ABSTRACT
Species exhibit a mixture of unique and common characters. Unique characters permit an appreciation of the fit of the organism’s form and function to its environment. Uniqueness is illustrated by language, a distinctive human character that suggests adaptation to social environments. Common characters allow the tracing of phylogenies, that is, lineages of ancestors and descendants. The pentadactyl (five-digit) hand is an instance of a character shared between humans and other tetrapods, one widely considered to be primitive among extant tetrapods.
