nt
implies artificial arrangement), and the process is too slow for
observation, is, nevertheless, to some extent confirmed by the practice
of gardeners and breeders of animals: since, by taking advantage of
accidental variations of form and colour in the plants or animals under
their care, and relying on the inheritability of these variations they
obtain extensive modifications of the original stocks, and adapt them to
the various purposes for which flowers and cereals, poultry, dogs and
cattle are domesticated. This shows, at least, that living forms are
plastic, and extensively modifiable in a comparatively short time.

Sec. 4. Suppose, however, that, in verifying a deductive argument, the
effect as computed from the laws of the causes assigned, does not
correspond with the facts observed: there must then be an error
somewhere. If the fact has been accurately observed, the error must lie
either in the process of deduction and computation, or else in the
premises. As to the process of deduction, it may be very simple and
easily revised, as in the above explanation of the common pump; or it
may be very involved and comprise long trains of mathematical
calculation. If, however, on re-examining the computations, we find them
correct, it remains to look for some mistake in the premises.

(1) We may not have accurately ascertained the laws, or the modes of
operation, or the amounts of the forces present. Thus, the rate at which
bodies fall was formerly believed to vary in proportion to their
relative weights; and any estimate based upon this belief cannot agree
with the facts. Again, the corpuscular theory of light, namely, that
the physical cause of light is a stream of fine particles projected in
straight lines from the luminous object, though it seemed adequate to
the explanation of many optical phenomena, could not be made to agree
with the facts of interference and double refraction.

(2) The circumstances in which the agents are combined may not have been
correctly conceived. When Newton began to inquire whether the attraction
of the earth determined the orbit of the moon, he was at first
disappointed. "According to Newton's calculations, made at this time,"
says Whewell, "the moon, by her motion in her orbit, was deflected from
the tangent every minute through a space of thirteen feet. But by
noticing the space which bodies would fall in one minute at the earth's
surface, and supposing this to be diminished in the