lO 



now consider these barriers, and, in tlie same connection, the 

 degree to which they may be overcome. 



Least important of these are the barriers which may exist 

 within the Hmits of any single basin, and which tend to prevent 

 a free diffusion through its waters of species inhabiting any por- 

 tion of it. In streams flowing southward, 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 communication is per- 

 fectly free for most of the length of the Mississippi, there is a 

 material difference between the faunai of the stream in Minne- 

 sota 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 water commu- 

 nication, has no appreciable importance, is shown by the almost 

 absolute identity of the fish-faunae of Lake Winnebago and 

 Lake Champlain. While many large fishes range freely up and 

 down the Mississippi, a majority of the species do not do so, 

 and the fauna of the upper Mississippi has more in common 

 with that of the tributaries of Lake Michigan than it has with 

 that of the Red river or the Arkansas. The influence of 

 climate 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 Southern waters do, though the number of indi- 

 viduals 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 



