rydberg: vegetative life zones of rocky mountains 491 



Erioneuron pilosum, which are found in Colorado only south of 

 the divide. The cactuses also are here represented by genera 

 not found north of the divide, as Echinocactus, Echinocereus, and 

 the section Cylindropuntia of Opuntia. The tree cactus, Opuntia 

 arborescens, is found on the Arkansas Divide, but not north 

 thereof. Even the other sections of Opuntia and the genus 

 Cactus are mostly represented by different species. The yuccas 

 so common in the Upper Sonoran Zone are represented north of the 

 divide by only one species, Yucca glauca, which ranges north to 

 South Dakota. The genera Dasylirion and Nolina are found 

 only on the south side. Several species of Padus and Rhus are 

 added south of the divide. Genera characteristic of the plains, 

 such as Lesquerella, Eriogonum, etc. are mostly represented by 

 different species south of the line. Atriplex confertifolia, the 

 Shad-Scale of the Great Basin, is not uncommon south of the 

 divide, but is not found north thereof. 



It is therefore evident that the line between the northern 

 plains and the southern should not be drawn in Montana but in 

 Colorado. Looking at Dr. Merriam's map of 1898, one may 

 wonder how it happens that the Upper Austral Zone should 

 swing northward towards the Rockies where the plain becomes 

 higher and that northern Iowa and northern Illinois should be 

 counted to the Transition Zone, while eastern Montana 4° or 5° 

 further north and 2,000-3,000 feet higher, or central Wyoming 

 on the same latitude and over 4,000 feet higher, should be counted 

 to the Upper Sonoran, the zone next below it. 



If we let the plains as far south as central Colorado belong to 

 the Transition Zone, i. e., if we let a region 3° or 4° degrees 

 further south and 4,000 or 4,500 feet higher than northern Iowa 

 and northern Illinois be on the southern boundary line of the 

 same life zone, it seems much more reasonable. When my obser- 

 vations relative to the native flora and the generally accepted 

 relation between latitude and altitude point to the same conclu- 

 sion, I can not see any reason why we should not draw the line 

 between the so-called Transition Zone and the Upper Sonoran 

 on the plains where I have drawn it. 



I know very little about the distribution of animals, but I think 

 that several illustrations may be found supporting my theory. 

 I know at least of one, viz. the distribution of the two species of 



