Darwin devoted one chapter of his book The Origin of Species to the "difficulties" of his theory. One concern he had was how natural selection could account for the development of a structure as complex as the eye.
In this section, you'll read an excerpt from The Origin of Species and watch several video segments to examine how natural selection could create a complex organ such as the eye.
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 degree. 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.
(From Charles Darwin, The Origin of Species. [New York City: The Modern Library, 1993] p. 227-228)
Watch Evolution Show One video segment "Evolution of the Eye" and Evolution Show Two video segment "Genetic Toolkit."
Based on your viewing, what is the likelihood of the eye being developed by natural selection? Why do you think the evidence of intermediate stages and master control genes supports or does not support the possible evolution of the eye by natural selection?