292 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



Sea) and slightly higher temperature (the air temperature in southern Sweden 

 was 2° to 2-25° higher than at present) lasted for about 4,000 years. A new 

 fauna appeared, with Littorina littorea, Cardium edule, Mytilus edulis, etc. 

 Subsequently, as a result of the rising of the sea-floor in the region of the 

 straits during the last 4,000 years, the inflow of the ocean waters slackened 

 and the Baltic Sea acquired a salinity approaching that of today. In the fol- 

 lowing period further distinction has been made between the Limnae Sea 

 (Lymnaea peregra) and the Mya Sea (Mva arenarid) ; the difference between 

 these two phases and the present phase is small as regards hydrology, and 

 it consists mainly of a change of fauna. 



V. FLORA AND FAUNA 



The present population of the Baltic Sea was evolved during the post-glacial 

 period and is very varied in its composition. It consists of three main com- 

 ponents — marine, fresh-water and brackish-water (in the narrow sense of 

 this word). In so far as the Baltic Sea has a low salinity all its population can 

 be considered as brackish in the broad sense of the word ; however, brackish- 

 water fauna in the narrow sense — the population of the Ice Age and the Arctic 

 basin — are also included in its composition. 



It is essential, therefore, to make a distinction between the population of a 

 brackish body of water and a brackish fauna, retaining this term only for the 

 fauna which is brackish in the narrow sense of the word, i.e. fauna which 

 is the result of a (geologically) prolonged development of a fauna which is 

 marine in origin and partly also fresh-water under conditions of consider- 

 ably lowered salinity. The population of the Baltic Sea consists of the follow- 

 ing groups (Fig. 137): 



(7) Marine euryhaline forms. The main part of the present population of the 

 Baltic Sea. 



(7) Taxonomic unseparable 

 (2) Taxonomic separable 

 (5) Marine relicts of former geological periods 

 (4) Immigrants from distant seas. 

 (//) Fresh-water euryhaline forms. These form a considerable part of the 

 population of the Baltic Sea. 

 (7) Taxonomic unseparable 

 (2) Taxonomic separable. 

 (777) True brackish-water forms. These also form a considerable part of the 

 population of the Baltic Sea. 



(7) Ancient brackish-water Arctic relicts (pseudo-relicts — immigrants) 

 formed during the Ice Age in the less saline parts of the Arctic 

 basin. They penetrated into the Baltic Sea during the post-glacial 

 period from the northeast and the east, possibly, via fresh-water 

 systems. 

 (2) Brackish-water forms which had originated from the fresh-water 

 ones. 



