CHAPTER I 



ON THE VARIABILITY OF BACTERIA AND THE DEVELOPMENT 



OP RACES 1 



(1892) 



From a medical as from a biological point of view, the variability 

 of micro-organisms is a subject of the highest interest. If it 

 can be clearly demonstrated that modifications of varying degrees 

 of permanency can be induced in sundry species of bacteria, 

 that " new races " are capable of being developed under the 

 influence of conditions that are easily recognizable and easily 

 controlled, then not only do we gain further knowledge of the 

 laws governing evolution, but — what for us is of more immediate 

 import — we become enabled to clear up not a few of the difficulties 

 constantly presenting themselves in the study of zymotic, or 

 mvcotic, disease. The class of difficulties to which I refer is 

 so familiar that here I need give but the briefest indication 

 thereof. It is notorious, for instance, that successive epidemics 

 of one disease, say, scarlatina or influenza, are characterized 

 by well-marked differences in symptomatology, and in the 

 intensity of effects upon the individual ; it is notorious also 

 that during the progress of one epidemic remarkable changes 

 are to be made out in the virulence of the malady. Often at 

 the onset the cases are so mild, the symptoms so vague, that 

 diagnosis can only be pronounced with exceeding caution. 

 Such doubtful early cases may be followed by a succession in 

 which the disease is typical and of increasing virulence, and 

 eventually is reached the period of decline of the epidemic, and 

 now again the cases assume a dubious aspect with indefinite 

 atypical symptoms. Or take a disease which now unfortunately 



1 Reprinted from the Medical Chronicle, September 1892. 



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