Practically, we are not much further
advanced. It is very probable that the combination of hydraulic
purification with a forced cultivation of the soil has sometimes
determined changes in its composition by which it has been rendered
sterile as regards malaria. If that has happened, it has happened by
chance, and we are unable to reproduce the result at will; for we have not
all the data which might enable us to understand how it has come about.
Most of the purifications obtained in ancient times, by means of forced
cultivation, continued during centuries, have not been definite at all,
but the production of malaria has been simply suspended. Hardly was the
regular cultivation of the fields interrupted than the production of
malaria recommenced. Among the numerous examples that I might cite in this
connection, I will limit myself to that of the Roman Campagna. This seemed
to have been made permanently healthy under the Antonii, but after the
fall of the empire it began again to produce malaria, as if the forced
cultivation through so many centuries had never been.
One might, strictly speaking, be content with such a result, and boldly
undertake forced cultivation of all malarious districts, without stopping
to ascertain whether the freedom from malaria so obtained would be
definite, or whether the production of the poison were only suspended.
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