. a 
Barriers to Dispersion of River Fishes 115 
How Fishes Cross Watersheds. —It is easy to account for 
this separation of the faune; but how shall we explain the 
almost universal diffusion of the whitefish 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 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 very broad, and we may imagine that at some time spawn 
or even young 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 forms 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 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 south- 
western corner, equally distinctly, came the waters which fed 
the Lara. This lake, like similar 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 
