It is also important to recognize that sociocultural evolution operates on two distinct levels simultaneously.  First, it occurs within individual societies.  Second, it occurs within the world system of societies.  While these to processes are separate and distinct, they are also related, because the changes that occur in individual societies produce the variations on which the process of intersocietal selection operates.  This is the process that determines which societies and which cultures survive and which become extinct, and the role that each of the survivors plays within the world system (1991, p. 66).