Blindness has repeatedly evolved in the animal kingdom. Thousands of blind cave-dwelling and underground and species are found throughout nature.
Originally Charles Darwin suggested that the loss of sight could be accounted to "disuse" over time. Currently, Reed Cartwright, a researcher, has re-studied the matter.
He explains that sight is not lost by disuse, but due to natural selection- another fundamental theory of Darwin.
Cartwright and team chose the Mexican tetra (Astyanax mexicanus) for their study. It is a docile, small, pink-hued, blind cavefish which can easily adapt in an aquarium. It inhabited caves for 2-3 million years, which gave it a time worth of 5 million generations to evolve blindness.
There also exists a surface-dwelling form of it which has retained its sight. This offered the researchers with a built-in comparative power which allowed for further exploration.
Their study found that "If sighted fish swim towards the light, the only fish that stay in the cave are blind fish. They aren't trying to get to the light anymore because they can't see it.” This hypothesis has already been derived in 1925 by E. Ray Lankester in his letter to the editor of Nature journal.