cation or power of common action between State and State. It was in the more open countries that larger kingdoms and empires tended to be formed. Then, as population increased, and commerce and industry grew, as naval and military forces developed, and as larger political aggregations began to supersede the smaller units, natural boundaries were found no longer to suffice. It became necessary to supplement or to replace them by artificial Frontiers, finding their origin in the complex operations of race, language, trade, religion, and war.
Here, however, I must pause to define the factors and to indicate with greater precision the various classes of Frontiers with which we are called upon to deal. I have already accepted the broad distinction between Natural and Artificial Frontiers, both as generally recognized, and as scientifically the most exact. Of all Natural Frontiers the sea is the most uncompromising, the least alterable, and the most effective. The defensive attributes of the sea receive the poet's testimony in his description of England as
compassed by the inviolate sea;
its exclusive qualities are celebrated in a not less familiar phrase:
the unplumbed, salt, estranging sea.
It is true that in one aspect the sea may be regarded as a connecting link by the artificial aid of navigation,
- Hence the failure of the attempt to defend Thessaly against the armies of Xerxes in 480 and the small number of troops collected to defend Thermopylae.