Homology is the similarity in traits between two species that indicate their common ancestry. For example, the general characteristics of cheetahs, lions, tigers, and house cats are whiskers, retractable claws, tooth structure, and so forth. These similarities indicate that each of these traits was inherited from a feline ancestor. To an evolutionist, an analogous structure is one that looks similar or has the same purpose but is definitely not the result of common inheritance. For example, bats and birds both use wings to fly, and the wings have the same general shape (thin but broad in width). However, the structures were not inherited from the same ancestors. Bats were four-legged mammals before their front limbs became modified for flight, while birds are not descended from mammals at all. Scientists can determine whether a trait is homologous or analogous by comparing it in species thought to be of common origin and contrasting it to traits of unrelated species in similar habitats.