Quinton’s central theoretical claim, articulated at length in his 1904 monograph and elaborated in the 1912 second edition. The claim built on the comparative physiology of blood and seawater mineral composition and combined it with an evolutionary-constancy argument: life originated in the sea, the internal milieu of higher animals preserves marine mineral ratios, and therefore the ideal therapeutic fluid is isotonic seawater rather than synthetic saline.

The mechanism is not supported by modern physiology. Seawater and human blood plasma share a similar total salt concentration when diluted to isotonicity, but the ion ratios differ in clinically relevant ways: seawater contains magnesium, sulfate, and other ions at concentrations that do not match plasma. Modern intravenous fluids (isotonic saline, Ringer’s lactate, Plasma-Lyte) are formulated from purified pharmaceutical-grade salts to match physiological ion concentrations, not from filtered seawater. The Law of Marine Constancy as a mechanistic theory has no foothold in contemporary physiology or evolutionary biology textbooks.