86 A CHEMICAL SIGN OF LIFE 



nothing else than, a propagation of chemical changes, 

 the propagation in wave form being due to the restora- 

 tion of an unstable equilibrium disturbed by the increase 

 of metabolism at the point of stimulus. This prop- 

 agation is always toward the point where there is less 

 chemical activity, as measured by the carbon dioxide 

 output. 



