MODIFICATION OF FISHES. 



239 



solved and capitalized, in his essay, " for Darwin." Of 

 the sirens, commonly but erroneously reckoned among 

 the Cetacea, and of which the majority prefer remain- 

 ing 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 trans- 

 formation, 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 modi- 

 fications 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 Scan- 

 dinavian 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 present, 

 however, these two great animal -groups, as regards the 

 higher, warm-blooded classes, are somewhat sharply con- 

 trasted. It is only from scanty remains that we know 



