Although fitness is generally perceived as the central concept of Darwinism, Darwin used the word only once in the first to the fifth, and twice in the sixth, editions of the Origin of Species, and then only in the meaning of adaptedness. In the fifth edition, he, on the recommendation of Wallace, added the phrase ''survival of the fittest,'' due to Herbert Spencer, as synonym for natural selection. However, Darwin's reasoning was throughout qualitative and he nowhere considered quantitative measures that might determine the speed of evolutionary change.
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