Barriers to Dispersion of River Fishes 305 



ness of its faunal list, the Yukon agrees with the Mackenzie 

 River, and with Arctic rivers generally. 



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 water- 

 sheds, or barriers which separate one river basin from an- 

 other. 



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 * 

 and the Rocky Mountain whitefishf 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 | of the Columbia River 

 and its affluents does not cross to the east side of the moun- 

 tains, nor does the Montana grayling ever make its way to the 

 West. In Northern Mexico, however, numerous Eastern river 

 fishes have crossed the main chain of the Sierra Madre. 



* Salmo clarki Richardson. % Salvelinus malma (Walbaum). 



f Coregonus williamsoni Girard. Thymallus tricolor Cope. 



