380 



The JoiRXAL of Heredity 



small arboreal creatures were thus free 

 to develo]3 their brains and maintain all 

 the plasticity of the generalized struc- 

 ture, which eventually enabled them to 

 go far in the process of adaptation to 

 almost any circiunstances that presented 

 themselves. 



Amongst the members of this group, 

 as in all the other mammalian phyla, 

 the potency of the forces of natural 

 selection was immensely enhanced by 

 the fact that the inquisitiveness of an 

 animal which can learn by experience — 

 i.e., is endowed with intelligence — was 

 leading these plastic insectivores into 

 all kinds of situations which were favor- 

 able for the operation of selection. 

 Various members of the group became 

 specialized in different ways. Of such 

 specialized strains the one of chief in- 

 terest to us is that in which the sense of 

 vision became especially sharpened. 



THE ORIGIN OF PRIMATES. 



Toward the close of the Cretaceous 

 period some small, arboreal, shrew-like 

 creature took another step in advance, 

 which was fraught with the most far- 

 reaching consequences, for it marked 

 the birth of the primates and the 

 definite branching off from the other 

 mammals of the line of man's ancestry. 



A noteworthy further reduction in 

 the size of the olfactory jjarts of the 

 brain, such as is seen in that of Tarsius,'* 

 quite emancipated the creature from 

 the dominating influence of olfactory 

 impressions, the sway of which was 

 already shaken, but not quite overcome 

 when its tupaioid ancestor took to an 

 arboreal life. This change was asso- 

 ciated with an enormous develoj^ment 

 of the visual cortex in the neopallium, 

 which not only increased in extent so 

 far as to exceed that of Tu])aia, but also 

 became more highly sjx'cialized in 

 structure. Thus, in the i)rimitive jjri- 

 mate, vision entirely usurjjcd the con- 

 trolling ]jlace once occupied by smell; 

 but the significance of this change is not 

 to be measured merely as the substitu- 

 tion of one sense for another. The 



visual area of cortex, unlike the olfac- 

 tory, is part of the neopallium, and 

 when its importance thus became en- 

 hanced 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 kinaesthetic sensibility, the im- 

 portance of which to an agile tree-livini; 

 animal is obvious, assist vision in the 

 conscious appreciation of the nature 

 and the various properties of the things 

 seen, and in learning to perform agil: 

 actions which are guided by vision. 



An arboreal life also added to the 

 importance of the sense of hearing; and 

 the cortical representation of this sense 

 exhibits a noteworthy increase in th'^ 

 primates, the significance of which it 

 would be difficult to exaggerate in the 

 later stages, when the simian are givin;: 

 place to the distinctively human charac- 

 teristics. 



The high specialization of the sense of 

 sight awakened in the creature the 

 curio.sity to examine the objects around 

 it with closer minuteness, and supplied 

 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 kinaesthetic 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 acti\-ities of the whole 

 neoijallium 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 wa\' is ojjcned 

 for all the muscles of the body to act 

 harmoniously so as lo ])ermit the con- 

 centration of their action for the per- 

 formance at one moment of some 

 delicate and fuielv adiusted movement. 



* "On the M(jrph )l()Ky of the Brain in the Mammalia, witli .Special Reference to that of the 

 Lemurs, Recent and li.xtinct," Trans. Linn. Soe. Lond., second series; Zoology, V^ol. 8, part 10, 

 Feb., I'KM. 



