not why they rise just where we find
them: Economists explain the general course of a commercial crisis, but
not why the great crises recurred at intervals of about ten years.

Derivative Laws make up the body of the exact sciences, having been
assimilated and organised; whilst Empirical Laws are the undigested
materials of science. The theorems of Euclid are good examples of
derivative laws in Mathematics; in Astronomy, Kepler's laws and the laws
of the tides; in Physics, the laws of shadows, of perspective, of
harmony; in Biology, the law of protective coloration; in Economics, the
laws of prices, wages, interest, and rent.

Empirical Laws are such as Bode's law of the planetary distances; the
laws of the expansion of different bodies by heat, and formulae
expressing the electrical conductivity of each substance as a function
of the temperature. Strictly speaking, I suppose, all the laws of
chemical combination are empirical: the law of definite proportions is
verifiable in all cases that have been examined, except for variations
that may be ascribed to errors of experiment. Much the same is true in
Biology; most of the secondary laws are empirical, except so far as
structures or functions may be regarded as specialised cases in Physics
or Chemistry and deducible from these sciences. The theory of Natural
Selection, however, has been the means of rendering many laws, that were
once wholly empirical, at least partially derivative; namely, the laws
of the geographical distribution of plants and animals, and of their
adaptation in organisation, form and colour, habits and instincts, to
their various conditions of life. The laws that remain empirical in
Biology are of all degrees of generality from that of the tendency to
variation in size and in every other character shown by every species
(though as to the reason of this there are promising hypotheses), down
to such curious cases as that the colour of roses and carnations never
varies into blue, that scarlet flowers are never sweet-scented, that
bullfinches fed on hemp-seed turn black, that the young of white, yellow
and dun pigeons are born almost naked (whilst others have plenty of
down); and so on. The derivation of empirical laws is the greater part
of the explanation of Nature (Sec.Sec. 5, 6).

A 'Fact,' in the common use of the word, is a particular observation: it
is the material of science in its rawest state. As perceived by a mind,
it is, of course, never ab