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; 4 
(7) Catadromous fishes, like 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 aspecies. 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 Linneus. 2 Acipenser, sp. 
3 Clupea sapidissima Wilson. 4 Morone lineata Bloch. 
5 Anguilla anguilla Linneus. 
& Thus the Chub-sucker (Zrimyzon sucetia) 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. 
