BARRIERS TO FOREST EXTENSION 89 
been of benefit to mankind is the silkworm, which 
feeds upon the leaves of the mulberry. When the 
vitality of the forest has been reduced by fire it falls 
an easy prey to the ravages of insects and other dis- 
ease-causing organisms. Injurious insects are usually 
kept in check by their enemies. These enemies are 
other insects, birds, toads, and other animals. 
Weeds are often a hindrance to forest-growth. 
In many instances, grass, climbing vines, and dense 
thickets of bushes, have complete. possession of the 
soil, so that young growth is seriously hindered, es- 
pecially in the south. In the north this kind of 
growth on the ground is often a benefit, in that it may 
protect young trees against the action of frost. 
Then, too, between the species of the forest and 
between the individuals of the species there is constant 
war. In tropical countries where the struggle is most 
intense, there are mortal hand-to-hand combats. 
Perhaps the greatest barrier to tree-growth is 
man. In addition to causing fires, he uses those spe- 
cies best suited to his needs, and leaves the most un- 
desirable kinds, and thus often causes forest deterio- 
ration. On the other hand, in some instances, he 
uses skill and foresight. The forest is constantly 
improved in consequence. Mankind, in his process 
of development, has accomplished a great work in 
subjugating and bringing to his household many use- 
