The term hom ology refers to a commonality of source between two structures or behaviors. For example, in its elements of role practice, explo ration, and mock aggression, the similarity of the social play of humans to play exhibited in Old World monkeys and apes is held to reflect the independently established genetic similarity between these groups (Wilson, 1980). Nearly every author who discusses homology concedes that aspects of the definition are controversial. The foci of contention include the following questions: (1) Does homology require the struc tures or behaviors compared to be similar on an observable level? and (2) Does homology imply commonality of descent (i.e., heredity), or does it only imply that there is a common source of information?