204 
POLITICAL ECONOMY 
when rents were created for land of quality D, 
the rents of fields of higher quality would 
advance, and the amounts by which they 
advanced would exactly equal the amount of 
the rent which had to be paid for D, on the 
assumption that the quantity of wheat obtain 
able from each kind of land is constant. 
In the paragraph above I have spoken of 
any annual payment for land of quality D as a 
rent, but it is only correct to call it so if 
rent embraces every payment for land, how- 
every determined, which is not a recompense 
for any capital invested in it. Should we 
elect to confine the conception of rent to 
payments for the differential advantages 
possessed by the super-marginal things of a 
class, then the payment made for land of 
quality D is not a rent, because it is not a 
payment for such differential advantages. 
It is, on the contrary, a payment which 
expresses the marginal worth#of land. Now 
land can only have a marginal worth when 
it is limited in quantity so that nobody can 
get as much of it as he likes. When the 
amount of land is unlimited with reference 
to the population, it is naturally used 
until its marginal worth becomes zero, just 
as air is, and water when the water can 
be obtained direct from nature and is not