FISHES OF THE MISSOURI RIVER BASIN. 383 



The explanation of their absence is practically the same as that which 

 accounts for the absence of spiny-rayed fishes. Land barriers have 

 evidently proved competent to prevent trout getting in from the head- 

 waters 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 speci- 

 mens of Salmonidct} are not or have not been indigenous. The absence 

 of trout and all other species offish from the various lakes and streams 

 of Yellowstone National Park (i. e., Lewis and Shoshone lakes, Gibbon, 

 Firehole, and Little Firehole rivers, and Indian, Glen, ISTez Perce, and 

 Sentinel creeks) is undoubtedly accounted for by the presence of impass- 

 able 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, not- 

 withstanding the fact that the outlet of Yellowstone Lake (Yellowstone 

 Kiver) has two enormous falls which wholly prevent the ascent of fish, 

 is quite evidently due to the most interesting and curious fact that 

 there is a continuous waterway furnishing easy passage for trout from 

 the upper tributaries of Snake River by way of Two-Ocean Pass into 

 the Upper Yellowstone Kiver. That Yellowstone Lake could have been ? 

 and almost certainly was, stocked in this way from the Columbia Basin 

 was demonstrated by the investigations which were made by Professor 

 Evermaun during a visit to Two-Ocean Pass in August, 1891. 1 



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

 Grande, Arkansas, and South 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 subspecies of Salmo found in those different basins 

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

 certain that they are all descended from a form which came up from 

 the Pacific Coast and that the headwaters of the Columbia, Colorado, 

 Rio Grande, Arkansas, and South Platte have been connected in some 

 way at some time or other, thus permitting the trout to spread into these 

 various basins. 2 



That there are no trout in the Cheyenne Basin would seem to indicate 

 that the streams of this system became separated and differentiated as 

 a distinct drainage system earlier than did those of the South Platte, 

 Arkansas, Rio Grande, Colorado, and Columbia; or else that they are 



'See Bull. U. S. Fish Coinm., xi, 1891, 24-28; also, Popular Science Monthly for 

 June, 189;"). 



2 For an interesting discussion of the origin of the varieties of Salmo of our western 

 waters see "How the trout came to California," by Dr. Jordan, in Recreation, for 

 October, 1891. 



