GENERAL CHARACTERISTICS AND GEOLOGICAL HISTORY 379 



by the fact that pelagic fish in the Caspian Sea, in contrast to those of the 

 Black and Azov Seas, have no powerful competitors ; it might also be the effect 

 of higher temperature. The other peculiarity to which Svetovidov drew atten- 

 tion lies in the fact that in the Caspian Sea they form a larger number of 

 species and a considerably larger number of smaller taxonomic subdivisions. 

 Six species of herring and one species of the Clupeonella genera live in the 

 Black Sea; in the Caspian there are eight species and sixteen smaller sub- 

 divisions of herring of the genera Caspiolosa. This difference is also ex- 

 plained by Svetovidov by the absence from the Caspian Sea of competitive 

 members of pelagic herring and other genera (in the Black Sea there are 

 Spratella, Sardina, Sardinella and Alosa), which has furthered the evolution of 

 the species. However, this might be rather more due to changes of salinity which 

 repeatedly occurred in the Caspian basin during the Tertiary and Quaternary 

 periods, during which a part of the Clupeidae must have died out and the 

 remainder have gone through a period of vigorous development of forms. 



Finally Svetovidov also notes a third very characteristic feature of the 

 Caspian Clupeidae — a large number of purely 'marine' species and forms 

 which do not enter fresh waters, but which migrate great distances within the 

 sea and multiply in sea water. This relates both to the three Caspian species 

 of the genus Clupeonella and to the species of the genus Caspialosa (C. 

 brashnikovi, C. saposhnikovi, С sphaerocephala). Svetovidov thinks that in 

 the Black Sea such forms were 'pushed into the least saline parts of the Black 

 and Azov Seas by more vitally active Mediterranean immigrants'. Both 

 Caspian Clupeidae forms, which make long migrations, and the purely 

 ' marine ' forms are absent from the Black and Azov Seas. These most curious 

 facts and the explanations given for the phenomena discussed above require 

 further research and additional speculation. 



Zoogeographical affinity 



The marked differences between the fauna of the Mediterranean and Caspian 

 Seas makes it impossible to include both in the same zoogeographical unit. 

 The Black Sea and the Sea of Azov must be included, as the Black Sea-Azov 

 province, in the Mediterranean-Lusitanian subregion of the boreal region ; 

 as for the Caspian Sea it should not be included as part of a Pontic-Caspian- 

 Aral province of the Mediterranean subregion as was done by V. Sovinsky 

 (1902), neither should it be considered as the Caspian province, as was done 

 by A. Derzhavin (1925). The Caspian fauna is too original and has little in 

 common with the Mediterranean fauna. Therefore it is more correct to give 

 to the Caspian Sea a separate zoographical place of its own as the Caspian 

 relict region. 



