160 LUTHER BURBANK 



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 

 which now, age on age, became less and less 

 bright as the earth may have changed the direc- 

 tion of its axis. 



Of course there were other trees that did 

 not undergo this modification. But these were 

 forced either to make more rapid migrations to 

 the south or to give up the fight altogether and 

 to submit to extermination. The only evergreen 

 trees that were able to maintain existence in the 

 regions where the climate became exceedingly 

 cold were those that had developed the new type 

 of leaf form, and had learned to conserve their 

 energies to the last degree. 



But of course the trees that took on this new 

 habit varied among themselves, and as they 

 spread to different regions such variations were 

 developed and fixed under the influence of dif- 

 ferent environments, until many tribes of needle- 

 leafed trees were developed so differently as to 

 constitute the races that the modern botanist 

 terms pine and spruce and cypress and juniper 

 and hemlock and yew and cedar, etc. 



