Darwin's theory of evolution by natural selection is broadly accepted among biologists, but its implications for the study of cognition are far from clear. Few within the scientific pale would argue against the proposition that life on Earth has evolved and that this general principle can be extended to the process of thought. But in taking an evolutionary approach, biologists have tended to assume that species with shared ancestry will have similar cognitive abilities, and that the evolutionary history of traits can be used to reveal how we and other animals perform certain mental tasks. A closer analysis suggests things aren't so simple. In The Descent of Man, Darwin proposed that there is "no fundamental difference between man and the higher mammals in their mental faculties" on the basis of his belief that all living species were descended from a common ancestor. He also suggested that "there is a much wider interval in mental power between one of the lowest fishes... and one of the higher apes, than between an ape and man".
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