[Sidenote: Differential advantage of agents in the quality of their products] 1. _Rent varies with the quality of the products yielded by agents, other things being equal._ Let us take first a simple case where the agent is the sole condition of the product. If there is but one tree bearing a certain luscious fruit, or but one spring yielding a mineral water, the rent of the tree or spring being equal to the value of the products must vary as the quality of the products varies. If two or more trees are standing side by side, they will be compared with regard to the difference in the quality of their fruits. If two fields differ in quality, greater importance will be attached to the field capable of producing the better grade or variety of fruit or product. A peculiar mineral quality in the soil may impart to wine a choice flavor that can at once be recognized by experts; while other fields, distant but a few rods, cannot by any effort be made to produce wine of the same rare quality. There is said to be a marked difference in the success of vineyards lying only a short distance apart on the shores of the larger lakes of New York. Nearness to the water moderates the temperature, often prevents frosts, and hence insures the ripening and quality of the fruit. In the Santa Clara valley, as in other parts of California, there is a frostless belt, sharply marked off from the lands where it is unsafe to attempt to cultivate the delicate orange-tree and other semi-tropical plants. In manifold ways differences in geological formation affect the use of land and the success of many industries. On one side of a little creek is limestone land, on the other shale, the limestone producing a crop larger and of better quality. When the peculiar nature of the one field is found to be the cause of the exceptional quality of its fruits, the difference in value is attributed to it. [Sidenote: The lower grade limits the value of the higher grade]