In each of these sheets of water, the Gulf of Mexico and the Caribbean, there is one position of pre-eminent commercial importance. In the Gulf the mouth of the Mississippi is the point where meet all the exports and imports, by water, of the Mississippi Valley. However diverse the directions from which they come, or the destinations to which they proceed, all come together here as at a great crossroads, or as the highways of an empire converge on the metropolis. Whatever value the Mississippi and the myriad miles of its subsidiary water-courses represent to the United States, as a facile means of communication from the remote interior to the ocean highways of the world, all centres here at the mouth of the river. The existence of the smaller though important cities of the Gulf coast--Mobile, Galveston, or the Mexican ports--does not diminish, but rather emphasizes by contrast, the importance of the Mississippi entrance. They all share its fortunes, in that all alike communicate with the outside world through the Strait of Florida or the Yucatan Channel. In the Caribbean, likewise, the existence of numerous important ports, and a busy traffic in tropical produce grown within the region itself, do but make more striking the predominance in interest of that one position known comprehensively, but up to the present somewhat indeterminately, as the Isthmus. Here again the element of decisive value is the crossing of the roads, the meeting of the ways, which, whether imposed by nature itself, as in the cases before us, or induced, as sometimes happens, in a less degree, by simple human dispositions, are prime factors in mercantile or strategic consequence. For these reasons the Isthmus, even under the disadvantages of land carriage and transshipment of goods, has ever been an important link in the communications from East to West, from the days of the first discoverers and throughout all subsequent centuries, though fluctuating in degree from age to age; but when it shall be pierced by a canal, it will present a maritime centre analogous to the mouth of the Mississippi. They will differ in this, that in the latter case the converging water routes on one side are interior to a great state whose resources they bear, whereas the roads which on either side converge upon the Isthmus lie wholly upon the ocean, the common possession of all nations. Control of the latter, therefore, rests either upon local control of the Isthmus itself, or, indirectly, upon control of its approaches, or upon a distinctly preponderant navy. In naval questions the latter is always the dominant factor, exactly as on land the mobile army--the army in the field--must dominate the question of fortresses, unless war is to be impotent.