BARRIERS TO FOREST EXTENSION 85 
only prevents the penetration of roots, but, because 
of imperfect drainage, the ground is sour and stag- 
nant in nature, and therefore not conducive to tree- 
growth. 
Although the winds occasion the spread of seeds, 
they also hinder tree-growth, if constant and severe. 
Although winds are checked by belts of trees, large 
areas of forest are often leveled to the ground by the 
action of gales. 
Fire has been for many years, even before the 
advent of mankind, and is still, a very potent agent 
in molding the nature of the surface of the earth. 
Lightning and volcanic fire have often set it. The 
lowlands have suffered less than the highlands, be- 
cause of their moistness. Glades with open pasture, 
with here and there a tree, are, no doubt, due in many 
cases to fire. Fire on a heavy soil tends toward the 
formation of grass-lands. The weaker species of the 
forest suffer first, then after a succession of fires only 
the hardiest remain. In the course of time even the 
hardiest forms are killed, and a prairie or desert re- 
sults. Where fires have been stopped, the forest is 
invading the prairie-land. It is said that in the dry 
parts of Australia there are species of trees with seeds 
which will not germinate until their outer husk is 
cracked by fire. The following is from the Agricul- 
tural Gazette of New South Wales: 
