354 AGRICULTURAL BACTERIOLOGY. 



derived from cattle and from the human species. Cultures 

 obtained from cattle behave very differently from those de- 

 rived from man. If both are injected into rabbits animals 

 whose susceptibility to the disease is not very great the 

 bovine bacillus is found to be the most active. If both are 

 inoculated into calves it is found that the human bacillus pro- 

 duces only a slight local infection which soon heals, while the 

 bacillus from the cow produces commonly a serious disease 

 which progresses, perhaps, to death. This fact has been veri- 

 fied sufficiently to place it beyond doubt ; but the interpreta- 

 tion to be placed upon it is not quite so clear. 



Some have thought these facts indicate that each animal 

 has a variety of bacillus especially adapted to itself, and that 

 bacilli from other animals are not so likely to produce the in- 

 fection as those from other individuals of the same species. 

 This suggestion led to the opinion that each species of animal 

 is chiefly infected from other individuals of the same species, 

 and only exceptionally from individuals of other species. This 

 would, of course, make the passage of the disease from cattle 

 to man an exceptional matter, and indicate that the human 

 infection comes practically always from other men. Others 

 have insisted that these facts show that the bovine variety of 

 the bacillus is decidedly more virulent, and hence is more 

 dangerous to man than the human variety. This would make 

 the passage of the disease from cattle to man easy, while the 

 reverse infection could hardly occur. The latest evidence on 

 the question has shown that the bovine variety is more viru- 

 lent than the human variety for several species of test animals, 

 including rabbits, swine and cattle. Experiments to test its 

 virulence upon man are manifestly impracticable. It is clear 

 that the conclusion upon this matter is of the greatest impor- 

 tance. If the bovine bacillus is more virulent than the human 

 variety for man as well as other animals, it will evidently 

 point out a great danger of distribution of the disease from 



