122 



NATURE 



[September 26, 19 12 



other senses received due representation in the neopal- 

 lium, the animal's behaviour was still influenced to a 

 much greater extent by smell impressions than by 

 those of the other senses. 



This was due not only to the fact that the sense of 

 smell had already installed its instruments in, and 

 taken firm possession of, the cerebral hemisphere long 

 before the advent in this dominant part of the brain 

 of any adequate representation of the other senses, but 

 also, and chiefly, because to a small land-grubbing 

 animal the guidance of smell impressions, whether in 

 the search for food or as a means of recognition of 

 friends or enemies, was much more serviceable than 

 all the other senses. Thus the small creature's mental 

 life was lived essentially in an atmosphere of odours, 

 and every object in the outside world was judged 

 primarily and predominantly by its smell; the senses 

 of touch, vision, and hearing were merely auxiliary 

 to the compelling influence of smell. 



Once such a creature left the solid earth and took 

 to an arboreal life all this was changed, for away 

 from the ground the guidance of the olfactory sense 

 lost much of its usefulness. Life amidst the branches 

 of trees limits the usefulness of olfactory organs, but 

 it is favourable to the high development of vision, 

 touch, and hearing. Moreover, it demands an agility 

 and quickness of movement that necessitates an 

 efficient motor cortex to control and co-ordinate such 

 actions as an arboreal mode of life demands (and 

 secures, by the survival only of those so fitted), and 

 also a well-developed muscular sensibility to enable 

 such acts to be carried out with precision and quick- 

 ness. In the struggle for existence, therefore, all 

 arboreal mammals, such as the tree-shrews, suffer a 

 marked diminution of their olfactory apparatus, and 

 develop a considerable neopallium, in which relatively 

 large areas are given up to visual, tactile, acoustic, 

 kinsesthetic, and motor functions, as well as to the 

 purpose of providing a mechanism for mutually blend- 

 ing in consciousness the effects of the impressions 

 pouring in through the avenues of these senses. 



Thus a more equable balance of the representation 

 of the senses is brought about in the large brain of the 

 arboreal animal; and its mode of life encourages and 

 makes indispensable the acquisition of agility. More- 

 ov'er, these modifications do not interfere with the 

 primitive characters of limb and body. These small 

 arboreal creatures were thus free to develop their brains 

 and maintain all the plasticity of a generalised struc- 

 ture, which eventually enabled them to go far in the 

 process of adaptation to almost any circumstances 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 favourable for the operation of 

 selection. Various members of the group became 

 specialised in different ways. Of such specialised 

 strains the one of chief interest to us is that in which 

 the sense of vision became especiall}' sharpened. 



'the Origin of Primates. 



Towards 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. 



.\ noteworthy further reduction in the size of the 

 olfactory parts of the brain, such as is seen in that of 

 NO. 2239, VOL. 90] 



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 associated with an enormous develop- 

 ment of the visual cortex in the neopallium, which 

 not only increased in extent so as far to exceed that 

 of Tupaia, but also became more highly specialised 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 visual area of cortex, unlike the 

 olfactory, is part of the neopallium, and when its 

 importance thus became enhanced the whole of the 

 neopallium felt the influence of the changed condi- 

 tions. The sense of touch also shared in the effects, 

 for tactile impressions and the related kinEesthetic 

 sensibility, tlie importance of which to an agile tree- 

 living 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 agile 

 actions which are guided by vision. 



.'\n arboreal life also added to the importance of the 

 sense of hearing; and the cortical representation of 

 this sense exhibits a noteworthy increase in the pri- 

 mates, the significance of which it would be difficult 

 to exaggerate in the later stages, when the simian 

 are giving place to the distinctively human character- 

 istics. 



The high specialisation of the sense of sight 

 awakened in the creature the curiosity 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 kinajsthetic senses, 

 and linked up their cortical areas in bonds of more 

 intimate associations with the visual corlex, but they 

 stimulated the process of specialisation within or 

 alongside the motor cortex of a mechanism for regu- 

 lating the action of that cortex itself — an organ of 

 attention which co-ordinated the activities of the whole 

 neopallium so as the more efliciently to regulate the 

 various centres 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 per- 

 formance 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 

 placed at first immediately in front of it, a formation, 

 which attains much larger dimensions and a more 

 pronounced specialisation 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 

 co-ordination 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 distinc- 

 tion of the human fabric. 



[By means of lantern slides representing Dr. Scharff's 

 convincing elucidation of the modifications of the land 

 connections during Tertiary times, a drmonstration 

 was given of the wanderings of the primates, which 

 the farts of pateontolog^y and comparative anatomy 

 demand ; the object being to direct attention to the fact 



^ "On the Morphology cf the Br.-iin in the Mammalia, with Special 

 Reference to that of the Lemurs, Recent and Extinct," Tran-;. Linn. Soc. 

 Lond., second series ; Zoology, vol. viii., Pait i >, February. IQ03- 



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



