Darwin’s view of vision
In Chapter six of Origins Darwin addressed what he saw as the chief difficulties with his new theory. A good example to begin with was the eye. “To suppose,” wrote Darwin, “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.”
But Darwin then abruptly pivoted. Anticipating the twentieth century’s non intuitive findings such as quantum mechanics and chaos theory, Darwin’s justification for evolution was that science is not always intuitive. Does the earth really hurtle at breakneck speed around the stationary sun? Vox populi, vox Dei, advised the sage of Kent, cannot be trusted in science. The next pivot was even easier:
Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but I may remark that, as some of the lowest organisms, in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility.
It is incredible that such non scientific conjecture continues to be taken seriously by anyone today. This nineteenth century thought experiment was conveniently oblivious to the biochemistry underlying vision. Even the relatively simple third eye vision cascade dwarfs anything of which Darwin could have dreamt. And it shows no sign of having evolved.
As if sensing a problem Darwin employed a deeply metaphysical argument from one of his favorite authors, David Hume’s anthropomorphic warning:
It is scarcely possible to avoid comparing the eye with a 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? If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with spaces filled with fluid, and with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. Further we must suppose that there is a power, represented by natural selection or the survival of the fittest, always intently watching each slight alteration in the transparent layers; and carefully preserving each which, under varied circumstances, in any way or in any degree, tends to produce a distincter image. We must suppose each new state of the instrument to be multiplied by the million; each to be preserved until a better one is produced, and then the old ones to be all destroyed. In living bodies, variation will cause the slight alterations, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man?
The Enlightenment supplied the gnosticism and Darwin supplied the just-so story. We must suppose “natural selection or the survival of the fittest” created the eye, our religion depends on it.
Dawkins and our backward retina
But Darwin didn’t have the best argument of all. What Darwin did not know was that our eye’s retina is a kluge—it is backwards. Over and over, as evolutionists such as Ernst Mayr and Elliott Sober have pointed out, Darwin argued that nature’s absurdities mandate evolution. The backward retina would have been a gem for Darwin. Needless to say when the inverted design was found, it became yet another metaphysical mandate for evolution. As Richard Dawkins has explained:
Like any nerve, the optic nerve is a trunk cable, a bundle of separate “insulated” wires, in this case about three million of them. Each of the three million wires leads from one cell in the retina to the brain. You can think of them as the wires leading from a bank of three million photocells (actually three million relay stations gathering information from an even larger number of photocells) to the computer that is to process the information in the brain. They are gathered together from all over the retina into a single bundle, which is the optic nerve for the eye.
Any engineer would naturally assume that the photocells would point towards the light, with their wires leading backwards towards the brain. He would laugh at any suggestion that the photocells might point away from the light, with their wires departing on the side nearest the light. Yet this is exactly what happens in all vertebrate retinas. Each photocell is, in effect, wired in backwards, with its write sticking out on the side nearest the light. The wire has to travel over the surface of the retina, to a point where it dives through a hole in the retina (the so-called “blind spot”) to join the optic nerve. This means that the light , instead of being granted an unrestricted passage to the photocells, has to pass through a forest of connecting wires, presumably suffering at least some attenuation and distortion (actually probably not much but, still, it is the principle of the thing that would offend any tidy-minded engineer!).
This much celebrated inverted design has served as a canonical example of why design is false, and evolution true. It isn’t that the design is not complex (indeed it is). It is that the design is not right. No designer would have done it that way. Dawkins must admit that the attenuation and distortion may not be significant, but so what? It is “the principle of the thing.” The retina obviously is not intelligently designed. This is evolutionary metaphysics.
Not junk after all
Now that evolution has been safely established, new research is showing that this icon of dysteleology actually improves vision. As one report explained:
in 2007 researchers analyzing the retinas of guinea pigs reported that the glial cells which nourish and physically support the bed of neurons also act as optical fibers for the rods and cones. These Müller cells are funnel-shaped, with wide tops that cover the surface of the retina and a long slender body that guides light to the receptors below. … findings suggest that sending light via the Müller cells offers several advantages. ... This suggests the cells act as light filters, keeping images clear. … The researchers also found that light that had leaked out of one Müller cell was unlikely to be taken up by a neighbor, because the surrounding nerve cells help disperse it. What’s more, the intrinsic optical properties of Müller cells seemed to be tuned to visible light, leaking wavelengths outside and on the edges of the visible spectrum to a greater extent.
The cells also seem to help keep colors in focus. Just as light separates in a prism, the lenses in our eyes separate different colors, causing some frequencies to be out of focus at the retina. The simulations showed that Müller cells’ wide tops allow them to “collect” any separated colors and refocus them onto the same cone cell, ensuring that all the colors from an image are in focus.
“It suggests that light-coupling by Müller cells is a crucial event that contributes to vision as we know it,” says Kristian Franze, a neurophysicist at the University of Cambridge and co-author of the 2007 study.
Optical fibers, light filters, autofocus, light-coupling? Is the evolution of such design not in question? Is this another just-so story? Is this not absurd in the highest degree?
It isn’t right
Of course not. It may have a fancy function, but the design still isn’t right. It must have evolved. There remain the blind spot and the wires going off in the wrong direction. Whatever enhancements we discover to the retina are just that, enhancements. It surely could have been done better.
As evolutionist Ken Miller cautions, none of this means that the backwards retina itself helps us to see. Rather, it emphasizes the extent to which evolution has fixed the problem:
The shape, orientation and structure of the Müller cells help the retina to overcome one of the principal shortcomings of its inside-out wiring.
This is evolution. A genre rich in metaphysical pronouncements that otherwise are enshrined in scientific terminology. Miller isn't making any new claims. He is simply being consistent with the traditions of evolutionary thought to which he subscribes. Religion drives science and it matters.