DISPERSION OF FRESH-WATER FISHES. 95 



(6) Anadromous fishes, or those which run up 

 from the sea to spawn in fresh waters, as the 

 Salmon,^ Sturgeon,^ Shad,^ and Striped Bass ; * 



(7) Catadromous fishes, hke the Eel,''' which pass 

 down to spawn in the sea ; and (8) brackish-water 

 fishes, which thrive best in the debatable waters 

 of the river-mouths, as most of the Sticklebacks 

 and the Killifishes. 



As regards the range of species, we have every 

 possible gradation from those which seem to be 

 confined to a single river, and are rare even in 

 their restricted habitat, to those which are in a 

 measure cosmopolitan,^ ranging everywhere in 

 suitable waters. * 



Still, again, we have all degrees of constancy and 

 inconstancy in what we regard as the characters 

 of a species. Those found only in a single river- 

 basin are usually uniform enough ; but the species 

 having a wide range usually vary much in different 

 localities. Such variations have at different times 

 been taken to be the indications of as many differ- 

 ent species. Continued explorations bring to light, 

 from year to year, new species ; but the number of 

 new forms now discovered each year is usually less 

 than the number of recognized species which are 

 yearly proved to be intenable. Three complete 

 lists of the fresh-water fishes of the United States 



1 Salmo salar Linnaeus. 2 Acipenser, sp. 



3 Clupea sapidissimaV^Wsow. * Morone liiteata'BXoch.. 



5 Angicilla anguilla Linnaeus. 



6 Thus the Chub-sucker [Erimyzon siicetta) in some of its varie- 

 ties ranges everywhere from Maine to Dakota, Florida, and Texas ; 

 while a number of other species are scarcely less widely distributed. 



