The quality which makes society change is that it is composed, not of inert objects which can be put away in a museum and kept forever, but of living individuals who spend their years and then pass away to make room for others. The succession of generations makes society a dynamic thing. Population is never stationary; it either increases or decreases, and usually in the same direction for a long time, thus giving an example of continuous variation.

. . . Human population tends to increase up to the limit of the supporting power of the environment, on a given stage of the arts, and for a given standard of living - that is, for a given stage of civilization. - Keller, Societal Evolution, p. 24.

The most rapid increase in population comes about through immigration such as the United States has received in great waves: from Ireland beginning in 1845, from Germany in 1848, from the Scandinavian countries after the Civil War, and from southeastern Europe after 1900.

Then change in population becomes a factor in effecting changes in social organization. When England had a scanty population, she exported wool; when population became more dense, the wool was manufactured at home and more was imported until England led the world in the manufacture of cloth. In the sixteenth and seventeenth centuries, when the population of England began to press upon the food supply, much cultivated land was turned into sheep pastures; grain could easily be imported but sheep could not; thereafter the additions to the population had to follow other industries than agriculture. The landowners long kept up protective duties, especially on grain, in order to keep up the value and rental of their land. But toward the middle of the nineteenth century the manufacturing and commercial interests became strong enough to sweep away the protective system and establish free trade. The fall of Rome was due not alone to the barbarian invasions. The population of the Empire had been declining from internal causes for centuries before the overthrow came; for more than a hundred years the Germans had been coming in to settle on the vacant lands. In the United States during the nineteenth century the scarcity of population in comparison with resources was a powerful stimulus to the introduction of labor-saving machinery. But in China and Japan, with their dense populations, tourists are still drawn about in jinrikishas, and ships are loaded with coal passed up in buckets by hand. Even the building of railroads has been opposed in China because they would take away work from the people. In Chapter I (Population) were noted the changes that occur in the educational system of a country as the population changes from sparsity to density.