AMERICAN BATS — HANDLE Y 217 



"With the retreat of the IlUnoian icecap and the warming during 

 Sangamon time, Plecotus could have expanded its range again from 

 coast to coast and perhaps northward into Canada and Alaska if 

 caves were available. By this time the southwestern and south- 

 eastern segments had differentiated to the species level and remained 

 genetically distinct when their expanding ranges came into contact. 

 In the meantime, northward advance of warm climate floras and 

 faunas isolated the Plecotus of the Mexican mountains from the 

 remainder of the southwestern segment. 



Southward advance of the Wisconsin icecaps and cooling clim.ates 

 once again contracted the range of Plecotus and broke its wide, con- 

 tinuous distribution into several isolated segments, remnants of which 

 now occur in the southeastern lowlands, in the Appalachians, in the 

 Ozarks, and in southwestern North America. Contact was reestab- 

 lished between the southwestern segment and the Mexican segment 

 which had by this time diverged to the species level. 



"Withdrawal of the Wisconsin icesheets and amelioration of climate, 

 still proceeding, make it possible once more for the ranges of Plecotus 

 to expand. Plotting the present distribution of American Plecotus 

 on a map (fig. 27) showing the maximum southward extension of 

 Wisconsin glaciation lends emphasis to the probability that range 

 expansion has progressed slowly. The present distribution of Plecotus 

 probably bears considerable resemblance to its distribution in the 

 Wisconsin age. It is significant that, in the several thousand years 

 since the last glacial retreat, bats of several genera have moved north- 

 ward, far into the glaciated area, but Plecotus has barely penetrated 

 it. No sharp biotic division marks the drift border, and there is no 

 lack of caves north of the present range of Plecotus (caves in Wisconsin, 

 Pennsylvania, New England, and Ontario, for example) to prevent 

 its northward spread. To assume a relatively slow rate of dispersal 

 for Plecotus could explain its present pattern of distribution, and 

 furnish an im.portant mechanism for speciation. 



