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- Fic. 15.—The arctic 
uiring sunshine, or birch, Suletind, 
q § ae Norway. (After 
long time for their nature.) 
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 “ Hundsdire, 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 
