DEMAND AND VALUE

31

utility of each and every amount supplied. The two kinds
of change are quite distinct in nature, and are worth distinguishing
 in discussing the problems which are presented
for consideration in practice. The one kind of change may
occur simultaneously with the other, and often does so, but
confusion is likely to arise if the explanation of observed
facts be sought in a reference to one of these causes of change
alone, especially if the other cause should happen to have
been the more important in the actual case presented.