e
article by General Steinberg, "Yellow Fever and Mosquitoes" (p. 251).
In that case first Dr. Carlos Finlay of Havana, and then Dr. Sternberg
himself, had become convinced by comparing many cases of yellow fever
that there was some intermediate host for the bacillus that caused the
disease. This conclusion they reached through the method of agreement.
Dr. Finlay's experiments by the method of difference had failed,
however, indisputably to establish the cause, since he did not see that
it was necessary to allow the bacillus at least twelve days for
incubation in the body of the mosquito. The final and definitive proof,
which came through the splendid self-devotion of the surgeons in charge
of the experiment and of certain enlisted men who volunteered to be made
the subject of the experiment, was by the method of difference. These
brave men allowed themselves to be exposed to mosquitoes which had
already bitten patients suffering from the fever, and they promptly came
down with the disease; one of them, Dr. Lazear, gave his life for his
devotion to the cause of his fellow men. Then other men were exposed in
a mosquito-proof room to clothes and other articles brought directly
from yellow-fever patients, and showed no ill effects. Thus it was
absolutely proved, though the bacillus itself had not been found, that
yellow fever is carried by mosquitoes, and is not carried by ordinary
contagion.

The unsuccessful experiments of Dr. Finlay and the later success of
Major Reed show how science advances by refinement of analysis in the
use of the method. The hypothesis on which the former worked was that
all mosquitoes who had bitten a yellow-fever patient can carry the
disease. Dr. Reed and his associates analyzed the phenomenon more
closely and tried their experiments on the hypothesis that only
mosquitoes who have lived twelve days after biting the patient are
capable of passing on the disease. This refinement of analysis and
observation is the chief mode of advance in the sciences which depend on
experiment.

Scientific arguments, therefore, make constant use of both methods.
Medical research frequently begins with the gathering of statistics from
reported cases, and the theory or theories suggested by the method of
agreement working on these facts leads to the application of the method
of difference through some series of critical experiments. In general
the conclusions of science where experiment cannot be used depend on