Darwinian Populations and Natural Selection is a superb work, introducing an important analytical technique, and applying it to a range of difficult and contested issues within evolutionary theory. The core idea of the book is that of a Darwinian Population (and, derivatively, a Darwinian individual). In developing this concept, Peter Godfrey-Smith builds on a long-standing tradition within evolutionary theory, that of developing a spare, bare-bones specification of the machine of selective change, both to better articulate the core causal processes of biological evolution, and to explore the possibility that selective regimes explain change in other domains (see also . Godfrey-Smith's ingredients are familiar: Darwinian Populations are populations of interacting individuals that (potentially) reproduce; that vary from one another in ways that sometimes influence their reproductive potential; and when reproduction does take place, offspring resemble their parents.Nothing new here, but Godfrey-Smith handles these ingredients in a distinctive way. First, everyone recognises that heritability-the tendency of offspring to resemble their parents-comes in degrees. Godfrey-Smith explicitly extends this recognition of gradients to the other constituent features of Darwinian Populations. He shows that there are clear and less clear cases of reproduction. For example, with asexual clone lines, the distinction between growth and reproduction is complex and contested (Dawkins 1982). Similarly, with social groups, there is no obvious dividing line between cases in which the individuals within the group have reproduced, and cases in which the group itself has founded colonies. So Godfrey-Smith