204 FISHES. 



etc. Tlie two former evidently belonged originally to the 

 freshwater series, and it was only in the course of their 

 existence that they acquired the habit of descending to the 

 sea, perhaps because their freshwater home did not offer a 

 sufficient supply of food. These migrations of freshwater 

 fishes have l)een compared with the migrations of birds ; 

 but they are much more limited in extent, and do not im- 

 part an additional element to the fauna of the place to which 

 they migrate, as is the case witli the distant countries to 

 which birds migrate. 



The distinction between freshwater and marine fishes is 

 further obscured by geological changes, in consequence of 

 which the salt Avater is gradually being changed into fresh, or 

 vice versa. These changes are so gradual and spread over so 

 long a time, that many of the fishes inhabiting such localities 

 accommodate themselves to the new conditions. One of the 

 most remarkable and best studied instances of such an altera- 

 tion is the Baltic, which, during the second half of the Glacial 

 period, was in open and wide communication with the Arctic 

 Ocean, and evidently had the same marine fauna as the 

 White Sea. Since then, by the rising of the land of Northern 

 Scandinavia and Finland, this great gulf of the Arctic Ocean 

 has become an inland sea, with a narrow outlet into the 

 l^orth Sea, and its water, in consequence of the excess of the 

 fresh water pouring into it over the loss by evaporation, has 

 been so much diluted as to be nearly fresh at its northern 

 extremities : and yet nine species, the origin of wliich from the 

 Arctic Ocean can be proved, have survived the changes, propa- 

 gating their species, agreeing with their brethren in the Arctic 

 Ocean in every point, but remaining comparatively smaller. 

 On the other hand, fishes which we must regard as true fresh- 

 water fishes, like the Eudd, Eoach, Pike, Perch, enter freely 

 the brackish water of the Baltic.^ Instances of marine fishes 



^ Ekstroni, Fisclie in den Sclieeren von j\Iorko. 



