118 SCIENCE SKETCHES. 
the fact that it is nowhere broken by the passage 
ofariver. But two species — the Red-throated, or 
Rocky Mountain Trout,! and the Rocky Mountain 
White-fish 2— 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 faunz 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 
faunze; but how shall we explain the almost uni- 
versal diffusion 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 Salmo mykiss Walbaum (= purpuratus Pallas). 
2 Coregonus williamsoni Girard. 
8 Salvelinus malma Walbaum. 
4 Salvelinus namaycush Walbaum. 
5 Thymallus signifer ontariensis Valenciennes. 
