The food supply of a country increases at a much slower rate. It increases in arithmetical progression because of the operation of law of diminishing returns in land. Arithmetical progression means increase by addition, as in the series 1, 2, 3, 4 etc. or 3, 5, 7, 9 etc.
As a result of the difference between the rate of growth of population and the rate of growth of food supply we find that in every country, sooner or later, food shortage occurs. This is the stage of over-population.
4. The Malthusian Cycle:
When this stage comes, grave problems arise. Shortage of food causes malnutrition, famine, disease and pestilence. Hunger leads to invasions and war.
These are called by Malthus, the Positive Checks. As a result of the operation of the Positive Checks, population is reduced and the balance with food is restored. But the balance is a short-lived one. For, the inherent urge to multiply soon asserts itself and population again increases and overtakes the food-supply.
5. Preventive Checks:
The only way of escaping this unfortunate sequence of events is to exercise control over the growth of population. Malthus suggested two methods of doing so, viz.
, late marriage and celibacy (abstinence from marriage). He called them Preventive Checks.