714 THE HIGHER ANTHROPOIDS 



primates. Notable rxccptions must he taken m llie cases of tarsius and 

 myeetes whieli exceed all ol the antliropoids m the size ot Deiters' nucleus. 

 This high specialization results from the lU'W responsibilities imposi'd upon 

 the Ixilancing mechanism by the addition to the animal's motor organization 

 of the |)reheiisile tail or its llight-like leaps. Although some variations do 

 occur in the degree of differentiation manifested by the neural elements of 

 the balancing mechanism, this functional acti\it\- is in reality a basic and 

 ancient one which has impressed itsell upon the entire vertebrate phylum. It 

 has had its chief incentives to exiJansion in tlu' assumption o( those loco- 

 motor characteristics which become neccssar\ to lift the bod\ and support it 

 off tlie ground by means of the extremities. 



The motor responses determined b\- the essential receptors ol this 

 activitx l)elong to the most ancient type of mol)ilit\ called paleokinesis. 

 Although age alone does not necessarily impart inllexibility, it is true that 

 those elements in the nervous system which ha\e the greatest antiquity 

 represent functional needs whose proper satisfaction demands a maximum of 

 structural development. This, for exampk-, applies particularly to such 

 archaic neural control as that regulating respiration, cardiac acti\ity or the 

 reactions of the gastrointestinal tract, flence tlu' binction inherent in the 

 balancing mechanism ol' the body would need to de\elop its full cjuota of 

 efficienc\- to be of any service w hatsoex'er. In this light it nia\ beclainu'd that 

 the vestibular compkvx in the central nersous system ol \ertebrates is of 

 little \alue as indicating the evolutional process. To a certain extent there is 

 truth in this xiew. On tin- other hand, those pi'diliai' exceptions in which the 

 fundamental organization manifests marked lluctuations either above the 

 usual standard or something below it, haxc a spi'cial significance in the prob- 

 lem of adaptation. They afford illuminating txamples of adaptive radiation 

 in highly specialized functional departments ol the organism. Par, therelore, 

 from being insignificant as bearing uijon the e\(ilutionai\ ]:)rocess in the 



