IMMUNITY IN TUBERCULOSIS—FLEXNER. 631 
artificial conditions of cultivation, into each other. Into the disputed 
questions of variation due to environment I can not afford to enter. 
But I would have you believe that transformations of avian. bovine, 
and human bacilli into each other have probably not been accom- 
plished by experimentation. The cultivation of one variety of bacilli 
in the body of an alien species has been. said to alter profoundly the 
properties of the bacilli; but the observations upon this point are in 
my opinion far from convincing. The mere fact that avian and bo- 
vine varieties of bacilli preserve their peculiar properties when occur- 
ring naturally in the diseased body of an alien species—man, for ex- 
ample—tends to discredit the experimental transmutations referred to. 
Bovine tubercle bacilli are characterized, as ascertained by Smith, 
by a greater degree of pathogenic power for mammals in general than 
human bacilli, with which fact is correlated certain peculiarities of 
cultural and physiological properties serving further to separate the 
bovine from the human bacilli. The bacilli of mammalian origin are, 
perhaps, closely related and less removed from each other by the sum 
of their properties than they are from the avian bacillus. With the 
few exceptions mentioned all forms of mammalian tuberculosis are 
caused by either the human or the bovine bacillus. 
In view of the general fact that the bovine bacilli show a greater 
degree of pathogenic action for the lower mammals than the human 
bacilli, it was natural to assume that bovine bacilli would be power- 
fully pathogenic for man also. To test this probability directly by 
experiment is, of course, not permissible. But the belief that tubercu- 
losis in cattle is a menace to man is expressed in the many regulations 
by which it is aimed to control and prevent the use as food of products 
derived from tuberculous animals. It was not until Koch’s address 
was delivered in 1901 that any serious doubt existed in the minds of 
sanitarians and pathologists that tuberculous cattle offered a source of 
danger to man. The specific knowledge which has accumulated since 
that date has served to establish the transmissibility in some degree 
of bovine tuberculosis to the human subject. The inherent difficulty 
and tediousness of the investigation of the specific types of tubercle 
bacilli existing in human cases of tuberculosis necessarily limit the 
total number of instances in which it has been established, beyond 
peradventure, that the bovine type of bacillus does occur in tubercu- 
lous processes in man. In this country the responsibility of refuting 
the too general statement of Koch has fallen chiefly upon Ravenel and 
Theobald Smith, whose admirable studies in this direction are of a 
convincing nature. 
If we pause for a moment to consider upon what data Koch based 
his statement of the independence and noncommunicability of tuber- 
culosis in cattle and man, we shall appreciate that, in so far as he dealt 
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