214 THE LOWER PRIMATES 



It is difficult to estimate all of the far-reaching consequences of this pro- 

 gressive differentiation of the upper extremity. The advent of the hand not 

 only brings into existence an acquisitive explorer in the environment, but, by 

 releasing the forehmbs from the responsibihty of locomotion, it inlluences 

 profoundly both the posture and the method of locomotion itself. In such a 

 capacity it dictates new tendencies in the selection of habitat. It provides 

 new means of defense and offense and thus may aflect the matter of food 

 supply and metabohsm. It becomes an instrument of investigation and con- 

 trivance, the creator of a wide range of gestures and hence of symbols, and 

 finally through its agency as a means of communication, leads on to the vocal 

 accompaniments which ultimately eventuate in verbal speech. Having such 

 importance in the synthesis of reactions \\hich characterize the complex 

 output of behavioral performances in the highest form, the significance of 

 this dorsal sensory field in the oblongata cannot be overestimated. There may 

 be certain difficulties, perhaps, in maintaining that the progressive differen- 

 tiation of the hand has been one of the chief factors in the later expansions 

 of consciousness. Yet it must be clear that the animal possessed of such a 

 discriminating organ requires more extensive sensory areas in the brain than 

 the animal not similarly equipped. The increase of these sensory areas which 

 represent the acquisitions of manual discrimination cannot fail to have a 

 widespread influence by amplifying the sensory syntheses which enter into 

 consciousness. The growing importance of the hand as a sensory organ is 

 witnessed by another interesting fact. The portion of the dorsal sensory field 

 in mycetes allotted to the transmission of sensory impulses from the head and 

 face is not proportionally large when compared with the areas for receiving 

 impressions from the extremities. Both the substantia gelatinosa and the 

 descending trigeminal tract (Trd) are relatively smaller than in lemur or 

 marmoset. The apparent reason for this relative decrease in prominence 

 may be sought in the fact that the face and head have lost some of their 



