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The idea that taxa should be monophyletic had its beginning in the writings of Charles Darwin. He stressed that "all true classification is genealogical" (Darwin 1959:656) but that "genealogy by itself does not give the classification" (F. Darwin 1888:247). Expanding the idea, he wrote, "The arrangement of groups within each class, in due subordination and relation to other groups, though allied in the same degree in blood to their common progenitor, may differ greatly, being due to the different degrees of modification which they have undergone; and this is expressed by forms being ranked under different genera, families, sections, or orders" (Darwin 1959:656). In his classification of the barnacles, Darwin consistently determined rank by degree of divergence, and many of his groups have been shown to have evolved from within other groups (Ghiselin and Jaffe 1973;Mayr 1974).

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