ROBERT KOCH 3^3 



If two drops of putrefying blood be injected Into such an animal 

 there is introduced not only a number of totally distinct species of bac- 

 teria, but also a certain amount of dissolved putrid poison (sepsin), 

 not sufficient to produce a fatal effect, but yet certainly not without 

 influence on the health of the animal. Different factors must there- 

 fore be considered as affecting the health of the animal. On the one 

 hand there is the dissolved poison, on the other the different species of 

 bacteria, of which, however, perhaps only two, as in the example 

 before us, can multiply in the body of the mouse and there exert a 

 continuous noxious influence. Only one of these two species can pene- 

 trate into the blood, and if the blood alone be used for further inocula- 

 tions, only this one variety will come victorious out of the battle for 

 existence. The further development of the experiment depends en- 

 tirely on the quantity of the putrid poison, and on the relation of the 

 two forms of bacteria to each other in point of numbers. If one in- 

 jects a large amount of septic poison and a large number of that va- 

 riety of bacteria which increase locally (in this case the chain-like 

 micrococci causing the gangrene of the tissue), but only a very small 

 number of the bacteria which pass into the blood (here the bacilli), 

 the first animal experimented on will die, as a result of the prepondera- 

 tion influence of the first two factors before many bacilli can have got 

 into the blood and multiplied there. Of the blood of this first animal, 

 containing, as it does, proportionately very few bacilli, one-fifth to 

 one-tenth of a drop must be inoculated in order to convey the disease 

 with certainty. In the second animal, however, only the bacilli are in- 

 troduced, and these develop undisturbed in the blood. For the infec- 

 tion of the third animal the smallest quantity of this blood which can 

 produce an effect is then sufficient, and after this third generation the 

 virulence of the blood remains uniform. 



We may also imagine another case in which the increase of the 

 virulence may go on through more than two generations without any 

 modification resulting from natural selection and transmission from 

 animal to animal. This would take place if several species of bacteria 

 capable of passing into the blood were introduced into the animal at 

 the first injection. Let us suppose, for example, that in the same 

 putrefying blood which served for the foregoing experiment, the bacilli 

 of anthrax were also present, there would then be contained in the 

 blood of the first animal not only the septicaemic bacillus, but also 



