DIFFERENT KINDS OF RESPONSE 149 



injure the tissues. The worm now responds to a repetition 

 of the stimulus (and often when the new stimulus is only 

 slight) by lifting the anterior fourth of the body into a 

 vertical position, and waving it about in a frantic manner. 

 ,This behavior is usually alternated with the right-about-face 

 reactions, and with persistent rapid crawling forward and 

 backward. ..." These reactions do not exhaust the 

 list but are simply more typical features of an almost endless 

 variety of modifications. 



The causes of the change of response to a given stimulus 

 may be many. The factor of fatigue is doubtless an im- 

 portant one, especially in the nervous centers. An afferent 

 fiber may be connected with several motor pathways, so 

 that slight variations in the ease with which impulses may 

 travel along certain lines may determine the direction of 

 motor discharge. A very slight degree of fatigue of any 

 pathway might then be a cause of a change of response after 

 one or a very few repetitions. An increase of transmitting 

 function as a result of previous exercise may also play its 

 part, so that we may say that the factors which produce an 

 increase or decrease of responsiveness also effect the replace- 

 ment of one response by another of a different kind. In the 

 one case they modify the intensity of the reaction, in the 

 other they act so as to switch off the action to other lines. 



The fluctuations of tonus here and there in the nervous and 

 muscular systems afford, as Uexkull has shown, a mechanism 

 by which the responses of the organism may be varied to an 

 indefinite degree. Every act of an organism alters the dis- 

 tribution of tonus in its body, and the way in which the organ- 

 ism responds to the next stimulation is naturally affected 

 by this circumstance. Then variations in assimilation, 

 respiration, excretion and other functions are constantly inter- 

 fering directly or indirectly with the outcome. There is 



