analogy, in biology, the similarities in function, but differences in evolutionary origin, of body structures in different organisms. For example, the wing of a bird is analogous to the wing of an insect, since both are used for flight. However, there is no common ancestral origin in the evolution of these structures: While the wings of birds are modified skeletal forelimbs, insect wings are extensions of the body wall. Although insects and birds do have a very remote common ancestry (more than 600 million years ago), the wings of the two groups evolved after their ancestries had separated. See also homology.