204 
K. KOCH. 
purely hypothetical view it can, nevertheless, not claim, for no facts speak for 
it, but many against it. A certainly proved example of a breeding of harm¬ 
ful bacteria from harmless ones, as is well known, does not yet exist, and 
there is, therefore, no ground for ascribing to tuberculous bacilli origination 
of that sort from indifferent bacteria. There is so much the less reason for 
this, since among the numberless experiments in anipials with disease-pro¬ 
ducing and non-disease-producing bacteria, it has never occurred that in the 
so very favorable breeding ground of the bodies of rabbits and guinea pigs, 
tuberculous bacilli have developed from other bacteria. On the contrary, all 
experiments undertaken with the necessary precautionary measures teach 
that tuberculosis only arises when genuine, that is to say, complete tubercu¬ 
lous bacilli are united with the animal body. 
The relations are different in regard to an eventual weakening of the tu¬ 
berculous bacilli, since the weakening of the anthrax-bacilli can be cited for 
the possibility of such a proceeding. Although the possibility of such a 
change in the virulence is not to be disputed, nevertheless it must be con¬ 
sidered that the weakening of the anthrax-bacilli is completed under circum¬ 
stances which can only be brought about artificially, but which do not come 
into play in ordinary circumstances either in the body or out of it. More¬ 
over, against such a supposition the fact speaks, that tuberculous bacilli do 
not show the slightest change in their qualities, especially in their virulence, 
when successive breedings have been carried on in cultures, that is to say out 
of the animal body, and on a dead breeding substratum almost two years. 
Also in the experiments of Fischer and Schill, which are reported in another 
part of this paper, when tuberculous bacilli had been exposed for six weeks 
to the influence of decay, no weakening of the virulence occurred. All this 
speaks with decision against the supposition of an easily occurring change in 
the virulent properties of the tuberculous bacilli. It is, perhaps, inconceiv¬ 
able that the bacilli did not at some time proceed from other bacteria. But 
after they had once become genuine parasites, they appeared to have the 
peculiarity common to other parasites of holding to their qualities with great 
obstinacy. 
The only source for the origin remains, therefore, the animal or human 
organism, and opportunity is not lacking to these parasites, owing to the ex¬ 
traordinary diffusion of tuberculosis, to reproduce themselves in this field in 
enormous masses, to develop the permanent form, to get into the outer world, 
and to attack other victims. 
Among the various forms of tuberculosis there are, to be sure, only cer¬ 
tain ones which admit an easy transference of the bacilli. But these are ex¬ 
actly the most frequently occtirring forms, namely, phthisis and the tubercu¬ 
lous diseases of the domestic animals. The other sorts of tuberculosis play 
almost no part in reference to infection, partly because they remain so hidden 
that they can only exceptionally cause infection. 
If we ask first in how far phthisis can cause a transference of tuberculous 
bacilli from diseased to healthy people, it is quite evident that here all the 
conditions for the spread of the infectious material are present in fullest 
measure. One only needs to remind himself that on the average one-seventh 
