114 Barriers to Dispersion of River Fishes 
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 whitefisht—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. t Salvelinus malma (Walbaum), 
+ Coregonus williamsont Girard. § Thymallus tricolor Cope. 
