CHAPTER XI. MODE OF LIFE OF THE BACTERIA. 487 



occurring in a living or dead body are not necessarily the inciters of disease, and the 

 attempt to decide the point experimentally often encounters great difficulties. To 

 expatiate further in the domain of pathology would carry us beyond the limits to 

 which we are confined. But we shall perhaps contribute something of value to the 

 above-mentioned discussion as well as to the determination of the recognised cases 

 if we append a few short general remarks to the foregoing account of anthrax. For 

 further details the reader is referred to medical works and to the compilations of 

 Marpmann and Zopf, which are not however as complete as might be wished. 



So far as can be judged from the accounts before us, all the Bacteria which are 

 suspected of being or are proved to be parasites with the power of inciting disease, 

 with one exception which will be noticed again below, are capable of vegetating and 

 being bred in dead organic matter ; some form their spores chiefly or exclusively in ' 

 this saprophytic stage of their development. The Bacteria of the latter category 

 are therefore facultative parasites like the Bacillus of anthrax, and the rest perhaps 

 are so too ; if not, they are at any rate facultative saprophytes. Hence they can both 

 vegetate, like the Bacillus of anthrax, outside the living animal ; the localities must 

 be ascertained in each separate case, and it follows that the danger of infection is 

 different in this case and in that of obligate parasitism. 



Further it will depend on the species, race and individual among the Bacteria in 

 question in their quality of parasites what hosts they will choose, as is the case with 

 the Bacillus of anthrax ; or conversely, some species of animals or some individuals 

 will be more liable to be attacked by a given species of Bacterium, while others will 

 be secure from it. It is at the same time conceivable that this condition may vary in 

 individuals, an individual not before susceptible may for instance become susceptible ; 

 this may be due to external causes whether otherwise injurious or apparently 

 indifferent. We have sufficient proof that such changes do actually occur. 



As the disposition of the host may vary, so also a change may take place in the 

 qualities, and specially in the virulence of the parasite, as we see in the case of the 

 attenuated Bacillus of anthrax. This may perhaps be assumed to be the general rule 

 in the forms which approach near to that species and of which we are here speaking. 

 The change may be in the direction of loss of virulence, or on the contrary of its 

 recovery. It may therefore also happen, that experiments in artificial infection with 

 the same forms of Bacterium may, caeteris paribus, yield different results, some of a 

 positive, others of a negative character. It is possible that the great difficulty or 

 impossibility of obtaining animals that are exactly alike for experiments may at least 

 help to heighten the apparent contradictions. 



The investigation of the Bacillus of anthrax has further shown that the changes 

 just mentioned may be accomplished in a form which would be considered by a 

 naturalist as distinctly specific, and which at the same time maintains its specific 

 characters within the limits of variation to which it is subject. Such changes there- 

 fore afford no ground for doubting the existence of distinct parasitic species. From 

 all other trustworthy sources of knowledge we obtain the same testimony, that real 

 species can and must be distinguished in the Bacteria exactly as in other groups of 

 plants and animals, and that the parasitic forms which incite disease do not differ in 

 this respect from the rest. Nageli's words *, ' if my view is correct, the same species 



1 Niedere Pilze (1877), p. 64. 



