avalanche on the mountain slope, and for the planets of the solar system,
moving in celestial coördination.

Such a principle as the law of gravitation, like evolution, is true if the
basic facts are true, if they are reasonably related, and if the
conclusion is drawn reasonably from them. It is true for all persons who
possess normal minds, and this is why Huxley speaks of science as "common
sense,"--that is, something which is a reasonable and sensible part of the
mental make-up of thinking persons that they can hold in common. The form
and method of science are fully set forth by these definitions, and the
purpose also is clearly revealed. For the results of investigation are not
merely formulæ which summarize experience as so much "conceptual
shorthand," as Karl Pearson puts it, but they must serve also to describe
what will probably be the orderly workings of nature as future experience
unfolds. Human endeavor based upon a knowledge of scientific principles
must be far more reliable than where it is guided by mere intuition or
unreasoned belief, which may or may not harmonize with the everyday world
laws. Just as the law of gravitation based upon past experience provides
the bridge builder and the architect with a statement of conditions to be
met, so we shall find that the principles of evolution demonstrate the
best means of meeting the circumstances of life.

Evolution has developed, like all sciences, as the method we have
described has been employed. Alchemy became chemistry when the so-called
facts of the medievalist were scrutinized and the false were discarded.
Astrology was reorganized into astronomy when real facts about the planets
and stars were separated from the belief that human lives were influenced
by the heavenly bodies. Likewise the science of life has undergone
far-reaching changes in coming down to its present form. All the principles
of these sciences are complete only in so far as they sum up in the best