The water a factor in man's mobility.
Man must be grouped with the air and water as part of the mobile envelope of the earth's surface. The mobility which maintains the unity of air and water has caused the unity of the human race. Abundant facilities of dispersal often give animal forms a wide or cosmopolitan distribution. Man, by appropriating the mobile forces in the air and water to increase his own powers of locomotion, has become a cosmopolitan being, and made the human race reflect the unity of atmosphere and hydrosphere.
Always the eternal unrest of the moving waters has knocked at the door of human inertia to arouse the sleeper within; always the flow of stream and the ebb of tide have sooner or later stirred the curiosity of the land-born barbarian about the unseen destination of these marching waters. Rivers by the mere force of gravity have carried him to the shores of their common ocean, and placed him on this highway of the world. Then from his sea-girt home, whether island or continent, he has timidly or involuntarily followed the track which headland-dotted coast, or ocean current, or monsoon, or trade wind marked out for him across the pathless waters, so that at the gray dawn of history he appears as a cosmopolite, occupying every part of the habitable earth.
These sporadic oversea wanderings, with intervals of centuries or milleniums between, opened to his occupancy strange and remote lands, in whose isolation and new environment he developed fresh variations of mind, body and cultural achievements, to arm him with new weapons in the struggle for existence. The sea which brought him bars him for a few ages from his old home, till the tradition of his coming even is lost. Then with higher nautical development, the sea loses its barrier nature; movements of people, and trade recross its surface to unite those who have been long severed and much differentiated in their mutual remoteness. The ensuing friction and mingling weed out the less fit variations of each, and combine in the new race the qualities able to fortify a higher type of man. Not only seas and oceans, but also mountains and deserts serve to isolate the migrant people who once has crossed them; but wastes of water raise up the most effective barriers.
Oceans and seas in universal history.
The transformation of the ocean into a highway by the development of navigation is a late occurrence in the history of man and is perhaps the highest phase of his adaptation to environment, because an adaptation which has placed at his disposal that vast water area constituting three-fourths of the earth's surface from which he had previously been excluded. Moreover, it was adaptation to an alien and hostile element, whose violent displays of power recurrently stimulated the human adjustment between attack and defense.
Because adaptation to the sea has been vastly more difficult than to the land, commensurate with the harder struggle it has brought greater intellectual and material rewards. This conquest of the sea is entitled to a peculiarly high place in history, because it has contributed to the union of the various peoples of the world, has formed a significant part of the history of man, whether that history is economic, social, political or intellectual. Hence history has always staged its most dramatic acts upon the margin of seas and oceans; here always the plot thickens and gives promise of striking development. Rome of the seven hills pales before England of the "Seven Seas."