September 23, 1909] 



NATURE 



S^7 



predominance was the part situated in the neighbourhood 

 of the mouth. This, in animals which move about, is the 

 part which always precedes the rest of the body, and 

 therefore the part which first experiences the sense 

 impressions, favourable or dangerous, arising from the 

 environment. It is this end that has to appreciate the 

 presence or approach of food material, as well as the 

 nature of the medium into which the animal is being 

 driven by the movements of its body. Thus a predomin- 

 ance of the front end of the nervous system was deter- 

 mined by the special development at this end of those 

 sense organs or sensory cells which are projicient — i.e. 

 are stimulated by changes in the environment proceeding 

 from disturbances at a distance from the animal. The 

 sensory organs of vision and the organs which correspond 

 to our olfactory sense organs and are aroused by minute 

 changes in chemical composition of the surrounding 

 medium, are always found especially at the front or mouth 

 end of the organism. The chances of an animal in the 

 struggle for existence are determined by the degree to 

 which the responses of the animal to the immediate 

 environment are held in check in consequence of stimuli 

 arising from approaching events. The animal, without 

 power to see or smell or hear its enemy, will receive no 

 impulse to fly until it is already within its enemy's jaws. 

 It must therefore be an advantage to any animal that the 

 whole of its nervous system should be subservient to those 

 ganglia or central collections of nerve cells which are in 

 direct connection with the projicient sense organs in the 

 head. This subservience is secured by endowing the head 

 centre with a power, firstly, of controlling and abolishing 

 the activities (i.e. all those aroused by external stimuli) 

 of all other parts of the central nervous system, and, 

 secondly, of arousing these parts to a reaction immediately 

 determined by the impression received from the projicient 

 sense organs of the head and originated by some change 

 in the surroundings of the animal which has not yet 

 affected the actual surface of its body. 



Education by Experience. 



The factors which so far determine success in the 

 struggle for predominance are, in the first place, foresight 

 and power to react to coming events, and, in the second 

 place, control of the whole activities of the organism by 

 that part of the central nervous system which presides 

 over the reaction. The animal therefore profits most which 

 can subordinate the impulses of the present to the 

 exigencies of the future. 



.^n organism thus endowed is still, however, in the range 

 of its reactions, a long way behind the type which has 

 attained dominance to-day. The machinery we have 

 described, when present in its simplest form, suflices for 

 the carrying out of reactions or adaptations which are 

 determined immediately by sense impressions, advantage 

 being given to those reactions which are initiated by 

 afferent stimuli affecting the projicient sense organs at 

 the head end of the animal. With the formation of the 

 vertebrate type, and probably even before, a new faculty 

 makes its appearance. Up to this point the reactions of 

 an animal have been what is termed " fatal," not in the 

 sense of bringing death to the animal, but as inexorably 

 fixed by the structure of the nervous system inherited by 

 the animal from its precursors. Thus it is of advantage 

 to a moth that it should be attracted by, and fly towards 

 light objects — e.g. white flowers — and such a reactivity is 

 a function of the structure of its nervous system. When 

 the light object happens to be a candle flame the same 

 response takes place. The first time that the moth flies 

 into and through the candle flame, it may only be scorched. 

 It does not, however, learn wisdom, but the reaction is 

 repeated so long as the moth can receive the light stimuli, 

 so that the response, which in the average of cases is for 

 the good of the race, destroys the individual under an 

 environment which is different from that under which it 

 was evolved. There is in this case no possibility of 

 educating the individual. The race has to be educated to 

 new conditions by the ruthless destruction of millions of 

 individuals, until only those survive and impress their 

 stamp on future generations whose machinery, by 

 the accumulation and selection of minute variations, has 

 undergone sufficient modification to determine their auto- 

 matic and " fatal " avoidance of the harmful stimulus. 

 NO. 2082, VOL. 81] 



The next great step in the evolution of our race was 

 the modification of the nervous system which should render 

 possible the education of the individual. The mechanism 

 for this educatability was supplied by the addition, to the 

 controlling sensory ganglia of the head, of a mass of 

 nervous matter which could act, so to speak, as an 

 accessory circuit to the various reflex paths already exist- 

 ing in the original collection of nerve ganglia. This 

 accessory circuit, or upper brain, comes to act as an organ 

 of memory. Without it a child might, like the moth, be 

 attracted by a candle flame and approach it with its hand. 

 The injury ensuing on contact with the flame would inhibit 

 the first movement and cause a drawing back of the hand. 

 In the simple reflex mechanism there is no reason why 

 the same series of events should not be repeated indefinitely, 

 as in the case of the moth. The central nervous system, 

 however, is so constituted that every passage of an impulse 

 along any given channel makes it easier for subsequent 

 impulses to follow the same path. In the new nerve 

 centre, which presents a derived circuit for all impulses 

 traversing the lower centres, the response to the attractive 

 impulse of the flame is succeeded immediately by the 

 strong inhibitory impulses set up by the pain of the burn. 

 Painful impressions are always predominant. Since they 

 are harmful, the continued existence of the animal depends 

 on the reaction caused by such impressions taking the 

 precedence of and inhibiting all others. The effect there- 

 fore of such a painful experience on the new upper brain 

 must far outweigh that of the previous impulse of attrac- 

 tion. The next time that a similar attractive impression 

 is experienced the derived impulse traversing the upper 

 brain arouses, not the previous primary reaction, but the 

 secondary one, viz. that determined by the painful 

 impressions attending contact with the flame. As a result, 

 the whole of the lower tracts, along which the primary 

 reaction would have travelled, are blocked, and the re- 

 action — now an educated one — consists in withdrawal from 

 or avoidance of the formerly attractive object. The burnt 

 child has learnt to dread the fire. 



The upper brain represents a nerve mechanism without 

 distinct paths, or rather with numberless paths present- 

 ing at first equal resistance in the various directions. As 

 a result of experience, definite tracts are laid down in 

 this system, so that the individual has the advantage not 

 only of his lower reflex machinery for reaction, but also 

 of a machinery which with advance in life is adapted more 

 and more to the environment in which he happens to be. 

 This educable part of the nervous system — i.e. the one in 

 which the direction of impulses depends on past experience 

 and on habit — is represented in vertebrates by the cerebral 

 hemispheres. From their first appearance they increase 

 steadily in size as we ascend the animal scale, until in 

 man they exceed by many times in bulk the whole of the 

 rest of the nervous system. 



We have thus, laid down automatically, increased power 

 of foresight, founded on the Law of Uniformity. The 

 candle flame injures the skin once when the finger is 

 brought in contact with it. We assume that the same 

 result will follow each time that this operation is repeated. 

 This uniformity is also assumed in the growth of the 

 central nervous system and furnishes the basis on which the 

 nerve paths in the brain are laid down. The one act of 

 injury which has followed the first trial of contact suffices 

 in most cases to inhibit and to prevent any subsequent 

 repetition of the act. 



The Faculty of Speech. 



If we consider for a moment the vastness and com- 

 plexity of the stream of impressions which must be con- 

 stantly pouring into the central nervous system from all 

 the sense organs of the body, and the fact that,_ at any 

 rate in the growing animal, every one of these impulses 

 is, so to speak, stored in the upper brain, and affects the 

 whole future behaviour of the animal, even the millions 

 of nerve cells and fibres which are to be found in the 

 human nervous system would seem to be insufficient to 

 carry out the task thrown upon them. Further develop- 

 ment of the adaptive powers of the animal would probably 

 have been rendered impossible by the very exigencies of 

 space and nutrition, had it not been for the development 

 of the power of speech. A word is a fairly simple motor 

 act and produces a correspondingly simple sensory 



