Knowledge concerning the sea and what lives within it has inevitably grown very gradually. This alien, darkly obscure medium was a supreme mystery to early man who sought food from it but seldom, except by accident, passed in his primitive vessel beyond the reassuring sight of land. For western man, the beginnings of knowledge came largely from the Mediterranean basin and chiefly from the Greeks, who first sought for answers to the problems of the Universe within the nature of what the Universe was composed. In this connection we need look only very briefly at the work of one person, Aristotle. His Meteorologica, although owing much to earlier authors, represents the first comprehensive attempt to explain the major problems of physical oceanography. It is concerned with salinity—then thought to be highest at the surface—with water movements including waves, and with the relation of the sea to rivers and to the atmosphere as well as to the possibilities of some subterranean source of water. To these was added the more recently encountered problem of tides, effectively absent in the Mediterranean but encountered in the Indian Ocean by the soldiers of Alexander the Great and then in the Atlantic by the explorer Pytheas.