THE DISTRIBUTION OF SPECIES. 



209 



keep down their size, they must carry as little foliage 

 as possible, and their stems must be tough enough to 

 resist snow and hardy enough to withstand almost per- 

 petual frost. Their year's growth must be finished 

 in a very short time, 

 and leaves, flowers, 

 and seeds must fol- 

 low in the most rapid 

 succession. In short, 

 there is room for 

 birch trees here, if 

 only the trees can be 

 reduced to their low- 

 est terms. And so 

 birch trees have crept 

 up the mountain sides 

 even to the very 

 edges of the perpet- 

 ual snow. But such 

 trees! All trees re- 

 quiring sunshine, or 

 long time for their 

 summer's growth, are 

 rigidly kept away by 

 " natural selection." 



The cold climate dwarfs the individual, and the hard 

 conditions exclude every individual not dwarfed. I have 

 before me three birch trees from a Norwegian mountain 

 called the " Suletind "—the little trees known to the 

 Norwegian peasants as " Hundsoire, or " dogs'-ears." 

 The trunk of each tree is barely an inch in height. 

 There are no branches and but three leaves. Half in- 

 closed by the uppermost leaf is the single little catkin 

 of flowers. Leaves in June, blossoms in July, fruit in 

 August, and then the little tree is ready for its nine 



Fig. 15. — The arctic 

 birch, Suletind, 

 Norway. (After 

 nature.) 



