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 Mitchill. 
3 Notropis rubricroceus Cope; Rhinichthys atronasus Mitchill; 
etc. 
