<P> Agrarian societies transition into industrial societies when less than half of their population is directly engaged in agricultural production . Such societies started appearing because of the Commercial and Industrial Revolution which can be seen beginning in the Mediterranean city - states of 1000 - 1500 C.E. As European societies developed during the Middle Ages, classical knowledge was reacquired from scattered sources, and a new series of maritime commercial societies developed again in Europe . The initial developments were centered in Northern Italy, in the city - states of Venice, Florence, Milan, and Genoa . By about 1500 a few of these city - states probably met the requirements of having half of their populations engaged in non-agricultural pursuits and became commercial societies . These small states were highly urbanized, imported much food, and were centers of trade and manufacture to a degree quite unlike typical agrarian societies . </P> <P> The culminating development, still in progress, was the development of industrial technology, the application of mechanical sources of energy to an ever - increasing number of production problems . By about 1800, the agricultural population of Britain had sunk to about 1 / 3 of the total . By mid-19th Century, all the countries of Western Europe, plus the United States of America had more than half their populations in non-farm occupations . Even today, the Industrial Revolution is far from completely replacing agrarianism with industrialism . Only a minority of the world's people today live in industrialized societies although most predominantly agrarian societies have a significant industrial sector . </P> <P> The use of crop breeding, better management of soil nutrients, and improved weed control have greatly increased yields per unit area . At the same time, the use of mechanization has decreased labor input . The developing world generally produces lower yields, having less of the latest science, capital, and technology base . More people in the world are involved in agriculture as their primary economic activity than in any other, yet it only accounts for four percent of the world's GDP . The rapid rise of mechanization in the 20th century, especially in the form of the tractor, reduced the necessity of humans performing the demanding tasks of sowing, harvesting, and threshing . With mechanization, these tasks could be performed with a speed and on a scale barely imaginable before . These advances have resulted in a substantial increase in the yield of agricultural techniques that have also translated into a decline in the percentage of populations in developed countries that are required to work in agriculture to feed the rest of the population . </P> <P> The main demographic consequences of agrarian technology were simply a continuation of the trend toward higher population densities and larger settlements . The latter is probably a more secure consequence of agrarian technology than the former . In principle livestock compete with humans for food and in some environments, advanced horticultural techniques can probably support more people per square kilometer than agrarian techniques . </P>

The two fundamental constraints on agrarian age hierarchies were