Chap. I.] OF A NERVOUS SYSTEM. 21 



shape of the part struck, or owing to the fact that an 

 impression made upon one region — say a tentacle — is 

 usually followed pretty quickly by a second impression 

 made by the same moving object upon another surface 

 region, that an impression or stimulus comes, as Herbert 

 Spencer points out, habitually to traverse a certain path. 

 Much of the molecular motion consequent upon the 'stim- 

 ulus' is drafted along this path. This being so, the stim- 

 ulus necessarily tends to excite contractions in particular 

 parts, and thus leads to the differentiation of the pro- 

 toplasm of such parts into the more or less definite 

 Muscular Tissue found in some of the lowest animal 

 organisms. 



This, however, is not all. The localization of the path 

 of the stimulus leads to structural results of another 

 kind. Whenever external impressions produce molecular 

 movements which traverse with frequency some definite 

 path, the transference of such movements is made easier 

 by each repetition, and there is a tendency to the initia- 

 tion of a structural change along this path. Just as the 

 frequent repetition of contractions in certain parts of the 

 protoplasm leads to the production of distinct muscular 

 tissues, so the frequent passage of a wave of molecular 

 movement along a definite track through protoplasm or 

 through juxtaposed plastides, leads to the differentiation 

 of the protoplasm thus acted upon. At first the actual 

 structural change may be unrecognizable, although a * line 

 of discharge ' may have become established along which 

 impressions are habitually transmitted with ease, as seems 

 to be the case with the majority of Medusae. Ultimately, 

 however, by the constant repetition of such a process, we 

 should have the gradual formation of an actual ' Nerve 

 Fibre' — this being a tissue element whose special use and 

 duty is to transmit molecular movement, and which may 



