3. _In a series of consumption goods of different qualities, the lower grades acquire value only as scarcity increases in the higher grades._ If difference in quality between two grades of apples is marked and there is a superabundant supply of the best grade, no importance is attached to the poorer. But if the better grade becomes scarce, the appetite for the poorer grade increases, and finally it, too, will be consumed. In some years the small, knotty apples are allowed to rot on the ground; in other years they are gathered and are sold at good prices. But if there is an abrupt difference in quality, and hence in the marginal utility of the two grades, the value of the better goods may rise considerably before there is any recourse to the poorer. If the differences in quality are very slight, the presence of the lower grades has the effect of limiting the increase of value of the higher grades. Practically in almost all kinds of goods there are gradations in quality. Complete uniformity is of the rarest occurrence. When did one ever see a basket of peaches that were all of the same size, ripeness, color, flavor, and perfection? If the step from the higher to the lower grade is very slight, resort is immediately made to the next lower grade, some of which is substituted for the higher. There is an independent reason for the value of each grade of goods; each grade would have value if there were none of the other, but they mutually affect each other's value when they exist, side by side, in the same market. The marginal utility of each is lessened by the presence of the other. And thus, two or ten grades constitute for many purposes a single supply as they shade into each other or are merged by substitution. [Illustration: _Grades of Consumption Goods by Quality_] [Sidenote: Free goods are on the margin of utilization] 4. _Goods of the lowest grades, having no marginal utility, are free goods._ This is a simple truth, but it has important bearings. There may be said to be an "extensive margin of utilization" of many consumption goods. The poorer grades of apples, rotting on the ground, the multitudes of waste things not valued, are on the margin of utilization. When a lower grade is used, the margin is extended. The value of goods is measured upward from the margin of utilization, but this is simply to say that their value is measured from zero upward. Likewise, there is an intensive marginal utility in consumption goods. As the better grade of apples becomes more scarce, they will be used more sparingly and kept to satisfy only the intenser wants. The superiority of some consumption goods, either in quantity or quality, often is exactly analogous to the "differential advantage" spoken of by economists in the case of productive agents. The differential advantage of the highest grade over the grade of free goods, whose value is zero, evidently is the whole value of the highest grade. § II. DIFFERENTIAL ADVANTAGES IN INDIRECT GOODS