194 LIFE AND DEATH. 



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 nniversal 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 in- 

 crease, and disappears when it has reached a certain 

 limiting value (maximum)." This law, proved by 

 Sachs, the German botanist, in i860, 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 nimisl' the p/Sei' ayav of 

 Theognis, and the biblical phrase " omnia in mensura 

 et niimero et pondered 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 sub- 

 stances, which can stand no extremes either from the 

 physical or from the chemical points of view. 



§ 2. Intrinsic Conditions. The Law of the 

 Constitution of Organs and Apparatus. 



Laiv of the Constitution of Organs and Apparatus. — 

 If we consider more highly organized beings, the 



