If four portions of capital, be employed on the land, giving each the same results, and any improvement in the formation of such capital should enable me to withdraw from each, so that they should be and no alteration would take place in the corn rent; but if the improvements were such as to enable me to make the whole saving on the largest portion of capital, that portion which is least productively employed, corn rent would immediately fall, because the difference between the capital most productive and the capital least productive would be diminished; and it is this difference which constitutes rent.