THE STRUCTURE AND FUXCTI0X3 OF XERVE-TISSUE. 33 



movement takes place with sufficient frequency, it cannot by 

 any effort of the will become again dissociated ; as is the case, 

 for instance, with the associated movement of the eyeballs, 

 which does not begin to obtain till some days after birth, but 

 which then soon becomes as closely organized as any of the 

 associated movements in the nmscles of the limbs * 



And if this is the case even in the life-time of individuals, 

 we can scarcely wonder that in the life-time of species heredity 

 with natural selection sliould still more completely adapt the 

 anatomical plan of ganglia, with tlieir attached nerves, to the 

 performance of the most useful — i.e., the most habitual — 

 actions. Thus we may see in a general way how such nei'vous 

 machinery may at last come to be differentiated into specially 

 distributed anatomical structures, wdiich, on account of their 

 special distribution, are adapted to minister only to particular 

 co-ordinations of muscular movements. That is to say, we 

 are thus able to understand the rise and development of 

 Keflex Action. 



* Mr. Darwin called my attention to the following passage in the writings 

 of Lamarck (Phil. Zool., tom. ii, pp. 318-19) : — " Dans toute action, le 

 fluicle des nerfs qui la proToque, subit un mouvement de deplacement qui y 

 donne lieu. Or, lorsque cette action a ete plusieurs fois repetee, il n'est pas 

 douteux que le fluide qui I'a executee, ne se soit fraye uue route, qui lui 

 derient alors d'autant plus facile a parcourir, qu'il I'a effectivenient plus 

 souvant francliie, et qu'il n'ait lui-meme une aptitude plus grand a suirre 

 cette route frayee que celles qui le sout moins." 



