BARRIERS TO FOREST EXTENSION 79 
often the case, there is of course no activity; but in 
warmer times, in the spring and fall, while broad- 
leaved species have no leaves, they are of course 
more or less active. At any rate, the cessation of 
growth in winter, as with drought in summer, is an 
enforced idleness which can serve no purpose, and is 
therefore not rest. The amount of wood which is 
added in so short a time is therefore small in quantity. 
In the tropics there is continuous growth in re- 
gions where there is a commensurate amount of 
moisture. In many instances the leaves fall, the 
flowers and fruit suddenly appear, and then in a short 
time follows again a new crop of leaves. In many 
instances, if not in the majority of cases, in warm, 
moist regions, there is no periodicity of leaf-fall. The 
trees are there evergreens, and growth is constant. 
The peach is an evergreen in Ceylon. 
Many northern species suddenly transported 
southward would die, no doubt, in short order. 
Northern trees have become more or less adapted to 
the conditions of the environment in which they live. 
Their mechanism is not fitted for a sudden change, 
although a northern tree moved south is more likely 
to thrive than a southern species moved north. It 
would be dazzled by the excessive sunlight, and its 
tender leaves would be shattered by tropical hurri- 
canes and downpours. It might be stifled by its more 
