Small wonder that it troubled Darwin 

If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous successive slight modifications, my theory would absolutely break down. 

- Charles Darwin, The Origin of Species

     There were a number of nature's wonders that gave Darwin the shudders. A feather in a peacock's tail was one. 'Small trifling particulars of structure often make me very uncomfortable,' he confessed not long after publication of Origin. But most worrying of all was that marvel of construction, the human eye. 

For the eye to work the following minimum perfectly coordinated steps have to take place (there are many others happening simultaneously, but even a grossly simplified description is enough to point up the problems for Darwinian theory). The eye must be clean and moist, maintained in this state by the interaction of the tear gland and movable eyelids, whose eyelashes also act as a crude filter against the sun. The light then passes through a small transparent section of the protective outer coating (the cornea), and continues via a self-adjusting aperture (the pupil), and a similarly automatic lens which focuses it on the back of the retina. Here 130 million light-sensitive rods and cones cause photochemical reactions, which transform the light into electrical impulses. Some 1,000 million of these are transmitted every second, by means that are not properly understood, to a brain which then takes appropriate action. 

Now it is quite evident that if the slightest thing goes wrong en route - if the cornea is fuzzy, or the pupil fails to dilate, or the lens becomes opaque, or the focusing goes wrong – then a recognizable image is not formed. The eye either functions as a whole, or not at all. So how did it come to evolve by slow, steady, infinitesimally small Darwinian improvements? Is it really possible that thousands upon thousands of lucky chance mutations happened coincidentally so that the lens and the retina, which cannot work without each other, evolved in synchrony? What survival value can there be in an eye that doesn't see? 

Small wonder that it troubled Darwin. ‘To this day the eye makes me shudder,’ he wrote to his botanist friend Asa Gray in February 1860. 

The eye is, in fact, merely an extreme example of a large number of evolutionary novelties, as they have come to be termed - structures that, logically, have either to be perfect, or perfectly useless. Darwin himself called them ‘organs of extreme perfection and complication’. With each of them the difficulty for Darwinians is twofold. Theory demands that successive steps of a gradually improving nature build towards a final product perfectly adapted to its environment. But many of the proposed intermediate steps seem impractical or even harmful. What use would be half a jaw? Or half a lung? Natural selection would surely eliminate creatures with such oddities, not preserve them. 

Secondly, simultaneous advantageous mutations seemingly have to take place. Otherwise the organ, even half-formed, would not work at all. In the eye, for instance, the pinhole opening (the pupil) and the lens have to work together. Statisticians call this a system of coordinated variables. 

It is extremely difficult for two variables to function in harmony - and in the eye, as we have seen, there are many more than two. The problem is coordination. Indeed, calculations have been made about the odds against the eye having evolved by chance alone. They turned out to be of an astronomical order - at least ten billion to one against, and perhaps many orders of magnitude more improbable even than that. ¹