642 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
discovered properties to a practical test of immunity occurred to the 
several independent workers in bacteriology. There can, I think, 
be no doubt that Behring deserves the credit of making the pro- 
tection of cattle from tuberculosis a feasible, practical object of 
study. This alone is a merit of no small order. 
From the mere fact that cattle have been successfully protected 
from infection by the tubercle bacillus, even under the severest condi- 
tions of laboratory experiment, it can not be concluded that they will 
be equally refractory when exposed to the natural sources and modes 
of infection. In the laboratory the virulent infectious agent is 
brought into the animal by injection, under the skin, into the serous 
cavities or into the circulation, which are avenues through which in 
the natural disease infection rarely if ever takes place. And while 
this mode of introduction of the virulent bacilli into the body may, 
theoretically, be more severe than their.introduction into the lungs 
with inhaled air, or into the stomach through infected stalls and food, 
yet the profound differences in the defenses of the body with which 
the bacilli come into conflict, under these different circumstances, may, 
after all, determine the issue in a manner quite contrary to our expec- 
tations. It is, therefore, of the highest interest to learn that in their 
later tests Behring and his coworkers exposed vaccinated cattle to 
stalls and herds which were known to be badly infected, with the re- 
sult that at the time of the report, they had apparently escaped infec- 
tion. Iam enabled through the courtesy of a private communication 
from Doctor Pearson to state that cattle vaccinated by himself and 
Gilliland which were kept for two years under natural conditions of 
infection have not contracted tuberculosis, while the control animals, 
exposed to the same conditions, have all developed the disease, some 
dying spontaneously by reason of the severity of the infection. Doc- 
tor Pearson also informs me that their experiments indicate that the 
degree of resistance bears a rather definite relation to the number of 
vaccinations given the cattle. No cattle vaccinated three times with 
their standard vaccine—a living culture of tubercle bacilli of human 
origin—have developed tuberculous lesions even after two years’ 
severe exposure. In their experience, two injections of Behring’s 
vaccine do not always suffice for such heavy exposure as they em- 
ployed. 
As regards the question of duration of the protection, it may be 
said that Behring, basing his views on results of vaccination made 
three years before, expressed the belief in 1904 that it would endure 
during the life of the animal. As young healthy cattle are vaccinated 
before they fall victims to infected stalls and herds, it would seem as 
if infected herds might therefore gradually be replaced by healthy 
ones. The gain, this being true, would be almost incalculable to 
agriculture. 
