14 



BULLETIN 144, UNITED STATES NATIONAL MUSEUM 



ably as wide ranging is the more highly specialized M. aR>escen8^ 

 The fifth species, M. shnus^ one of the most strikingly characterized 

 American members of the genus, is known from two localities about 

 250 miles apart on the lowlands east of the Andes. 



A feature which is noticeable among the American members of 

 this group is the usually inverse ratio between distribution and dif- 

 ferentiation. The four nearly related and slightly specialized species 

 Myotis lucifugus^ M. yumanensis, M. chiloen^sis, and M. nigHccms 

 cover practically the entire American area inhabited by these bats; 

 at the opposite extreme of structural modification we find Pizonyx, 

 an animal whose area of dispersal is restricted, apparently, to the 

 coast and islands of the Gulf of California. The range of Myotis 

 velifer includes most of Mexico and the adjoining part of the United 

 States ; that of the more highly specialized M. ocauZtus is, according 

 to present knowledge at least, decidedly less extensive. The seem- 

 ingly narrow ranges of such aberrant species as Myotis grisescens 

 and M. simvus are in noticeable contrast with the large areas occupied 

 by M. keerm and M. subulatus. On the other hand the range of 

 the highly specialized M. volans is practically coincident with that 

 of the rather primitive M. yumanensis. 



Where species spread over areas of strikingly different climatic 

 conditions, they usually show a corresponding color variation. This 

 seems to be chiefly a response to the degree of atmospheric saturation? 

 or dryness. Thus along the humid coast of the Northwest, from 

 southern Alaska to California, no less than five different Myotis are 

 represented by darkened (" saturate ") local forms. In the semiarid 

 regions farther eastward, the color becomes less intense over a wide 

 area of country from the interior of California northeastward, while 

 under the desert conditions of the Southwest and parts of the interior 

 of the United States, the same species become extremely pale. Still 

 farther south, in southern Mexico, they again darken in color ; while 

 in the eastern United States, they may be of a different color still, 

 neither saturate nor pallid. 



In the following table are listed the geographical races of six North 

 American species of Myotis grouped according to the general climatic- 

 areas inhabited. 



