2/2 SCIENCE SKETCHES. 



called Salmo mykiss leivisi. How the lewisi 

 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. E^ermann 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. 1 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 lewisi by Girard and Salmo carinatus by Cope. 

 It uoes 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. 



