EVOLUTION OF MAN SMITH. 563 



of Tiipaia, but also bccamo more highly specialized in structure. 

 Thus, in the primitive primate, vision entirely usurped the controlling 

 place once occupied by smell; but the significance of this change is 

 not to be measured merely as the substitution of one sense for another. 

 The A-isual area of cortex, unlike the olfactory, is part of the neo- 

 pallium, and when its importance thus became enhanced the whole 

 of the neopallium felt the influence of the changed conditions. The 

 sense of touch also shared in the effects, for tactile impressions and 

 the related kinsesthetic sensibility, the importance of which to an 

 agile tree-living animal is obvious, assist vision in the conscious ap- 

 preciation of the nature and the various properties of the things seen, 

 and in learning to perform agile actions which are guided by vision. 



An arboreal life also added to the importance of the sense of hear- 

 ing; and the cortical representation of this sense exhibits a note- 

 worthy increase in the primates, the significance of which it would 

 be difficult to exaggerate in the later stages, when the simian are 

 gi^^g place to the distinctively human characteristics. 



The high specialization of the sense of sight awakened in the 

 creature the curiosity to examine the objects around it witli closer 

 minuteness, and su])plied guidance to the hands in executing more 

 precise and more skilled movements than the tree shrew attempts. 

 Such habits not only tended to develop the motor cortex itself, 

 trained the tactile and knisesthetic senses, and linked up their cortical 

 areas in bonds of more intimate associations with the visual cortex, 

 but they stimulated the process of specialization within or alongside 

 the motor cortex of a mechanism for regulating the action of that 

 cortex itself — an organ of attention which coordinated the activities 

 of the whole neopalHum so as the more efficiently to regulate the 

 various centers controlling the muscles of the whole body. In this 

 way not only is the guidance of all the senses secured, but the way is 

 opened for all the muscles of the body to act harmoniously so as to 

 permit the concentration of their action for the performance at one 

 moment of some delicate and finely adjusted movement. 



In some such way as this there was evolved from the motor area 

 itself, in the form of an outgrowth })laced at first immediately in front 

 of it, a formation, which attains much larger dimensions and a more 

 pronounced specialization of structure in the primates than in any 

 other order; it is the germ of that great prefrontal area of the human 

 brain which is said to be " concerned with attention and the general 

 orderly coordination of psychic processes,"^ and as such is, in far 

 greater measure than any other part of the brain, deserving of being 

 regarded as the seat of the higher mental faculties and the crowning 

 glory and distinction of the human fabric. 



• J. S. Bolton "The Functions of the Frontal Lobes," Brain, 1903. 



