218 R. R. MILLER 



(of the Columbia complex) and the upper Missouri River. Moun- 

 tain whitefish {Prosopium williamsoni), cutthroat trout (Salmo 

 clarki), and the mountain sucker (Pantosleus) have moved east- 

 ward; and the longnose sucker {Catostomus catostomus), longnose 

 dace {Rhinichthys cataractae), and mottled sculpin {Cottus hairdi) 

 have moved westward. Farther north, the Continental Divide has 

 been crossed at several points, notably between the upper Fraser or 

 Skeena basins and the MacKenzie Valley in British Columbia 

 (Lindsey, 1956), where at least 8 and likely 10 species have crossed 

 from east to west and 7 species have moved from west to east. 



Multiple crossings of the Continental Divide are evident from the 

 distribution patterns of such western species as Prosopium william- 

 soni, Salmo clarki, and Salvelinus malma, and of the eastern 

 Catostomus catostomus and possibly Rhinichthys cataractae. Most 

 species have not spread far after entering eastern or western waters 

 but the cutthroat trout, mountain whitefish, and mottled sculpin 

 (Cottus hairdi), all of which ascend tributaries, have extended their 

 ranges into isolated basins probably via stream captures, at least in 

 part. Two northern species are not included in the above discussion 

 since they are unknown from Pacific streams in the area covered by 

 this report. Round whitefish, Prosopium cylindraceum, and northern 

 pike, Esox lucius, have invaded the Alsek River, a Pacific tributary 

 in extreme northwestern British Columbia, which also contains 

 Arctic grayling (Lindsey, 1956, p. 789) ; Esox and Thymallus also 

 inhabit the Taku River, next to the south. Headwater transfer from 

 the MacKenzie drainage furnishes a plausible explanation of their 

 limited occurrence on the Pacific slope. Most of these crossings of the 

 Continental Divide probably took place in Postglacial times, at 

 least in the northern Rockies, and no crossing is thought to be earlier 

 than the latter part of the Pleistocene. The identity of most or all of 

 the now separated species on each side of the Divide supports the 

 view that the transfers were recent. 



CONCLUSION 



Of the 21 families of primary freshwater fishes inhabiting North 

 America (Fig. 1), about 30 per cent of the species are judged to be of 

 North American origin, 55 per cent of Eurasian ancestry, and 15 

 per cent of South American affinities. Two relict families, the 

 Polyodontidae and the Amiidae, with a single species each in eastern 



