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　 In recent years, the incidence of vision defects are also rising rapidly, People are always attributing it to the lack of light or improper use of the eyes and other external reasons. However, despite of the popularity and improvement of health care, the incidence rate does not decline as expected, but rather rises and changes with the passing days. Those various kinds of eye lesions can by no means be explained by external reasons. If we know the eye development history of the blind fish, it should be not difficult to understand the experience process of human eye. The difference lies in that the human eye become useless not because of entering the darkness and they still need vision. As the vision defects do not affect survival, it might be deemed "useless" from the perspective of evolution.　

　 The activities or all kinds of behavior of higher animals are all controlled by the brain. Under natural conditions, animal behavior is not allowed to deviate, because it is impossible to get outside help for their survival but to rely on themselves. In case that a wild animal fails to understand what it shall understand, or is unable to do what it shall do, or the outside information deforms after getting into the brain, or behavior and action do not match with the circumstances, then there would be no reason for it to survive. For example, if it mistakenly thinks its own kind is threating, challenging, or attacking itself, hears some sound that does not exist, sees some bizarre fantasy images, smells the strange smell that does not exist, and becomes somehow dread, nervous, excessively excited, or indifferent, or think they have extraordinary magic skills, or feel controlled by some kind of invisible power ... ... and so on, if these symptoms happen to the wild animal living in nature, it will be by no means saved. Contrary to that, normal people in the human society live hard while mental patients have been greatly tolerated and preferentially treated. As long as the dementia or silly can eat, they live a more carefree life than anyone else. The normal people will be headed if commit some crimes, the mad and the crazy can do whatever they want and enjoy immunity. Therefore, morbid thinking beyond control has survival space in the human society, and naturally the morbidity rises. madness and dementia have become the common phenomenon in the human population, which is unique among the creatures on earth. It should be noted that madness and dementia only account for a small part of mental illness, while more mental diseases do not manifest so obvious and strong symptoms, therefore the total of them cannot be ignored.