as 
salt as that of the open ocean. The Crustacea, described by 
Sars, belong to undoubtedly marine groups, eg. the Mysidae, 
Cumacea, and Amphipoda Crevettina, but the remarkable feature 
of these Caspian Crustacea is the great variety of peculiar species 
representing marine genera which are very poorly represented in 
the sea, thus indicating that the variety of the fauna is not due 
to a great variety of species having been shut up in the Caspian 
Sea to begin with, but rather that, after the separation from the 
sea, the isolated species began to vary and branch out in the 
most luxuriant way—whether from lack of competition or owing 
to the changing conditions of salinity it is difficult to say. As 
an example, the Cumacea of the Caspian Sea are ten in number, 
all belonging to peculiar genera related to Pseudocwma, except 
one species which is included in that genus. These Caspian 
forms make up the Family Pseudocumidae, which contains in 
addition only two marine forms of the genus Pseudocuma (see p. 
121). <A very similar condition is found in the numerous 
Amphipods of the Caspian Sea. Considering the enormous changes 
that must have taken place in the distribution of land and 
water even during Tertiary times, it is astonishing that the 
fresh-waters of the world do not contain more species in 
common with the ocean, but it must be considered that the 
limited area and comparatively uniform conditions of fresh- 
water lakes and streams would only permit a limited number of 
these forms to survive which could most easily adapt themselves 
to the changed conditions. And these would in all probability 
be the littoral species that were in the habit of passing up into 
the brackish waters of estuaries and lagoons, so that the uniform 
and limited nature of the fresh-water fauna can be accounted for 
to a certain extent by this hypothesis. 
We have seen in dealing with the marine Crustacea of the 
littoral zone that the chief condition determining their distribution 
is temperature, and that the world may be divided into three chief 
1G. O. Sars, ‘‘ Crustacea Caspia,’ Bull. Acad. Imp. Sc. St. Pétersbowrg (4), 
XXXvi., 1893-4, pp. 51 and 297; (5) i., 1894, pp. 179 and 243; also Crustacea of 
Norway, vol. ii. Isopoda, 1900, p. 73. 
