European predecessors of ecological economics include K. William Kapp (1950)[14] Karl Polanyi (1944),[15] and Romanian economist Nicholas Georgescu-Roegen (1971). Georgescu-Roegen, who would later mentor Herman Daly at Vanderbilt University, provided ecological economics with a modern conceptual framework based on the material and energy flows of economic production and consumption. His magnum opus, The Entropy Law and the Economic Process (1971), is credited by Daly as a fundamental text of the field, alongside Soddy's Wealth, Virtual Wealth and Debt.[16] Some key concepts of what is now ecological economics are evident in the writings of Kenneth Boulding and E.F. Schumacher, whose book Small Is Beautiful – A Study of Economics as if People Mattered (1973) was published just a few years before the first edition of Herman Daly's comprehensive and persuasive Steady-State Economics (1977).[17][18]