DISPERSION OF FRESH-WATER FISHES. Ill 



common with that of the tributaries of Lake Michi- 

 gan than it has with that of the Red River or the 

 Arkansas. The influence of cHmate is again shown 

 in the paucity of the fauna of the cold waters of 

 Lake Superior, as compared with that of Lake 

 Michigan. The majority of our species cannot 

 endure the cold. In general, therefore, cold or 

 Northern waters contain fewer species than South- 

 ern waters do, though the number of individuals 

 of any one kind may be greater. This is shown 

 in all waters, fresh or salt. The fisheries of the 

 Northern seas are more extensive than those of 

 the Tropics. There are more fishes there, but 

 they are far less varied in kind. The writer 

 once caught seventy-five species of fishes in a 

 single haul of the seine at Key West, while 

 on Cape Cod he obtained with the same net 

 but forty-five species in the course of a week's 

 work. Thus it comes that the angler, contented 

 with many fishes of few kinds, goes to Northern 

 streams to fish, while the naturalist goes to the 

 South. 



But in most streams the difference in latitude is 

 insignificant, and the chief differences in tempera- 

 ture come from differences in elevation, or from 

 the distance of the waters from the colder source. 

 Often the lowland waters are so different in charac- 

 ter as to produce a marked change in the quality 

 of their fauna. These lowland waters may form a 

 barrier to the free movements of upland fishes ; but 

 that this barrier is not impassable is shown by 

 the identity of the fishes in the streams ^ of the 

 1 For example, Elk River, Duck River, etc. 



