CHAPTEE XVII. 



THE DISTEIBUTION OF FEESHWATER FISHES. 



Having shown above that mimerous marine fishes enter fresh 

 waters, and that some of them have permanently established 

 themselves therein, we have to eliminate from the category of 

 freshwater fishes all such adventitious elements. They are 

 derived from forms, the distribution of which is regulated by 

 other agencies, and which, therefore, would obscure the rela- 

 tions of the faunae of terrestrial regions if they were included 

 in them. They will be mentioned with greater propriety 

 along with the fishes constituting the fauna of the brackish 



