The Chipmunks of the Genus Eutamias. 201 



two forms in question — spcciosus and callipejjlas — are separated, 

 if at all, by a gap only 15 miles in width. 



In view of these facts it is important to bear in mind that the 

 southern part of the lofty Sierra is split lengthwise by the upper 

 valle}'' of Kern River into two parallel ridges, one of which (the 

 eastern) is inhabited by true speciomts, the other (the western) 

 by callipeplus. It should be remembered also that the Mt. Piiios 

 colony of callipeplus lies southwest of the Sierra colony, and that 

 the San Bernardino Mt. colony of speclosas lies southeast of the 

 Sierra colon\^ of the same form. These facts, taken in connec- 

 tion with the close relationship of speciosus with quadrivittutus of 

 Colorado, point to the former continuous range of the group 

 across the south end of the Great Basin from the Rocky Mts. to 

 the Sierra, San Bernardino Mt., and San Jacinto Peak ; Avhile the 

 presence of a distinct form {callipeplus) on Mt. Pinos at the ex- 

 treme west end of the Mohave Desert and on the western range 

 of the Sierra not only points to a former connection between the 

 now separated colonies of this form by way of Tehachapi and 

 the intervening low mountains, but also seems to show that the 

 two existing colonies of true speciosus now separated by the Mo- 

 have Desert were never united — unless in very remote times — 

 by continuity of range along the horseshoe of mountains which 

 connect the San Bernardino range with the Sierra. These facts 

 argue great antiquity for the spechsus-quadriviUatus type and 

 seem to show that very little change has taken place during the 

 nian}^ thousands of years that have elapsed since the climate 

 was cool enough to admit of continuity of Boreal forest across 

 what are now the torrid Sonoran deserts of eastern California, 

 southern Nevada, and southwestern Utah. Tliis view receives 

 additional support from the large amount of differentiation 

 undergone by the colonies of clearly derivative forms of these 

 animals now stranded on isolated mountains within the area 

 of former continuous range of quadrivittatus-speciosus across the 

 southern part of the Great Basin. These forms are the subspe- 

 cies inyoensis of the Inyo and White Mts., and tlie very distinct 

 species pa^ium of the Charleston Mts., Ijoth of which must have 

 developed their distinctive i)eculiarities since the great change 

 in climate took i)lace. And it is interesting to note that the 

 degree of differentiation of these forms is proportionate to the 

 climatic isolation of their homes. 



