EXCHANGE AND MARKETS 33

five plums. At this rate, suppose three apples to be given
for fifteen plums. This exchange appreciably diminishes
the store which each has of the fruit he gathered, and, at
the same time, diminishes the eagerness to obtain some of
the other kind. Perhaps, at the start, A would have been
willing to accept two plums for an apple, while B would have
been willing to give as many as twelve in order to procure
an apple. When B has procured three apples, he no longer
cares so much to have more that he would give twelve plums
for another apple, but would be quite ready to give nine
each for one or two more. A, however, has secured a fair
supply of plums, and does not care to accept for further
apples fewer than six plumseach. A new rate of seven plums
for an apple presents advantage to both, and two more
apples may change hands at this rate. After this, B does
not care for more apples at any rate less than eight plums
for an apple, but, if A wishes for more plums at this rate, one
more apple may be exchanged for eight plums. B has now
six apples, and his store of plums has been seriously diminished,
 He will not give more than seven plums at the most
for even one more apple. A has thirty-seven plums, and his
apples are fewer by six than at the start. He may have had
a dozen in all, while B had, say, sixty plums. A will not
part with any further apples at a less rate than ten plums
for an apple. Further exchange is, therefore, out of the
question. The marginal utility of apples has risen for A,
while that of plums has fallen, and vice versa for B, For
zach, the marginal utility of the purchased fruit, which
started by being high, and that of his own stock, which was
low at first, have become approximately equal. It is to be
remarked that no comparison is made between the utilities
of either fruit to the two boys, but only between the utilities
of the two fruits to A and to B respectively.
This exchange might have proceeded differently if,
instead of beginning at a rate of five to one it had begun
at a rate of, say, twenty-five plums for three apples. At
this rate, suppose an exchange of three apples to be
made. After this exchange, A may be willing to take
more plums at any rate not less than five for an apple,