But, whatever may happen in the future, the great changes which have taken place in the relative importance of the peninsulas and islands of Europe in the short span of twenty centuries, sufficiently prove that geographical features exercise a varying influence at different epochs. That which at one time was looked upon as a great natural advantage may become, in course of time, a serious disadvantage. Thus the numerous inlets and gulfs enclosed by mountain chains, which favoured the rise of the cities of Greece, and gave to Athens the dominion of the Mediterranean, now constitute as many obstacles to their connection with the existing system of European communications. That which in former times constituted the strength of the country has become its weakness. In primitive times, before man ventured upon the seas, these bays and gulfs formed insurmountable obstacles to the migration of nations; at a later date, when the art of navigation had been acquired, they became commercial high-roads, and were favourable to the development of civilisation; and at the present time they are again obstacles in the way of our road-builders and railway engineers. {16} V.—CLIMATE. The influence exercised by the relief of the land and the configuration of the coasts varies in different ages, but that of climate is permanent. In this respect Europe is the most favoured region of the earth, for during a cycle of unknown length it has enjoyed a climate at once the most temperate, the most equable, and the most healthy of all continents. Owing to the inland seas which penetrate far into the land, the whole of Europe is exposed to the modifying influence of the ocean. With the exception of Central Russia, no part of Europe is more than 400 miles from the sea, and, as most of the mountains slope from the centre of the continent towards its circumference, the influence of the sea breezes is felt throughout. And thus continental Europe, in spite of its great extent, enjoys the advantages of an insular climate throughout, the winds passing over the ocean moderating the heat of summer and tempering the cold of winter. The continuous north-easterly movement of the waters of the Atlantic likewise has a favourable effect upon the climate of Europe. After having been heated by a tropical sun in the Gulf of Mexico, the gulf-stream issues through the Strait of Florida, and, spreading over the Atlantic, takes its course towards the coasts of Europe. This enormous mass of warm water, equal in volume to twenty million rivers as large as the Rhone, brings the warmth of southern latitudes to the western and northern shores of Europe. Its influence is felt not only in the maritime countries of Western Europe, but to some extent as far as the Caspian and the Ural Mountains.