loio EVOLUTIONAL MODIFICATIONS 



stage to stage through the prmiati' order until it reaches its highest expression 

 in the human brain. The acl\ anee as indicated by increments in the pyram- 

 idal system is clearly seen in tlie lower primates. The marmoset may 

 express this progress somewhat leebl\ , but the new-world monkeys reassert 

 the growing imjjortanci' ol ncokmetie organization. Passing upward to the 

 intermediate primates, the acKanee is more apparent, with the possible 

 exception of thi- gibbon in w hich there appears to be some hesitation. This 

 seeming indecision is due no doubt to the lact that this animal has become 

 largely ckpendent upon its upper extremities lor locomotion and thus has 

 deprived its manual dillerentiation of a real opportunity to progress. In 

 olhtM" words, the gibbon has carru'd brachiation to an extreme degree. 



In the higher anthropoids the upward progress is resumed with unmis- 

 takable decisi\eness. The orang, the chimpanzee and the gorilla all show 

 marked acKanee in the [nramidal system which hnally reaches its highest 

 pro])ortions m the Ijrain ol man. 1 he progressi\'e expansion allecting this 

 iieokinetic s\ stem represents a characteristic de\elo])ment in the primates. 

 It is gra|:)lHcall\ (k'lnonstrated in tabulations ol tlu' planimetric coeflicients 

 of the pyramid in which man occupies the highest position, while bom him 

 the line descends through mcasurabk- gradations to the higher anthropoids, 

 to the intermediate primates, and linall\ reaches its lowest expression in the 

 tarsier. The tlillerence betwot'ii the latter and man is approximatelx lilteen 

 hundre'dt hs, which leprtst'iits a remarkable mcix'nicnt in tlu' \olunu' ol the 

 human pyramid. 1 hat the dillerenee betwoen these two contrasted lorms 

 does not appear more pronounced is due to the lact that the pxramidal 

 tract must accommodate itsell to a portion ol the neuraxis through which 

 many other ec|ually im|:)oi"tanl conduction systems make their wa\. The 

 pyramidal system, lor (.■xample, represt'iits a conduction ])athwa\ trom the 

 motor or preeeiitral area ol the hemispheri'. Its axons arise in tlu- giant 

 cells ol" Bctz, leave the cortex, become cou\ergenl in the internal capsule 



