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 way for the chemical composition of the vital medium—and for the other ambient physical conditions, such as atmospheric pressure.

It is therefore a law of universal scope, a regulating law, as it were, of life. Life is a function of extrinsic variables, water, air, heat, the chemical composition of the medium, and pressure. "Every vital phenomenon begins to be produced, starting from a certain stage of the variable (minimum), becomes more and more vigorous as it increases up to a determinate value (optimum), weakens if the variable continues to increase, and disappears when it has reached a certain limiting value (maximum)." This law, proved by Sachs, the German botanist, in 1860, apropos of the action of temperature on the germination of plants, by Paul Bert in 1875, apropos of the action of oxygen and of atmospheric pressure on animals, and already formulated at that time by Claude Bernard, was illustrated by Leo Errera in 1895. It is a law of moderation. It expresses La Fontaine's "rien de trop" Terence's "ne quid nimis," the [Greek: mêden agan] of Theognis, and the biblical phrase "omnia in mensura et numero et pondere." L. Errera sees the profound cause of this optimum law in the properties of the living protoplasm, which are mean properties. It is semi-liquid. It is composed of albuminoid substances, which can stand no extremes either from the physical or from the chemical points of view.

§ 2.

Law of the Constitution of Organs and Apparatus.—If we consider more highly organized beings, the