Spatially Determined Reactions 191 



generally characterize reactions to contact with moving ob- 

 jects. In eliciting the scratch-reflex of dogs, an object drawn 

 along the skin is decidedly more effective than one pressed 

 against the skin for the same length of time (382, p. 184). 

 The physiological effect is probably, Sherrington says, the 

 same as that involved in the "summation" of successive slight 

 stimuli applied at the same point. As is well known, the latter 

 will bring about a response of considerable violence, though 

 each one acting alone would apparently be without effect. 



Is it likely that these responses to moving stimuli in contact 

 with the skin involve the perception of movement as a form 

 of space perception; that is, a perception of the successive 

 positions occupied by the stimulus and their relative direc- 

 tion ? I think we may say that they probably do not, in the 

 lower animal forms at least. And a chief reason for saying 

 so lies in the fact that the reactions are so rapid. To perceive 

 the spatial relations of stimuli, or any other relations, is a 

 process not favored by great speed of response. The quicker 

 the reaction, the less clear the perception of its cause : such 

 seems to be the general law. The sensation accompanying 

 contact with a moving object may differ in intensity from that 

 accompanying a resting stimulus ; it may, in the lower forms, 

 differ qualitatively in some way not represented in our own 

 experience, but it can hardly be connected with the more 

 complex psychic processes involved in any form of space per- 

 ception. 



In vision, also, there are special arrangements for reacting 

 to moving stimulation. The sensitiveness of many animals 

 to changes of light intensity, although not a direct adaptation 

 to the spatial characteristics of a stimulus, serves the same 

 purpose, for changes in light intensity are oftenest brought 

 about by objects in motion. In the mollusk Pecten varius, 

 a transition from shadow vision to movement vision is illus- 



