140 MODIFICATIONS OF BEHAVIOR 



time. An electric current which would cause a Paramoecium 

 if swimming freely in the water to turn to the cathode, will 

 usually produce no movement in a specimen in contact 

 with a solid. The reactions of Paramoecium to chemical, 

 thermal and mechanical stimuli are also greatly influenced 

 by the same factor. Earthworms, as Darwin has observed, 

 usually fail to react to light when mating or feeding. 

 A striking illustration of the influence of the substratum 

 in reactions to light is afforded by the leech Branchellion. 

 When in a dish of water the leech reacts to a passing shadow 

 by raising its anterior end and swaying it about. If the 

 leech is in contact with its host, the torpedo, it pays not the 

 slightest heed to passing shadows. In Ranatra the positive 

 response to light is checked when the animal is feeding or 

 cleaning itself and is quickly resumed when these operations 

 are completed. And in fiddler crabs the positive reaction 

 to light may be overcome by fear of an approaching object, 

 although with longer exposure to light the phototactic re- 

 sponse becomes the more potent one. Contact in many lower 

 animals inhibits, as we have seen hi a previous chapter, re- 

 actions to light and many other stimuli, and in higher forms 

 it may profoundly modify behavior in relation to enemies. 

 To a considerable degree these changes of behavior are the 

 results of the simple interference of reactions; but stimuli 

 may act in such a way that they produce a marked physio- 

 logical change in the organism, as when a contact stimulus 

 brings about the death feint, and the lack of responsiveness 

 is due then to the induced condition rather than to an an- 

 tagonistic movement. 



DIMINUTION OF REACTION TO REPEATED STIMULATION 



Modifications of reaction due to the simultaneous reception 

 of other stimuli are closely affiliated with modifications due 



