Such artificial checks upon population are more conspicuous in natural regions with sharply defined boundaries, like islands and oases, as Malthus observed;[123] but they are visible also among savage tribes whose boundaries are fixed not by natural features but by the mutual repulsion and rivalry characterizing the stage of development, and whose limit of population is reduced by their low economic status.
[Sidenote: Extra-territorial relations.] There is a great difference between those states whose inhabitants subsist exclusively from the products of their own country and those which rely more or less upon other lands.

Great industrial states, like England and Germany, which derive only a portion of their food and raw material from their own territory, supply their dense populations through international trade.

Interruption of such foreign commerce is disastrous to the population at home; hence the state by a navy protects the lines of communication with those far-away lands of wheat fields and cattle ranch.

This is no purely modern development.

Athens in the time of Pericles used her navy not only to secure her political domination in the Aegean, but also her connections with the colonial wheat lands about the Euxine.
The modern state strives to render this circle of trade both large and permanent by means of commercial treaties, customs-unions, trading-posts and colonies.