The conditions that obtained when life had not yet emerged from the oceans have not subsequently changed a great deal for the cells of the human body, bathed by the primordial wave which continues to flow in the arteries. Our blood in fact has a chemical composition analogous to that of the sea of our origins, from which the first living cells and the first multicellular beings derived the oxygen and the other elements necessary to life. With the evolution of more complex organisms, the problem of maintaining a maximum number of cells in contact with the liquid environment could not be solved simply by the expansion of the exterior surface: those organisms endowed with hollow structures, into which the sea water could flow, found themselves at an advantage. But it was only with the ramification of these cavities into a system of blood circulation that distribution of oxygen was guaranteed to the complex of cells, thus making terrestrial life possible. The sea where living creatures were at one time immersed is now enclosed within their bodies.