 For helpful suggestions, we are grateful to Mark Alfano, C. Daniel Batson, William J. FitzPatrick, Adam Lerner, Joshua May, Samir Okasha, Gualtiero Piccinini, Alejandro Rosas, Thomas Schramme, Armin Schulz, Elliott Sober, Kim Sterelny, Valerie Tiberius and David Sloan Wilson. Our thanks to Zhao Wang who helped assemble and check the references.