 Lo 2001 provides an overview and critique of Callicott’s changing position over two decades, while Ouderkirk and Hill (eds.) 2002 gives an overview of debates between Callicott and others concerning the metaethical and metaphysical foundations for the land ethic and also its historical antecedents. As Lo points out, the final modified version of the land ethic needs more than two second-order principles, since a third-order principle is needed to specify Callicott’s implicit view that the second second-order principle generally countermands the first one when they come into conflict (Lo 2001, 345). In later work, Callicott follows Lo’s suggestion, while cautioning against aiming for too much precision in specifying the demands of the land ethic (Callicott 2013, 66–7). While Callicott’s reading of Leopold is widely regarded as authoritative, later writers have queried whether Leopold might be better interpreted a a moral pluralist (Dixon 2017) and have also raised doubts about the form of Darwinism that Leopold is supposed to have espoused (Millstein 2015). For further critique of Callicott on Leopold, see also Newman, Varner and Linquist 2017, ch.10. 