lained; and I was taught
that, in speculating on the greater effects which the direct action of
rain may once have produced on the surface of certain parts of England,
we need not revert to periods when the heat of the climate was
_tropical_.

In the torrid zone the degradation of land is generally more rapid; but
the waste is by no means proportioned to the superior quantity of rain
or the suddenness of its fall, the transporting power of water being
counteracted by a greater luxuriance of vegetation. A geologist who is
no stranger to tropical countries observes, that the softer rocks would
speedily be washed away in such regions, if the numerous roots of plants
were not matted together in such a manner as to produce considerable
resistance to the destructive power of the rains. The parasitical and
creeping plants also entwine in every possible direction, so as to
render the forests nearly impervious, and the trees possess forms and
leaves best calculated to shoot off the heavy rains; which, when they
have thus been broken in their fall, are quickly absorbed by the ground
beneath, or, when thrown into the drainage depressions, give rise to
furious torrents.[997]

_Influence of Man in modifying the Physical Geography of the Globe._

Before concluding this chapter, I shall offer a few observations on the
influence of man in modifying the physical geography of the globe; for
we must class his agency among the powers of organic nature.

_Felling of forests._--The felling of forests has been attended, in many
countries, by a diminution of rain, as in Barbadoes and Jamaica.[998]
For in tropical countries, where the quantity of aqueous vapor in the
atmosphere is great, but where, on the other hand, the direct rays of
the sun are most powerful, any impediment to the free circulation of
air, or any screen which shades the earth from the solar rays, becomes a
source of humidity; and wherever dampness and cold have begun to be
generated by such causes, the condensation of vapor continues. The
leaves, moreover, of all plants are alembics, and some of those in the
torrid zone have the remarkable property of distilling water, thus
contributing to prevent the earth from becoming parched up.

_Distribution of the American forests._--There can be no doubt then,
that the state of the climate, especially the humidity of the
atmosphere, influences vegetation, and that, in its turn, vegetation
re-acts upon the climate: but some writ