484 THIRD PART.—BACTERIA OR SCHIZOMYCETES, 
raised above the surface of the soil by the floods and spread over the plants which are 
subsequently eaten by the cattle. It is at the same time possible that the Bacillus 
may also be conveyed to the localities by the bodies of animals which have died of 
anthrax. This would at all events be more likely to happen in a district which has 
once been much infected with the fever than in any other. Mice are very susceptible 
to the disease, and the dead bodies of these creatures and of other small rodents 
would be especially calculated to propagate the infection. But Pasteur’s sensational 
hypothesis, that the Bacillus is introduced into the soil by the burial of the bodies of 
infected animals, and that its spores are then conveyed by earth-worms from beneath 
to the surface, is not necessary for the explanation of the phenomena in these or any 
other localities; it is moreover open to the objection that the formation of spores 
never or scarcely ever takes place, as is urged by Koch, in the unopened body of an 
animal in the temperature of the deeper layers of soils and with the small amount 
of oxygen in the air which they contain. 
The Bacillus here described is shown by its life-history to be a strictly facultative 
parasite, which only reaches the highest stage of its development in the non-parasitic 
state, and not only can but actually often does go through the entire course of 
its development in this state during many generations and even many years. It has 
been shown that it has a virulent effect at least on the animals above mentioned. 
Whether it can vegetate in other species of animals without doing them harm is not 
yet ascertained. But its virulent effects on those animals which suffer from its 
presence may be diminished by certain methods of breeding, and indeed be weakened 
till they become quite innocuous. Pasteur first discovered through his experiments 
on fowl-cholera, and Koch? confirmed his results, that this takes place when the 
Bacillus is grown in a neutralised nutrient solution, such as a meat-broth, with 
a plentiful supply of oxygen and at a high temperature. The attenuation of the 
effects may be carried so far that a mouse, the most susceptible animal with which 
experiments have been made, can suffer inoculation without being made ill; the 
attenuation is induced rapidly when the temperature is raised to nearly 43°C. and may 
be completed in 6 days; at a temperature of 42°C. the cultivation may require to be 
continued during 30 days, and the process is still slower in the temperature of an 
ordinary room. The Bacillus vegetates under these conditions and multiplies without 
alteration of its morphological characters, but 27 does not produce spores. Cultures 
kept at a temperature of 42°-43°C. perish in about a month’s time, but fresh cultures 
can be obtained from them from 1-2 days before that time has expired. The Bacillus 
may recover its virulent properties after a certain degree of attenuation, if it finds its 
way into an animal which is susceptible to the infection and kills it. There is 
a degree of attenuation in which it is innocuous to full-grown guinea-pigs, but not to 
very young ones; if the latter are inoculated with the attenuated matter, the Bacillus 
returns to its state of greater virulence. The data before us do not show whether a 
return to virulence is possible from the highest degree of attenuation, nor have we any 
distinct experiments to prove whether the attenuated Bacillus developes at all in the 
animal which is inoculated with it but continues healthy, or in what manner it developes. 
It has been assumed that it does develope there, but there are no precise facts on 

1 See his essay, Ueber d. Milzbrandimpfung, Cassel, 1883, p. 17. 
