128 Proceeding* of the Biological Society of Washington. 



probably represent the definite results of the coming and going 

 of the glaciers. Prior to the glacial epoch, if Ochotona existed 

 at all in Colorado, the genns was likely confined to the highest 

 mountains. During that epoch glaciers pushed down all the 

 valleys from the higher mountains, and climatic conditions 

 were favorable for the extension of the range of pikas southward 

 and to lower altitudes, thus enabling them to span the low 

 lands separating our mountain masses in the west and south- 

 west. With the next change of climate which caused the 

 retreat of glaciers, the lower limit of favorable habitat must 

 have again risen, leaving the Pagoda Peak stock and many 

 others in similar situations, stranded, isolated from the parental 

 stock by the intervening unfavorable territory. The alpine and 

 subalpine plants and animals of the isolated mountain masses 

 of the West afford great opportunities for the study of the effect 

 of isolation upon variation and the origin of new species. 



