|Schematic diagram of the human eye|
The vertebrate eye consists of five parts: cornea (a transparent insulating layer); aqueous humor; crystalline lens (a lens surrounded by muscles that make it possible to achieve a variable focal length); vitreous humor; and retina. The light passes through the cornea and the aqueous humor, is focused by the lens, goes through the vitreous humor and impacts on the nerve cells of the retina, which generate electric signals that the optic nerve transmits to the brain, which forms from them an image of the external world, where the light rays came from. The brain even turns the image around, as it is inverted when projected on the retina.
The complex structure of the eye has always been a problem for evolutionists, and an argument for those opposed to the theory of evolution. Darwin, in Chapter 6 of The Origin of Species, whose title is significant (Difficulties of the theory) dealt with the problem of the evolution of the eye in the following words:
Organs of extreme perfection and complication. 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 possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor...
In looking for the gradations by which an organ in any species has been perfected, we ought to look exclusively to its lineal ancestors; but this is scarcely ever possible, and we are forced in each case to look to species of the same group, that is to the collateral descendants from the same original parent-form, in order to see what gradations are possible... Amongst existing Vertebrata, we find but a small amount of gradation in the structure of the eye, and from fossil species we can learn nothing on this head...
It is scarcely possible to avoid comparing the eye to 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... 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 on 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?
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. But I can find out no such case... We should be extremely cautious in concluding that an organ could not have been formed by transitional gradations of some kind.
Despite Darwin's reasoning, the eye remains one of the organs that the supporters of the philosophical theory that has come to be called intelligent design present as an example of irreducible complexity, arguing that such a complex organ cannot have arisen by natural selection from simpler intermediate phases, because these would not have been functional, and therefore would not have provided selective advantages to their owners.
Stanley Jaki attacked intelligent design with these words:
The shortcomings, often very serious, of Darwinian theory cannot be remedied with Intelligent Design theory, which philosophically cannot cope with design and purpose. Moreover, it is a subtle rehash of the doctrine of special creation. Even worse, as it claims to be a "scientific" theory of evolution, it implies that design, insofar as it means purpose (and indeed divine purpose) can be the object of measurements, which is the touchstone of truth in science. S. L. Jaki, Intelligent Design?, Real View Books, 2007.
However, Jaki himself uses the eye to assert that the theory of evolution is far from complete, in his book The savior of science (1988-2000):
[The eye] has been studied with far greater accuracy than any other organ. Exact measurements and theories of physics have been added to anatomical studies of the eye to such extent as to deprive Darwinists of their chief strategy: general if not vague qualitative speculations about the specific steps of evolution. Yet they cannot help admitting, without contradicting the exactness of physics, that, in the words of one of them, “even if the slightest thing is wrong—if the retina is missing, or the lens opaque, or the dimensions in error—the eye fails to form a recognizable image and is consequently useless.” The quotation in the text is from G. Hardin, Nature and Man’s Fate, New York: Rinehart and Company, pp.73-74.
The truth is that, in that quotation, Harding did not act as a competent evolutionist. Please! Can we call slightest things a missing retina or an opaque lens? It is evident that any of these "slightest things" would turn the eye into a useless instrument. Compare this with asserting that, in a car, if the slightest thing were wrong, such as missing the engine, or having non-round wheels, the car could not fulfill its function.
Can we imagine in detail the evolution of the eye, beyond general if not vague qualitative speculations about the specific steps of evolution? Yes, we can. But that will be the subject of the next post.
I thank John Beaumont, who suggested this series of four posts.