Animal classification is a matter of sorting out similarities and differences, of placing animals in groups and then breaking those groups apart into subgroups. The whole endeavor creates a structure—a hierarchy in which the large high-level groups sort out bold and obvious differences, while the low-level groups tease apart subtle, almost imperceptible, variations. This sorting process enables scientists to describe evolutionary relationships, identify shared traits, and highlight unique characteristics down through the various levels of animal groups and subgroups.