SECTION OF COVEKED-EYED MEDUS.E. 87 



intelligence, and the difficulty which intelligence 

 now experiences in trying to conceive of such 

 relations as disjoined. Thus it is that, even during 

 the history of an individual intelligence, 'practice 

 makes perfect,' by frequently repeating the needful 

 stimulations along the same lines of cerebral dis- 

 charge, so rendering the latter ever more and more 

 permeable by use. Thus it is that a child learns 

 its lessons by frequently repeating them ; and thus 

 it is that all our knowledge is accumulated."* 



Rate of Transmission of Stiniull. 



The rate at which contraction-waves traverse 

 spiral strips of Aurelia is variable. It is largely 

 determined by the length and width of the strip ; so 



* I have associated the above theory of nerve-genesis with the 

 name of Mr. Spencer, because it occupies so prominent a place in 

 his " Principles of Psychology." But from what I have said in 

 the text, I think it is clear that the theory, as presented by Mr. 

 Spencer, consists of two essentially distinct hypotheses — the one 

 relating to the formation of nerve-tissue out of protoplasm, and 

 the other to the increase of functional capacity in a nerve-fibre 

 by use (a third hypothesis of Mr. Spencer relating to the 

 formation of ganglion-tissue does not here concern us). The 

 latter hypothesis, however, ought not to be associated with Mr. 

 Spencer's name without explaining that it has likewise occurred to 

 other writers, the first of which, so far as I can ascertain, was La- 

 marck, who says, "Dans toute action, le fluide des nerfs qui la 

 provoque, subit un mouvement de deplacement qui y donne lieu. 

 Or, lorsque cette action a ete plusieurs fois repe'tce, il n'est pas 

 douteux que le fluid qui I'a exe'cutee, ne se soit fraye une route, 

 qui lui devient alors d'autanfc plus facile a parcourir, qu'il l*a 

 effectivement plus souvant francliie, et qu'il n'ait lui-meme une 

 aptitude plus grand a suivre cette route frayee que celles qui le 

 sunt niuins." ("Phil. ZooL," torn ii. pp. 318-19.) 



