12 INTRODUCTION 



former of energy which integrates all parts into a harmoniously acting 

 machine. It interprets ail impressions from without. It projects all impulses 

 Irom withni. In its dual capacity of servant and master the brain has been 

 peculiarly susceptible to the inlluence of that combination of factors 

 described by Professor Osborn in his tetraplastic theory of evolution. The 

 cerebrum has felt more, perhaps, than any other organ, the etlects of action, 

 reaction and interaction. It has responded more extensively to its inorganic 

 and vital environment because it comprises the most highly ditlerentiated 

 tissue of the organism. It has been ecjually sensitive to the inlluence of hered- 

 ity-chromatin and lile (ecological) environment. 



The Comparison of the Human Brain with 

 THE Brains of Other Primates 



A structural comparison ol the human brain with the brains of other 

 members ot the primate order illustrates in detail the progressive modifica- 

 tions in this evolutionary process. In this comparison the brain ol man for 

 many obvious reasons is placed at the head of this order. It has been com- 

 pared with the brains of Troglodytes gorilla (gorilla), Troglodytes niger 

 (chimpanzee), Simla satyrus (orang-outang), Hylobates hoolock (gibbon), 

 Papio cynocephalus (baboon), Macacus rhesus (common old-world monkey), 

 Mycetes seniculus (South American howling monkey), Callathrix jacchus 

 (marmoset), Tarsius spectrum (tarsier) and Lemur mongoz (lemur). These 

 species comprise representatives of the highest and lowest primates. 



The lemur represents the transition from some mammalian form which 

 was predominantly arboreal. The weight, size and celerity of this animal 

 adapt it especially to a lofty arboreal habitat. 



In most of the new-world monkeys the development of a prehensile tail 

 greatly extends the range of motor activity. Due to increasing size and weight 

 of body, the larger apes have shown a progressive tendency to desert their 



