206 BOTANY. [CHAP. vii. 



Polar Region of alpine plants, a few species of flower- 

 ing plants, mosses, and lichens, the last-named usually 

 being the last to appear in both latitude and altitude. 



It must be clearly understood that the altitudes given 

 are merely approximate, and not of universal application ; 

 in fact, as a rule, when the same species occur on both 

 sides of a mountain, the altitude at which they occur 

 varies considerably on opposite sides. This will be 

 readily understood when the variation in the snow-line 

 is remembered. This in the Peruvian Andes exceeds 

 18,000 feet; in the Himalayas, in lat. 26 N., the snow- 

 line is about 18,000 feet on the north side and 15,500 feet 

 on the south side ; whereas in Norway it is under 

 4,000 feet, and close to the Arctic Ocean it is 2,000 feet. 



