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 guttatus ; Esox masquinongy. 
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. 
4 Coregonus williamsoni. 
