Once carried across in favorable circumstances, the species 

 might survive and spread. 



I have seen an example of how such transfer of species 

 may be accomplished, which shows that we need not be left 

 to draw on the imagination to invent possible means of transit. 



There are few water-sheds in the world better defined than 

 the mountain range which forms the "backbone" of Norway. 

 I lately climbed a peak in this range, the Suletind. From its 

 summit I could look down into the valleys of the Lara and the 

 Bagna, flowing in opposite directions to opposite sides of the 

 peninsula. To the north of the Suletind is a large double lake 

 called the Sletningenvand. The maps show this lake to be one 

 of the chief sources of the westward-flowing river Lara. This 

 lake is in August swollen by the melting of the snows, and at 

 the time of my visit it was visibly the source of both these 

 rivers. From its southeastern side flowed a large brook into 

 the valley of the Bagna, and from its southwestern corner, 

 equally distinctly, came the waters which fed the Lara. This 

 lake, like similar mountain ponds in all northern countries, 

 abounds in trout ; and these trout certainly have for 

 part of the year an uninterrupted line of water com- 

 munication from the Sognefjord on the west of Norway 

 to the Christianiatjord on the southeast, — from the North 

 Sea to the Baltic. Part of the year the lake has probably 

 but a single outlet through the Liira. A higher temperature 

 would entirely cut off the flow into the Biigna, and a still 

 higher one might dry up the lake altogether. This Sletnin- 

 genvand,* 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 



* Since the above was written I have been informed by Professor John M. Coulter, who was one 

 of the first explorers of the Yellowstone Park, that such a condition still exists on the Rocky Moun- 

 tain 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 Mon- 

 tana and in Wyoming. 



Professor John C. Brannsr calls my attention to a marshy upland which separates the valley of 

 the La Plata from that of the Amazon, 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. 



