Extracted from a Washington Post article 24th August 2011 by Richard Dawkins (Article Link here)
The power of a scientific theory may be measured as a ratio: the number of facts that it explains divided by the number of assumptions it needs to postulate in order to do the explaining. A theory that assumes most of what it is trying to explain is a bad theory. That is why the creationist or ‘intelligent design’ theory is such a rotten theory.
What any theory of life needs to explain is functional complexity. Complexity can be measured as statistical improbability, and living things are statistically improbable in a very particular direction: the direction of functional efficiency. The body of a bird is not just a prodigiously complicated machine, with its trillions of cells – each one in itself a marvel of miniaturized complexity – all conspiring together to make muscle or bone, kidney or brain. Its interlocking parts also conspire to make it good for something – in the case of most birds, good for flying. An aero-engineer is struck dumb with admiration for the bird as flying machine: its feathered flight-surfaces and ailerons sensitively adjusted in real time by the on-board computer which is the brain; the breast muscles, which are the engines, the ligaments, tendons and lightweight bony struts all exactly suited to the task. And the whole machine is immensely improbable in the sense that, if you randomly shook up the parts over and over again, never in a million years would they fall into the right shape to fly like a swallow, soar like a vulture, or ride the oceanic up-draughts like a wandering albatross. Any theory of life has to explain how the laws of physics can give rise to a complex flying machine like a bird or a bat or a pterosaur, a complex swimming machine like a tarpon or a dolphin, a complex burrowing machine like a mole, a complex climbing machine like a monkey, or a complex thinking machine like a person.
Darwin explained all of this with one brilliantly simple idea – natural selection, driving gradual evolution over immensities of geological time. His is a good theory because of the huge ratio of what it explains (all the complexity of life) divided by what it needs to assume (simply the nonrandom survival of hereditary information through many generations). The rival theory to explain the functional complexity of life – creationism – is about as bad a theory as has ever been proposed. What it postulates (an intelligent designer) is even more complex, even more statistically improbable than what it explains. In fact it is such a bad theory it doesn’t deserve to be called a theory at all, and it certainly doesn’t deserve to be taught alongside evolution in science classes.
The simplicity of Darwin’s idea, then, is a virtue for three reasons. First, and most important, it is the signature of its immense power as a theory, when compared with the mass of disparate facts that it explains – everything about life including our own existence. Second, it makes it easy for children to understand (in addition to the obvious virtue of being true!), which means that it could be taught in the early years of school. And finally, it makes it extremely beautiful, one of the most beautiful ideas anyone ever had as well as arguably the most powerful. To die in ignorance of its elegance, and power to explain our own existence, is a tragic loss, comparable to dying without ever having experienced great music, great literature, or a beautiful sunset.