The term "social Darwinism" owes its currency and many of its connotations to Richard Hofstadter's influential Social Darwinism in American Thought, 1860-1915 (SDAT). The post-SDAT meanings of "social Darwinism" are the product of an unresolved Whiggish tension in SDAT: Hofstadter championed economic reform over free markets, but he also condemned biology in social science, this while many progressive social scientists surveyed in SDAT offered biological justifications for economic reform. As a consequence, there are, in effect, two Hofstadters in SDAT. The first (call him Hofstadter(1)) disparaged as "social Darwinism" biological justification of laissez-faire, for this was, in his view, doubly wrong. The second Hofstadter (call him Hofstadter(2)) documented, however incompletely, the underside of progressive reform: racism, eugenics and imperialism, and even devised a term for it, "Darwinian collectivism." This essay documents and explains Hofstadter's ambivalence in SDAT, especially where, as with Progressive Era eugenics, the "two Hofstadters" were at odds with each other. It explores the historiographic and semantic consequences of Hofstadter's ambivalence, including its connection with the Left's longstanding mistrust of Darwinism as apology for Malthusian political economy. (C) 2009 Elsevier B.V. All rights reserved.