If there is but one grade of agent, it is, of course, valued without reference to any lower grade. The effect of the presence of lower grades of agents is to lower the value of the higher, inasmuch as the lower grades are substituted for the higher. There may be at first enough of the higher grade of agents to produce all the fruit wanted of the better quality. If, then, there is an increasing demand, and the additional yield can be secured only with greater effort, the value of the product will rise. The presence of poorer grades, however, checks that rise, because use can be shifted to them. The value of grade one is not high because grades two, three, and four, which are worse than it, are available, but because they are not of better quality than they are. Poor as they are, their presence reduces somewhat the intensity of demand for the best grade. Indirect agents, therefore, are seen to be subject to just the same comparisons, substitutions, and estimates, when their value is considered, as are direct consumption goods.
Differential advantage of agents in the amount of their products
2. The rents of two agents differ as do the quantities of goods yielded by them, other things being equal. In the case just considered, the quantity remained the same while the quality differed; now is to be considered the case where the quantity differs while the quality remains the same. It is possible that one grade of agents is "poorer" because it produces less fruit, not fruit of poorer quality. Consider first the static problem. If both agents yield fruits exactly alike, the value of equal units at the same place and time must be equal, and the usufructs would vary in just proportion with the quantity of product. Now consider the dynamic problem. If the desire for that fruit increases, rent would grow as scarcity became more felt. The agents yielding, under the prevailing conditions, the largest product, would first be used; later, the poorer agents. The possibility of resorting to the poorer agents would keep the better from rising so high.
Grades of Agents by amount of Product of Uniform Quality
Complementary agents unite to form a product
3. When two agents are necessary to secure a product, the value attributed to each is influenced by competing uses. The thought of one agent independently producing a certain product is far too simple to correspond with reality. Two or more agents unite to produce a single product, and each agent at the same time can be used for acquiring other products. Complex as the problem appears, it is solved according to the principle of marginal utility at every moment in every market. The different uses, figuratively speaking, bid for an agent, and thus its marginal utility is determined just as is the price of a good by the bidding of buyers. Indeed, it is the bidding of buyers, indirectly. The more urgent the use, the higher the bid. The felt importance is reflected from the consumption goods that are sought, to the agent that will aid to get them. Two or more agents that are mutually needed for the acquiring of a product are complementary goods. A complementary agent may be either other material agents or labor.
Complementary agents used intensively show diminishing returns
When labor is applied to an agent, either to improve the Quality or to increase the quantity, it is subject to the law of diminishing returns. In the effort to increase the quantity of products, labor is applied first more intensively to the better agents. If it meets with resistance, if returns diminish, it is transferred to any of the poorer agents that have in them uses of as high grade as those still in the better agent. The superior effectiveness of the earlier over the later units of the added agent is called the "differential advantage" of the two fixed agents. The result of a day's labor applied to a field may be represented by 100, a second day's labor by 90 (it being only ninety per cent, as effectual), a third day's labor by 75; but it is more usual to say that the first field produces 10 more than the second and 25 more than the third, the second 15 more than the third. To the agent fixed in supply is attributed the difference in the effectiveness of the agent that is applied.