es more than the third; and
that, if we continue to supply units one at a time, the last unit in
the series produces the least of all. Wages are fixed by the amount
that one unit of labor produces when the working force is complete,
and that is what is contributed to the general product by the unit of
labor which comes last in the imaginary series by which the force is
built up. Owing to the more favorable conditions under which, in their
time, the earlier units worked, they were able to produce surpluses
above the amount produced by the last one. When they entered the field
they were supplied with excessive amounts of capital. The first one
had the whole fund cooeperating with it, till it had to share it with
the second; and after that each had a half of it till they had to
share evenly with a third, etc. We have seen that all the surpluses
appearing in connection with the earlier units are attributable in
reality to capital. The area _BCD_ (page 139) represents the amount by
which the presence of an excess of capital increases the products
attributable to the earlier units of labor. It represents the sum of
all the differences between the products of the earlier units and the
product of that final one which in the end sets the standard of
productivity of labor. It might be called the rent of the fund of
capital. It is composed of a sum of differences exactly like those
which constitute the rent of a piece of land.

_The Rent of a Permanent Force of Labor._--In the figure on page 148,
the working force was supposed to be fixed in amount, the capital
increasing by increments, or as some earlier economists would have
said, by "doses" along the line _A'E'_. The last unit of capital
produces the amount _D'E'_, and all the capital produces _A'B'D'E'_,
while products of the earlier units of capital, as they come
successively into the field and are used by an excessively large labor
force, are represented by the area _B'C'D'_. Here this area represents
what may be called the rent of the force of labor, since it is a sum
of surpluses that, again, are entirely akin to those that constitute
the rent of a piece of land.

_A Question of Nomenclature._--It may be an open question, as a matter
of mere nomenclature, whether these surpluses which are thus traceable
to a permanent fund of capital, on the one hand, and to a permanent
force of labor, on the other, can with advantage be called rents. In
this treatise we do not think it