PART III.—Of the variations in the Proportion between the respective Values of that sort of Produce which always affords Rent, and of that which sometimes does, and sometimes does not, afford Rent. The increasing abundance of food, in consequence of the increasing improvement and cultivation, must necessarily increase the demand for every part of the produce of land which is not food, and which can be applied either to use or to ornament. In the whole progress of improvement, it might, therefore, be expected there should be only one variation in the comparative values of those two different sorts of produce. The value of that sort which sometimes does, and sometimes does not afford rent, should constantly rise in proportion to that which always affords some rent. As art and industry advance, the materials of clothing and lodging, the useful fossils and materials of the earth, the precious metals and the precious stones, should gradually come to be more and more in demand, should gradually exchange for a greater and a greater quantity of food; or, in other words, should gradually become dearer and dearer. This, accordingly, has been the case with most of these things upon most occasions, and would have been the case with all of them upon all occasions, if particular accidents had not, upon some occasions, increased the supply of some of them in a still greater proportion than the demand.