396 



SCIENCE 



[N. S. Vol. XXX. No. 76 



ever, 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 

 center, which presents a derived circuit for 

 all impulses traversing the lower centers, 

 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 attraction. The 

 next time that a similar attractive impres- 

 sion 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 re- 

 action would have traveled, are blocked, 

 and the reaction — now an educated one- 

 consists in withdrawal from or avoidance 

 of the formerly attractive object. The 

 burnt child has learned to dread the fire. 

 The upper brain represents a nerve 

 mechanism without distinct paths, or 

 rather with numberless paths presenting 

 at first equal resistance in the various di- 

 rections. As a result of experience, defi- 

 nite tracts are laid down in this system, 

 so that the individual has the advantage 

 not only of his lower reflex machineiy for 

 reaction, but also of a machinery which 

 with advance in life is adapted more and 

 more to the environment in which he hap- 

 pens to be. This educable part of the 

 nervous system— i. e., the one in which the 

 direction of impulses depends on past ex- 



perience 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 uni- 

 formity 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 in- 

 jury which has followed the first trial of 

 contact sufSces 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 complexity 

 of the stream of impressions which must be 

 constantly pouring into the central nerv- 

 ous system from all the sense organs of 

 the body, and the fact that, at any rate in 

 the growing animal, every one of these im- 

 pulses is, so to speak, stored in the upper 

 brain, and affects the whole future behav- 

 ior of the animal, even the millions of 

 nerve cells and fibers which are to be found 

 in the human nervous system would seem 

 to be insufficient to carry out the task 

 thrown upon them. Further development 

 of the adaptive powers of the animal would 

 probably have been rendered impossible 

 by the very exigencies of space and nutri- 

 tion, had it not been for the development 

 of the power of speech. A word is a fairly 

 simple motor act and produces a corre- 

 spondingly simple sensory impression. 

 Every word, however, is a shorthand ex- 

 pression of a vast sum of experience, and 



