¹ The successive growing-in of regional populations, for instance, gives some indication of increments to the natural resource base. Subject to the assumptions of constant factor proportions in similar production processes throughout the USA, of constant labor force participation from a given population, of reasonably similar areal occupational structure, the measure holds fairly well. Further as the additions to total population by decade and newly incorporated region fall off so rapidly towards the end of the 19th century, the assumptions are not damaging for if we could adjust for them the amount of resource injection would still fall rapidly.

² Shannon, Fred A., “A Post Mortem on the Labor-Safety-Valve Theory,” The Turner Thesis, Taylor, George Rogers, ed. (Boston: D. C. Heath, 1949), p. 52.Google Scholar

³ We quoted Professor Higgins because he stressed sequential movement into broad production possibilities.

⁴ Charles Kindelberger, International Economics (Revised Edition), p. 127 doubts whether the supply of land increased after 1860. Even were this true spatial expansion permitted substitution of parcels of the factor of production one for another.

⁵ See Robertson, Ross M., History of the American Economy (New York: Harcourt Brace, 1955). PP. 357–67.Google Scholar