140 MODIFICATIONS OF BEHAVIOR 
time. An electric current which would cause a Paramecium 
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 Paramcecium 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 in 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 
