INTRODUCTION 13 



more inaccessible arboreal retreats and approach nearer to the ground. 

 The transition IVom semi-arboreal habitat to completely terrestrial life has 

 culminated m the development of the erect posture and plantigrade loco- 

 motion with tree climbing but an incident in bipedal adjustment. 



CERTAIN FUNDAMENTAL ADJUSTMENTS OF BEHA\TOR 



In making the comparison of these primate brains, the chief object has 

 been to note the structural changes whereby the evolutional process has 

 been advanced. Certain fundamental components of behavior have been 

 simultaneously studied in relation to these structural modifications. These 

 components include the following: 



The development of the prehensile tail, its recession and linal dis- 

 appearance 

 The progressive adaptation of the hand for manual performances other 



than those concerned in locomotion 

 The adjustment of simultaneous movements in the eyes, head and hands 



necessary to the execution of skilled acts 

 The gradual assumption of the erect posture 

 The increase of volitional control of the arms and the legs 

 The amplification ot postural regulation in the body and extremities 

 The development and perfection of binocular vision 

 The progressive readjustments of visual, auditory and equilibratory 



reactions in passing from arboreal to ground-living habits 

 The modification of the automatic associated movements induced by 



such adjustments. 



In comparing these species one with another, it is possible to observe 



the reflection of certain behavioral adjustments as they are mirrored by the 



organic constituents of the brain. In some cases, the range of variation in 



these behavioral reactions is so wide as to create most outspoken differences. 



