long the lines
of least resistance or greatest attraction, long streamers of
occupation; so that the frontier takes on the form of a fringe of
settlement, whose interstices are occupied by a corresponding fringe of
the displaced people. Such was its aspect in early colonial America,
where population spread up every fertile river valley across a zone of
Indian land; and such it is in northern Russia to-day, where long narrow
Slav bands run out from the area of continuous Slav settlement across a
wide belt of Mongoloid territory to the shores of the White Sea and
Arctic Ocean.[335] [See maps pages 103 and 225.]

The border zone is further broadened by the formation of ethnic islands
beyond the base line of continuous settlement, which then advances more
or less rapidly, if expansion is unchecked, till it coalesces with these
outposts, just as the forest line on the mountains may reach, under
advantageous conditions, its farthest outlying tree. Such ethnic
peninsulas and islands we see in the early western frontiers of the
United States from 1790 to 1840, when that frontier was daily moving
westward.[336] [See map page 156.]

[Sidenote: Breadth of the boundary zone.]

The breadth of the frontier zone is indicative of the activity of growth
on the one side and the corresponding decline on the other, because
extensive encroachment in the same degree disintegrates the territory of
the neighbor at whose cost such encroachment is made. A straight, narrow
race boundary, especially if it is nearly coincident with a political
boundary, points to an equilibrium of forces which means, for the time
being at least, a cessation of growth. Such boundaries are found in old,
thickly populated countries, while the wide, ragged border zone belongs
to new, and especially to colonial peoples. In the oldest and most
densely populated seats of the Germans, where they are found in the
Rhine Valley, the boundaries of race and empire are straight and simple;
but the younger, eastern border, which for centuries has been steadily
advancing at the cost of the unequally matched Slavs, has the ragged
outline and sparse population of a true colonial frontier. Between two
peoples who have had a long period of growth behind them, the
oscillations of the boundary decrease in amplitude, as it were, and
finally approach a state of rest. Each people tends to fill out its area
evenly; every advance in civilization, every increase of population,
increases th