experiments made to demonstrate on animals either the poisonous nature, or, on the other hand, the barmlessness, of the fungi commonly found in rotting refuse. But real contradictions do not exist in science; they are only apparent, because the results in any opposite cases were not obtained under identical conditions. The explanation of the variable effects of common putrefaction-germs upon animals is self-evident as soon as we admit that each parasitic disease is due to a separate species of bacteria, characteristic of the disease, producing only this form and no other affection; while, on the other hand, the same disease can not be caused by any other but its special parasite. It can be affirmed, on the basis of decisive experiments, that the bacteria characteristic of various diseases float in the air, in many localities at least. Hence rotting material, teeming with bacterial life, may or may not contain disease-producing germs, according to whether the latter have settled upon it by accident or not. Even if these disease-producing species were as numerous in the dust as the common bacteria of putrefaction, which we do not know, they would be at a disadvantage, as far as their increase is concerned. For experience has shown that the germs of most diseases require a special soil for their growth, and can not live, like the agents of putrefaction, upon any organic refuse. In some cases, indeed, these microscopic parasites are so fastidious in their demands that they can not grow at all outside of the animal body which they are adapted to invade. Hence, if a decomposing fluid does contain them, they form at least a minority of the inhabitants, being crowded out by the more energetically growing forms. In the microscopic world there occurs as bitter a struggle for existence as is ever witnessed between the most highly organized beings. The species best adapted to the soil crowds out all its competitors.
Though the putrefaction-bacteria, or, as Dumas calls them, the agents of corruption, are not identical with disease-producing germs, they are yet not harmless by themselves. Putrid fluids cause grave sickness when introduced into the blood of animals in any quantity. But this is not a bacterial disease proper; it is an instance of poisoning by certain substances produced by the life-agency of the bacteria while decomposing their soil. The latter themselves do not increase in the blood of the animal; they are killed in their struggle with the living animal cells. The putrefaction-bacteria need not be further present in the putrid solution to produce the poisonous effect on animals. They may be killed by boiling, if only the poisonous substances there formed remain.
In order to prove the bacterial origin of a disease two requirements are necessary: First, we must detect the characteristic bacteria in every case of that disease; secondly, we must reproduce a disease in other individuals by means of the isolated bacteria of that disease. Both these demonstrations may be very difficult. Some species of