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, which 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 
