red merely as consumers of a certain quantity of organic
matter, must necessarily cause.

_Whether man increases the productive powers of the earth._--It may
perhaps, be said, that man has, in some degree, compensated for the
appropriation to himself of so much food, by artificially improving the
natural productiveness of soils, by irrigation, manure, and a judicious
intermixture of mineral ingredients conveyed from different localities.
But it admits of reasonable doubt whether, upon the whole, we fertilize
or impoverish the lands which we occupy. This assertion may seem
startling to many; because they are so much in the habit of regarding
the sterility or productiveness of land in relation to the wants of man,
and not as regards the organic world generally. It is difficult, at
first, to conceive, if a morass is converted into arable land, and made
to yield a crop of grain, even of moderate abundance, that we have not
improved the capabilities of the habitable surface--that we have not
empowered it to support a larger quantity of organic life. In such
cases, however, a tract, before of no utility to man, may be reclaimed,
and become of high agricultural importance, though it may, nevertheless,
yield a scantier vegetation. If a lake be drained, and turned into a
meadow, the space will provide sustenance to man, and many terrestrial
animals serviceable to him, but not, perhaps, so much food as it
previously yielded to the aquatic races.

If the pestiferous Pontine marshes were drained, and covered with corn,
like the plains of the Po, they might, perhaps, feed a smaller number of
animals than they do now; for these morasses are filled with herds of
buffaloes and swine, and they swarm with birds, reptiles, and insects.

The felling of dense and lofty forests, which covered, even within the
records of history, a considerable space on the globe, now tenanted by
civilized man, must generally have lessened the amount of vegetable food
throughout the space where these woods grew. We must also take into our
account the area covered by towns, and a still larger surface occupied
by roads.

If we force the soil to bear extraordinary crops one year, we are,
perhaps, compelled to let it lie fallow the next. But nothing so much
counterbalances the fertilizing effects of human art as the extensive
cultivation of foreign herbs and shrubs, which, although they are often
more nutritious to man, seldom thrive with the same rank luxuria