[Sidenote:  Political factors in this decline.]

Not only that far-reaching readjustment of maritime ascendency which in the sixteenth century followed the advance from thalassic to oceanic fields of commerce, but also purely local political events may for a time produce striking changes in the use or importance of coasts.  The Piraeus, which had been the heart of ancient Athens, almost wholly lost its value in the checkered political history of the country during the Middle Ages, when naval power and merchant marine almost vanished; but with the restoration of Grecian independence in 1832, much of its pristine activity was restored.  Up to the beginning of the seventeenth century, Japan had exploited her advantageous location and her richly indented coast to develop a maritime trade which extended from Kamchatka to India; but in 1624 an imperial order withdrew every Japanese vessel from the high seas, and for over two hundred years robbed her busy littoral of all its historical significance.  The real life of the Pacific coast of the United States began only with its incorporation into the territory of the Republic, but it failed to attain its full importance until our acquisition of Alaska, Hawaii, and the Philippines.  So the coast of the Persian Gulf has had periods of activity alternating with periods of deathlike quiet.  Its conquest by the Saracens in the seventh century inaugurated an era of intense maritime enterprise along its drowsy shores.  What new awakening may it experience, if it should one day become a Russian littoral!

[Sidenote:  Physical causes of decline.]

Sometimes the decline in historical importance is due to physical modifications in the coast itself, especially when, the mud transported by a great river to the sea is constantly pushing forward the outer shoreline.  The control of the Adriatic passed in turn from Spina to Adria, Ravenna, Aquileia, Venice, and Trieste, owing to a steady silting up of the coast.[525] Strabo records that Spina, originally a port, was in his time 90 stadia, or 10 miles, from the sea.[526] Bruges, once the great entrepot of the Hanseatic League, was originally on an arm of the sea, with which it was later connected by canal, and which has been silted up since 1432, so that its commerce, disturbed too by local wars, was transferred to Antwerp on the Scheldt.[527] Many early English ports on the coast of Kent and on the old solid rim of the Fenland marshes now lie miles inland from the Channel and the Wash.

A people never utilizes all parts of its coast with equal intensity, or any part with equal intensity in all periods of its development; but, according to the law of differentiation, it gradually concentrates its energies in a few favored ports, whose maritime business tends to become specialized.  Then every extension of the subsidiary territory and intensification of production with advancing civilization increases the mass of men and wares passing through these ocean