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Evolution is a powerful force that applies to ideas as well as to species. This was recognised before Darwin wrote On the Origin of Species. He compared the evolution of species by natural selection with the previously recognised evolution of western languages from what is now called proto-Indo-European. Neurological diagnoses also evolve as do their treatments, the selective forces being clinical trials, new investigative mechanisms and insights into pathogenesis. As with the evolution of species, these changes are usually gradual and long term but sometimes come all at once.