account* for the absence of spiny-rayed fishes. Land barriers have evi- 

 dently proved competent to prevent trout getting in from the headwaters 

 of the trout streams to the westward, and the mud and alkali which 

 they encountered in the lower portion of the Yellowstone, the Missouri 

 and the Big Cheyenne have as certainly proved an impassable barrier 

 from that direction. Among the many regions of the United States 

 which possess the necessary natural conditions for trout, the Black Hills 

 district is the only one of any considerable area, if we except portions of 

 the Yellowstone National Park, in which one or more species of Salmon- 

 idee are not or have not been indigenous. The absence of trout and all 

 other species of fish from the various lakes and streams of the Yellow- 

 stone National Park (e. g. Lewis and Shoshone lakes, Gibbon, Firehole 

 and Little Firehole rivers, and Indian, Glen, Nez Perce 1 and Sentinel 

 creeks) is undoubtedly accounted for by the presence of impassable falls 

 where these waters leave the great rhyolite sheet which covers the Park, 

 as shown by the investigations made by Dr. Jordan in 1889. The presence 

 of trout in Yellowstone Lake and tributary streams, notwithstanding the 

 fact that the outlet of Yellowstone lake (Yellowstone River) has two enor- 

 mous falls which wholly prevent the ascent of fish, is quite evidently due 

 to the most interesting and curious fact that there is a continuous water- 

 way furnishing easy passage for trout from the upper tributaries of Snake 

 River, by way of Two-Ocean Pass, into the upper Yellowstone River. That 

 Yellowstone Lake could have been, and almost certainly way, stocked in 

 this way from the Columbia basin, was demonstrated by the investiga- 

 tions which I made during my visit to Two-Ocean Pass in August, 1891. 



The presence of trout in the upper tributaries of the Colorado, Rio 

 Grande, Arkansas, and Platte, whose lower courses are, in some cases at 

 least, not unlike those of the Cheyenne and Missouri, is a matter whose 

 explanation is not without some difficulties. The relationships of the 

 various species or sub-species of Salmo found in these different basins 

 are very close and indicate a common origin at no remote date. Whether 

 they are all descended from a form which came up from the Pacific coast or 

 one from the Atlantic cannot be certainly known, though the bulk of the 

 evidence points to the former view. But whatever may have been the 

 fact, it is certain that the headwaters of the Columbia, Colorado, Rio 

 Grande, Arkansas, and Platte have been connected in some way at some 

 time or other, thus permitting the trout to spread into these various basins. 

 That there are no trout in the Chevenne basin would seem to indicate that 



