298 Barriers to Dispersion of River Fishes 



St. John's Rivers have not a single species in common; and 

 with one or two exceptions, not a species is common to any 

 two of them. None of these * has any species peculiar to itself, 

 and each shares a large part of its fish fauna with the water- 

 basin next to it. It is probably true that the faunas of no two 

 distinct hydrographic basins are wholly identical, while on 

 the other hand there are very few species confined to a single 

 one. The supposed cases of this character, some twenty in 

 number, occur chiefly in the streams of the South Atlantic 

 States and of Arizona. All of these need, however, the con- 

 firmation of further exploration. It is certain that in no case 

 has an entire river fauna f originated independently from the 

 divergence into separate species of the descendants of a single 

 type. 



The existence of boundaries to the range of species implies, 

 therefore, the existence of barriers to their diffusion. We may 

 now consider these barriers and in the same connection the 

 degree to which they may be overcome. 



Local Barriers. Least important to these are the barriers 

 which may exist within the limits of any single basin, and 

 which tend to prevent a free diffusion through its waters of 

 species inhabiting any portion of it. In streams flowing south- 

 ward, or across different parallels of latitude, the difference in 

 climate becomes a matter of importance. The distribution of 

 species is governed very largely by the temperature of the water. 

 Each species has its range in this respect, the free-swimming 

 fishes, notably the trout, being most affected by it; the mud- 

 loving or bottom fishes, like the catfishes, least. The latter can 

 reach the cool bottoms in hot weather, or the warm bottoms in 

 cold weather, thus keeping their own temperature more even than 

 that of the surface of the water. Although water communica- 

 tion is perfectly free for most of the length of the Mississippi, 

 there is a material difference between the faunae of the stream 

 in Minnesota and in Louisiana. This difference is caused chiefly 

 by the difference in temperature occupying the difference in 

 latitude. That a similar difference in longitude, with free 



* Except possibly the Sacramento. 



t Unless the fauna of certain cave streams in the United States and Cuba 

 be regarded as forming an exception. 



