Sec. 4] : UTILITY 45
whatever. Reference may be had to a specified group of
goods now existing, or to a specified group of goods in the
future, or to a specified flow of goods through a period of
time. For instance, the marginal desirability of coal to an
individual may be taken to refer to the particular stock of
coal in his bin at the present moment. If this stock con-
sists of fifteen tons, its marginal desirability is the desir-
ability of the fifteenth ton, or the difference to him between
the desirability of having fifteen and that of having four-
teen tons. Or, reference may be had to an intended pur-
chase of coal to be delivered in three months. If we con-
sider a possible purchase of future coal to the extent of
fifteen tons, its marginal desirability then represents the
present desire for the fifteenth ton, in exactly the same
way as though reference were had to an existing stock.
Again, if a person is consuming in his household fifteen tons
of coal a year, its marginal desirability at any instant is
the desirability of the fifteenth ton, or the sacrifice which
would be occasioned were he to reduce his yearly consump-
tion from fifteen tons to fourteen.
Again, the group of goods considered may consist of ar-
ticles all of which are of the same kind, or of a heterogeneous
collection. In the preceding examples the goods were of
exactly the same kind. As an example of the marginal
desirability of a group consisting of diverse kinds, we
may cite the desirability of an additional monthly magazine
or newspaper. If a subscriber is already taking ten periodi-
cals of different kinds, the desirability of a specified journal
additional to the existing assortment may be regarded as
the marginal desirability with reference to the entire group
of journals.
In the same way we may speak of the marginal desir-
ability of a series of characteristics or features connected with
any article or articles of wealth. A person contemplating
the building of a house may have to decide how many win-
dows he will putin. If he contemplates fifty windows, the
