Land, in the economic sense, does not increase in quantity, however changeful and progressive a society may be. The chief distinguishing mark of land--that of being fixed in amount--separates it from other things only in a dynamic state and because of the action of the forces which produce organic changes. These are subjects to be studied in the dynamic division of economic theory. _A Distinguishing Mark of Land which appears in a Static State of Industry._--In a static state there remains this difference between a piece of ground and a building, a tool, or any other instrument: the ground is not artificially made and does not perish in the using; while the building or the tool or other appliance is so made and does so perish. It must in wearing itself out create in the indirect way which we have described its own successor. The engine must, by a part of its product, pay the men who will make another engine and so perpetuate the series of engines. This makes it necessary for the owner of the engine to save some of its gross rent to pay for depreciation and renewal, while he can safely use the whole rent of land. _This Mark of Distinction not Applicable when Land is contrasted with a Permanent Stock of Capital Goods._--If we look, not at one particular instrument, but at an entire series of them,--if we take into view, not only the engine which is now driving the mill, but also the one that will succeed it, and again the one which will succeed that second engine, and so on forever,--this difference between land and the artificial instrumentality vanishes. _The series of engines, like land itself, yields only a net rent._ The remainder of its gross product is not a true rent at all, since any one of the engines creating it has to consume it on itself and cannot give it to the owner as an income. This remainder pays certain men for keeping the series of engines intact, and what is given to them as pay for their services cannot accrue to any one as an income from the series of instruments so maintained. It is the earnings of the corps of maintenance created by their own labor and capital. What the series of engines yields over and above what it expends in maintaining itself it gives to its owners as an income. This is their net return and they can use it without trenching on their property. The analogy between the returns from land and those from a self-perpetuating series of made capital goods is in this particular complete.