This brings us face to face with the fact that it may be less expensive to establish and protect a naval base situated on a little island, even if an artificial harbor has to be constructed, than to establish and protect a base on a large island, even if the base on the large island has a large natural harbor and can be more easily defended against bombardment from the sea. It would be cheaper, for instance, to protect a base on Culebra than one at Guantanamo, or even Samana, if the enemy commanded the sea; and cheaper to protect a base on the forbidding rocks of Polillo or Guam than on the large and fertile island of Luzon, with its extensive gulfs and bays, in many of which a fleet in command of the sea could land its force; because protecting a base on a large island would require covering a very large area, and perhaps a long extent of coast. Aircraft may exercise an important influence on the choice of the position of a base, perhaps in the direction of choosing a base on a large island rather than on a small one; since the great speed of aircraft tends to lessen the importance of having the base out a great distance from home--so far as purposes of scouting are concerned. It seems probable also that aircraft will soon be recognized as inherently adapted to preventing the landing of hostile troops, by dropping bombs on the troops, while they are in process of disembarkation, while proceeding in small boats to the shore, and while in the act of landing on the beach, with their guns, ammunition, supplies, horses, and impedimenta of various kinds. _Co-operating Bases_.--Discussion of the relative values of positions for bases, say in the Caribbean, should not blind our eyes to the fact, however, that no nation is prevented from establishing as many bases as it needs, wherever its flag may float; that the United States, for instance, is not debarred from establishing permanent naval bases at both Guantanamo and Culebra, should such a procedure seem desirable. The fact that each locality has advantages that the other does not have, suggests the idea that two bases, placed in those localities, would form a powerful combination. In fact, the great value of the position of Culebra being its distance toward the enemy, which necessitates a great distance away from our continental coast, and a long line of communications from that coast suggest an intermediate base as a support and stepping-stone. Analogous cases are seen in all the countries of Europe, in the fortresses that are behind their boundary-lines--the fortresses existing less as individuals than as supporting members of a comprehensive scheme.