6 THE ORIGIN AND EVOLUTION OF LIFE 



more or less clearly interpretable in terms of action, reaction, 

 and interaction, we are compelled to believe that complex forms 

 will also prove to be interpretable in the same terms. None 

 the less, if we affirm that the entire trend of our observation 

 is in the direction of physicochemical explanations rather than 

 of vitalism and vitalistic hypotheses, this is very far from 

 affirming that the explanation of life is purely materialistic, 

 or purely mechanistic, or that any of the present physico- 

 chemical explanations are final or satisfying to our reason. 



Chemists and biological chemists have very much more to 

 discover. May there not be in the assemblage of cosmic chem- 

 ical elements necessary to life, which we shall distinguish as 

 the "/i/c element s,^^ some knoivn element which thus far has 

 not betrayed itself in chemical analysis ? This is not impossi- 

 ble, because a known element like radium, for example, might 

 well be wrapped up in living matter but remain as yet unde- 

 tected, owing to its suffusion or presence in excessively small 

 quantities or to its possession of properties that have escaped 

 notice. Or, again, some unknown chemical element, to which 

 the hypothetical term bion might be given, may lie awaiting 

 discovery within this complex of known elements. Or an 

 unknown source of energy may be active here. 



It is, however, far more probable from our present state of 

 knowledge that unknown principles of action, reaction, and 

 interaction between living forms await discovery; such prin- 

 ciples are indeed adumbrated in the as yet partially explored 

 activities of various chemical messengers in the bodies of 

 plants and animals. 



We are now prepared for the fourth of our leading questions. 

 If it be determined that the evolution of non-living matter 

 follows certain physical laws, and that the living world con- 



