 The reference to “organ” in Owen’s definition is indicative of a structure (an entity) found in an organism that may vary in its shape and composition (form) or what it is for (function) in the species where it occurs. Translated into an evolutionary context, sameness is cashed out by reference to common ancestry. Since structures also can be similar by virtue of natural selection operating in similar environments, homology is contrasted with analogy. Homologous structures are the same by virtue of descent from a common ancestor, regardless of what functions these structures are involved in, whereas analogous structures are similar by virtue of selection processes favoring comparable functional outcomes, regardless of common descent  (Figure 6).  