AMERICAN BATS — ^HANDLEY 211 



as far as food supply is concerned. For antisocial, nonmigratory, 

 cool climate inhabitants, such as some Plecotus are, not only is the 

 presence of caves suitable for hibernation essential, but the caves 

 must be sufficiently abundant to allow a degree of privacy and isola- 

 tion from other species of bats. Unfortunately, it is not possible to 

 index all of these factors through time. Otherwise, a more satisfactory 

 pictm*e of the differentiation and dispersal of these bats might be 

 drawn with less recourse to speculation. 



The present distribution of Euderma and Plecotus and their relative, 

 Barhastella, coincides rather closely with the North Temperate zone. 

 Thus, it may be assumed that tropical and boreal climates have 

 served as barriers to their dispersal. The center of origin for the 

 group may have been either Nearctic or Palearctic. Since distantly 

 related primitive forms and closely related progressive forms occur in 

 both regions, a hypothesis of multiple invasion is required to explain 

 present distributions regardless of which region may have served as 

 the center of origin. No paleontological record is available for the 

 more primitive forms, so paleontology gives no hint of origin or an- 

 cestry. If it be assumed that the area of most active differentiation 

 and greatest elaboration of forms indicates the center of origin of a 

 group of animals, and that the most advanced, most progressive, and 

 most specialized form is to be found about the center of origin, while 

 the most primitive, most conservative, and least progressive is to be 

 found on the peripheries (Matthew, 1939, p. 32), then it must be 

 reckoned that this group of bats originated in the Nearctic. 



Ancestral. Forms 



It is probable that the ancestral forms were derived early in the 

 Cenozoic, since the related living genus Myotis is recognizable at least 

 as far back as Middle Oligocene. During the Eocene and Oligocene 

 epochs, low relief of the northern continents and lack of climatic 

 barriers permitted a moist temperate forest to extend from Greenland 

 across northern America to Alaska, Siberia, northern Europe, and 

 Spitzbergen. A land bridge at Bering Strait intermittently connected 

 North America and Eurasia [indicated by strong Lower Eocene, 

 Upper Eocene, and Lower Oligocene migrations of mammals (Simp- 

 son, 1947, p. 625)]. The temperate forest of this region was char- 

 acterized by redwood (Sequoia), and included such other trees and 

 shrubs as alder (Alnus), dogwood {Cornus), maple {Acer), oak {Quer- 

 cus), ash (Fraximus), madrone (Arbutus), beech (Fragus), and elm 

 (JJlmus) (Cain, 1944, p. 107). During this period the ancestors of 

 Euderma and Plecotus possibly could have had a Holarctic distribution. 



Continental uplift, beginning in the Oligocene epoch and continuing 

 to the present time, led to a gradual southward spread of the temperate 



