Barriers to Dispersion of River Fishes 125 
dates from the time when these small deep lakes of Indiana 
and Wisconsin were connected with Lake Michigan. The 
changes in habits which the cisco has undergone are consider- 
able. The changes in external characters are but trifling. The 
presence of the cisco in these lakes and its periodical disappear- 
ance—that is, retreat into deep water when not in the breeding 
season—have given rise to much nonsensical discussion as to 
whether any or all of 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 Great Basin of Utah.—The similarity of the fishes in the 
different streams and lakes of the Great Basin is doubtless to be 
attributed 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 over- 
flow 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 uninhabitable. In some of these lakes 
certain species may now have become extinct which still remain 
in others. In some cases, perhaps, the differences in surround- 
ings may have caused divergence into distinct species of what was 
once one parent stock. The suckers in Lake Tahoe 7 and those 
in Utah Lake are certainly now different from each other and from 
those in the Columbia. The trout t in the same waters can be 
regarded as more or less tangible species, while the white- 
fishes§ show no differences at all. The differences in the present 
faunas of Lake Tahoe and Utah Lake must be chiefly due to 
influences which have acted since the Glacial Epoch, when the 
whole Utah Basin was part of the drainage of the Columbia. 
Arctic Species in Lakes.—Connected perhaps with changes 
* As Lota maculosa; Percopsis guttata; Esox masquinongy. 
} Catostomus tahoensis, in Lake Tahoe; Catostomus macrocheilus and dis- 
cobolus, in the Columbia; Catostomus fecundus, Catostomus ardens; Chasmistes 
liorus and Pantosteus generosus, in Utah Lake. 
} Salmo henshawi and virginalis. 
§ Coregonus williamsoni. 
