This is the best, most thought-provoking book in the philosophy of biology that I have read in a long time. It is rigorously argued, deeply informed, full of wonderful examples, and it has more novel ideas within its pages than any two other recent books in the field that I can think of. It has opened my mind on several key issues and changed my mind as well; it is also provoked me to come up with what I hope are better defenses of some of the points of my position criticized in the book. One of the most important things we have learned from Darwin (but some philosophers, alas, still don’t get it), is that essentialism is simply a mistake. There is no mystery about why many philosophers resist this verdict: their method, going back to Socrates, demands exceptionless definitions and self-evident axioms, from which deductive consequences can be made to flow. Philosophers are not alone in their weakness for essentialism. Even evolutionary theorists have often succumbed to the temptation to define the essence of Darwinism, the necessary and sufficient conditions for natural selection to occur. For one thing, this makes for effective pedagogy. But a good Darwinian should attempt to honor Darwinism about Darwinism itself—in Glenn Adelson’s apt phrase—and Peter Godfrey Smith (henceforth PGS) shows us how to think about all the many marginal cases—the semi-Darwinian processes, proto-Darwinian phenomena, quasi-Darwinian relationships—without marginalizing them. Therapsids, after all, were just as real as their ancestor reptiles and their mammalian descendants, and the question of whether viruses are alive (or not—do we have to fish or cut bait?) is less important than seeing which Darwinian features viruses share with bacteria, with aspen groves, with us— and why. PGS sets out with a deliberately neutral, maximally latitudinarian, basic category, Darwinian populations. Literal populations of organisms are populations,