14 CONDITIONS MODIFYING [oh. ii 



found that in Cleveland U.S.A., during the 12 months covered 

 by the investigation, "barred" forms of the diphtheria bacillus 

 had almost disappeared; during the same period, in Boston 

 and Providence, another observer noted that "barred" forms 

 were unusually common while "granular" forms were very 

 rarely met with. 



3. Many organisms after prolonged cultivation on artificial 

 media display variation in character. 



In some of these cases the length of the period of cultiva- 

 tion is not the cause of the modification, it merely extends the 

 survey over a large number of generations and so enables the 

 observer to detect variations spontaneously occurring. 



In other cases the length oitm\Q permits" natural selection" 

 to play its part and produce modifications which, in a shorter 

 interval, would not have advanced far enough to be apparent. 



In other cases, again, the prolonged exclusion from animal 

 tissues does lead directly to a modification in character which 

 is proportionate, as regards its extent and its permanence, to 

 the duration of such exile, but disappears when the organism 

 is again "passed" through the body of an animal. This is true 

 more particularly of the property of virulence {q. v.). 



As an example of the influence exerted by prolonged 

 cultivation in modifying the character of an organism may be 

 cited the statement of Mohler and Washburn (1906) that 

 a strain of bovine tubercle bacilli after cultivation for 11 years 

 was found to have become modified in morphological and 

 cultural characters to the human type. 



Lentz (quoted by Bahr, 1912) found that a "Flexner" 

 type of B. dysenteriae after 9 years' laboratory cultivation 

 completely lost the power to ferment maltose. Arkwright 

 (1909) mentions a strain of the meningococcus which when 

 first isolated did not ferment glucose but after ten months' 

 artificial cultivation developed power to do so. Rettger and 

 Sherrick (1911) describe the gradual loss of power to produce 

 pigment on the part of an old stock culture of B.pyocyaneus 

 after 5 years' artificial growth. 



Prolonged cultivation in a medium containing a particular 

 carbohydrate may develop in a strain of bacteria the ability 



