September 26, 1912] 



NATURE 



12 1 



circumstances it may seem, is essentially instinctive; 

 and the main problem we have to solve, in attempting 

 to explain the emergence of the distinctive attributes of 

 the creature which m greater measure than any other 

 has succeeded in subordinating its instincts to reason, 

 i> the means by which it has become possible for the 

 effects of individual experience to be brought to bear 

 upon conduct. 



The ability to learn by experience necessarily implies 

 the development, somewhere in the brain, of a some- 

 thing which can act not only as a receptive organ for 

 impressions of the senses and a means for securing 

 that their influence will find expression in modifying" 

 behaviour, but also serve in a sense as a recording 

 apparatus for storing such impressions, so that they 

 may be revived in memory at some future time in 

 association with other impressions received simul- 

 taneously, the state of consciousness they evoked, and 

 the response they called forth. 



Such an organ of associative memory is actually 

 found in the brain of mammals. It is the cortical area 

 til which eleven years ago I applied the term "neo- 

 pallium." " Into it pathways lead from all the sense 

 organs ; and each of its territories, which receives a 

 definite kind of impression, visual, acoustic, tactile, 

 or an\' other, is linked by the most intimate bonds 

 with all the others. In spite of the disapproval of the 

 psychologists, we can indeed regard this neopallium 

 as fulfilling all the conditions of the sensorium com- 

 mune, which Aristotle and many generations of 

 philosophers have sought for twenty centuries; for it 

 is unquestionably a "unitary organ the physical pro- 

 cesses of which might be regarded as corresponding 

 to the unity of consciousness " (Wm. MacDougall). 



Nothing that happens in this area in the course of 

 its enormous expansion and differentiation in the 

 higher mammals materially affects this fundamental 

 purpose of the neopallium, which continues to remain 

 a unifying organ that acts as a whole, though each 

 part is favourably placed to receive and transmit to 

 the rest its special quota to the sum-total of what we 

 mav call the materials of conscious life. 



The consciousness which resides, so to speak, in 

 tliis neopallium, and is fed by the continual stream 

 of sensory impressions pouring into it and awakening 

 memories of past sensations, can express itself directly 

 in the behaviour of the animal through the inter- 

 mediation of a part of the neopallium itself, the so- 

 called motor area, which is not only kept in intimate 

 relation with the muscles, tendons, and skin by 

 sensorv impressions, but controls tlie voluntary re- 

 sponses of the muscles of the opposite side of the 

 body. 



TJie Differentiation of Mammals and the Effects of 

 Specialisation. 



The possession of this higher type of brain enor- 

 mously widened the scope for the conscious and in- 

 telligent adaptation of the animal to varying surround- 

 ings ; and in the exercise of this newly acquired ability 

 to K-arn from individual experience, and so appreciate 

 the possibilities of fresh sources of food supply and 

 new modes of life, the way was opened for an infinite 

 series of adaptations to varying environments, entail- 

 ing structural modifications in which the enhanced 

 plasticitv of the new type of animal found expression. 



Nature tried innumerable experiments with the new 

 tvpe of brain almost as soon as the humble Therapsid- 

 like mammal felt the impetus of its new-found power 

 of adaptation. In turn, the Prototherian and Meta- 

 therian tvpes of brain were tried before the more adapt- 

 able scheme of the Eutherian brain was evoU'ed. 



" "The Natural Subdivision of tl'e Cerebr.-il Hemisphere,"/"'"". Annt. 

 and I'ltys., vol. ,\xxv., 1901, p 431 : Arris and Gale Lectures on the 

 Kvolution of the Brain. Laucet, January t5, 1910, p. 153. 



The new breed of intelligent creatures rapidly spread 

 from their South -African home throughout the whole 

 world, and exploited every mode of livelihood. 'J"he 

 power of adaptation to the particular kind of life each 

 group chose to pursue soon came to be expressed in 

 a bewildering variety of specialisations in structure, 

 some for living on the earth or burrowing in it, others 

 for living in trees or even for flight ; others, again, 

 for an aquatic existence. Some mammals became fleet 

 of foot, and developed limbs specially adapted to 

 enhance their powers of rapid movement. They at- 

 tained an early pre-eminence, and were qble to grow 

 to large dimensions in the slow-moving world at the 

 dawn of the age of mammals. Others developed limbs 

 specially adapted for swift attack and habits of stealth, 

 successfully to prey upon their defenceless relatives. 



Most of these groups attained the immediate success 

 that often follows upon early specialisation ; but they 

 also paid the inevitable penalty. They became defi- 

 nitely committed to one particular kind of life ; and 

 in so doing they had sacrificed their primitive simpli- 

 city and plasticity of structure, and in great measure 

 their adaptability to new conditions. The retention of 

 primitive characters,, which so many writers upon bio- 

 logical subjects, and especially upon anthropology, 

 assume to be a sign of degradation, is not really 

 an indication of lowliness. We should rather look 

 upon high specialisation of limbs and the narrowing of 

 the manner of living to one particular groove as con- 

 fessions of weakness, the renunciation of the wider 

 life for one that is sharply circumscribed. 



The stock from which man eventually emerged 

 played a very humble role for long ages after many 

 other mammalian orders had waxed great and strong. 

 But the race is not always to the swift, and the lowly- 

 group of mammals which took advantage of its in- 

 1 significance to develop its powers evenly and very 

 I gradually, without sacrificing in narrow specialisation 

 j any of its possibilities of future achievement, ev'entually 

 gave birth to the dominant and most intelligent of all 

 ! living creatures. 



{ The tree-shrews are small squirrel-like animals 

 I which feed on " insects and fruit, which they usually 

 seek in trees, but also occasionally on the ground. 

 When feeding they often sit on their haunches, hold- 

 I ing the food, after the manner of squirrels, in their 

 i forepaws." ' They are of "lively disposition and great 

 1 agility." * These vivacious, large-brained little insec- 

 I tivores, linked by manifold bonds of relationship to 

 some of the lowliest and most primitive mammals, 

 present in the structure of their skull, teeth, and limbs 

 j undoubted evidence of a kinship, remote though none 

 the less sure, with their compatriots the Malaysian 

 lemurs ; and it is singularly fortunate for us in this 

 inquiry that side by side there should have been pre- 

 served' from the remote Eocene times, and possibly 

 earlier still, these insectivores, which had almost 

 become primates, and a little primitive lemuroid, the 

 spectral tarsier, which had only just assumed the char- 

 acters of the primate stock, when nature fixed their 

 types and preserved them throughout the ages, with 

 relatively slight change, for us to study at the present 

 dav. 



Thus we are able to investigate the influence of an 



I arboreal mode of life in stimulating the progressive 



I development of a primitive mammal, and to appreciate 



i orecisely what changes were necessary to convert the 



iivelv, agile Ptilocercus-like ancestor of the primates 



into a real primate. 



In the forerunners of the mammalia the cerebral 



hemisphere was predominantly olfactory in function; 



I and even when the true mammal emerged, and all the 



" Flower and Lydekker, "M.-iinmals, Living and Extinct," 1891 .618 

 » W. K. Gregory, of), cit., p. 269, and pp. 279, 280. 



NO. 2239, VOL. 90] 



