chance to rise, and serves only as an emergency harbor; while on the Weser, maritime leadership has oscillated between Bremen and Bremerhaven.[419] So the whole German coast and the Russian Baltic have seen a more or less irregular shifting backward and forward of maritime importance between the inner and the outer edges. [Sidenote: Artificial extension of inner edge.] The width of the coast zone is not only prevented from contracting by dredging and canaling, but it is even increased. By deepening the channel, the chief port of the St. Lawrence River has been removed from Quebec 180 miles upstream to Montreal, and that of the Clyde from Port Glasgow 16 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.]