Suppose we measure the power of a scientific theory as a ratio: how much it explains divided by how much it needs to assume in order to do that explaining. By this criterion, Darwin's theory of evolution by natural selection is second to none. Think of what it explains and I really mean explains in the fullest sense of the word: your existence and mine; the form, diversity, and apparently designed complexity and elegance of all living things, not only on this planet but probably wherever in the universe organized complexity may be found. The explanatory work that the theory does, then the numerator of the ratio is immense. But the theory itself the denominator could hardly be smaller or more simple; you can write it out in a phrase: 'Non-random survival of randomly varying hereditary elements.' That isn't quite how Darwin himself would have put it (he phrased it in terms of the survival and reproduction of individual organisms). But it captures the essence of his idea in a way that I am convinced he would recognize and, indeed, enjoy if he were alive today. We can probably pare the denominator down even further, to a single essential: heredity. Given that there exists a system of high fidelity copying, two things will follow immediately: there will be a population of the entities copied; and, since no copying process is perfect (there is mutation, in other words), they won't all be exactly the same. It then follows that successful hereditary types will tend to increase in number at the expense of unsuccessful hereditary types. That is probably all that is needed for life to get going and, given enough time, to evolve living machinery whose complexity and diversity progresses without obvious limit. Consider where we would be without Darwin's idea. We'd presumably have some sort of science of biology. Students would take degrees in the subject, books would be written, Nobel prizes in physiology and medicine would be won. We might know in some detail how living organisms work; we'd know that a human body is a teeming army of cells, a thousand times more numerous than the people in the world; we'd know that every one of these cells is a mass-production moleculefactory packed with the membranous equivalent of miles of sophisticated conveyor belt. We'd know a great deal about how our bodies work, and how the bodies of shrimps, elephants and redwood trees work. We'd be compelled to recognize how complicatedly organisms are fitted to survive in their particular worlds. But we wouldn't have the foggiest idea why. We'd read volumes about living things but we wouldn't have a clue about where they came from originally nor why they work so efficiently and purposefully. It would undoubtedly be the most baffling problem in biology probably in the whole of science and the whole of philosophy. This was the problem that Darwin decisively solved. We are now as certain as one can ever be in science that a version of Darwin's solution is the correct one. The Darwinian solution to the riddle of existence is so powerfully simple, so felicitous to the modern mind, that it is hard for us to understand why it had to wait until the midnineteenth century before anyone thought of it. It is further surprising and probably telling that this great inspiration, which looks so elegant from the modern armchair, eluded centuries of philosophers, mathematicians and polymaths. Plato and Aristotle never even got close, fooling about with 'essences' and 'ideal forms'; Leibnitz and Newton gave us calculus (which might seem a more exacting intellectual achievement) but not an inkling of the reason for our own existence; Hume would surely haveDarwin, Charles is the author of 'Origin of Species and the Voyage of the Beagle And, the Voyage of the Beagle' with ISBN 9781400041275 and ISBN 1400041279.