Successional change can be caused by either endogenous or exogenous factors depending upon whether the change is caused by the actions of the plants themselves or by external factors. Changes caused by endogenous factors (the plants themselves) are termed autogenic, where as changes caused by exogenous factors are termed allogenic. Primary succession is the classic case of autogenic change, in that the vegetation is part of the reason that soils develop. Allogenic success is driven by periodic disturbances.