The world is divided into things that look as though somebody designed them (wings and wagon-wheels, hearts and televisions), and things that just happened through the unintended workings of physics (mountains and rivers, sand dunes, and solar systems). Mount Rushmore belonged firmly in the second category until the sculptor Gutzon Borglum carved it into the first. Charles Darwin moved in the other direction. He discovered a way in which the unaided laws of physics — the laws according to which things “just happen” — could, in the fullness of geologic time, come to mimic deliberate design. The illusion of design is so successful that to this day most Americans (including, significantly, many influential and rich Americans) stubbornly refuse to believe it is an illusion. To such people, if a heart (or an eye or a bacterial flagellum) looks designed, that’s proof enough that it is designed.

No wonder Thomas Henry Huxley, “Darwin’s bulldog,” was moved to chide himself on reading the Origin of Species: “How extremely stupid not to have thought of that.” And Huxley was the least stupid of men. The breathtaking power and reach of Darwin’s idea [...] is matched by its audacious simplicity. You can write it out in a phrase: nonrandom survival of randomly varying hereditary instructions for building embryos. Yet, given the opportunities afforded by deep time, this simple little algorithm generates prodigies of complexity, elegance, and diversity of apparent design. True design, the kind we see in a knapped flint, a jet plane, or a personal computer, turns out to be a manifestation of an entity — the human brain — that itself was never designed, but is an evolved product of Darwin’s mill.

Paradoxically, the extreme simplicity of what the philosopher Daniel C. Dennett called Darwin’s dangerous idea may be its greatest barrier to acceptance. People have a hard time believing that so simple a mechanism could deliver such powerful results.

The arguments of creationists, including those creationists who cloak their pretensions under the politically devious phrase “intelligent-design theory,” repeatedly return to the same big fallacy. Such-and-such looks designed. Therefore it was designed. To pursue my paradox, there is a sense in which the skepticism that often greets Darwin’s idea is a measure of its greatness.

Paraphrasing the twentieth-century population geneticist Ronald A. Fisher, natural selection is a mechanism for generating improbability on an enormous scale. Improbable is pretty much a synonym for unbelievable. Any theory that explains the highly improbable is asking to be disbelieved by those who don’t understand it.

Yet the highly improbable does exist in the real world, and it must be explained. Adaptive improbability — complexity — is precisely the problem that any theory of life must solve and that natural selection, uniquely as far as science knows, does solve.

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In any developing science there are disagreements. But scientists — and here is what separates real scientists from the pseudoscientists of the school of intelligent design — always know what evidence it would take to change their minds. One thing all real scientists agree upon is the fact of evolution itself. It is a fact that we are cousins of gorillas, kangaroos, starfish, and bacteria. Evolution is as much a fact as the heat of the sun. It is not a theory, and for pity’s sake, let’s stop confusing the philosophically naive by calling it so. Evolution is a fact.

That should be our bumper sticker: