In order to interpret these figures correctly we must remember that we are dealing with two different antagonisms, one between the salts with univalent and bivalent metals and the other between Mg and Ca. The former antagonism is satisfied by the addition of Mg, inasmuch as enough Mg was present for this purpose in all solutions. What was lacking was the balance between Mg and Ca. The experiments in Table XIX therefore answer the question of the ratio between Mg and Ca. If we consider only the concentrations of Mg between 2.5 and 10.0 c.c. 3/8 m MgCl{ 2}--which are those closest to the normal concentration of Mg in the sea water--we notice that C{ Ca} must vary in proportion to C{ Mg}. If we now combine the results of this and the previous paragraph we may express them in the form of the _theory of physiologically balanced salt solutions, by which we mean that in the ocean (and in the blood or lymph) the salts exist in such ratio that they mutually antagonize the injurious action which one or several of them would have if they were alone in solution._[273] This law of physiologically balanced solutions seems to be the general expression of the effect of changes in the constitution of the salt solutions for marine or all aquatic organisms.
[273] This theory was first expressed by the writer in _Am. Jour. Physiol._, 1900, iii., 434.
This chapter would not be complete without an intimation of the rôle of buffers in the sea water and the blood, by which the reaction of these media is prevented from changing in a way injurious to the organism. These buffers are the carbonates and phosphates. Instead of saying that the organisms are adapted to the medium, L. Henderson has pointed out the fitness of the environment for the development of organisms and one of these elements of fitness are the buffers against alterations of the hydrogen ion concentration.[274] The ratio in which the salts of the different metals exist in the sea water is another. It is obvious that the quantitative laws prevailing in the effect of environment upon organisms leave no more room for the interference of a "directing force" of the vitalist than do the laws of the motion of the solar system.
[274] Henderson, L., _The Fitness of the Environment_. See also Michaelis, L., _Die Wasserstoffionenconzentration_. Berlin. 1914.