Distribution of to their distribution in the streams, the fresh water fishes may be subdivided as follows: 1. Lowland fishes; as the bowfin, pirate perch, large-mouthed black bass, sun-fishes and some catfishes.
2. Channel fishes; as the channel catfish, the mooneye, the goldeye, garpike, buffalo fishes and drum.
3. Upland fishes; as many of the darters, shiners and suckers, and the small-mouthed black bass.
4. Mountain fishes; as the brook-trout, and many of the darters and minnows.
To these we may add, (5) the more or less distinct classes of lake fishes, inhabiting only the waters which are deep, clear and coloi, as the various species of whitefish, lake herring, cisco and the Great-Lake trout; (6) anadro mous fishes, or those which run up from the sea to spawn in fresh waters, as the salmon, stur geon, shad and striped bass; (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 every where in suitable waters.
Barriers to River The existence of boundaries to the range of species implies the existence of harriers to their diffusion. We may now consider these barriers and, in the same connection, the degree to which they may be overcome.
Least important of these are the barriers which may exist within the limits of any single basin, and which tend to prevent a free diffu sion through its waters of species inhabiting any portion of it. In streams flowing south ward, or across different parallels of latitude, the difference in climate becomes a matter of Importance. The distribution of species is gov erned very largely by the temperature of the water. Each species has its range in this re spect — the free-swimming fishes, notably the trout, being most affected by it; the mud-loving or bottom fishes, like the catfishes, least. The latter can reach the cool bottoms in hot weather, or the warm bottoms in cold weather, thus keeping their own temperature more even than that of the surface of the water. Al though water communication is perfectly free for most of the length of the Mississippi, there is a material difference between the faunas of the streams in Minnesota and in Louisiana. This difference is caused chiefly by the differ ent temperature occupying the difference in lati tude. That a similar difference in longitude,
with free water communication, has no appre ciable importance, is shown by the almost ab solute identity of the fish-faunas of Lake Win nebago and Lake Champlain. While many large fishes range freely up and down the Mis sissippi, a majority of the species do not do so, and the fauna of the upper Mississippi has more in common with that of the tributaries of Lake Michigan than it has with that of the Red River or the Arkansas. The influence of climate is again shown in the paucity of the fauna of the cold waters of Lake Superior, as compared with that of Lake Michigan. The majority of our species cannot endure the cold. In general, therefore, cold or northern waters contain fewer species than southern waters do, though the number of individuals of any one kind may be greater. This is shown in all waters, fresh or salt. The fisheries of the northern seas are more extensive than those of the tropics. There are more fishes there, but they are far less varied in kind.
But in most streams the difference in lati tude is insignificant, and the chief differences in temperature come from differences in elevation, or from the distance of the waters from the colder source. Often the lowland waters are so different in character as to produce a marked change in the quality of their faunas. These lowland waters may form a barrier to the free movements of upland fishes; but that this bar rier is not impassable is shown by the identity of the fishes in the streams on the uplands of middle Tennessee with those of the Holston and French Broad. Again, streams of the Ozark Mountains, similar in character to the rivers of East Tennessee, have an essentially similar fish fauna, although between the Ozarks and the Cumberland Range lies an area of lowland bayous, into which such fishes are never known to penetrate. We can, however, imagine that these upland fishes may be sometimes swept down from one side or the other into the Mis sissippi, from which they might ascend on the other side. But such transfers certainly do not often happen. This is apparent from the fact that the two faunas are not quite identical, and in some cases the same species are represented by perceptibly different varieties on one side and the other. The time of commingling of these faunas is perhaps now past, and it may have occurred only when the climate of the in tervening regions was colder than at present.