(i) Initially, primary sector was the main economic activity.
(ii) Dueto improved methods of farming, productivity of agriculture sector increased. Therefore, more persons were taking up other activities like trading, transport, army, administration etc.
(iii) Secondary sector (or manufacturing sector) became the most important sector for production and employment, over a long period of time. This was a shift from primary to secondary sector as more and more factories were started.
(iv) In developed countries a further shift from secondary sector to tertiary sector is observed in past 100 years. Major portion of the working population is engaged in service sector.
In conclusion, in developed economies, tertiary sector has gained an important position.