CHAPTER XX 



THEORETIC CONSmERATIONS 



The following points have been established in the pre- 

 ceding pages: 



(i) Movement and change in movement, both in rate 

 and direction, may take place without any immediate exter- 

 nal change. 



(2) Sudden changes in light intensity on any sensitive 

 structure in an organism may cause reactions ; ^ for exam- 

 ple, the orientation of Euglena and the retraction of the 

 tubicolous annelids. 



(3) Continued illumination without any variation of 

 intensity probably affects the rate of locomotion in all 

 organisms which respond to light, and it may cause changes 

 in direction of movement by inducing a reversal in the 

 sense of reaction. The time of exposure, as well as the 

 absolute intensity, is functional in this. In fact the prod- 

 uct of the time of exposure and the intensity is probably, 

 within certain limits, constant in producing a given stimu- 

 lus, no matter what the relative value of the two factors is. 



(4) A sudden increase and a sudden decrease of light 

 intensity may under certain conditions produce the same 

 reaction, e.g., the contraction of Helix hortensis, the avoid- 

 ing reaction in Euglena and the raising and throwing of 

 the anterior end from side to side in planarians. Such 

 responses are more striking in some cases of stimulation 

 by temperature than in case of stimulation by light. Para- 

 mecium, for instance, gives the avoiding reaction to de- 



^ In this discussion we shall consider anything which causes a change 

 of movement a stimulus, and any response to a stimulus a reaction. A 

 reaction, then, is either a change in rate of movement or in direction of 

 movement. 



366 



