On the occasion of Darwin's 200th birthday, I'd like to put Darwin in his philosophical context. Most of the time we think of the empiricism of Anglophone philosophy as the doctrine that knowledge is gained through observation and experiment, an epistemological formulation that was stressed by the greatest empiricist, David Hume. But there is another important aspect to empiricism that is often overlooked, partly because empiricists themselves have tended to spurn the idea of metaphysics (Hume aimed to do away with metaphysics altogether, and the early 20th century "positivists" such as A. J. Ayer also explicitly embraced that program). Empiricism (using that label broadly: liberal Enlightenment thought, English-language philosophy since Hobbes) represents a revolution in systems dynamics: the model of transformative processes in nature, which is ultimately a cosmological topic. Explaining the persistence of the identity of a thing across changes to the properties of that thing was a basic issue for the Greeks. Heraclitus simply denied persistence, Parmenides simply denied change. The Platonic solution was to bifurcate the world into an eternally unchanging component (form) and a transient polymorphic component (matter). (And I'm not so sure whether this is all wrong, by the way.) Thus change was explained as the (metaphysically problematic) interaction of the earthly with the divine (to put it in neoplatonic Christian terms). This model persisted beyond the Christian era in the Rationalist tradition through Descartes and Spinoza to Kant and Hegel. I call this the "top-down" model: the particular states of affairs at the micro level are explained by appeal to a macro transcendental force (Platonic universals, the Christian personal God, Kant's noumenal rationality, Hegel's Absolute Spirit, etc.).
Modern empiricism's development of an alternative model (the "bottom-up" model) is, I think, one of the most important developments in the history of philosophy, perhaps the greatest revolution in thought since Plato's metaphysics. The idea is that complex systems organize themselves through the iteration of simple algorithms at the micro level. In Locke this was the organization of society through the repetition of consensual behavior of self-interested individuals (democracy). In Hume this was the organization of a system of knowledge through regularities of observation (science). The kinship between Enlightenment democracy and science cannot be overstressed. One of Darwin's principal influences was Adam Smith's Wealth of Nations, published in 1776. Smith argued that complex economies organized themselves through the iteration of exchanges between individuals. His economics is an example of an application of the law of effect: actions resulting in negative consequences tend to be extinguished, actions resulting in positive consequences tend to be reinforced (Note: I think that one of Daniel Dennett's best articles is "Why the Law of Effect Won't Go Away"). Of course this is also the basis of behaviorist and other operationalist approaches to psychology. It is also the premise of Pragmatism, a thoroughgoing empiricist development of a theory of truth. Darwin's theory of natural selection is another application of the law of effect.
What did Darwin "discover," or what "theory" did Darwin develop? He pointed out that a proof in mathematical logic applies to all transformative processes in nature: given a set of replicators, the members of that set will have an average probability of reproductive success. That goes for conspecific breeding animals, flea market swaps and jokes. Any individual member with an above-average probability will tend to have more descendants in the next generation. That's not circular reasoning, because it leaves open the question of the reasons for the above-average probability. Selection of the fittest (most adaptive) from a variegated set. This is a proof that can be formalized. It's logically valid, which is not the same thing as empirically true. That is, it's not the kind of argument that can even potentially be false. Darwin did not develop a "theory." He simply pointed out the indubitable operation of a homely truth about the world, the law of effect, and in doing so he is coming straight out of the Scottish Enlightenment thought of Hume and Smith.
One more point: the law of effect operates on all levels. That is, genes, actions, beliefs, species, tribes, nations, come-on lines, ecosystems and all manner of replicating things, biological, cultural, and otherwise, come under this principle. Thus group selection and indeed selection at any level of organization whatever occurs, as Darwin himself recognized (for example in his discussion of the altruistic warriors of Tierra del Fuego). Thus the "selfish gene" doctrine of Dawkins is false. Not a question of science, it is a question of logic.
Happy birthday Charlie!