EVOLUTION THE MASTER-KEY 



visible, each demanding that the eyeball be so 

 moved that the most sensitive part of the retina 

 be directed towards the object, so that it may be 

 the more clearly denned. Now if these two stimuli 

 were added together, so to speak, the eyeball 

 would be swung too far round, and neither object 

 would be clearly seen. On the other hand, if an 

 average or mean were struck between the two 

 stimuli the eyeball would swing round not far 

 enough for clear vision of the one object, but too 

 far for clear vision of the other. Neither of these 

 results is observed. On the contrary, one of the 

 stimuli definitely inhibits or arrests the action of 

 the other, and the eyeball is swung just so far as 

 will make the image of one of the two objects fall 

 exactly on the most sensitive spot of the retina. 



This discovery, typical of all action, we owe to 

 Professor Sherrington, who discussed his years of 

 work upon this subject in his Presidential Address 

 to the section of Physiology at the meeting of the 

 British Association at Cambridge in 1904. By 

 discovering that reflexes inhibit one another he has 

 not only explained how it is that this amazingly 

 complex nervous system of ours acts as a unity, 

 but he has gone very far to explain that phenome- 

 non which most strikingly illustrates this unity 

 namely, the phenomenon of attention. When we 

 attend, one series of sensory fibres such as those 

 of the auditory nerve when we listen with individ- 

 ual attention to a sermon or a song has taken 

 possession of what Professor Sherrington calls the 

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