190 CHARLES DARWIN. 



The third principle is " the direct action of the 

 excited nervous system on the body, independently 

 of the will, and independently, in large part, of 



habit." 



. 



By showing that the expressions of emotions could 

 thus be explained naturally, Darwin undermined the 

 position taken up by Sir Charles Bell, that the muscles 

 used in producing expression were created for this 

 special end. 



In 1876 he re-commenced geological work, bringing 

 out his previous works on " Volcanic Islands/' and on 

 " South America," as a single volume. In this year 

 too he wrote (November 16th) a most interesting 

 letter to James Geikie, offering an explanation of the 

 large stones standing in an upright position in the 

 drift of the south of England. He had noticed the 

 same thing with the flints in the red clay left upon the 

 chalk as a residuum after the action of solvent agencies 

 on the latter. This position he explained was due to 

 the movement following the slow subsidence of parts of 

 the clay as the chalk beneath dissolved, the flints ar- 

 ranging themselves along the lines of least resistance. 

 This suggested to him the view that the flints in the 

 drift are to be explained by the subsidence, during the 

 warmer climate which followed the glacial period, of 

 alternate layers of snow and drift accumulated during 

 the winters and summers respectively, of the cold 

 period itself. 



This interesting view will, Geikie believes, come to 

 be accepted as the truth. 



