"The notion of a 'movement' is more complicated than the ideas of groups and events. By a social movement we often mean a group of people identified by their attachment to some particular set of beliefs. In that case, the population in question can change drastically, but so long as some group of people is still working with the same beliefs, we consider the movement to survive. Thus the Women's Movement survives major changes in composition and internal organization. But the movement also commonly means action. People writing histories of the women's movement are quite likely to include past heroines who were quite different in beliefs and personal characteristics from current activists, just so long as their actions were similar or had similar effects. The fact that population, belief, and action do not always change together causes serious problems for students of social movements. When they diverge, should we follow the beliefs, whatever populations and actions they become associated with? Should we follow the population, whatever beliefs and actions it adopts? Should we follow the action, regardless of who does it and with what ideas?"
Charles Tilly From Mobilization to RevolutionNew York: Random House 1978 pp9-10