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 fewrepetitions. 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 Uexkiill 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 
” 
