The evidence for the conception that the inorganic composition of the blood of animals with closed circulatory systems is a reproduction of the sea water in remote geological time is reviewed. The postulated changes that have occurred in the concentrations of inorganic salts in the ocean during the history of the earth serve to explain the differences in the composition of blood and sea water of the present day. It is estimated, with restrictions, that 1/12 of all geological time had intervened between the beginning of oceanic history and the fixation of the inorganic concentrations and ratios in the cytoplasm.