The principles of the evolution of navigation, of agriculture, of trade, as also the theory of population, can never reach their correct and final statement, unless the data for the conclusions are drawn from every part of the world, and each fact interpreted in the light of the local conditions whence it sprang.
Therefore anthropology, sociology and history should be permeated by geography.
[Sidenote: Political geography and history.] In history, the question of territory,--by which is meant mere area in contrast to specific geographic conditions--has constantly come to the front, because a state obviously involved land and boundaries, and assumed as its chief function the defence and extension of these.
Therefore political geography developed early as an offshoot of history.
Political science has often formulated its principles without regard to the geographic conditions of states, but as a matter of fact, the most fruitful political policies of nations have almost invariably had a geographic core.

Witness the colonial policy of Holland, England, France and Portugal, the free-trade policy of England, the militantism of Germany, the whole complex question of European balance of power and the Bosporus, and the Monroe Doctrine of the United States.

Dividing lines between political parties tend to follow approximately geographic lines of cleavage; and these make themselves apparent at recurring intervals of national upheaval, perhaps with, centuries between, like a submarine volcanic rift.

In England the southeastern plain and the northwestern uplands have been repeatedly arrayed against each other, from the Roman conquest which embraced the lowlands up to about the 500-foot contour line,[79] through the War of the Roses and the Civil War,[80] to the struggle for the repeal of the Corn Laws and the great Reform Bill of 1832.[81] Though the boundary lines have been only roughly the same and each district has contained opponents of the dominant local party, nevertheless the geographic core has been plain enough.
[Sidenote: Political versus social geography.] The land is a more conspicuous factor in the history of states than in the history of society, but not more necessary and potent.

Wars, which constitute so large a part of political history, have usually aimed more or less directly at acquisition or retention of territory; they have made every petty quarrel the pretext for mulcting the weaker nation of part of its land.