There can be no doubt that the general tendency is for each species to extend its range more and more widely until all localities suitable 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 the various modifications in the habitat of certain species have been caused by human agencies is of course too well known to need discussion here. =Watersheds.=--We may next consider the question of watersheds, or barriers which separate one river basin from another. Of such barriers 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 it is nowhere broken by the passage of a river. But two species--the red-throated or Rocky Mountain trout[113] and the Rocky Mountain whitefish[114]--are found on both sides of it, at least within the limits of the United States; while many genera, and even several families, find in it either an eastern or a western limit to their range. In a few instances representative species, probably modifications or separated branches of the same stock, occur on opposite sides of the range, but there are not many cases of correspondence even thus close. The two faunas are practically distinct. Even the widely distributed red-spotted or "dolly varden" trout[115] of the Columbia River and its affluents does not cross to the east side of the mountains, nor does the Montana grayling[116] ever make its way to the West. In Northern Mexico, however, numerous Eastern river fishes have crossed the main chain of the Sierra Madre. =How Fishes Cross Watersheds.=--It is easy to account for this separation of the faunæ; but how shall we explain the almost universal diffusion of the whitefish and the trout in suitable waters on both sides of the dividing ridge? We may notice that these two are the species which ascend highest in the mountains, the whitefish inhabiting the mountain pools and lakes, the trout ascending all brooks and rapids in search of their fountainheads. In many cases the ultimate dividing ridge is not very broad, and we may imagine that at some time spawn or even young fishes may have been carried across by birds or other animals, or by man, or more likely by the dash of some summer whirlwind. Once carried across in favorable circumstances, the species might survive and spread.