Long-term trends in population are often discussed with what is termed the demographic transition. The phrase has acquired several meanings, of which the simplest can be briefly stated: the shift accompanying modernization is from Stage I, a relatively static population with high fertility and mortality, to Stage II, a rapid population growth based on a continuing high fertility and falling mortality, and then to Stage III, a relatively static population based on a new balance between low fertility and low mortality. In short, the initial dynamic factor was the control of mortality at early ages, which propelled the population surge of modern times, followed by efforts to control fertility.