INTRODUCTORY 
21 
value of goods by small changes in their 
supply, the differences made to the worth 
of capital by slight accessions to its quan 
tity, the differences made to the burden of 
labour by some lengthening of the hours of 
work, and so forth. The marginal theory has 
been caustically described as the theory that 
the tail wags the dog. There is a certain 
truth in the remark; but the intended jibe, 
instead of nailing a fallacy to the counter, 
draws attention to a highly significant fact. 
In the determination of value it is the last 
steps that count—the last step on the side 
of demand, and the last step on the side of 
supply. 
Foreshadowings and even partial applica 
tions of the marginal theory may be dis 
covered in the old economics ; but it is 
one thing to understand the essential nature 
of a particular kind of causation and detect 
its workings in apparently dissimilar actions 
and reactions, and another thing to give now 
and then a reason for phenomena implying the 
same kind of causation, without being alive 
to its peculiarity and far-reaching influence. 
“Demand” and “supply” used to be the 
magic wands for solving all economic problems, 
—and in the worst days of error they were 
indiscriminately applied to everything “from