INOCULATION WITH DILUTED VIRUS 
67 
inoculations was a slight circumscribed inflammation, which was 
noticed in each case at the point of inoculation the seventh day 
after the operation. This local lesion is very apparent from the 
increased size of th6 blood-vessels, and a noticeable swelling from 
one-fourth to three-fourths of an inch in diameter. 
At the time of these experiments, I underestimated the impor¬ 
tance of the lesion just described, for, while 1 thought it possible 
that a slight degree of immunity might be obtained by means of 
it, I could not conceive that a complete insuseptibility would re¬ 
sult without the yellow urates and other symptoms of a constitu¬ 
tional affection. That is, it was naturally expected that a mild 
attack of the fever was necessary to protect against the subse¬ 
quent recurrence of the disease. 
Being desirous of producing the general fever of as mild a 
type as possible, I now inoculated the four fowls with the same 
virus diluted as 1 to 1,000. What was my surprise to see each 
one of these birds succumb to the disease in its most virulent and 
fatal form ! I was now brought face to face with the great ob¬ 
stacle which has always prevented the physiological investigator 
from developing an exact science, and which had proved an in¬ 
surmountable difficulty to those who had previously attempted 
this line of investigation—that is, the individual peculiarities of 
living animals. 
It was not to be supposed that the birds in the first experi¬ 
ments, escaped because no germs were introduced with the diluted 
virus, since in a dilution of 1 to 2,500 there would be more than 
600 germs to every drop, and in the dilution of 1 to 50 these 
would be increased to more than 30,000; besides there was the 
local inflammation which developed after the punctures had healed, 
and with two of the birds a sufficient coloration of the urates to 
denote constitutional disturbance. 
My experiments have demonstrated, conclusively, that suseep 
tibility and insusceptibility are only relative and never absolute 
conditions. A certain proportion of fowls will resist inocu¬ 
lation with a drop of strong virus, but if we increase the 
dose to ten, twenty, thirty, or sixty drops, even these may be 
made to contract the disease. As we decrease the dose to one- 
