128 SCIENCE SKETCHES. 



these lakes are still joined to Lake Michigan by 

 subterranean channels. Several of the larger fishes, 

 properly characteristic of the Great Lake Region,^ 

 are occasionally taken in the Ohio River, where 

 they are usually recognized as rare stragglers. 

 The difference in physical conditions is probably 

 the sole cause of their scarcity in the Ohio basin. 



The similarity of the fishes in the different streams 

 and lakes of the Great Basin is doubtless to be at- 

 tributed to the general mingling of their waters 

 which took place during and after the glacial epoch. 

 Since that period the climate in that region has 

 grown hotter and drier, until the overflow of the 

 various lakes into the Columbia basin through the 

 Snake River has long since ceased. These lakes 

 have become isolated from each other, and many of 

 them have become salt or alkaline and therefore un- 

 inhabitable. In some of these lakes certain species 

 may now have become extinct which still remain 

 in others. In some cases, perhaps, the differences 

 in surrounding may have caused divergence into 

 distinct species of what was once one parent stock. 

 The Suckers in Lake Tahoe^ and those in Utah 

 Lake are certainly now different from each other 

 and from those in the Columbia. The Trout ^ in 

 the same waters can be regarded as more or less 

 tangible varieties only, while the White-fishes ^ show 

 no differences at all. The differences in the present 



1 As, Lota lota maculosa ; Percopsis giittatus ; Esox masqiiino7igy. 



2 Catostomus tahoensis, in Lake Tahoe ; Catostomus macrocheilus 

 and discobolus, in the Columbia ; Catostomus fecundus, Catostomus 

 ardens; Chasmistes liorus and Pantosteus generosus, in Utah Lake. 



3 Salmo mykiss, et vars. henshawi and virginalis. 

 * Coregonus williamsoni. 



