more certain and more marked, the more difficult it is to vary the
proportions in which the things are supplied.


§7. _Composite Supply and Composite Demand_. Joint Demand and Joint
Supply do not complete the list of relations between the demand and
supply of different things. Between tea and coffee, or beef and mutton
there is a relation of a different kind. These things are in large
measure what we call "substitutes" for one another. An increased
supply, and a lower price of mutton, will probably induce us to
consume less beef. This relation it is convenient to describe as
Composite Supply. Beef and mutton make up a composite supply of meat;
tea and coffee a composite supply of a certain type of beverage. For
any group of things, between which the relation of Composite Supply
exists, we can say, with complete generality, that an increased supply
of one of them will tend to diminish the demand for the
others. Parallel to the relation of Composite Supply is that of
Composite Demand. There are frequently several alternative uses in
which a commodity or service can be employed; and these alternative
uses make up a composite demand for the thing in question. Thus
railways, gasworks, private households and a great variety of
industries contribute to a Composite Demand for coal. It is worth
noting that there is frequently an association in practice between
Joint Demand and Composite Supply on the one hand; and between Joint
Supply and Composite Demand on the other. Wool and mutton, for
instance, we have described as an instance of Joint Supply; but, in so
far as the proportions of wool and mutton can be varied, we can regard
these things as constituting a Composite Demand for sheep. And this
conception may help us to retain a clearer and more orderly picture of
the problems we have discussed above. We can regard the fact that wool