ica does not hesitate
to admit that when a swampy tract is heated by the sun's rays to the
necessary point for the putrid decomposition of the organic matters
contained in it, the "chemical ferment," or rather the "mephitic gases,"
to which is attributed the morbific action, are developed, whatever may be
the distance from the equator at which this marshy region lies. But since
it has been ascertained that malaria is produced in soils of the most
varied chemical composition, _the persistent identity of this product_ has
become chemically inexplicable; while it is however readily conceivable,
if one admits that malaria is an organized ferment which easily finds the
necessary conditions for its life and multiplication in the most varied
soils, as is the case with millions of other organisms vastly superior to
the rudimentary vegetables which constitute the living ferments.

The same thing may be said of _the progressive intensity of the morbific
production in abandoned malarious districts_. This fact has been
historically proved in several parts of the earth, and especially in
Italy. A large number of Grecian, Etruscan, and Latin cities, even Rome
itself, sprang up in malarious territories and attained a high state of
prosperity. First among the reasons for this success must be placed the
works undertaken with a view of rendering these places more salubrious,
and which lessened the evil production, _but almost never extinguished it
completely_. After the abandonment of these localities, the production of
malaria recommenced in a degree which went on increasing from age to age,
and which has rendered some of these places actually uninhabitable. This
was seen, in the time of the ancient Romans, in Etruria, when it was
conquered and laid waste, and in several parts of Magna Graecia, and of
Sicily. From the fall of Rome even to the present day, this phenomenon has
been manifested in a very evident manner in the Roman Campagna, in certain
parts of which, even up to the time of the Renaissance, it was possible to
maintain pleasure houses, but which are now unhabitable during the hot
season. In many cases the physical conditions of the soil have undergone
no appreciable change during centuries, so that it is impossible to
attribute so enormous an augmentation of malaria to an increase in its
annual production, itself increased by a progressive alteration of the
chemical composition of the soil. But if, on the contrary, it be admit