Il8 SCIENCE SKETCHES. 



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 

 White-fish^ — 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 faunae 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 

 mountains; nor does the Great Lake Trout ^ nor 

 the Montana Grayling ^ ever make its way to the 

 West. 



It is easy to account for this separation of the 

 faunae ; but how shall we explain the almost uni- 

 versal diff"usion of the White-fish 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 White-fish in- 

 habiting the mountain pools and lakes, the Trout 

 ascending all brooks and rapids in search of their 

 fountain-heads. In many cases the ultimate divid- 

 ing ridge is not very broad, and we may imagine 



1 Sahno mykiss Walbaum {=piirpuratus Pallas). 



2 Coregonus zvilliamsoni Girard. 



3 Salvelmns 77ialma Walbaum. 



4 Salvelinus jiamayaish Walbaum. 



^ Thyniallus signifer ontariensis Valenciennes. 



