15 



jties 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 various modifications in the 

 habitat of certain species have been caused by human agencies 

 is of course too well known to need discussion here. 



We may next consider the question of water-sheds, or bar- 

 riers 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 {Sahno 

 mykiss Walbaum \j=^pnrpicratiis Pallas],) and the Rock}' Moun- 

 tain whitefish [Coregonus williamsom Girard) — 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. 



It is easy to account for the separation of the faunae ; 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 fountain-heads. 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. 



