BARRIERS TO FOREST EXTENSION 81 
also, that many of the trees of the Eastern United 
States reach their southern limit. 
The great cold wave which covered a large part 
of this continent during the glacial epoch must have 
exercised a gigantic influence on the northern dis- 
tribution of many species of trees. It could not 
have driven them southward, because trees, unlike 
animals, can not be easily driven. Tropical life once 
existed in arctic and temperate regions. Although 
the seeds of many trees will retain their potency a 
long time when embedded solidly in ice, it is sate to 
assume that none survived in those regions which 
were actually covered with the grinding ice-mass of 
the glacial epoch. Its chilling effects must have been 
felt far southward. Since that time, no doubt, spe- 
cies have been moving slowly northward, depending 
mainly upon the various natural agencies of dissemi- 
nation, especially the wind. The species with heavy 
seeds, such as oaks, walnuts, and hickories, have lin- 
gered behind. These changes will continue, no doubt, 
until every species finds the northernmost limit of its 
possible range. Those species with the lightest seeds 
are not always those of the broadest range. This de- 
pends upon the ability of the tree to endure adverse 
conditions. The giant Sequoia, of California, is lim- 
ited in its range, although its seeds are very small 
and light, and widely scattered by the wind. The 
