Some of these circumstances are of a nature to strike every serious observer, and deserve a few moments’ attention.  How could one maintain, for example, that this ferment is a product of chemical reactions taking place in the ground, when it is seen to remain constantly the same whatever may be the composition of the soil from which it emanates!  As long as the paludal theory held sway, the chemical interpretation of this identity of the product in every latitude was easy.  Rica 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 admitted that malaria is caused by a living organism whose successive generations accumulate in the soil, the interpretation of this fact becomes very simple.