MICRO-ORGANISMS 



the avian, the bovine, and the human varie- 

 ties of tubercle bacilli. ,But these are so 

 little removed from one another that their 

 title to distinct species is still doubtful. 

 The same may be said of the colon group, 

 to which the typhoid bacillus belongs. 

 More can be said of the changes in the 

 variola agent in vaccinia by change in its 

 soil or medium of growth. When changes 

 are artificially produced in them by alter- 

 ing the medium or soil in which they grow, 

 no new species is thus formed, for they 

 either die out or revert to their original 

 characters so soon as their original native 

 conditions are restored. Hence, no per- 

 manent and general modification takes 

 place in them, as in visible plant forms like 

 the botanical family of the solanacea?, for 

 example, which includes such divers forms 

 as the potato, the eggplant, the thorn ap- 

 ple or stinkweed growing about our barn- 

 yards, the hyoscyamus, and the belladonna 

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