Introduction. 



It has been shown by many observers that changes in the chemical 

 and physical environment of micro-organisms lead to marked variations, 

 not only in the morphology of the cell, but also in its physiological 

 activities. The variations in the form and staining characteristics of 

 B. diphtheria e, the formation of long and short chains in the Strepto- 

 cocci, the appearance of capsules, the production of spores and devel- 

 opment of involution forms in old cultures indicate the readiness with 

 which unicellular organisms respond to their external environment. 



The facility with which micro-organisms change their form and the 

 mechanical difficulties involved in the observation of variations in their 

 morphology, renders any classification of bacteria based on morphology 

 alone very unsatisfactory. Recently, the bio-chemical characteristics of 

 bacteria have played an important part in the study of bacterial species. 

 The physiological activities of bacteria in different cultural media are 

 easily studied, and under a uniform set of conditions many of the 

 characteristics of species exhibit a remarkable degree of constancy. If 

 uniform conditions are not adhered to, however, variations may be 

 obtained. 



Pere" (1) in his researches on lactic acid produced by micro- 

 organisms found that the same organism may produce lactic acid of the 

 opposite optical activity, according to the quantity and the quality of the 

 protein used. The correctness of this observation has, however, been 

 questioned by later workers. Kruse (2) found that Staphylococci 

 lose their liquefying power after prolonged cultivation under anaerobic 

 conditions. Andrewes and H o r d e r (3) record a case where Strepto- 

 cocci refused to attack lactose under ordinary conditions, but did so 

 readily under anaerobic conditions. Conn (4) starting with a pure 

 culture of a micrococcus was -able to obtain by simply replating many 

 times and selecting from the number of colonies on the plate, the one 

 which liquefied most rapidly and the one which liquefied most slowly, 

 a rapidly liquefying culture and one which hardly liquefied at all. 

 Smith (5) records similar results with Proteus vulgaris. Peckham 

 (6) has shown that organisms that may not have the power to produce 

 indol may develop this power if allowed to grow in suitable media. 

 T w o r t (7), by growing organisms in a fluid medium containing a sugar 

 that they had not been previously able to ferment, was able to cause 

 these organisms to ferment the sugar. Goodman (8) states that he 

 was able by a gradual process of selection of impressed variations, 



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