DISTRIBUTION AND LOCATION 265 



are also marine, but the three families of the Labyrinthici, the 

 serpent-heads, climbing perches, and gouramis, are adapted 

 to live exclusively in fresh water. These last must be 

 regarded, like the fresh-water Acanthopterygii (spine-finned 

 fishes), as fishes which have returned from the sea to inland 

 waters. Of fresh-water Acanthopterygii the most characteristic 

 are the Percida; (perches) of the northern hemisphere, evi- 

 dently related to the Serranidae, justly called sea-perches. The 

 Centrarchidae or sun-fishes of North America are another fresh- 

 water family allied to the perches. There can be no doubt 

 that the Cichlidae of the tropics are derived from Labridae 

 (wrasses) which have ascended from the sea. In cases where 

 a single species of a family, as the miller's thumb among the 

 bull-heads (Cottidae) and the burbot among the cod-family 

 (Gadidae), lives in fresh water, it is obvious that it is recently 

 derived from marine ancestors. Lastly fresh-water forms 

 include the Mastacembelidae, placed in a separate Sub-Order, 

 and a few species of Plectognathi ; these also have been secon- 

 darily adapted to their present habitat. 



The distinguishing features of the fresh-water fish-fauna of 

 the great continents may here be mentioned. (See Map, Plate 

 XXII.) In Europe and northern Asia there are no lung-fishes 

 (Dipnoi) and of the ganoid forms only the sturgeons (Chond- 

 rostei) are represented ; of these the true sturgeons are abund- 

 ant in Europe. Acipenser sturz'o, the common sturgeon (Plate 

 XX., B) is frequently caught in the North Sea and occurs also 

 in the Mediterranean, ascending all the great rivers; the great 

 Russian sturgeon, A. huso, is common in the Volga and the 

 little sterlet inhabits the Danube and other rivers flowing into 

 the Black Sea and Caspian. Of the soft-finned fishes (Mala- 

 copterygii) the characteristic families are the carps (Cyprinidae) 

 and Salmonidae ; among the former the loaches are almost 

 confined to this region, and of other members of the carp 

 family there are numerous species which are abundant in 

 individuals such as our roach, dace, chub, bream, etc. Of 

 Salmonidne we have the trout, salmon, and numerous species 

 of char and white-fish or Coregonus, Siluridae or cat-fishes, 

 are almost entirely absent, the few species which occur being 

 immigrants from India or North America ; in Europe there is 

 only one species, Siluris giants, the wels, which lives in the 



