We study the similar myths of various societies by comparing them to one another. We can compare these myths on the basis of their generic, genetic, or historical relationships.
Generic relationships among myths are based on the way people react to common features in their environment. For example, the Maori of New Zealand and the ancient Greeks both had myths that described how the earth became separated from the sky.
Genetic relationships. A large society may develop a particular myth. Then, for some reason, the society breaks up into several separate societies, each of which develops its own version of the myth. These myths have a genetic relationship.