ON BACTERIOLOGY IN ITS RELATIONS TO CHEMICAL SCIENCE. 453 



■bacteria resembling the particular one in question in almost every 

 respect, but differing in one or more details — certainly not more important 

 details than those which we have seen can be artificially produced in the 

 laboratory. Let me cite a few examples of such natural varieties, as we 

 may call them. 



The hacillus of anthrax we know under natural conditions may, and 

 frequently does, temporarily reside in the soil ; it would not be surprising, 

 'therefore, to find in the soil some organism presenting more or less 

 likeness to this bacillus. As a matter of fact, not only has an organism 

 indistinguishable from anthrax in all save its pathogenic properties been 

 discovered in the soil by Hueppe and Cartwrigbt Wood, but these investi- 

 gators further proved the excessively close relationship of this soil bacillus 

 to the anthrax bacillus by the observation that rabbits and even mice 

 inoculated with the soil bacillus were protected against subsequent 

 inoculation with virulent anthrax, as though they had been vaccinated 

 with an attenuated anthrax virus (' Lancet,' February, 1889 ; ' Berlin, 

 klin. Wochenschrift,' No. 16, 1889). 



The dipJitheria bacillus of Loeffler ('Centralbl. f. Bakteriol.,' ii. [1887], 

 p. 105) was found by him in the false membranes of the throat associated 

 tvith another bacillus, almost indistinguishable from it, excepting that it 

 had no toxic effect on animals. Roux and Yersin ('Ann. de I'lnst. Pasteur,' 

 iv. [1890], p. 385) have, moreover, found that this Sacillus pseudo- 

 diphtheric'us, as it is called, is frequently present in the pharyngeal mucous 

 •membrane of healthy children. 



The cholera hacillus of Koch, again, as we have already seen, is not 

 only subject to very considerable variations in form and functions accord- 

 ing to the particular place or epidemic from which it has been obtained, 

 but its natural habitats — -the human intestine and natural waters — have 

 both been found to yield forms which are distinguishable from it only 

 with the greatest diflficulty. 



The typhoid bacillus of Eberth-Gaffky, again, is distinguishable only 

 with the greatest difficulty from a number of pseudo-forms occurring in 

 its natural habitats — the human intestine and natural waters. 



Closely connected with these phenomena are doubtless also aiirohic 

 and anaerobic growth. As is well known, bacteria may be divided into 

 three classes, according to their relationship to oxygen : — ■ 



(1) Compulsorily aerobic, or those organisms which will only grow 

 in the presence of free oxygen; (2) facultatively aerobic and anaerobic, 

 or those organisms which can grow either in the presence or absence of 

 free oxygen ; (3) compulsorily anaerobic, or those organisms which will 

 only grow in the absence of free oxygen. The phenomenon of aerobic 

 growth would appear, of course, to be the normal one ; but in many of 

 the decompositions brought about by bacteria such large quantities of 

 gases — especially carbonic anhydride and hydrogen — are evolved that 

 all free oxygen is rapidly swept out of the medium in which the bacteria 

 are carrying on their operations. Under these circumstances, then, any 

 bacteria which are entirely dependent on oxygen would have their 

 ■vitality either destroyed or suspended, whilst those which can maintain 

 themselves either temporarily or permanently in the absence of oxygen 

 must be at a great advantage, inasmuch as they can continue their vital 

 processes in the oxygen-deprived medium which they have themselves 

 created. In this way we can imagine how originally aerobic organisms 

 endowed with the capacity of decomposing certain substances with the 



