352 SCIENCE PROGRESS 



one hand and the majority obtained from bovine sources on 

 the other. 



The human strains grow readily on artificial media outside 

 the body and when injected into animals produce lesions which 

 are neither severe nor acute ; bovine strains are often extremely 

 difficult to cultivate on artificial media but when injected into 

 animals produce widespread and rapidly fatal disease. 



The elaborate investigations of the Royal Commission have 

 proved, as might have been anticipated, that tubercle bacilli 

 from either source show remarkable constancy in form and 

 behaviour during a number of generations, whether propagated 

 by passage from animal to animal of the same or different species 

 or by repeated subcultivations on artificial media. 



There is, however, abundant intrinsic evidence in the 

 Report that the tubercle bacillus, stable as it is, shows almost 

 every mutation between the bovine type on the one hand and 

 the typical human variety on the other. 



Bacilli from bovine sources produce in one calf lesions of a 

 mild or chronic type, in a second acute tuberculosis ; when the 

 infecting agent is passed from the chronic case into another calf, 

 acute lesions are produced, showing that there has been a 

 gradual increase in virulence of the bacilli. 



Similarly, bacilli which at first are difficult to cultivate on 

 media will flourish after repeated subcultivation where at first 

 they pined. In either case, the aptitude to flourish under one or 

 other set of conditions pre-existed in some, so that the organism 

 only required appropriate treatment to assume corresponding 

 characters. 



It is specially noteworthy that bacilli of typical bovine 

 character have been isolated from human sources, whilst bacilli 

 have been obtained from oxen showing a variable degree of 

 virulence for animals and ready growth on media. 



The bacilli from bovine sources show considerable con- 

 stancy of character. Bacilli from human sources often vary and 

 are placed in the Second Report of the Royal Commission 

 in three groups, one approximating in its character to the 

 typical bovine form, the second to the typical human form ; 

 the strains of "group three" manifest various mutations and 

 combinations of the characters accepted as typical in the two 

 strains together with a marked variability in cultures and when 

 inoculated into animals. 



