The first sentence of Darwin's section on the eye is often cited by his enemies as the naturalist throwing in the towel. The idea that the exquisite, complex structure of the eye could have evolved by accumulated random mutations, Darwin says, seems "absurd to the highest possible degree". But read on and Darwin cajoles his reader to embrace the full power of natural selection.

If you accept the logic that variation between individuals is heritable and those individuals with the most successful variations survive to reproduce, then you have to accept that vast chasms of adaptation can be traversed by the slow plodding accumulation of inherited changes. Natural selection can travel huge distances if given time. "[The reader's] reason ought to conquer his imagination," writes Darwin, throwing in a few types of simple eye to show some of the possible steps along the way to a complex eye.

It is this failure of reason to conquer imagination that lies at the heart of the modern-day notion of intelligent design. Proponents of ID make the very same argument that Darwin is trying to counter here, namely that complex structures in nature are simply too intricate to have come about by natural selection, therefore they must have been the work of a designer.

One favourite example is the bacterial flagellum, a whip-like tail that propels the beast along. It looks beautifully designed, with a drive shaft and motor made up of over 40 proteins. If you take one away, the flagellum stops working. To ID proponents, this "irreducible complexity" means it cannot have evolved.

But this crude attempt at re-running evolution is meaningless. It turns out that the flagellum's complexity can be reduced. One bacterial species that lives in our stomach, for example, has 33 proteins instead of the full complement. What's more, many of the components of the flagellum have turned up doing separate jobs elsewhere in bacteria. So the notion of natural selection bodging together the tail using bits already present in bacteria is plausible. No need for a designer.

Again, Darwin was a century and a half ahead of the ID crowd. He acknowledges the temptation to liken the eye to a human-produced telescope: "We know that this instrument has been perfected by the long continued efforts of the highest human intellects; and we naturally infer that the eye has been formed by a somewhat analogous process. But may not this inference be presumptuous? Have we any right to assume that the Creator works by intellectual powers like those of man?"