A central contention of this book is that our understanding of biology, and the very nature and history of living things, hinges crucially on our understanding of development as the basic biological process. Development is what distinguishes biological systems from other sorts of systems, and it is the material source of evolutionary change.

In unpacking my claims about development, it is useful to explore a number of metaphors used in attempts to explain development: these include ‘information’, ‘programme’, and ‘triggering’. Rather than addressing these in turn, I instead offer an account of how these metaphors, and others, come together in what I call the ‘modern consensus’ on development.

Our very sense of biological possibilities, and of the nature of gene action and activation, is constrained by our conceptions, whether implicit or explicit, of epigenesis and preformation. The idea of epigenesis has a rich history dating back to Aristotle, and it is typically understood as the antipode of ‘preformation’; thus, even though there are no pure preformationists or epigenecists still with us, both epigenesis and preformation will initially occupy me in this chapter.