298 PROCEEDINGS OF THE ACADEMY OF [May, 



THE OCCURRENCE OF BUFO COLUMBIENSIS EAST OF THE ROCKY 



MOUNTAINS. 



BY ROBERT T. YOUNG. 



High mountain chains, especially those whose crests extend above 

 timber line, are more or less efficient barriers to the spread of animals. 

 This is more particularly so in the case of species which do not migrate 

 easily from place to place. 



The capture of Bufo columbiensis on the eastern slope of the Conti- 

 nental Divide in Colorado is, therefore, of some interest. 



In August, 1902, I obtained a specimen of this toad on the eastern 

 slopes of the Arapahoe Peaks, about 28 km. west of Boulder, Colo., at 

 an altitude of about 3,100 m., and it has since been reported to me by 

 Prof. Beardsley, of the Colorado State Normal School, from the eastern 

 slope of the mountains west of Greeley. 



Cope 1 gives this species as a characteristic of the Pacific district of 

 the Western sub-region of the Medicolumbian region, included in the 

 Transition of Merriam. 2 My records, however, show it to occur in the 

 Hudsonian zone, far above the upper limit of the Transition, where it 

 has doubtless migrated from its original habitat. 



There are several passes in northern Colorado over the Continental 

 Divide which cross the mountains below timber line, affording possible 

 paths of migration for toads from the western to the eastern slope. 



Further observations on the distribution of this species on both 

 slopes of the mountains, taken from year to year, would doubtless be 

 of interest as affording some data for a determination of the conditions 

 and rate of migration of toads. 



1 The Geographical Distribution of Batrachia and Reptilia in North America, 

 Am. Nat., XXX, 886-902 and 1003-1026. 



2 Laws of Temperature Control of the Geographic Distribution of Terrestrial 

 Animals and Plants, Nat. Geog. Mag., VI, 229-238. 



