RESPONSIVE LIFE OF ORGANISMS 469 



j. Types of sensory phenomena. 



The powers of animals find expression in the responses 

 they make to the external world.* The great differences 

 we have seen in complexity of nervous organization are 

 more or less closely accompanied by differences in mental 

 powers, manifest in behavior. Every step in integration of 

 the nerve paths of the body is doubtless accompanied by 

 improvement in action. Without attempting, however, 

 to draw a close parallel between organization and behavior, 

 we will now examine some types of animal activity, that at 

 least show steps of progress. 



Automatic unvarying activities. --Two examples of invary- 

 ing response have already been before us for consideration. 

 The avoiding reaction of the protozoan Paramcecium as we 

 have seen, constitutes almost the entire stock-in-trade of 

 reactions for that animal. The distance of movement back- 

 ward upon stimulation and the extent of the swing to the 

 aboral side will depend somewhat on the strength of the 

 stimulus; but the one sort of reaction follows upon almost 

 every sort of vigorous stimulus that the environment of the 

 animal normally offers. And with repetition of the stimu- 

 lus, the reaction is repeated indefinitely, and in essentially 

 the same manner. The reaction of the moth to the candle 

 flame is essentially of the same invarying type, although it 

 is performed by the aid of a complicated nervous system. 

 The moth, however, has other less inflexible responses that 

 it makes to other less dominating stimuli. 



Responses automatically varying. The repetition of a 

 stimulus usually brings about a change in the behavior of an 

 organism, sometimes an augmentation of the reaction, 

 sometimes the entire cessation of it. A stentor, for exam- 

 ple, if stimulated by mechanical contact with the cilia of its 



*The essence of an animal is in what it does. Aristotle. 



