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tuted for any of quality A, it must be at the best 
that of quality B ; and, therefore, the inferior 
will have to be substituted for the superior. 
Such a substitution would not prove profitable 
until the rent of land A became something 
greater than what would be lost by evacuating 
some of A and occupying in its place some of 
B. Hence rent of land A can be no more than 
10 bushels an acre. And, broadly speaking, 
it can be no less, because the competition of 
farmers for the better land would force up 
its annual value until the substitution of 
land B for land A (in view of the rent of land 
A) was a matter of indifference. 
Let us now take a further step in the 
demonstration and suppose that the demand 
for the produce of the soil is such that some of 
land C must be occupied. Then eventually the 
rent of B would be 15 bushels, the difference 
between the yield of B, 50 bushels, and the 
yield of C, 35 bushels ; and the rent of 
A would be 25 bushels per acre, the difference 
between its yield, 60 bushels, and the yield of 
C, 35 bushels. Finally when D, which bears 
15 bushels an acre, is tilled, C with a produce 
of 35 bushels an acre will bear a rent of 20 
bushels, that is 35 minus 15. B, with its output 
of 50 bushels an acre, will earn a rent of 35 
bushels, that is 50 minus 15 ; and the rent of