CHAPTER XI. 



Systematic study of an organism— Points to be considered in identifying an 

 organism as a definite species. 



After isolating an organism by the plate method, 

 considerable work is necessary in order to establish its 

 identity as a definite species. 



It must possess certain morphological and cultural 

 peculiarities, which must be constant under constant 

 conditions. 



Its form at different stages must always be the same. 

 Its ability or inability to produce spores must not vary 

 under proper conditions. Its growth upon the different 

 media under constant conditions of temperature and 

 reaction must always present the same outward appear- 

 ances. The reactions given by it to the media in which 

 it is growing must follow a fixed rule. Its power to 

 produce liquefaction of the gelatin, or to grow upon it 

 without bringing about this change, must always be the 

 same. Its motility or non-motility, and, if motile, the 

 number and position of its organs of locomotion, must 

 be determined. Its production of certain chemical pro- 

 ducts must be detected by chemical analysis. Its be- 

 havior toward oxygen — i. e., does it require this gas for 

 its growth ? is this gas an indifferent factor ? or by its 

 presence are the life processes of the organism checked ? 

 — must be decided. Its behavior under varying condi- 

 tions of temperature and under the influence of different 

 chemical bodies, as well as its growth in media of 



