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. [Sidenote: 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. [Illustration: _Grades of Agents by amount of Product of Uniform Quality_] [Sidenote: Complementary agents unite to form a product]