LUTHER BURBANK 
ences of warmth and moisture being supplanted 
by the chill and drought that presaged the onset 
of perpetual winter, a premium was put on the 
conservation of plant energies. Whereas before 
the elements favored the tree that could raise its 
head highest and thrust out the most luxuriant 
growth of spreading leaves to absorb the carbon 
from the heavily laden atmosphere, the time now 
came when the tree that had a smaller system of 
branches to nourish and a less expansive leaf 
system had better chance of maintaining existence. 
So in the lapse of ages, the conditions becoming 
more and more hard, the trees that varied in the 
direction of smaller size and narrower leaves had 
an ever-increasing advantage. These survived 
where their more rank-growing and luxuriant- 
leaved fellows perished. 
Thus generation after generation natural selec- 
tion operated to modify the size of the trees and 
to develop a race of trees with narrow leaves, 
which ultimately were reduced to the form of 
needles. 
Such leaves, offering the largest possible sur- 
face in proportion to their bulk, could gain nour- 
ishment from an impoverished atmosphere, and 
at the same time would obstruct the rays of the 
sun but little, so that the entire foliage of the tree 
might secure a share of the all-essential light 
[284] 
