Darwin’s discussion of the eye in the theory of evolution by natural selection
In the discussion on fallacies in reasoning, I considered the technique of quote-mining, the use of a quotation taken out of context in an effort to make the author appear to be saying something other than what was intended — in many cases, the opposite of the original statement.
One infamous example of this is taken from Darwin’s own writing. Creationists love to quote a passage from The Origin of Species about the evolution of the eye. To demonstrate what is being done, consider the part that is quoted first:
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 degree.
This is taken from the sixth chapter, titled, “Difficulties of the Theory.” Thus far, it appears that Darwin is agreeing with the creationist claim that complex structures must have been designed, rather than having arisen through the process of natural selection. And this is where the act of quoting leaves off for too many. But finish the paragraph:
When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. 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 [protoplasm] should become aggregated and developed into nerves, endowed with this special sensibility.
Darwin makes several important points here that show that he was not making an admission of failure regarding his theory. He asks us to recall the time before our era of modern astronomy in which we realize that the Earth is not the center of the universe. While a stationary world with the sky turning above us made sense when we did not have enough evidence, new facts led to a better understanding. As he explains, lots of people believing something is of no consequence to the idea’s value. What is required for science is logic and evidence.
And Darwin gives us both. First, he reminds us of his theory of evolution by natural selection. In every generation, the members of a given species will have variations, making each member subtly different from others. Those variations are inheritable — Darwin did not know about genes, but his theory predicts them. Any variation that gives an advantage to the individual organism that has it will give that individual a better chance in the competition for resources and for reproduction. More copies of the underlying genetic material that cause that advantage will be passed down to descendants than will copies of genes that produce less advantageous characteristics. In other words, healthier or more skillful organisms have more children. Over time, the advantageous gene becomes the dominant one in the general population. And given enough time, the accumulation of changes means that what was once one species has drifted into a new species. The development of eyes only requires that in each generation, some measure of sensitivity to light exists — and that sensitivity could simply be the by-product or a side-effect of something else — and that slight changes result in improvements.
All of this would sound like speculation, except Darwin offers examples, first in the paragraph that I quoted, but also in the following pages in specific detail. In the more than a century and a half since the publication of The Origin of the Species, we have found many more examples. Richard Dawkins devotes a chapter, “The Forty-fold Path to Enlightenment,” in his book, Climbing Mount Improbable, to examples of different types of eyes — eyespots, cups, pinholes, and lenses — found among animal species. One study by József Gál and others is particularly interesting to me as my own eyes age. The authors found that trilobites, a group of arthropods living between 520 and 250 million years ago, had bifocal vision, the only organisms known to possess this trait. The advantage to the trilobite would be the ability to see food particles passing close by, while remaining able to spot predators approaching from farther away.
What does this all mean? The claim made by advocates of intelligent design is that there are structures in living organisms that cannot have arisen without outside direction, since some fraction of the structure, some precursor to it would serve no purpose. Michael Behe of the Discovery Institute calls this irreducible complexity. And yet in the case of eyes, this line of reasoning falls apart. Eyespots, organs that can discern the difference between light and dark, give some advantage. Cup eyes, muliplications of light-sensitive spots that fold inward, add an ability to sense the direction of a shadow. Pinholes and lenses sharpen the image. But each kind of eye is better for the organism possesing it in an illuminated environment than being blind. And given all the examples and variations of eyes that are to be found in the fossil record and among species alive today, it is clear that organs for vision are easy to arrive at through evolution by natural selection, and when creationists claim that Darwin doubted this, they are engaging in a case of quote mining, along with a willful rejection of the abundant evidence.
1. Darwin, Charles. The Origin of the Species. The Harvard Classics. Ed. Charles W. Elliot. New York: Collier, 1909. Print.
2. Dawkins, Richard. Climbing Mount Improbable. New York: Norton, 1996. Print.
3. Gál, József, Gábor Horváth, Euan N. K. Clarkson, and Ottó Haiman. “Image formation by bifocal lenses in a trilobite eye?” Vision Research 40 (2000) 843–853. Print.
This will appear as a chapter in my book, Insufficient Faith: Reason in Science and Religion. For more of my writing now, go here.