54 ECONOMIC PRINCIPLES

arise from changes of taste or of habit, and education and
surroundings constantly produce profound changes of this
kind. The influence of judicious advertisement must be
sought, in great part, in the fact that it induces a belief in
the utility of a commodity in minds previously not much
impressed with that belief. As to habit, the fact that a
demand for a commodity can be relied upon to last, if the
habit of using it be once formed, is sufficiently well known,
and that efforts to effect changes in such habits are made by
astute dealers is also well known. When such changes in
the estimated utility of a commodity have taken place,
unless a corresponding change has taken place in its supply,
a change of value is sure to result, in fact this change is the
index of the change of utility. The gradual growth of the
habit of using tramcars requires, for example, a great increase
of accommodation to carry all who wish to travel, especially
if the desire to travel is stimulated by lowering fares. The
growing habit of desiring to use a tramcar probably counts
for at least as much as the cheap rates nowadays demanded
for that accommodation, and the improvement of the accommodation
 itself.
Changes in conditions of supply arise from new discoveries,
 either as to natural sources of necessary elements
for the production, or from inventions affecting the elaboration
 of those elements, improvements in chemical or
mechanical processes, for example, or in business organisation.
Changes tending to restrict a supply may, of course, also
take place.
A value once established for a commodity, is not, then,
something permanent in its nature. If the term “normal
value” be applied to it, it is only so long as the conditions
of which it expresses the normal outcome are unchanged.
Normal values undergo progressive changes as time passes.
This is very necessary to bear in mind, because of an
involuntary tendency to regard a normal value as a point in
some way fixed. What is normal under one set of conditions
may be far other than normal under a changed régime.