102 VARIATIONS IN PATHOGENICITY [ch. tii 



consolidation of the lung and streptococcal invasion of the 

 skin, respectively. 



It is however common experience that even in the post 

 mortem room a certain diagnosis of the nature of the infec- 

 tion cannot always be made. Sydney Martin, in speaking of 

 tuberculosis, says "There is, with the exception of the presence 

 of the tubercle bacillus, no element in the structure of the tu- 

 berculous lesion which is diagnostic of the disease." In other 

 words the lesions regarded as charoiCteristic of infection hy 

 one species of organism may he produced hy infection hy a 

 totally different species. 



Such departures from what experience has taught us to 

 regard as the normal or characteristic lesion in the case of a 

 given organism may be accounted for by the influence of other 

 factors beside the nature of the organism itself — such factors, 

 for example, as the age of the patient, the route of invasion, 

 the presence of a secondary infection, the eflect of treatment, 

 and many others. The question arises, how far, if it were 

 possible to exclude such disturbing influences, would the 

 lesions retain their specific character ? 



2. This leads us to a consideration of the second method 

 of studying the question — ^by observing the lesions produced 

 by artificial inoculation of animals, both at the site of inocu- 

 lation and elsewhere. Such a method enables one to, so to 

 speak, "standardise" the lesion. A healthy animal of the 

 same species, age and weight can be utilised at each experi- 

 ment, the inoculation made in the same manner, at the same 

 site, with the same number of organisms and these of the same 

 degree of virulence, and the animal can be killed after the 

 same interval of time. 



Many investigators maintain that under such conditions 

 the lesions produced by a certain species of organism are 

 constant in their appearance — that, however much the other 

 characters of an organism may vary, this character at any 

 rate is invariable and will establish beyond dispute to which 

 of two species a doubtful organism actually belongs. 



Thus, Klein as long ago as 1899 in describing the " bacillus 

 of pseudo-tuberculosis " stated that in cultural and morpho- 



