153 ^""^^ °" ^^® mountain top 



mean a drive along one of the automobile roads which 

 lead up a few of them — in summer, to cooler regions; in 

 winter, to snow half an hour away from a sunny 70 de- 

 grees in town. 



But to the student of nature they mean several distinct 

 worlds and the possibility of moving from one to the other 

 more quickly and easily than is possible in any but a rel- 

 atively few regions of the earth. Within a distance of a 

 few hundred miles one may move from a subtropical des- 

 ert near sea level at Yuma to the summit of the San Fran- 

 cisco Peaks whose more than twelve-thousand-foot altitude 

 carries the visitor into a region where some of the plants 

 are those which grow within the Arctic Circle. 



Even more spectacular because one need travel only tens 

 rather than hundreds of miles is the journey from the 

 Sonoran Desert around Tucson to the pine and fir forests 

 which correspond to those found near sea level in south- 

 ern Canada. To travel a thousand feet upward is, so far 

 as climate is concerned, the approximate equivalent of 

 traveling six hundred miles northward. Because this fact is 

 so spectacularly demonstrated in Arizona and eastern Cali- 

 fornia, studies there were largely responsible for the con- 

 ception of Life Zones which was formulated a little more 

 than half a century ago by C. Hart Merriam and has 

 now become fundamental in ecology. 



Probably the most striking illustration to be found in 

 America of what this zoning means is supphed by the 

 now famous Mount San Jacinto in southeastern Califor- 

 nia. It rises from its base near Palm Springs and almost 

 at sea level to nearly ten thousand feet. The eastern slope is 

 so steep that eight thousand feet of altitude is gained in 

 only three linear miles, so that zones ranging from the 



