rface of the ocean denies to man a dwelling place; every century,
however, the Dutch are pushing forward their northern boundary by
reclamation of land from the sea; but repeatedly they have had to drop
back for a time when the water has again overwhelmed their hand-made
territory.

[Sidenote: Peoples as barriers.]

The boundaries of race and state which are subjected to greatest
fluctuations are those determined by the resistance of other peoples.
The westward sweep of the Slavs prior to eighth century carried them
beyond the Elbe into contact with the Germans; but as these increased in
numbers, outgrew their narrow territories and inaugurated a
counter-movement eastward, the Slavs began falling back to the Oder, to
the Vistula, and finally to the Niemen. Though the Mohawk Valley opened
an easy avenue of expansion westward for the early colonists of New
York, the advance of settlements up this valley for several decades went
on at only a snail's pace, because of the compact body of Iroquois
tribes holding this territory. In the unoccupied land farther south
between the Cumberland and Ohio rivers the frontier went forward with
leaps and bounds, pushed on by the expanding power of the young
Republic. [See map page 156.]

Anything which increases the expanding force of a people--the
establishment of a more satisfactory government by which the national
consciousness is developed, as in the American and French revolutions,
the prosecution of a successful war by which popular energies are
released from an old restraint, mere increase of population, or an
impulse communicated by some hostile and irresistible force behind--all
are registered in an advance of the boundary of the people in question
and a corresponding retrusion of their neighbor's frontier.

[Sidenote: Boundary zone as index of growth or decline.]

The border district is the periphery of the growing or declining race or
state. It runs the more irregularly, the greater are the variations in
the external conditions as represented by climate, soil, barriers, and
natural openings, according as these facilitate or obstruct advance.
When it is contiguous with the border of another state or race, the two
form a zone in which ascendency from one side or the other is being
established. The boundary fluctuates, for equilibrium of the contending
forces is established rarely and for only short periods. The more
aggressive people throws out across this debatable zone, a