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 ali 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 Lovén 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 
