. Climatic variation in historic and prehistoric time. sted as regards the other sea-fish: plaice, flounder, cod and sprat, etc. Southern forms of molluscs, too, such as Tapes, Littorina, Ost- rea, came with the Atlantic waters into the Baltic. Oyster-banks existed everywhere in Isefjord, and around the Danish Islands. This was the period of the »kitchen-middens»; the great fish-migration period, when the gates of the Baltic stood wide open to the fish from the Atlantic. All the species of fish found in the Baltic, except the eel, sea-pike, and mackerel, are to be considered as relicts from th
. Climatic variation in historic and prehistoric time. sted as regards the other sea-fish: plaice, flounder, cod and sprat, etc. Southern forms of molluscs, too, such as Tapes, Littorina, Ost- rea, came with the Atlantic waters into the Baltic. Oyster-banks existed everywhere in Isefjord, and around the Danish Islands. This was the period of the »kitchen-middens»; the great fish-migration period, when the gates of the Baltic stood wide open to the fish from the Atlantic. All the species of fish found in the Baltic, except the eel, sea-pike, and mackerel, are to be considered as relicts from the Littorina period, at the close of which began the rise of the land which restored the depth-conditions of the Sund and the Belt to about their present position. This geological alteration, which was completed about 3,000 years ago, had a far-reaching influence on the fish-races of the Baltic, which are now in a greater or less degree separated from their relations in the ocean and live und other hydrographical conditions,. Surface-map (present time). Surface-map (Littorina-epoch). which have gradually altered their physiological life-conditions and have even commenced to set their stamp on the bodily and exterior form of the fish. The expression »relict» must, however, be understood relatively, The ancient relicts from the days of the Polar Sea consist, at present, of some lower animal forms, mostly inhabiting the great depths and including too the arctic gray-seal, which still lives in the Baltic and in lake Ladoga in Russia. These are relicts in the original sense of the word, from a period dating 50,000—100,000 years back. Amongst them there is only one certain arctic species of fish, the Cottus quad- ricornis. Among the relicts of the Littorina period we reckon, in the first place, the stromling or small herring, the indigenous herring race of the central and northern Baltic, which now lives isolated from the North Sea-herring, whose migrations nowadays do not exten
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