638 THE WONDER OF LIFE 



determined ; the chains are of iron. In the succession 

 of organic changes there are alternatives, as a species may 

 show in splitting into two or more equally successful 

 species ; the creatures are genuine agents in a fashion 

 quite different from that of streams of water or ice which 

 diverge and combine ; in short, the mechanical categories 

 are transcended. We are not unaware of the analogies 

 between the inorganic sequence of changes and the evolu- 

 tion of organisms that have often been indicated, and that 

 Herbert Spencer made much of ; they are fascinating 

 but unconvincing. It is said, for instance, that ' the 

 process by which worlds emerge from the primal nebula 

 depends upon the conflict of attractive and repulsive 

 forces ', just as the process by which species emerge from a 

 primal stock depends upon the struggle for existence. But 

 ' the conflict of attractive and repulsive forces ' is a phrase 

 which must be used in a large and metaphorical sense 

 which is what Darwin said in reference to the phrase 

 ' struggle for existence '. 



What we have in the realm of organisms is a continual 

 creation and experimenting on the one hand, and a con- 

 tinual sifting on the other, but the sifting is often a very 

 gentle process. At the best, in comparing inorganic and 

 organic ' evolution ', we do not get beyond formal re- 

 semblances. 



The reasons why many biologists cannot accept as 

 adequate any mechanical description of organic evolution 

 centre in the nature of the organism. The organism plays 

 such an active part. It is active in its variations, which are 

 experiments in self-expression, though some environmental 

 stimulus may pull the trigger which liberates them. It is 

 in some measure active even in the process of natural 



