There are known at the present about twenty thousand species of fishes, which are distributed throughout the creeks, rivers, lakes, seas and oceans of the world. A few species of the open sea are cosmopolitan; the others are more or less restricted in their range. Northern Asia, Europe and North America have in common a few species of fresh water fishes. There are many others of close relationship, which indicates a somewhat common origin of the fish faunas. The same is largely true of the salt water shore fishes, which live well to the north. The fresh water fishes of South America, Africa and Australia are all different from each other, none being even closely related as are those we find in the countries of the northern hemisphere.

The fishes of our Atlantic coast are different from those of the Pacific, very few species being common to both coasts. The fishes of the Ohio river are entirely different from those of the Columbia, not a single species being common to both streams. The fishes of the Missouri river are very different from the Ohio, many of the larger species, as catfishes, buffalo fishes, black basses, and some of the sun fishes are common to both rivers. The difference between the fishes of these two rivers is chiefly in the smaller kinds, which do not migrate to any great extent, and is greater as you go toward their sources, or confine yourself to their smaller tributaries.

There are many reasons why the fishes of one region are not the same as those we find in another. Some of these reasons we may learn by making a careful study of the fishes of each region, and their environment. In addition we must learn all we can about the past history of the country, finding which streams were formed first, and how they became inhabited from the old ancient fish faunas of our earlier geological periods. If you visit streams in the Alleghenies, the Ozarks and the Black Hills you will find them much alike. All have clear, cool water, flowing over sand or gravel. The black bass, speckled trout, channel cat, and the eastern pickerel will live quite as well in streams of each locality. If you spend a day at each place collecting fishes all your catch will not be the same species. In the Allegheny region you will obtain about forty species, and a like number in the Ozarks.

Of these quite one-fourth, or one-fifth, will be the same species, and the others closely related. A large portion will consist of sunfishes and very small, perch-like fishes, which are called darters. These are spiny-rayed fishes; that is, nearly all of the fins are made partly of strong, sharp spines, such as you find on the back of sunfishes, black bass and the like. In the streams of the Black Hills you will not find more than fifteen species, and not more than one or two, if any, will be the same as in either of the other two catches. There are none of the spiny-rayed fishes in the Black Hills, and no trout, though the streams seem in every way well suited for them. The fishes of the Black Hills consist of two catfishes, four suckers, eight minnows, and one member of the cod family. Why are there no spiny-rayed fishes? If you examine a map you will find that the Black Hills is an isolated region, about seventy-five by one hundred miles in extent. It is covered with heavy pine forests and drained by a dozen or, more good sized creeks, which find, through the north and south forks of the Cheyenne, an outlet into the Missouri river. Surrounding the Black Hills is a broad plain one hundred or two hundred miles in width. It has no forests, and only a scant vegetation. Its streams are alkali and contain much solid matter in suspension. None of these streams flow over rocky or gravelly beds. Like all the streams of the great plains they are overloaded with sediment. All the streams can do with this sediment is to deposit it in places during falling or low water, and in time of freshets, pick it up, shift it about and redeposit it farther down the stream. Such streams are like the Platte, narrow and deep in a few places, but mostly wide and shallow, with a bottom of quicksand. The streams of the plains have in them but few species of fishes; especially is this true of the upper Missouri, and these are such species as we find in the Black Hills. It is thus evident that the fishes of this region migrated there, and only such fishes as were able or willing to live in the muddy, alkaline streams of the great plains could have ever reached the Black Hills. The minnows and suckers are ever preyed upon by sunfishes, bass and the like, and to escape them evidently sought retreat in the alkaline water, which was too much disliked by their enemies for them to follow. Once there and accustomed to such water they would migrate farther up stream until they reached the clear, cool streams of the Black Hills.