FIXITY OF BACTEEIAL SPECIES 105 



bacteriologists, and in this country by the teaching of Dr. Klein, 

 the tendency of the medical world has been to regard the species 

 of micro-organisms, and especially of pathogenic micro-organisms, 

 as fixed. If in each case of a given disease a given microbe is 

 discovered with specific cultural characteristics : if the more 

 closely zymotic disease is studied the more clear becomes the 

 evidence of its infectious nature, of its passage either directly or 

 indirectly from person to person, while the more the subject is 

 studied the weaker becomes the evidence of the spontaneous 

 origin of infectious disease : then the greater is the difficulty of 

 arriving at any other conclusion than that pathogenic microbes 

 belong to species that are fixed and possess unchanging attributes. 

 I need not say how valuable, not to say necessary, it has been 

 in a young science like bacteriology to hold this view of the 

 fixity of species, and, speaking broadly, this view must still 

 be held. In the great majority of cases the micro-organisms 

 gained from the tissues of the patient suffering from any one 

 zymotic disease approximate sufficiently closely to the type for 

 us to say with absolute confidence that no specific alteration is 

 to be discovered in them. But while acknowledging this much 

 it must be confessed that there has been a tendency to overlook 

 differencesin the mode of growth of pathogenic microbes — gener- 

 ally, it is true, minute, though not always so — and to dwell 

 upon differences in the resistance of the organism more than 

 upon differences in the pathogenic properties of the micro- 

 organisms whenever, in an individual case, or in an epidemic, 

 the symptoms have varied from the ordinary. While bacterio- 

 logists in general admit, to a greater or less extent, that bacteria 

 show variations, I know of no work in which the facts in con- 

 nexion with such variations have been brought together ; it may, 

 therefore, be useful at the present time if I gather together 

 some of the many researches into the variability of microbes, 

 and show that this variability can clearly be made out. 



Undoubtedly where a number of facts are brought together, 

 all pointing in one direction, there is a danger that by some 

 these facts will be held to support an extreme theory, and one 

 that cannot surely be gained from them — the theory, in this 

 case, that non-pathogenic bacteria can easily become pathogenic, 

 and that zymotic disease may, as a frequent event, arise de novo. 

 While, possibly, further research may prove that in a certain 



