72 THE ORIGIN AND EVOLUTION OF LIFE 



dioxide;^ in respiration carbon dioxide raises the hydrogen con- 

 centration of the blood; the phosphates restrain this tendency, 

 while the breathing apparatus, in response to stimuli from the 

 respiratory centres irritated by the hydrogen, throws out the 

 excess of this element. 



Thus there evolved step by step the function of coordinating 

 and correlating the activities of various parts of the life organ- 

 ism remote from each other by means of chemical messen- 

 gers adapted to effect not only a general interaction between 

 general parts, but also special interactions between special 

 parts; for it is now known that, as Huxley prophesied (see 

 p. 57), certain chemical messengers do reach particular groups 

 of living elements and leave others entirely untouched. For 

 example, the enzyme developed in the yeast ferment produces 

 a different result in each one of a series of closely related carbo- 

 hydrates.- 



These chemical messengers are doubtless highly diversified; 

 they are now known to exist in at least three or four forms, 

 as follows: 



First: The simplest forms of such chemical messengers are 

 those which originate as by-products of single chemical reactions. 

 For example, the carbon dioxide (CO2) liberated in the cell by 

 the reactions of respiration acts at a distance on other portions 

 of the cell and of the organism. Thus every cell of the body 

 furnishes in the carbon dioxide which it ehminates a chemical 

 messenger,'^ since under normal conditions the carbon dioxide 

 of the blood is one of the chief regulators of the respiratory 

 centre, influencing this centre by virtue of its acidic properties. 



Second: Of prime importance among the various ''chemical 

 messengers" are the organic catalyzers'^ known as enzymes, the 



' W. J. Gies. - Moore, F. J., 1915 p. 170; Loeb, Jacques, 1906, pp. 21, 22. 



^ Abel, John J., 1915, p. 168. ■'Loeb, Jacques, 1906, pp. 8, 28. 



