The Wealth of Nations by Part 1 Chapter 11 Page 292

more labour than corn, come much cheaper to market. Such are potatoes and maize, or what is called Indian corn, the two most important improvements which the agriculture of Europe, perhaps, which Europe itself has received from the great extension of its commerce and navigation. Many sorts of vegetable food, besides, which in the rude state of agriculture are confined to the kitchen-garden, and raised only by the spade, come in its improved state to be introduced into common fields, and to be raised by the plough: such as turnips, carrots, cabbages, etc.

If in the progress of improvement, therefore, the real price of one species of food necessarily rises, that of another as necessarily falls, and it becomes a matter of more nicety to judge how far the rise in the one may be compensated by the fall in the other.