86 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



they were not able, eventually they became able to 

 draft off enough molecular disturbance to carry on 

 a stimulus adequate to cause a muscular contraction. 

 It will be observed that the difference thus ex- 

 pressed is one of considerable importance; for now 

 the facts cease to lend any countenance to Mr. 

 Spencer's theory touching the formation of nerves 

 out of protoplasm, or other contractile material. 

 They continue, however, to countenance his views 

 touching the improvement of functional capacity 

 which nerve-fibres, when already formed, undergo 

 by use; and this, wliich is in itself an important 

 matter, is the point with which I was mainly con- 

 cerned in the lecture of the Royal Institution just 

 alluded to. For, as I then observed, in this theory 

 of nerve-fibres becoming more and more functionally 

 developed by use, we probably have a physical ex- 

 planation, which is as full and complete as such an 

 explanation can ever be, of the genesis of mind. 

 " For from the time that intelligence first dawned 

 upon the scene of life, whenever a new relation had 

 to be established in the region of mind, it could only 

 be so established in virtue of some new line of dis- 

 charge being excavated through the substance of 

 the brain. The more often this relation had to 

 be repeated in the mind, the more often would 

 this discharge require to take place in the brain, 

 and so the more easy would every repetition of 

 the process become. . . . Thus it is, according to 

 the theory, that there is always a precise proportion 

 between the constancy with which any relations 

 have been joined together during the history of 



