in some places have lost a little territory, but 

 along most of its front the timber-line is slowly 

 advancing into the heights. 



With this environment it would be natural 

 for these trees to evolve more hardiness than 

 the present trees have. This would mean trees 

 better fitted to contend with, and more likely to 

 triumph over, the harsh conditions. Evolu- 

 tionary development is the triumphing factor 

 at the timber-line. 



The highest timber-line in the world is prob- 

 ably on Mount Orizaba, Mexico. Frank M. 

 Chapman says that there are short-leaved pines 

 {Pinus MontezumcB) from thirty to forty feet 

 high, on the southern exposure of this peak at 

 an altitude of about 13,800 feet. In Switzer- 

 land, along the steep and snowy Alps, it is sixty- 

 four hundred ; on Mt. Washington, about forty- 

 five hundred feet. In the mountains of Colorado 

 and California it is of approximately equal alti- 

 tude, between eleven and twelve thousand feet. 

 Advancing northward from California along the 

 timber-line, one enters regions of heavy snow- 

 fall as well as of restricting latitude. Combined, 



59 



