3 I 8 Barriers to Dispersion of River Fishes 



of which abound in all waters of northern Asia, Europe, and 

 western North America, has nowhere crossed the basin of the 

 Mississippi, although one of its species finds no difficulty in passing 

 Bering Strait. The trout and whitefish of the Rocky Moun- 

 tain region are all species diiTerent from those of the Great Lakes 

 or the streams of the Alleghany system. To the grayling, the 

 trout, the whitefish, the pike, and to arctic and subarctic 

 species generally, Bering Strait has evidently proved no serious 

 obstacle to diffusion ; and it is not unlikely that much of the close 

 resemblance of the fresh-water faunse of northern Europe, Asia, 

 and North America is due to this fact. To attempt to decide 

 from which side the first migration came in regard to each group 

 of fishes might be interesting ; but without a wider range of facts 

 than is now in our possession, most such attempts, based on guess- 

 work, would have little value. The interlocking of the fish faunas 

 of Asia and North America presents, however, a number of inter- 

 esting problems, for migrations in both directions have doubtless 

 taken place. 



Causes of Dispersion Still in Operation. — One miglit go on 

 indefinitely with the discussion of special cases, each more or less 

 interesting or suggestive in itself, but the general conclusion is in 

 all cases the same. The present distribution of fishes is the result 

 of the long-continued action of forces still in operation. The 

 species have entered our waters in many invasions from the Old 

 World or from the sea. Each species has been subjected to the 

 various influences implied in the term "natural selection," and 

 under varying conditions its representatives have undergone 

 many different modifications. Each of the six hundred fresh- 

 water species we now know in the United States may be con- 

 ceived as making every year inroads on territory occupied by 

 other species. If these colonies are able to hold their own in 

 the struggle for possession, they will multiply in the new condi- 

 tions, and the range of the species becomes widened. If the 

 surroundings are different, new species or varieties may be formed 

 with time ; and these new forms may again invade the territory 

 of the parent species. Again, colony after colony of species 



Salmo clarki Richardson, throughotat the Rocky Mountain range to the Mexican 

 boundary and the headwaters of the Kansas, Platte, and Missouri. 



