Darwin singled out the eye as posing a particularly challenging problem: 'To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.' Creationists gleefully quote this sentence again and again. Needless to say, they never quote what follows. Darwin's fulsomely free confession turned out to be a rhetorical device. He was drawing his opponents towards him so that his punch, when it came, struck the harder. The punch, of course, was Darwin's effortless explanation of exactly how the eye evolved by gradual degrees. Darwin may not have used the phrase 'irreducible complexity', or 'the smooth gradient up Mount Improbable', but he clearly understood the principle of both. 'What is the use of half an eye?' and 'What is the use of half a wing?' are both instances of the argument from 'irreducible complexity'. A functioning unit is said to be irreducibly complex if the removal of one of its parts causes the whole to cease functioning. This has been assumed to be self-evident for both eyes and wings. But as soon as we give these assumptions a moment's thought, we immediately see the fallacy. A cataract patient with the lens of her eye surgically removed can't see clear images without glasses, but can see enough not to bump into a tree or fall over a cliff. Half a wing is indeed not as good as a whole wing, but it is certainly better than no wing at all. Half a wing could save your life by easing your fall from a tree of a certain height. And 51 per cent of a wing could save you if you fall from a slightly taller tree. Whatever fraction of a wing you have, there is a fall from which it will save your life where a slightly smaller winglet would not. The thought experiment of trees of different height, from which one might fall, is just one way to see, in theory, that there must be a smooth gradient of advantage all the way from 1 per cent of a wing to 100 per cent. The forests are replete with gliding or parachuting animals illustrating, in practice, every step of the way up that particular slope of Mount Improbable. By analogy with the trees of different height, it is easy to imagine situations in which half an eye would save the life of an animal where 49 per cent of an eye would not. Smooth gradients are provided by variations in lighting conditions, variations in the distance at which you catch sight of your prey—or your predators. And, as with wings and flight surfaces, plausible intermediates are not only easy to imagine: they are abundant all around the animal kingdom. A flatworm has an eye that, by any sensible measure, is less than half a human eye. Nautilus (and perhaps its extinct ammonite cousins who dominated Paleozoic and Mesozoic seas) has an eye that is intermediate in quality between flatworm and human. Unlike the flatworm eye, which can detect light and shade but see no image, the Nautilus 'pinhole camera' eye makes a real image; but it is a blurred and dim image compared to ours. It would be spurious precision to put numbers on the improvement, but nobody could sanely deny that these invertebrate eyes, and many others, are all better than no eye at all, and all lie on a continuous and shallow slope up Mount Improbable, with our eyes near a peak—not the highest peak but a high one.
Cope laughed. “I wouldn’t worry yourself, my friend. Eobasileus has been extinct for thirty-seven million years.”At this, the preacher could no longer contain himself. “Nonsense! Utter nonsense!”“Nonsense?” asked Cope.“The archbishop James Ussher, using the Holy Bible itself, worked back generation by generation, mathematically, and calculated that the Earth was created on Sunday, October 23, 4004 BC at precisely eight a.m.”“Did he, now? Eight a.m., precisely?”“Precisely,” the preacher insisted.Copy and Sternberg exchanged amused looks.“Well,” Cope replied, “since the rotation of the Earth assures us that it’s always eight a.m. somewhere in the world, I suppose I should applaud him for guessing the correct time, at least.” The cowboy couldn’t help but interject.“Pardon me, Preacher, but if I recall correctly, didn’t the Bible say something about the Lord resting on the seventh day?”The preacher looked confused. “What?”“I’m certain of it.” The cowboy quickly snatched the Bible from the preacher’s hands and opened it to the first page of Genesis. “Sure. Here it is. He got started on a Monday, making light and darkness. By the time he got around to creating the Earth it was well into the third day. I make that to be Wednesday, not Sunday.”Nonplussed and blushing, the preacher snatched his Bible back. The cowboy shrugged. “Looks to me like your archbishop pulled a fast one, Preacher. Or maybe he just wasn’t all that good at calculating.