By means of these experiments, as unexpected as they were conclusive, Pasteur had demonstrated that Jaillard and Leplat had not really inoculated their rabbits with an amorphous virus, liquid or solid, but with a virus constituted of a living microscopic organism--in other words, with a true ferment. By the side of the parasite of splenic fever we have thus a fresh example of a living animated virus, with germs forming dust. And the extraordinary thing is that among the microbes of special maladies--which they produce by penetrating and multiplying in the bodies of animals--are to be found aerobies like the bacilli of splenic fever, and anaerobies like the vibrios of acute septicæmia. II. In these two virulent maladies, then, splenic fever and septicæmia, the researches of Pasteur had clearly established the parasitic theory. A grand and novel opening was made for future studies on the origin of diseases. Yet, judging from the surprising differences which separate septicæmia and splenic fever, we can foresee that should the future, copying the past, in regard to this and still more recent discoveries, have in store, as it no doubt has, the knowledge of new microbes of disease, the specific properties of these microscopic organisms will demand, for each new exploration, ceaselessly repeated efforts, not only to make the existence of these organisms evident, but also to furnish decisive proofs of their morbific power. But the question which may be considered as already solved is the non-spontaneity of these infectious microbes. By what is called spontaneous disease is meant parasitic disease. But in the present state of science spontaneous disease has no more existence than spontaneous generation. Such aphorisms, however, are not allowed to pass without occasional contradictions, all the more vehement from their rarity. At the International Medical Congress held in London, August 1881, Dr. Bastian, who practises in one of the principal hospitals of London, declared that though he was unable to deny the existence of parasitic diseases, yet, in his opinion, the microbes were the effect and not the cause of these diseases. 'Is it possible,' cried Pasteur, who was present at the meeting, 'that at this day such a scientific heresy should be held? My answer to Dr. Bastian will be short. Take the limb of an animal and crush it in a mortar; let there be diffused in this limb, around these crushed bones, as much blood, or any other normal or abnormal liquid as you please. Take care only that the skin of the limb is neither torn nor laid open, and I defy you to exhibit on the following day, or during all the time the malady lasts, the least microscopic organism in the humours of this limb.' After the example of Liebig in 1870, Dr. Bastian did not accept the challenge.