-} I 6 Barriers to Dispersion of River Fishes 



dates from the time when these small deep lakes of Indiana 

 and AYisconsm were connected with Lake Michigan. The 

 changes m habits wliich 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 

 bv subterranean channels. Several of the larger fishes, properly 

 characteristic of the Great Lake region,* are occasionally taken 

 in the Ohio Ri\-cr, 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 

 dift'erent 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 rcgiim has grown hotter and drier, until the over- 

 flow fif tlie \Tirious lakes into the Columbia basin through the 

 Snake Ri\-er 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 dift'erences in surround- 

 ings may have caused divergence into distinct species of what was 

 once one parent stock. The suckers in Lake Tahoe f and those 

 in Utah Lake are certainly now dift'erent from each other and from 

 those in the Columbia. The trout + in the same waters can be 

 regarded as more or less tangible species, while the w^hite- 

 fishes§ show no differences at all. The clift'erences 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 Lola maculosa; Pcrcopsis gullata: Esox inasquinongy. 



t L alostoiHus tahoensis, in Lake Tahoe; Caiosiomns iiiacroclieilus and dis- 

 cobolus, in the Cokimbia; Calostoiiius jccundus, Catostonuis ardens; Chasmisies 

 liorus and Panlosteus gcticrosus. in Utah Lake. 



J Salmo henshawi and virginalis. 



\ Coregonus williamsoni. 



