The effects of waterfalls and cascades as barriers to the diffusion of most species is self evident ; but the importance of such obstacles is less, in the course of time, than might be ex pected. In one way or another very many spe cies have passed these barriers. The falls of the Cumberland limit the range of most of the larger fishes of the river, but the streams above it have their quota of darters and minnows. It is evident that the past history of the stream must enter as a factor into this discussion, but this past history it is not always possible to trace. Dams and artificial waterfalls now check the free movement of many species, especially those of migratory habits; while, conversely, numerous other species have extended their range through the agency of canals.
Every year fishes are swept down, the rivers by the winter floods; and in the spring, as the spawning season approaches, almost every spe cies is found working its way up the stream.
In some cases, notably that of the uinnat-sal mon and the red salmon, the length of these migrations is surprisingly great. To some spe cies rapids and shallows have proved a sufficient barrier, and other kinds have been kept back by unfavorable conditions of various sorts. Streams whose waters are charged with silt or sediment, as the Missouri, Arkansas or Brazos, do not invite fishes; and even the occasional floods of red mud, such as disfigure otherwise clear streams, like the Red River or the Colorado (of Texas), are unfavorable. Extremely unfavor able also is the condition which obtains in many rivers of the Southwest; as for example, the Red River, the Sabine and the Trinity, which are full from bank to bank in winter and spring, and which dwindle to mere rivulets in the autumn droughts.
In general, those streams which have condi tions most favorable to fish-life will be found to contain the greatest number of species.
There can be no doubt that the general tend ency is for each species to extend its range more and more widely until all localities suit able for its growth are included. The various agencies of dispersal which have existed in the past are still in operation. There is apparently no limit to their action. It is probable that new °colonies° of one species or another may be planted each year in waters not heretofore inhabited by such species. But such colonies become permanent only where the conditions are so favorable that the species can hold its own in the struggle for food and subsistence. That various modifications in the habitat of certain species have been caused by human agencies is of course too well known to need discussion here.
Of watersheds in the United States the most important and most effective is unquestionably that of the main chain of the Rocky Mountains.
This is due in part to its great height, still more to its great breadth, and most of all, perhaps, to the fact that (Two Ocean Pass excepted) it is nowhere broken by the passage of a river. In the few cases when species have crossed this barrier, some break in the chain (as the Two Ocean Pass in Wyoming connecting the Snake River with the Yellowstone) has now been re corded.
Habitat of Species.— Each species finds its habitat fitted to its life, and then in turn is forced to adapt itself to this habitat. Any other kind of habitat then appears as a barrier to its distribution. Thus to a fish of the ripples a stretch of still water becomes a barrier. A spe cies adapted to sandy bottoms will seldom force its way through swift waters or among weeds or rocks.
The stream that has the greatest variety of animals in it would be one (1) connected with a large river; (2) in a warm climate; (3) with clear water, and (4) little fluctuation from winter to summer; (5) with little change in the clearness of the water; (6) a gravelly bottom; (7) preferably of limestone, and (8) covered in its quiet reaches and its ripples with water weeds. These conditions are best realized in the United States in the tributaries of the Ohio, Cumberland, Tennessee and Ozark rivers among American streams, and it is in them that the greatest number of species of fresh-water animals (fishes, crayfishes, mussels, etc.) has been recorded. These streams approach most nearly to the ideal homes for animals of the fresh waters. The streams of Wisconsin, Michigan and the Columbia region have many advantages, but are too cold. Those of Illi nois, Iowa, northern Missouri and Kansas are too sluggish, and sometimes run muddy. Those of Texas and California shrink too much in the summer, and are too isolated. The streams of the Atlantic Coast are less isolated, but none connects with a great basin, and those of New England run too cold for the great mass of the species. For similar reasons the fresh-water animal life of Europe is relatively scanty, that of the Danube and Volga being richest. The animal life of the fresh water of South Amer ica centres in the Amazon, and that of Africa in the Nile, the Niger and the Kongo. The great rivers of Siberia, like the Yukon in Alaska and she Mackenzie River in British America, have but few species of fresh-water animals, though the kinds fitted for life in cold, clear water exist in great abundance. See FISH ; ICHTHYOLOGY. DAVID STARR JORDAN, Chancellor Emeritus Leland Stanford Junior University.