A Series of Eyes
How do we see? In the 19th century the anatomy of the eye was known in great detail and the sophisticated mechanisms it employs to deliver an accurate picture of the outside world astounded everyone who was familiar with them. Scientists of the 19th century correctly observed that if a person were so unfortunate as to be missing one of the eye's many integrated features, such as the lens, or iris, or ocular muscles, the inevitable result would be a severe loss of vision or outright blindness. Thus it was concluded that the eye could only function if it were nearly intact.
As Charles Darwin was considering possible objections to his theory of evolution by natural selection in The Origin of Species he discussed the problem of the eye in a section of the book appropriately entitled "Organs of Extreme Perfection and Complication." He realized that if in one generation an organ of the complexity of the eye suddenly appeared, the event would be tantamount to a miracle. Somehow, for Darwinian evolution to be believable, the difficulty that the public had in envisioning the gradual formation of complex organs had to be removed.
Darwin succeeded brilliantly, not by actually describing a real pathway that evolution might have used in constructing the eye, but rather by pointing to a variety of animals that were known to have eyes of various constructions, ranging from a simple light sensitive spot to the complex vertebrate camera eye, and suggesting that the evolution of the human eye might have involved similar organs as intermediates.
But the question remains, how do we see? Although Darwin was able to persuade much of the world that a modern eye could be produced gradually from a much simpler structure, he did not even attempt to explain how the simple light sensitive spot that was his starting point actually worked. When discussing the eye Darwin dismissed the question of its ultimate mechanism1:
How a nerve comes to be sensitive to light hardly concerns us more than how life itself originated.
He had an excellent reason for declining to answer the question: 19th century science had not progressed to the point where the matter could even be approached. The question of how the eye works--that is, what happens when a photon of light first impinges on the retina--simply could not be answered at that time. As a matter of fact, no question about the underlying mechanism of life could be answered at that time. How do animal muscles cause movement? How does photosynthesis work? How is energy extracted from food? How does the body fight infection? Nobody knew.
This paper was originally presented in the Summer of 1994 at the meeting ofthe C.S. Lewis Society, Cambridge University.
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