DEVELOPMENT BY NATURAL SELECTION. 109 



now well aware of the difficulty in forming any con- 

 ception of the mechanism of heredity. They put the 

 matter aside with the excuse that there are many 

 things of which we do not yet understand the mechanism, 

 and that this is one of them. They also argue, though 

 now somewhat faintly, that the success of mechanistic 

 explanations elsewhere in physiology justifies the belief 

 that some day a mechanistic conception of heredity 

 will come into view. This attitude would be justified 

 if it were possible to see any glimmer of a mechanistic 

 explanation of heredity, and if it were actually the case 

 that mechanistic explanations generally showed any 

 signs of fulfilling the hopes that were formed of them 

 about the middle of last century. But the difficulty 

 about heredity is, not that we do not know the details 

 of its mechanism, but that we cannot remotely conceive 

 any possible theory on mechanistic lines, unless it be 

 the inherently ridiculous " box-within-box " theory. 

 Moreover, the difficulty about a mechanistic theory of 

 heredity is only another aspect of difficulties which 

 loom clearer and clearer as physiology advances, and 

 which are inherent in mechanistic theories of all vital 

 processes. 



Before going further let me dwell shortly on this last 

 point. What distinguishes living organisms from other 

 things is that they actively maintain their characteristic 

 structure and activities during their life-history. On 

 superficial examination the structure of an organism 

 may appear to be a mere aggregate of material, as does 

 an ordinary inorganic structure. But closer examina- 

 tion shows that living structure is in a constant state 

 of flux — of breaking down and building up of material ; 



