THE CASPIAN SEA 571 



The free-living fauna of the Barents Sea comprises approximately 2,000 

 species, the Mediterranean 6,000, and the Caspian only 538 ; the last is about 

 27 per cent of that of the Barents Sea and 9 per cent of that of the Medi- 

 terranean. Moreover this low proportion differs greatly for various groups 

 {Table 238). 



Evidently the migration into brackish waters is much easier for the higher 

 crustaceans and fish than for other animals, since they can endure the sub- 

 sequent changes of salinity of the water body much more readily. This is due 

 to their integuments which protect their body from osmotic processes. 



Formation of species 



The process of vigorous species formation undergone by its many forms is a 

 characteristic peculiarity of the Caspian Sea fauna; groups of numerous 

 species were evolved here and their transitions are often indistinct. Such are 

 herrings, bullheads, Benthophilus, Amphipoda, mysids, Cumacea, Dreissena, 



and others. - . 



K. Kiselevitch (1923) considers that all the numerous forms of Caspian 

 herrings have evolved from the one species Caspialosa caspia. G. Sars (1927) 

 came to the conclusion, as a result of his study of Caspian crustaceans, that 

 all the members of the Cumacea species have evolved from the same ancestral 

 form, an immigrant from the Mediterranean. This feature is even more pro- 

 nounced for the remarkable faunas of Lakes Baikal and Tanganaika. 



A definite part of the autochthonous fauna of the Caspian Sea is a relict 

 of the Tertiary seas which had begun to evolve by the end of the middle Mio- 

 cene under the effect of the fall in salinity. 



Sovinsky points out the huge preponderance (89-39 per cent) of forms 

 peculiar to the Pontic-Caspian-Aral area in the Caspian Sea fauna, among 

 which almost three-quarters of the forms are found in the Caspian Sea only. 



