Growth of population is a function of birth rates and death rates. Number of persons (babies) born per thousand people during a certain period of time in a country is called birth rate (BRs).
Growth rate is the rate at which the population grows in a country during a certain period.It is the difference of the birth rate and death rate during the period. If birth rates exceed death rates population will grow and it is called growth of population.
During the early period of Human history, population growth was extremely slow because death rates were higher than birth rates. Reasons for high death rates are as follow:
(i) Scarcity of food and the cave man had to struggle hard for survival.
(ii) Hostile environment and prevalence of fatal diseases.
(iii) Absence of medical aid.
This happened during 1821. It was called period of great divide. Discovery of agriculture lead to more settle life. Famines and deaths due to scarcity of food registered a decline. All this leads to higher survival rate.
Population did increase but slowly. People migrated to new areas and more land was brought under-cultivation. These developments made world population 300 million. It happened at the beginning of the Christian era.
Industrial revolution marked the next important stage in the growth of human population followed by advancement in medical science.
Advances in the field of medicine resulted in the control of plague, smallpox, malaria and other deadly diseases. Protected water supply, sanitation and other preventive measures further reduces death rates.
Not only that, settlements encouraged population growth. This happened with the opening up of new lands in North-America, South America and Australia. So population increased by migration as well.