272 SCIENCE SKETCHES. 
called Salmo mykiss lewist. UHow the lewis? 
crossed the Great Divide over to the headwaters 
of the Missouri and spread itself where it could 
in the Yellowstone Park, I have already twice 
told in my way. Dr. Barton W. Evermann of 
the U. S. Fish Commission has told it in a still 
better way, for he has himself visited the Two 
Ocean Pass and caught it in the very act of 
crossing the Divide. Just south of the Yellow- 
stone Park is a great depression in the main 
divide of the Rocky Mountain chain which is re- 
duced to a quarter of a mile of low marshy 
ground. East of this marsh the Atlantic Creek 
flows eastward into the Yellowstone. West of it, 
Pacific Creek finds its way into Snake River. 
Across the marsh the streams become entangled, 
and each one sends a part of its water over into 
the other. In the spring the marsh is largely 
under water, and there is no obstacle to the pas- 
sage of the trout. For the greater part of the 
year one stream at least is open, and the trout 
can pass without hindrance from the Snake River 
to the Yellowstone, from the basin of the Colum- 
bia to that of the Missouri! Thus the trout 
came over into Yellowstone Lake and into the 
Yellowstone River, thence into the Missouri and 
its great clear affluents, the Jefferson, Madison, 
1 The Trout of the Upper Missouri (and Upper Columbia) has 
been called Salmo lewis’ by Girard and Salmo carinatus by Cope. 
It does not differ in any visible way from Salmo mykiss, although 
it is now isolated from the latter, its parent stock. Trout con- 
fined to rivers are always smaller than those of the same kind 
resident in lakes. Those which enter the sea grow to a still 
larger size. 
