24 ADAPTATION AND DISEASE 



Non-virulent diphtheroid bacilli are so common in the throat 

 as to be the curse of Army bacteriologists : the same may be 

 said regarding harmless diplococci which can only be differen- 

 tiated from meningococci by a relatively elaborate technique : 

 only within the last few weeks the frequent presence of these 

 " parameningococci," affording the " group agglutins " as dis- 

 tinct from the specific agglutins of the meningococci proper, 

 in one military district was on the verge of causing serious 

 trouble ; as Andrewes and others have pointed out, the majority 

 of streptococci obtainable from the throat are non-virulent, 

 and yet with them, as I shall point out later, there appear to 

 be an extraordinary number of strains of streptococci obtain- 

 able from the throat of varying grades of virulence ; non- virulent 

 spirilla flourish in water just as do the closely-related but virulent 

 cholera spirilla : from the alimentary canal there may be isolated 

 non-pathogenic members of the typhoid-coli group, only dis- 

 tinguishable by physiological or bio-chemical tests from typhoid 

 bacilli, certainly not by any morphological test : harmless gram- 

 negative diplococci morphologically indistinguishable from gono- 

 cocci are obtainable from the vulva : acid-fast bacilli closely 

 related to the tubercle bacillus of cattle are found in hay, butter, 

 etc. ; only after some years of study have the non-pathogenic 

 amoebae of the intestine been clearly differentiated from the 

 Entamoeba histolytica of dysentery and the list might be 

 greatly extended. 



This state of affairs in itself leads to the conclusion that patho- 

 genic microbes at some period, or periods, have originated from 

 the microbes saprophytic upon the body surfaces, or existing 

 commonly in the water and food-stuffs ; that they have origin- 

 ated by adaptation of these forms to growth, not merely on, 

 but within the tissues. 



THE DOCTRINE OF FIXITY OF BACTERIAL SPECIES 



This is the simplest, most rational conclusion, but I must 

 point out to you that for long years the dominant German 

 school of bacteriologists upheld vigorously the doctrine of fixity 

 of bacterial species, closing their eyes to the steadily growing 

 volume of data which tended to establish the potential varia- 

 bility of the bacteria. The great leader of German bacterio- 



