1. There floats in the lower strata of the atmosphere in all regions of the earth, but in very various proportions, for reasons already explained, a poison or poisons, generated by the processes which nature adopts for the destruction of past generations, and the reconstruction of those to come; the destruction of the aged, the worn-out, the nearly extinguished; the reconstruction of the organisms springing into life, to occupy the place of those that were! Whether the poison be one or many; whether it be a single species or one of a natural family, does not affect the general conclusions. The diversity of its effects is no proof of diversity in its essential nature or even origin; the living principle is supposed to be of one nature everywhere and for ever; yet see how varied are the results of this principle in moulding the vegetable and animal worlds; how slight are the modifications even in organic elements, which, when called into play, give rise to the most astonishing and unexpected diversity of results. Why should it not happen, then, with the poison, influence, or thing we call malaria, which, modified by a chemical action too subtle for the scientific man to observe, may yet, being so modified, give rise to an intermittent or a remittent, a plague, a cholera, a diphtherite, a scarlatina, a typhus, or a small-pox? Where did so many poisons come from? Whence came the murrains, the vine-plague, the potato-destroying poison, which was not at all new, neither was it confined to the potato? Whence came the pestilences which destroyed the ancient world? which exterminated at once whole species and genera now extinct? Of one thing we may be assured, they did not die a natural death.