184 ANIMAL BEHAVIOUR 



A fact of great interest has been established by Professor 

 Jennings, that the behaviour of unicellular organisms is 

 modifiable by experience. He has experimented, for instance, 

 with a trumpet-shaped ciliated Infusorian called Stentor 

 which abounds in marshy pools, attaching itself by the nar- 

 row end to a water-weed, and surrounding the lower half of 

 its body with a mucus-like sheath, the so-called tube. A 

 cloud of carmine particles is introduced into the water- 

 currents passing to the ciliated mouth of the Stentor. It 

 bends to the aboral side, twisting on its stalk two or three 

 times as it bends, and thus often avoids the cloud of particles. 

 That is answer one. But if the particles continue to come, 

 the ciliary movement is suddenly reversed and the water is 

 driven away from the mouth. This may be repeated two 

 or three times, and is answer two. If the Stentor does not 

 get rid of the obnoxious stimulation in either of these two 

 ways, it contracts into its tube and suspends activity, this 

 being answer three. After half a minute or so it re-expands, 

 and if the carmine particles still reach it, it contracts again. 

 It will do this many times, and after each contraction it 

 stays a little longer in its tube than it did before. Finally, 

 if no improvement in circumstances rewards its trials, it 

 breaks its attachment and swims forwards or backwards 

 away from its tube. And this is answer four. " The stim- 

 ulus and other external conditions remaining the same, the 

 organism responds by a series of reactions becoming of more 

 and more pronounced character, until by one of them it rids 

 itself of the stimulation" (p. 176). "The same individual 

 does not always behave in the same way under the same 

 external conditions, but the behaviour depends upon the 

 physiological condition of the animal. The reaction to any 

 given stimulus is modified by the past experience of the ani- 



