THE STRUCTURE AND FUNCTIONS OF NERVE-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 muscles 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 should still more completely adapt the 

 anatomical plan of ganglia, with their 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 nervous 

 machinery may at last come to be differentiated into specially 

 distributed anatomical structures, which, 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 

 Keilex Action. 



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

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

 fluide des nerl's qui la provoque, subit un mouvernent de deplacement qui y 

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

 douteux que le fluide qui l'a executee, ne se soit fraye une route, qui lui 

 devient alors d'autant plus facile a parcourir, qu'il l'a effectivement plus 

 souvant franchie, et qu'il n'ait lui-meme une aptitude plus grand a 6uivre 

 cette route frayee que celles qui le sont moins." 



