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scarcely assume that free oxygen has been always fatal to 

 the parent forms; for in that case how could we realise 

 the possibility of zymotic diseases having ever begun, 

 unless we trace them back to some time when poisonous 

 vapours enfolded the earth ? And, granted various stages 

 of vi.rulence from harmless to deadly in one and the same 

 species, how are we to define the difference of species ? 

 We may define species pathologically by the different 

 symptoms of the diseases with which they are associated, 

 and to some extent possibly by the forms of the microbes. 

 We may also classify microbes by the marked differences of 

 their own constitutions. For the susceptibility of these 

 organisms seems to differ in the most extraordinary fashion. 

 What is meat for one appears to be poison for another. 

 Thus, in his latest report on the cholera bacillus, Koch tells 

 us that the smallest proportion of acid is fatal to the life of 

 that organism, yet we know that other bacilli live and thrive 

 in strongly acid solutions. Ideas bearing upon these later 

 discoveries have long been current, witness Dr. Angus 

 Smith's remark in 1848, that a man might be capable of one 

 disease on one day and another disease on the following day. 

 Perhaps the peculiar susceptibilities of the microbes may be 

 developed and fixed as the peculiar virulence is developed 

 and fixed. The desirableness of further experiments on the 

 cultivation of organisms in various gases, and particularly on 

 the cultivation of the spores, is strongly suggested by these 

 considerations. It would also be worth while to test further 

 the specific consequences of the presence or absence of light. 

 1 4. — The presence or absence of oxygen is associated with 

 the question of spore formation. In this connection the 

 varying susceptibility or vigour of microbes, not only in 

 different animals, but in different milieux in the same 

 animal, must not be overlooked. Thus the virus of the 

 Maladie de Chabert, or Chai'hon symptomatique, formerly 

 supposed to be the same disease as anthrax, acts as a vaccine 



