ORIGIN AND MAINTEN. OF OPT. ACTIVITY 45 



by glucose as a component of the glucoside and by gen- 

 tiobiose as a component of the gentiobioside is responsible 

 for the fact that they are fomid in nature as indicated. 



4. Procedures Used by Nature for Mainfaiuing Optical 

 Purity and Establishing a ''Fixed Internal Milieu." The 

 evolution of living beings has consisted in a gradual in- 

 crease of the number of fixed parameters of the internal 

 milieu. For example, in the transition from poikilother- 

 mic into homoiothermic animals, the body temperature has 

 been fixed at a constant value. The non-dependence on 

 the temperature of the external medium has given the 

 homoiotherms important advantages over the cold-blooded 

 animals in natural selection. 



Considering such cases of specific fixity acquired by the 

 internal medium, Claude Bernard made his famous state- 

 ment : * ' La fixite du milieu interieur est la condition de la 

 vie libre " ( " The fixity of the internal medium is the con- 

 dition of independent living"). 



The elaboration of mechanisms in living matter to 

 maintain optical purity evidently contributes to the fixity 

 of the internal milieu. The spatial parameters which de- 

 termine the asymmetry of a substance are fixed in primary 

 constituents of protoplasm in such a way that optical 

 purity is maintained. 



Barcroft (1934) notes that two methods are used by 

 nature to secure the constancy of internal medium, the 

 method of evasion and the method of correction. 

 a) Widely Different Velocities in the 

 Formation of the Two Optical Iso- 

 mers. As a mechanism of evasion nature uses a high 

 ratio of the reaction velocities. A-,/ A',,. While, in the cases 

 of catalysis more commonly encountered, this ratio is of 

 the order of 1 to 2, in enzymatic reactions the ratio reaches 

 100, 1000, or even greater values. This ratio evidently 

 determines the degree of predominance of the right or of 

 the left isomer in the substance synthesized. A great dif- 

 ference between the velocity constants, Jii and k,i, makes that 



