Very suggestive evidence for common descent is also provided by the study of comparative anatomy. It was customary already in the eighteenth century to call certain organisms "related" when they were similar. At that time the French naturalist Comte Buffon described this for horses, donkeys, and zebras. The less similar that two kinds of organisms were, the less closely they were considered to be "related." The systematists, the students of classification, used the degree of similarity to establish a hierarchy of taxonomic categories. The most similar organisms were placed in the same species. Similar species were placed in the same genus, similar genera in the same family, and thus all the way up to the taxa of the highest category. This arrangement of organisms by the degree of their similarity and relationship is called the Linnaean hierarchy , after the Swedish botanist Carolus Linnaeus, who developed the system of binomial classification. Such a classification groups organisms into larger and larger taxa, finally comprising all the animals and all the plants. Beginning with a particular species, let us say the cat, one was able to construct this hierarchy. It was known that there were other species of cats rather similar to the house cat, which Linnaeus also placed in the genus Felts. This group of cats could be combined with the lion, the cheetah, and other genera of cats into the family Felidae. This family of catlike mammals could then be combined with other predatory mammals such as the Canidae (doglike), Ursidae (bears), Mustelidae (weasels), Viverridae (civets), and related groups into the order of Carnivora. In a similar manner, other mammals could be combined into the orders of Artiodactyla (deer and relatives), Perissodactyla (horses, etc.), Rodentia (rodents, etc.), and those of whales, bats, primates, marsupials, and so on to form the class Mammalia (mammals). A similar hierarchy exists for all other kinds of animals, such as birds and insects, and for plants. The nature and causation of this grouping, unless ascribed to creation, was a complete riddle until Darwin showed that it was evidently due to "common descent." Each taxon (group of organisms), Darwin demonstrated, could be explained as consisting of the descendants from the nearest common ancestor, and such descent required evolution.
The Linnaean hierarchy. Each category is nested within the next higher category,
such as the species in the genus.
