DISPERSION OF FRESH-WATER FISHES. II? 



is a considerable difference between the fauna of 

 the Columbia and that of the Sacramento. The 

 species which these two basins have in common 

 are chiefly those which at times pass out into the 

 sea. The rivers of Alaska contain but few species, 

 barely a dozen in all, most of these being found 

 also in Siberia and Kamtschatka. In the scanti- 

 ness of its faunal list, the Yukon agrees with the 

 Mackenzie River, and with Arctic rivers generally. 



There can be no doubt that the general ten- 

 dency is for each species to extend its range more 

 and more widely until all localities suitable for its 

 growth are included. The various agencies of 

 dispersal which have existed in the past are still 

 in operation. There is apparently no limit to 

 their action. It is probable that new " colonies " 

 of one species or another may be planted each 

 year in waters not heretofore inhabited by such 

 species. But such colonies become permanent 

 only where the conditions are so favorable that 

 the species can hold its own in the struggle for 

 food and subsistence. That various modifications 

 in the habitat of certain species have been caused 

 by human agencies is of course too well known to 

 need discussion here. 



We may next consider the question of water- 

 sheds, or barriers which separate one river basin 

 from another. 



Of such barriers in the United States, the most 

 important and most effective is unquestionably 

 that of the main chain of the Rocky Mountains. 

 This is due in part to its great height, still more 

 to its great breadth, and most of all, perhaps, to 



