Evolution of the animal eye
A description of the evolution of different eye types in animal species. Darwin had difficulty with his Theory of Evolution when considering the origin of the eye. His critics could not believe that an organ so intricate in mechanism could have evolved by a natural process. Darwin suggested that it could if progressions from a simple to complex eye were shown to exist, each useful to its possessor. There is a demonstration of the gradation from light sensitive cells in the earthworm, to other organisms where further refinements exist, to the ultimate visual ability of the eagle. Darwin believed there were no limits to what the process could produce.