The Wealth of Nations by Part 1 Chapter 11 Page 220

uncultivated countries, nature produces with such profuse abundance that they are of little or no value, and which, as cultivation advances are therefore forced to give place to some more profitable produce.

During a long period in the progress of improvement, the quantity of these is continually diminishing, while at the same time the demand for them is continually increasing. Their real value, therefore, the real quantity of labour which they will purchase or command, gradually rises, till at last it gets so high as to render them as profitable a produce as anything else which human industry can raise upon the most fertile and best cultivated land. When it has got so high it cannot well go higher. If it did, more land and more industry would soon be employed to increase their quantity.