TREES AND SHRUBS 159 



striking character to meet the exigencies of a 

 changing environment. 



Time was, doubtless, when the ancestors of the 

 conifers had flat, spreading leaves like the leaves 

 of other forms of vegetation. But when the 

 climatic conditions changed, the pampering influ- 

 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 sys- 

 tem had better chance of maintaining existence. 



So in the lapse of ages, the conditions becom- 

 ing more and more hard, the trees that varied in 

 the direction of smaller size and narrower leaves 

 had an ever-increasing advantage. These sur- 

 vived where their more rank-growing and luxu- 

 riant-leafed fellows perished. 



Thus generation after generation natural se- 

 lection 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. 



