When scuba diving I noticed that at 40 feet of depth and beyond, there is no visible light, it is absolutely black. That always made me wonder, “Why do deep water fish have eyes?” Darwin's theory states that if a biological function is not used, that function becomes a burden and will be lost. A great example of this selection process is the cave dwelling fish of the USA Southwest deserts. These cave fish live in underground pools of water devoid of sunlight. The cave fish have completely lost their eye structures. Their eyes have been replaced by fleshy membranes. According to Darwin, this eye structure loss is natural selection, reducing the blind fish’s biological burden of possessing a function which is not useful for survival or reproduction. Interestingly, the complete loss of their eye structures didn't take very long, probably occurring within a time period of several thousand years. Natural selection is a powerful force with rapid consequences. Then I ask again, “Why do fish have eyes?”

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