miles to Glasgow itself, so that now the largest ocean
steamers come to dock where fifty years ago children waded across the
stream at ebb tide. Such artificial modifications, however, are rare,
for they are made only where peculiarly rich resources or superior lines
of communication with the hinterland justify the expenditures; but they
find their logical conclusion in still farther extensions of sea
navigation into the interior by means of ship canals, where previously
no waterway existed. Instances are found in the Manchester ship canal
and the Welland, which, by means of the St. Lawrence and the Great
Lakes, makes Chicago accessible to ocean vessels. Though man
distinguishes between sea and inland navigation in his definitions, in
his practice he is bound by no formula and recognizes no fundamental
difference where rivers, lakes, and canals are deep enough to admit his
sea-going craft.

Such deep landward protrusions of the head of marine navigation at
certain favored points, as opposed to its recent coastward trend in most
inlets and rivers, increase the irregularity of the inner edge of the
coast zone by the marked discrepancy between its maximum and minimum
width. They are limited, however, to a few highly civilized countries,
and to a few points in those countries. But their presence testifies to
the fact that the evolution of the coast zone with the development of
civilization shows the persistent importance of this inner edge.

[Sidenote: Outer edge in original settlement.]

The outer edge finds its greatest significance, which is for the most
part ephemeral, in the earlier stages of navigation, maritime
colonization, and in some cases of original settlement. But this
importance persists only on steep coasts furnishing little or no level
ground for cultivation and barred from interior hunting or grazing land;
on many coral and volcanic islands of the Pacific Ocean whose outer rim
has the most fertile soil and furnishes the most abundant growth of coco
palms, and whose limited area only half suffices to support the
population; and in polar and sub-polar districts, where harsh climatic
conditions set a low limit to economic development. In all these regions
the sea must provide most of the food of the inhabitants, who can
therefore never lose contact with its waters. In mountainous Tierra del
Fuego, whose impenetrably forested slopes rise directly from the sea,
with only here and there a scanty stretch of ston