Pure agricultural lands of central Europe support 100 to the square mile, and those of southern Europe, 200; when combining some industry, from 250 to 300.  But these figures rise to 500 or more in lowland India and China.  Industrial districts of modern Europe, such as England, Belgium, Saxony, Departments Nord and Rhone in France, show a density of 500 to 800 to the square mile. [See maps pages 8 and 9.]

[Sidenote:  Density of population and government.]

With every increase of the population inhabiting a given area, and with the consequent multiplication and constriction of the bonds uniting society with its land, comes a growing necessity for a more highly organized government, both to reduce friction within and to secure to the people the land on which and by which they live.  Therefore protection becomes a prime function of the state.  It wards off outside attack which may aim at acquisition of its territory, or an invasion of its rights, or curtailment of its geographic sphere of activity.  The modern industrial state, furthermore, with the purpose of strengthening the nation, assists or itself undertakes the construction of highways, canals, and railroads, and the maintenance of steamship lines.  These encourage the development of natural resources and of commerce, and hence lay the foundation for an increased population, by multiplying the relations between land and people.

[Sidenote:  Territorial expansion of the state.]

A like object is attained by territorial expansion, which often follows in the wake of commercial expansion.  This strengthens the nation positively by enlarging its geographic base, and negatively by forcing back the boundaries of its neighbors.  The expansion of the Thirteen Colonies from the Atlantic slope to the Mississippi River and the Great Lakes by the treaty concluding the Revolution was a strong guarantee of the survival of the young Republic against future aggressions either of England or Spain, though it exchanged the scientific or protecting boundary of the Appalachian Mountains for the unscientific and exposed boundary of a river.  The expansion to the Rocky Mountains by the Louisiana purchase not only gave wider play to national energies, stimulated natural increase of population, and attracted immigration, but it eliminated a dangerous neighbor in the French, and placed a wide buffer of untenanted land between the United States and the petty aggressions of the Spanish in Mexico.  Rome’s expansion into the valley of the Po, as later into Trans-Alpine Gaul and Germany, had for its purpose the protection of the peninsula against barbarian inroads.  Japan’s recent aggression against the Russians in the Far East was actuated by the realization that she had to expand into Korea at the cost of Muscovite ascendency, or contract later at the cost of her own independence.

[Sidenote:  Checks to population.]