vii FAUNA OF THE CASPIAN SEA 215 



Sea 1 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, e.g. 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 Pseudocuma, 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 

 Arnphipods 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 



1 G. 0. Sars, "Crustacea Caspia," Bull. Acad. Imp. Sc. St. PUcrsbourg (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. 



