FOREST IN RELATION TO AGRICULTURE 491 
through the New Forest on the Bournemouth road. This is only 
the effect which is expressed in this difference of temperature: as with 
other effects, so this one affects the neighbouring lands near the forest. 
Even a dry wind on reaching a forest must take up some moisture, 
and thus be less dry when it reaches agricultural land farther on. 
It can often be seen how the grain in the drier parts of the country 
planted near the edge of a forest sprouts up sooner than that right in 
the open. Again, certain crops, such as cacao-yams, cocoa, and Cola 
trees thrive best amongst trees. Without the forests the Cola planta- 
tions of the Ilesha district would cease to exist, or at any rate bear 
very small crops. 
9. The forest increases the humidity of the air. According to 
accurate measurements taken over a period of forty years, it has been 
proved that the forest increases the humidity of the air up to 10 per 
cent. No doubt in many of the Southern parts of Nigeria and the 
Gold Coast, people travelling in the forests have often thought 
that there was far too much moisture, not only coming from above 
and below, but also in the air around them. However, this should 
not blind us to the fact that further up country this greater humidity 
of the air of the forest is spread over the adjacent lands, and thus makes 
possible the growth of agricultural crops, such as cocoa, Cola, yams, 
and cacao-yams, which would not otherwise be possible. This greater 
humidity also tends to induce local minor rainstorms, which in their 
turn benefit the adjacent agricultural land. This greater humidity 
of the air also tends to prevent the excessive drying in the dry season 
of the land situated very near, and taken in conjunction with the 
other factors, such as the regulating of the temperature and the rainfall, 
makes it possible to put in crops earlier and almost ensures their 
growing successfully. 
10. The forests prevent the spread of insect and fungoid pests. 
So often we read that the crops in South Africa or the Argentine have 
been destroyed by locusts, and on the Gold Coast we read that various 
pests are destroying cocoa-trees in large numbers. Here, again, a 
forest or a belt of forests with large numbers of different species of 
trees interpose a living barrier of green leaf food or woody tissue which 
these pests may attack, owing to the diversity of the species of trees. 
Most pests only find a few trees on which they can thrive or reproduce 
themselves; thus on reaching the forest the scourge tends to lessen, 
or even die out. Even in the worst case of large hordes of locusts, 
it is cheaper to allow them into a forest, perhaps sacrificing it, 
rather than they should spread further and further over agricultural 
lands. 
Having shown that it is necessary in a new and comparatively 
unsettled country to preserve a certain proportion of the forests, it 
remains to be considered what proportion is desirable and necessary 
