OF THE UNITY OF THOUGHT. 705 



it prevented him from clearly defining and accurately 

 gauging the validity of the purely mechanical concep- 

 tions which he had expounded in his ' First Principles.' 

 This task has since been performed by a large amount of 

 penetrating criticism, and not least by the change which 

 has come over scientific thought itself. 



The main result of this may be stated in the thesis 

 that all purely mechanical processes are reversible and, 

 as Spencer himself stated, rhythmical, and in the long- 

 run self-repeating. In consequence of this property 

 the property of all so-called conservative systems 

 mechanism, however complicated, is incapable of explain- 7i. 

 ing that peculiarity of organic and mental life which is mechanical 



phenomena. 



the main characteristic of progress, and which consists 

 in something more than mere change or rearrangement. 

 jSTow it was just this something more in which the 

 mechanical scheme of Spencer was deficient, but which 

 seemed to be supplied by the principles of natural 

 selection and adaptation introduced into biology by 

 Darwin and Lamarck. What] thinkers of the older and 

 idealistic schools had tried to define by various terms such 

 as " vital force " (Stahl and Bichat), " purpose and 

 finality " (Kant), " nisus formativus " (Blumenbach), 

 " the Idea " (Hegel, Lotze, Claude Bernard), " inherent 

 tendency " (' Zielstrebigkeit,' von Baer), seemed to be 

 supplied by the conception of an inherent teleology of 

 nature which found expression in an entirely novel 

 vocabulary such as " natural selection," " adaptation," 

 " survival of the fittest," " struggle for existence," &c., 

 terms which received a purely causal or mechanical 

 explanation on the foundation of two empirical facts 

 VOL. iv. 2 Y 



