Orleans in 1905, that the theory is now considered established. Thus
systematic study has relieved us of one of the most dreaded diseases
to which mankind has been subject.

_The principal factors in study_

An extensive study, like this investigation, into the cause of yellow
fever employs induction very plainly. It also employs deduction
extensively, inasmuch as hypotheses that have been reached more or
less inductively have to be widely applied and tested, and further
conclusions have to be drawn from them. Such a study, therefore,
involving both induction and deduction and their numerous short cuts,
contains the essential factors common to the investigation of other
topics, or to study in general; for different subjects cannot vary
greatly when it comes to the general method of their attack. An
analysis, therefore, which reveals the principal factors in this study
is likely to bring to light the main factors of study in general.

_1. The finding of specific purposes, as one factor in study_

If the search for the cause of yellow fever were traced more fully,
one striking feature discovered would be the fact that the
investigation was never aimless. The need of unraveling the mystery
was often very pressing, for we have had three great epidemics of
yellow fever in our own country since 1790, and scientists have been
eager to apply themselves to the problem. Yet a specific purpose, in
the form of a definite hypothesis of some sort, was felt to be
necessary before the study could proceed intelligently.

Thus, during the epidemic of 1793, the contagiousness of the disease