MODIFICATIONS OF FISHES. 239 



completely solved and capitalized, in his essay, " for 

 Darwin." Of the sirens, commonly but erroneously 

 reckoned among the Cetacea, and of which the majority 

 prefer remaining at the mouths of large estuaries, one 

 entire species has penetrated into the great inland lakes 

 of Africa ; and certain species of salmon as well as the 

 sturgeons, which alternate periodically between salt and 

 fresh water, are in the phase of gradually forsaking 

 ocean life. From my special experience, I may add 

 that the brackish-water sponges are certainly dependent 

 on the marine families, and that the fresh-water species 

 unmistakably point to these brackish forms. 



If in all these cases we are dealing with gradual 

 transformation, and more or less voluntary adaptation, 

 there is no lack of conspicuous instances of forcible 

 and almost sudden severance ; of upheavals by which 

 former sections of the ocean became inland seas. What 

 were the modifications undergone by the fish and crabs 

 secluded with them, is shown by the fine observations 

 of Loven on the animals of Lakes Wener and Wetter, 

 and of Malmgren on those of Ladoga. The latter brings 

 evidence that the salmon-trout of the Alps (Salmo 

 salvelinus) is derived from the Polar Sea, and is own 

 brother to the Scandinavian Salmo* alpinus. 



Riitimeyer pronounces the opinion that by more 

 minutely tracing the relations of the fresh-water fauna 

 to those of the denizens of the ocean, the cosmopoli- 

 tanism of fresh-water animals will be explained, as well 

 as the relation of antarctic to arctic life. For the pre- 

 sent, however, these two great animal groups, as regards 

 the higher, warm-blooded classes, are somewhat sharply 

 contrasted. It is only from scanty remains that we 



