PATHOGENIC MICROBES— INFECTION 109 



The great discovery of Pasteur of the attenuation of viruses 

 proves the plasticity of microbial species and the facility with 

 which they modify their primitive characters. The history of 

 bacterial diseases shows also the great role which these in- 

 finitesimal creatures have played in natural selection, for is it 

 not they which have caused to disappear in the course of ages 

 certain vegetable and animal species insufficiently armed to 

 resist them ? . . . Experimental medicine has studied the 

 adaptation of certain pathogenic microbes which permit them 

 to attack the body in spite of the defences opposed to them. 

 Here it is probably a question of a selection of individuals 

 endowed with particularly stable characters." ^ 



Starting with a bacterium almost non-virulent, Pasteur 

 succeeded in infecting with anthrax in succession, by the 

 method of passages, the new-born mouse, the adult mouse, the 

 young guinea-pig, the adult guinea-pig, the rabbit, and the 

 sheep. Vincent by introducing into the peritoneal cavity little 

 collodion sacs containing bacteria which are nourished by the 

 body fluids, while being protected from the cellular defences, 

 rendered pathogenic for the guinea-pig and the rabbit such 

 saprophytic microbes as B. megatherium and B. mesentericus 

 vulgatus, but the virulence thus acquired disappeared as soon 

 as the artifice which produced it was suspended. These 

 experiments do not permit of the conclusion that pathogenic 

 species can be created at will in the laboratory ; only more or 

 less stable variations are got. It is' not with such ease that we 

 are likely to reproduce what Nature has taken centuries to 

 accomplish. It is very probable that small-pox and vaccinia 

 are two modifications of the same virus. Yet the production 

 of vaccinia with small-pox has not yet been successfully 

 performed. The experiments said to have proved variolo- 

 vaccination are still disputed. Nothing has been able to 

 produce from B. coli a Typhoid bacillus. There exist various 

 families of the Tubercle bacillus, which may be secular adapta- 

 tions from the same strain, adaptations to the human species 



' Metchnikoff, address read to the Cambridge festival in honour of the 

 Darwin centenary, June, 1909. 



