CHAPTEE 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 

 pecidiarities, 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 changes brought about by it in the reaction 

 of 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 \vithout bringing about this change, 

 must always be the same. Its motility or non-motility, 

 and, if motile, the approximate number and position of 

 its organs of locomotion, must be determined. Its pro- 

 duction of certain chemical products must be detected 

 by chemical analysis. Its behavior 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 conditions of temperature 

 and under the influence of different chemical bodies, as 



