Each species of fishes is best fitted for a particular region into which it has been forced to live, either to escape its enemies or to be able to get a living easiest. In its migrations it has moved along lines of least resistance, and has colonized those streams where Mother Nature has been able to do the most for it. The darters are small, perch-like fishes, which seldom exceed a length of six inches, the average being about three. All are active and swift swimmers and well suited for a life among the rocks and swift water of our smaller streams.

All countries have small, swift, rocky streams, but few have darters. In their stead we find loaches, gobies, characins, sulpins, and the like. These fishes, have "become dwarfed and concentrated, taking the place in their respective habitats which the darters occupy in the waters of the Mississippi valley. By the same process of 'analogous variation' the cichlids of South America parallel the sunfishes of the United States, although in structure and in origin the two groups are diverse."

Dr. Jordan tells us that the trout of the Pacific coast came to America from Asia, and gradually spread eastward and southward until now it is found in all the streams of the Rocky Mountains, the Sierra Nevada, the Cascades and the Coast range. It is but a short distance from Kamchatka to Alaska, and this distance is traveled by trout to this day; once over, a fish able to spend much of its time in salt water could easily colonize all our coast streams. Whether or not all of our Pacific trout are descendants of one species, the cut-throat trout, is more or less uncertain, though it is quite certain that all have descended from not more than two or three species. In many places they have been able to pass from the head waters of one river to that of another, just as they now pass from the head waters of the Columbia to the Missouri by the way of Two Ocean Pass. The ancient lakes, Lahontan and Bonneville, no doubt assisted them in their migrations. Since these have disappeared each colony has had to remain more or less isolated. In time they have become somewhat changed, to better adapt themselves to their new environment.

These changes have developed certain peculiar characters, by means of which we can distinguish one kind of trout from another, just As the farmer distinguishes his Berkshire from his Poland China. Spread, as the trout are, over such a large area, in such an immense variety of streams and lakes, and with a vertical range of over one thousand feet, we would certainly expect as large a number of species and varieties of trout to be developed as we find at present in the streams of our west coast.

Fishes are found in the deepest parts of the ocean. Some of these are peculiar to the deep waters, none of the shore fishes resembling them. On the other hand, many deep sea fishes belong to families well represented in the shallow water. The flounders are found in water at all depths, and the same is true of the bat fishes, rock fishes and other shore fishes. It is easy to understand how these fishes have found their way to the deep water. It was either to escape their enemies or to extend their range for some reason; as Mr. Garman puts it, "they have slid down," as it were to the bottom of the ocean.

In general, animals migrating will always move along lines of least resistance. Some deep sea fishes have a considerable vertical range. It is thought that some move into shallower water to deposit their eggs or place their young in warmer water, and where the peculiar kind of food they need early in life is the most abundant. To study deep sea fishes is difficult, and so little has been done that we not only know them imperfectly but also know very little concerning their life histories.

In February, March and April of 1891 the United States Fish Commission steamer Albatross explored a portion of the region between the coasts of Mexico and Central America and the Galapagos Archipelago. Besides obtaining a large number of shore fishes, about nine hundred specimens of fishes were secured, ranging from a depth of one hundred to twenty-two hundred and twenty-three fathoms. This collection was carefully studied by Professor Garman, of Harvard. He found the collection to contain one hundred and eighty species, eighty five per cent. of which were new to science. The bottoms of the oceans are far from level, and each deep basin has its own peculiar fauna.