12 THE PROBLEMS OF EVOLUTION 



A number of phenomena are characteristic of 

 living matter. Ordinarily we think of its ability 

 to appropriate to itself selectively the materials of 

 its environment and to shape them definitely into 

 its own body, its ability to adjust itself to the 

 conditions of its environment, its ability to repro- 

 duce itself, and the physiological properties by 

 which these things are brought about. Yet in- 

 organic matter affords a number of phenomena 

 which show us that these properties may, after all, 

 be only a result of intricacy which makes possible 

 the association of a certain complex of actions. We 

 may mix substances dry without result, moisten 

 them and secure a reaction. We may mix other 

 things and secure reactions only by heating them 

 or striking them. Association of elements may be 

 accidental to the extent that they are distributed 

 by natural forces beyond our ken. One may be 

 wholly independent of another until the proper 

 conditions obtain and they may then unite. The 

 resulting compound is different from what existed 

 before. It may be able to avail itself of other con- 

 ditions and other neighboring materials to form 

 still other things. Nor must a complete change 

 always result. The mere presence of a substance 

 may enable others to react or may prevent reac- 

 tion; this phenomenon of catalysis is apparently 

 a marked acceleration or slowing of reactions 

 already possible, but under natural conditions 



