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06 Barriers to Dispersion of River Fishes 



How Fishes Cross Watersheds. — It is easy to account for 

 this separation of the faume; but how shall we explain the 

 almost uniA'ersal diffusion of the whitefish and the trout in 

 suitalde waters on both sides of the dividing ridge? We may 

 notice that these two are the species which ascend highest in 

 the mountains, the whitefish inhabiting the mountain pools 

 and lakes, the trout ascending all brooks and rapids in search of 

 their fountainheads. In many cases the ultimate dividing ridge 

 is not \-ery broad, and we may imagine that at some time spawn 

 or even A^oung fishes may have been carried across by birds or 

 other animals, or by man, or more likely by the dash of some 

 summer whirlwind. Once carried across in favorable circum- 

 stances, the species might survive and spread. 



The following is an example of how such transfer of spe- 

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

 left to draw on the imagination to invent possible means of 

 transit. 



The Suletind. — There are few watersheds in the world 

 better defined than the mountain range which f(3rms the "back- 

 bone" 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 op- 

 posite sides of the peninsula. To the north of the Suletind is 

 a large double lake called the Sletningen\-and. The maps shew 

 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 south- 

 western corner, equally distinctly, came the waters which fed 

 the Lara. This lake, like smiilar mountain ponds in all north- 

 ern countries, abounds in trout; and these trout certainly have 

 for part of the year an uninterrupted line of water communica- 

 tion from the Sognefjord on the west of Norway to the Chris- 

 tianiafjord on the southeast, — from the North Sea to the Baltic. 

 Part of the year the lake 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 



