scientific questions, such, for example, as the origin of our present
species of plants and animals, or the ultimate constitution of matter,
or the cause of yellow fever. In such arguments we start out with many
facts, already gained through observation and experiment, which need the
assumption of some other fact or facts attained through reasoning from
the others, to make them fit together into a coherent and intelligible
system. Every important new discovery in science makes necessary
arguments of this sort. When the minute forms of life that the layman
lumps together under the name "germs" were discovered there was a host
of arguments to explain their manner of life and the way some of them
cause disease and others carry on functions beneficent to mankind. A
notable example of the arguments concerning this kind of fact is that at
page 251 concerning the cause of yellow fever; and another is Huxley's
argument on evolution (p. 233), where he points out that "the question
is a question of historical fact." The element of uncertainty in the
settlement of such questions is due to the facts being too large or too
minute for human observation, or to their ranging through great ages of
time so that we must be contented with overwhelming probability rather
than with absolute proof. Furthermore the facts that are established in
arguments of this sort may have to be modified by new discoveries: for
many generations it was held to be a fact that malaria was caused by a
miasma; now we know that it is caused by a germ, which is carried by
mosquitoes. Arguments of this type tend to go through a curious cycle:
they begin their life as arguments, recognized as such; then becoming
the accepted explanation of the facts which are known, for a longer or
shorter time they flourish as statements of the truth; and then with the
uncovering of new facts they crumble away or are transformed into new
and larger theories. Darwin's great theory of the origin of species has
passed through two of these stages. He spoke of it as an argument, and
for a few years it was assailed with fierce counterarguments; we now