Illustrations of a Law of Evolution of Thought [pp. 373-393]

THE PRINCE TON RE VIEW.
Thus in the progress of knowledge in all the subjects thus far
mentioned we find, more or less marked, two stages of rational
embodiment; i.e., two distinct theories, the one provisional, the
other permanent; the one may be called an hypothesis, the other
a true theory. In the subject of geyser-eruption Mackenzie's.
is the provisional hypothesis and Bunsen's the permanent theory; in the subject of coral reefs the "crater theory" is the pro —
visional and the "subsidence theory" the permanent; in the
subject of slaty cleavage Sidgwick's "crystalline theory" is the
provisional and Sharpe's "pressure theory" the permanent. We
have selected these among many others because the two stages
are well marked and without the intermediate stages which obscure the law. In some other cases there are many successive
steps, and therefore the law, tho still observable, is less evident.
Thus in the difficult subject of glacial motion there are many successive theories, but De Saussure's, Agassiz's, and Moseley's may
be classed in the first category of provisional guesses, while
Forbes', Tyndall's, and Croll's fall into the second category of
more or less perfect inductive theories. As a general rule, tho
not always, in proportion as a subject is more complex there
will be more successive steps, the evolution will be more gradual. I might multiply illustrations of the law by examples taken
from every department of science, but this seems unnecessary.
Genera/ity of the Law in the domain of Science. But if this.
be indeed a true law, we ought to find it not only in the progress.
of evolution of each special subject, but also in the general progress of scientific knowledge. The law of the part must be also
the law of the whole; if we find it in the development of special
scientific problems, we ought to find it also in the general pro —
gress of the great departments of science. Now we do indeed so!
find it. In astronomy it is well marked; for we have here first:
the geocentric theory of the solar system, the provisional, and.
the heliocentric theory, the permanent. In optics we have first:
the corpuscular and then the undulatory theory Of light. In
botany (another excellent example) we have first the Linnaean orartificial system of classification, and then the natural system of
classification of plants. In each of these cases it would be easy'
to show that the spirit and the method of the earlier and the:
later theories differ precisely in the manner already pointed out..
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