CH. vi] VARIATIONS IN VIRULENCE 93 



misleading such a distinction may be, and in this case, more- 

 over, the difference in the appearance of colonies on agar 

 could by the adoption of certain precautions be entirely 

 eliminated. The rate of growth of organisms is always subject 

 to variation. Slight motility was therefore the only distin- 

 guishing feature to which any importance could be attached, 

 apart from the question of virulence. As regards the latter, 

 experiments indicated that this property could be increased 

 by passage. Nevertheless these observers regarded the dif- 

 ference in virulence as sufficiently fundamental to justify 

 their description of the organism as a new species quite dis- 

 tinct from the B. anthracis, although spores of the latter 

 organism were found with it. 



The Streptococcus erysipelatis was formerly considered, on 

 account of its greater virulence and certain minor differences, 

 to be a distinct species from S. pyogenes\ further investi- 

 gation has however shown this opinion to be untenable. 



Many other examples might be given but these will suffice 

 to show the difficulties that arise from regarding virulence as 

 a " specific " character. 



