   In a recent monograph entitled Natural Selection: Domains, Levels and Challenges in the Oxford Series in Ecology and Evolution, George C. Williams has vigorously defended Darwinian selection theory against a variety of challenges that have emerged over the last few decades. Those challenges can be placed into two broad categories: [i] proposed limitations on natural selection as an evolutionary force; and [ii] expansions of the scope of natural selection to include new ‘targets’ and ‘levels’. It will be noted that in neither case is it obvious that the theory itself requires modification in the face of such challenges—in principle these might be nothing more than challenges to the theory’s range of application.  However, if it turned out that most evolutionary change could be explained without recourse to natural selection, this would be grounds for arguing that evolutionary biology was no longer Darwinian. And if it turned out that the theory of natural selection could only be integrated with our new understanding of the processes of inheritance and development by a wholesale modification of its foundations, it might be best to see the new theory as a modified descendent of Darwinism, rather than Darwinism itself. Theories may need essences, as Gould claims; but if what is fundamental to the theory has changed, then so has its essence. To borrow a phrase from Paul Griffiths, perhaps it is not that theories need histories and essences—perhaps what they need are historical essences.