i8 INTRODUCTION 



of their forest home. They are partially equipped to cope with some of the 

 dangers hirking without. Their forehmbs are in part freed from the responsi- 

 bilities of locomotion and so made available for acts of self defense and even, 

 to a considerable degree, for exploration. 



Actual progress in the direction of human specialization had its l^egin- 

 nings in this recession of superarboreal tendencies and the estabhshment 

 of a terrestrio-arboreal mode of life. Such a gradual recession may be seen 

 in this panorama of the primates. It is apparent in the first indecisive yet 

 promising steps that led the anthropoid out of his ancient forest dominion 

 toward the inviting plain. And fnially, with the complete recession of arboreal 

 life, there began that long journey destined to lead over every sea and into 

 every land, until no region of the earth remained for further conquest; until 

 the complete acquisition of the upright posture and the full development of 

 the hand had more and more bent the forces of nature to the designs of the 

 races of man. 



Br.\in Development Parallelixg the Progressive 

 Development of Beha\ ior 



Step by step the brain has kept pace with these progressive alterations. 

 Its newer, recently acquired portions reveal a much more conspicuous 

 response to this progress than its less plastic, more archaic elements. But 

 old parts and new alike, according to their varying degrees of susceptibility, 

 bear the certain imprint of adaptive change. Among the primates the bio- 

 logical formula determined by quadrumanal specialization and arboreal 

 habitat has worked itself out through graded stages from lemur to man. 

 The evolutional process of this gradual transition is disclosed by structural 

 modifications of the nervous system. Beginning with the Lower Primates, 

 such as the tarsiers, the advance may be traced through many intermediate 

 phases of cerebral development to its ultimate goal in the human brain. 



