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theory of rent, which holds in this particular 
case, holds also of payments for differential 
advantages of quite another kind. The 
foregoing demonstration is in fact an 
exposition by the method of example of the 
general theory of rent when rent is understood 
as payment for differential advantages. It 
will be comprehended that the same forces 
come into play when human factors in 
production are being dealt with as well as 
when land is being dealt with. Just as there 
are differences in respect of fertility between 
different plots of land, so there are differences 
in respect of capacity, industry and quickness 
between different persons. Take the case 
of a particular industry, and, to make it a 
strong case, suppose that those who work in 
it must have special ability. Naturally it is 
impossible to obtain for the industry an 
unlimited supply of labour of the highest 
quality. Employers must have recourse to 
labour of lower qualities if the trade is to 
attain to any magnitude, just as farmers 
must have recourse to some land which is not 
of the best sort. The least efficient operative 
may be termed the marginal operative, just 
as the least valuable land may be termed the 
marginal land. Through the working of the 
principle of substitution it is plain that the