120 SCIENCE SKETCHES. 



has probably but a single outlet through the Lara. 

 A higher temperature would entirely cut off the 

 flow into the Bagna, and a still higher one might 

 dry up the lake altogether. This Sletningenvand,^ 

 with its two outlets on the summit of a sharp 

 water-shed, may serve to show us how other lakes, 

 permanent or temporary, may elsewhere have 

 acted as agencies for the transfer of fishes. We 

 can also see how it might be that certain mountain 

 fishes should be so transferred while the fishes of 

 the upland waters may be left behind. In some 

 such way as this we may imagine the Trout and 

 the White-fish to have attained their present wide 

 range in the Rocky Mountain region ; and in simi- 

 lar manner perhaps the Eastern Brook Trout ^ 

 and some other mountain species^ may have been 

 carried across the Alleghanies. 



1 Since the above was written I have been informed by Professor 

 John M. Coulter, who was one of the first explorers of the Yel- 

 lowstone Park, that such a condition still exists on the Rocky 

 Mountain Divide. In the Yellowstone Park is a marshy tract, 

 traversable by fishes in the rainy season, and known as the " Two- 

 Ocean Water." In this tract rise tributaries both of the Snake 

 River and of the Yellowstone. Similar conditions apparently 

 exist on other parts of the Divide, both in Montana and in 

 Wyoming. 



Professor John C. Branner calls my attention to a marshy upland 

 which separates the valley of the La Plata from that of the Ama- 

 zon, and which permits the free movement of fishes from the 

 Paraguay River to the Tapajos. It is well known that through 

 the Cassiquiare River the Rio Negro, another branch of the 

 Amazon, is joined to the Orinoco River. It is thus evident that 

 almost all the waters of eastern South America form a single 

 basin, so far as the fishes are concerned. 



2 Salvelinus fontinalis Mitch ill. 



3 Notropis rtibricroceiis Copej Rhinichthys atronasjcs Mitchill ; 

 etc. 



