644 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
can not be replaced by dead ones in bringing about immunity, the 
immunizing process is IN some way bound up with their survival and 
even, possibly, with a restricted multipheation. , Hence it is necessary 
that we ascertain, first, how long the human bacilli survive in the 
organs of the vaccinated animals, and second, whether they are ever 
eliminated with the milk of cows. The observations already made 
upon these points are so few as at present not to be useful for any sci- 
entific deductions. But before the method is too implicitly relied 
upon these questions should be answered. 
It is an interesting subject of speculation as to what the result will 
be when cattle in general, and possibly, man later, shall have been 
immunized to tuberculosis. Will the race of tubercle bacilli dis- 
appear in large measure from the world? This would indeed be a 
beneficent result. But Doctor Smith has pointed out in a recently 
delivered address that doubtless host and parasite eventually come to 
hold a kind of equilibrium to each other, and hence an increased de- 
gree or resistance in the former might tend to bring about that selec- 
tion among the parasites through which races of greatly augmented 
power for invasion would be produced. If this were true, and he 
even suggests that the natural process of weeding out the weaker 
among the human race tends to this result, the parasite would try 
to keep up with the host as his resistance increased until a point was 
reached beyond which further enhancement of power was impossible. 
Would the higher animal or the lower vegetable organism finally 
claim the victory? We need perhaps at this moment not to relax our 
efforts to achieve a practical immunity for man as well as for animals 
because of this future danger. I am not aware that the smallpox 
germ has increased measurably in virulence since vaccination became 
general, but I would also add that a century is a small period of time 
in the life history of any living organism. 
Before closing this address I should like to refer briefly to the new 
interest which has been excited in the use of tuberculin in the treat- 
ment of human tuberculosis by reason of the application to the study 
of. tuberculosis of a method introduced by A. E. Wright, of London, 
whereby it is held that the exact effect of the tuberculin injection can 
be measured and controlled. The method consists in the determina- 
tion of the capacity of the blood leucocytes to take up tubercle bacilli 
when the blood and the bacilli are brought together outside the body 
in a test tube. Wright and his pupils have worked out the normal 
power of the blood to cause the englobing of the bacilli; and they have 
noted a diminution of this capacity in the blood of many persons suf- 
fering from tuberculosis. They speak of this englobing capacity 
of the blood as “ opsonic index,” from the word meaning to prepare— 
to cater for; since the bacilli must first be prepared by substances in 
the blood serum before they can be ingested by leucocytes. The in- 
