1C INFECTION AND IMMUNITY. 



(Bacillus teiani, bacillus of malignant edema, 

 Amoeba dysenteries). 

 OH:* in anei It is futile to speculate on the ultimate origin of 



Variations . . J . . . & 



in organ- our various micro-organisms. It is sufficient to 

 appreciate that the principles of biology and evolu- 

 tion are broad enough to permit us to assume that 

 some of the species which we now recognize may 

 have arisen through the influences of environment 

 and selection from other more or less closely re- 

 lated species. However, investigations have shown 

 that the essential characters of bacterial species are 

 fixed more or less firmly, suggesting that new spe- 

 cies are likely to be developed only through a long 

 course of time, or, if more quickly, through rare 

 chance variations. Koch has suggested that among 

 the trypanosomes found in different diseases some 

 may still be too young in their differentiation to 

 represent fixed species, although this cannot apply 

 to the whole group of trypanosomes. 



Many micro-organisms do, indeed, show a great 

 deal of flexibility in their physiology and viru- 

 lence, with the result that they may approximate 

 species which are usually recognized as being dis- 

 tinct. Thus a strain of the diphtheria bacillus 

 which loses its virulence is similar to a pseudo- 

 diphtheria bacillus, and the cholera vibrio which 

 has become avirulent resembles a number of other 

 vibrios. The plague bacillus, whereas it commonly 

 causes acute death in rats, may undergo such a 

 change in the character of its virulence that it 

 causes a chronic nodular inflammation. Some 

 strains of the tetanus bacillus, which is habitually 

 anaerobic, acquire the power of growing in the 

 presence of atmospheric oxygen. By suitable 

 passage a species of the tubercle bacillus may be 



