Still another and very important variety of arguments of fact, which are often conveniently described as arguments of theory, includes large 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 hold it to be a masterful explanation of an enormous body of facts.  When it will pass on to the next stage we cannot foresee; but chemists and physicists darkly hint at the possibility of the evolution of inorganic as well as organic substances.

In arguments of fact, it will be noticed, there is little or no element of persuasion, for we deal with such matters almost wholly through our understanding and reason.  Huxley, in his argument on evolution, which was addressed to a popular audience, was careful to choose examples that would be familiar; but his treatment of the subject was strictly expository in tone.  In some arguments of this sort, which touch on the great forces of the universe and on the nature of the world of life of which we are an infinitesimal part, the tone of the discourse will take on warmth and eloquence; just as Webster in the White Murder Case, dealing with an issue of life and death, let the natural eloquence which always smoldered in his speech, burn up into a clear glow.  But both Huxley and Webster would have held any studied appeal to emotion to be an impertinence.