Some Aspects of Attention 317 



organic welfare than the stronger. For example, it would 

 often be important that an animal should be able to respond 

 to a very faint food stimulus rather than to any of the 

 stronger forces acting upon it. Evidently a prime need of 

 animal life is some arrangement whereby weak but im- 

 portant stimuli shall be given the preference in determining 

 reaction over stronger but less vitally necessary ones. 

 Sense organs are one such device. The comparatively 

 slight" amount of chemical energy coming from a bit of 

 food may have its effectiveness for the nervous system 

 greatly increased through its reception by a structure 

 adapted to use the whole of it to advantage. Light stimu- 

 lation involves a quantity of energy that is insignificant 

 in comparison with the grosser forces acting on an organism ; 

 yet falling on the retina, the energy is economized and 

 magnified through the stored-up chemical forces it sets 

 free. Thus a weak stimulus may by a sense organ be made 

 powerful to determine reaction. Another arrangement 

 to the same effect is the peculiarity of the nervous system 

 whereby, through an arrangement akin to the summation 

 of faint stimuli, a moving stimulus, one acting successively 

 upon neighboring points of a sensitive surface, produces an 

 effect disproportionate to its intensity. A moving stimulus is 

 a vitally important stimulus ; it means life, and hence may 

 mean food or danger. The response to it is in most cases 

 adapted rather to its importance than to its physical 

 strength. A third arrangement for the securing of reaction 

 to vitally important stimulation lies in the existence of 

 preformed connections in the nervous system, which bring 

 it abouttiiat the path of the excitation produced by one stimu- 

 lus is clear to the motor apparatus, while that of another is 

 closed. Reactions of this sort we call instinctive. The 

 nesting bird responds to the sight of building material rather 



