_To what Extent the Movement of Labor and that of Capital are Interdependent._--The early statements of the law of rent did not usually define the intensive margin of cultivation in connection with labor and capital separately, but spoke of these two agents as employed together upon land in quantities increasing up to a limit beyond which both labor and capital would best be employed elsewhere. The supposition that labor and capital go thus together from one grade of land to another is only approximately accurate. If we consider one man and five hundred dollars' worth of productive wealth as a dual unit of labor and capital, and add such units, one after another, to the forces at work on a tract of good land, we shall reach a point at which it will not be profitable to increase the amount of one of the agents, while it will still be profitable to increase the amount of the other. It will perhaps not pay to use any more capital, but it may still pay to add to the number of workers. On land that is tilled more and more intensively, labor and capital are not tied together in fixed proportions in such a way that, when there is more of one of them used, there is _proportionately_ more of the other. Moreover, when a unit of one of them abandons a piece of land and goes elsewhere, there is no probability that exactly one unit of the other will do the same. There is, indeed, no such thing as a dual unit of labor and capital that can be thought of as moving to and fro among different employments till it finds the point at which, as a dual unit, it can create its largest product. These two agents so locate themselves that a final unit of each one, separately considered, produces as much where it is as it can produce anywhere else.