580 THE POPULAR SCIENCE MONTHLY. 



experiments made to demonstrate on animals either the poisonous na- 

 ture, or, on the other hand, the barmlessness, of the fungi commonly 

 found in rotting refuse. But real contradictions do not exist in sci- 

 ence ; 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 sep- 

 arate 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 bac- 

 teria of putrefaction, which we do not know, they would be at a dis- 

 advantage, 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 de- 

 composing 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 com- 

 petitors. 



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 poison- 

 ing 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 ani- 

 mals. 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 



