John Bates Clark,
Essentials of Economic Theory
“ Summary of Facts concerning the Economic Center.—We may, then, regard a certain limited part of the world as a society in itself. It is modified by its environment, but, in an important sense, it has a self-contained life. The economic changes which go on within it can be grouped under the five generic heads: increase in the amount of labor, increase in the quantity of capital, improvement of method, improvement in organization, and changes in the wants of the individual consumers. ”
