t unbroken drift ice, and beyond this is an irregular
concentric zone of scattered icebergs which varies in breadth with
season, wind and local current; a persistent decrease in continuity from
solid pack to open sea. The line of perpetual snow on high mountains
advances or retreats from season to season, from year to year; it drops
low on chilly northern slopes and recedes to higher altitudes on a
southern exposure; sends down long icy tongues in dark gorges, and
leaves outlying patches of old snow in shaded spots or beneath a
covering of rock waste far below the margin of the snow fields.

In the struggle for existence in the vegetable world, the tree line
pushes as far up the mountain as conditions of climate and soil will
permit. Then comes a season of fiercer storms, intenser cold and
invading ice upon the peaks. Havoc is wrought, and the forest drops back
across a zone of border warfare--for war belongs to borders--leaving
behind it here and there a dwarfed pine or gnarled and twisted juniper
which has survived the onslaught of the enemy, Now these are stragglers
in the retreat, but are destined later in milder years to serve as
outposts in the advance of the forest to recover its lost ground. Here
we have a border scene which is typical in nature--the belt of unbroken
forest, growing thinner and more stunted toward its upper edge,
succeeded by a zone of scattered trees, which may form a cluster perhaps
in some sheltered gulch where soil has collected and north winds are
excluded, and higher still the whitened skeleton of a tree to show how
far the forest once invaded the domain of the waste.

[Sidenote: Oscillating boundaries]

The habitable area of the earth everywhere shows its boundaries to be
peripheral zones of varying width, now occupied and now deserted,
protruding or receding according to external conditions of climate and
soil, and subject to seasonal change. The distribution of human life
becomes sparser from the temperate regions toward the Arctic Circle,
foreshadowing the unpeopled wastes of the ice-fields beyond. The outward
movement from the Tropics poleward halts where life conditions
disappear, and there finds its boundary; but as life conditions advance
or retreat with the seasons, so does that boundary. On the west coast of
Greenland the Eskimo village of Etah, at about the seventy-eighth
parallel, marks the northern limit of permanent or winter settlement;
but in summer the Eskimo, in his kay