372 AGKICULTTJKAL AND INDUSTRIAL BACTERIOLOGY 



animals. Inasmuch as tuberculosis is a disease which may 

 attack many parts of the body, the clinical symptoms may 

 vary in different cases and the appearances on post mortem 

 examination may differ widely. In consequence until mod- 

 ern times tuberculosis of different types were regarded 

 as different diseases. Tuberculosis of the skin, tuberculosis 

 of the lungs, tuberculosis of the peritoneum and tubercu- 

 losis of the bones and joints were regarded as distinct 

 diseases. It was only after the discovery of the specific 

 organism in 1884 by Robert Koch that the essential identity 

 of the various types of the disease was established. He 

 succeeded in showing the characteristic organism always to 

 be present in nodules of the disease, and succeeded further 

 in growing the organism in pure culture in the laboratory 

 and reproducing the disease upon experimental inoculation. 

 The organisms isolated from man, cattle and birds appeared 

 so nearly alike that at first they were regarded as identical, 

 that is, it was assumed that tuberculosis in all animals was 

 caused by the same germ. In 1896, Theobald Smith ob- 

 served that there were certain differences to be noted in the 

 culture, morphology and physiology of the bacteria isolated 

 from cases of human and bovine tuberculosis. This was 

 further elaborated by Koch who in 1901 concluded that the 

 organism causing tuberculosis in man was quite distinct from 

 that of tuberculosis in cattle. At the present time at least 

 four distinct varieties of Mycobacterium tuberculosis have 

 been recognized. The variety most frequently causing 

 tuberculosis in man is variety hominis, the type pro- 

 ducing disease in cattle, variety bovis, the type com- 

 monly producing disease in birds, particularly domestic 

 fowls, variety avium, and the type producing disease in 

 cold-blooded animals, such as fishes and turtles, variety 

 piscium. It seems to be well demonstrated that it is difficult 

 if not impossible to transform one variety into another. 

 The preceding statement should not be misinterpreted; it 



