Eyes have long fascinated those who study the natural world. Cleanthes - the natural theologian protagonist of Hume's Dialogues Concerning Natural Religion - invites his interlocutor to 'consider, anatomize the eye: Survey its structure and contrivance; and tell me, from your own feeling, if the idea of a contriver does not immediately flow in upon you with a force like that of sensation' (1990, 65). Darwin, too, counted the eye among what he called 'organs of extreme perfection'. Placing himself squarely within the tradition that runs from natural theology, through Darwin, to a certain style of modern biology, Maynard Smith writes that 'the main task of any theory of evolution is to explain adaptive complexity, that is, to explain the same set of facts that Paley used as evidence of a creator' (1969, 82). More recently still, Dawkins (1986) is impressed, also, with a force like that of sensation, by how well suited - how well adapted, that is - the eye is to its purpose. Like Paley, he thinks eyes are better pieces of work than watches, although unlike Paley he regards their artificer as blind.

An essay on adaptation could fill volumes. One might begin by asking how adaptation is to be explained. Immediately we would need to answer the prior question of what the proper definition of adaptation is, and we would also have to get clear on the nature of the diverse candidate processes - natural selection, self-organisation, macromutation, development, divine design - sometimes tabled as potential explanations.