TEXT-BOOK OF BACTERIOLOGY. 13? 



velopment, and when the stone once begins to roll there is no stop- 

 ping it — the insusceptible individual has become susceptible. 



In other cases this change is brought about, not by an altera- 

 tion in the state of the body, but by the degree of virulence in the 

 bacteria. We have seen that the excretions are the chief agents 

 of infection, and that even the slight variations in their composition 

 can essentially influence their poisonous power. . 



In our eyes and for oar interests the virulence of the bacteria 

 is doubtless their most important property, while for the bacteria 

 themselves it is not of such paramount importance. In many spe- 

 cies of bacteria their pathogenic action is the most variable thing 

 about them, being subject to numerous external influences, stand- 

 ing on tottering foundations, and depending on slight modifications 

 of condition. If a small increase of alkali can turn the scale and 

 out of perfectly innocuous anthrax bacilli, for instance, produce a 

 modified breed which kills mice, then we must say that almost 

 everything is possible in this department. 



If we further consider that for general reasons, depending on 

 their natural history, every pathogenic species of bacteria must 

 once have been non-pathogenic ; that a micro-organism which is now 

 only capable of living as a parasite must once have led a sapro- 

 phytic life; that most of them at the present time still possess the 

 faculty of existing in both ways; that the pathogenic agency has 

 only proceeded from an adaptation to special conditions and a par- 

 ticular nourishment, we shall be able to see in the loss of this prop- 

 erty nothing but a return to former habits and customs. 



It will also be seen that the readiness with which different spe- 

 cies lay aside their virulence is very variable — that some hold more 

 tenaciously to the acquired faculty than others. In all of them, 

 however, it is an acquired faculty, an accessory, or, if you like, a 

 secondary faculty, which can be changed in any direction. 



Therefore differences of virulence must not induce us to separate 

 into different species bacteria which are otherwise identical. 



Virulent and attenuated anthrax baciUi, as already said, stand 

 to each other in the same relation as pathogenic and non -patho- 

 genic species. How would it be, then, if chance had made us first 

 acquainted witlti the harmless variety and afterward brought the 

 poisonous ones under our notice ? Should we not commit a great 

 error if we regarded them as two altogether different things? 

 How much less, then, ought we to insist on far slighter variations, 

 such as the difference of effect on different animals. 



For practical use such a way of proceeding — particularly when 

 the differences are of a somewhat enduring nature and of regular 



