"The ways in which the dispersal of fresh-water fishes has been affected were various. They are probably all still in operation, but most work so slowly and imperceptibly as to escape direct observation; perhaps they will be more conspicuous after science and scientific inquiry shall have reached a somewhat greater age. From the great number of fresh-water forms which we see at this present day acclimatized in, gradually acclimatizing themselves in, or periodically or sporadically migrating into, the sea, we must conclude that under certain circumstances salt water may cease to be a barrier at some period of the existence of fresh-water species, and that many of them have passed from one river through salt water into another. Secondly, the headwaters of some of the grandest rivers, the mouths of which are at opposite ends of the continents which they drain, are sometimes distant from each other a few miles only. The intervening space may have been easily bridged over for the passage of fishes by a slight geological change affecting the level of the watershed or even by temporary floods; and a communication of this kind, if existing for a limited period only, would afford the ready means of an exchange of a number of species previously peculiar to one or the other of these river or lake systems. Some fishes provided with gill-openings so narrow that the water moistening the gills cannot readily evaporate, and endowed, besides, with an extraordinary degree of vitality, like many Siluroids (_Chlarias_, _Callichthys_), eels, etc., are enabled to wander for some distance over land, and may thus reach a water-course leading them thousands of miles from their original home. Finally, fishes or their ova may be accidentally carried by water-spouts, by aquatic birds or insects, to considerable distances." =Fresh-water Fishes of North America.=--We now recognize about six hundred species[75] of fishes as found in the fresh waters of North America, north of the Tropic of Cancer, these representing thirty-four of the natural families. As to their habits, we can divide these species rather roughly into the four categories proposed by Prof. Cope, or, as we may call them, (1) Lowland fishes; as the bowfin,[76] pirate-perch,[77] large-mouthed black bass,[78] sunfishes, and some catfishes. (2) Channel-fishes; as the channel catfish,[79] the mooneye,[80] garpike,[81] buffalo-fishes,[82] and drum.[83] (3) Upland fishes; as many of the darters, shiners, and suckers, and the small-mouthed black bass.[84] (4) Mountain-fishes; as the brook trout and many of the darters and minnows.