ANIMAL BEHAVIOUR 213 



brainless, ganglionless starfish already referred to. And our 

 inclination to be parsimonious in our interpretation is of 

 course strengthened by the fact that the hermit-crab belongs 

 to the ' little brain ' type of organisation on quite a different 

 line of evolution from Vertebrates. Many spiders are read- 

 ily deceived if a vibrating tuning fork is brought near their 

 web. They rush out to deal with the situation — responding 

 to the familiar tremor stimulus. They may be cheated over 

 and over again. In one case, however, after a tantalising 

 deception extending over fifteen days, the spider ceased to 

 give any attention to the tuning fork. The question is 

 whether we must in such a case postulate memory and per- 

 ceptual inference, or whether some purely physiological in- 

 terpretation is adequate. Thus the ' getting used to ' a stim- 

 ulus may be in some cases due to fatigue, in the wide sense, 

 including dulled sensation. Our inclination to a parsimoni- 

 ous interpretation in such a case as this is strengthened by 

 the fact that even brainless and ganglionless animals illus- 

 trate a modification of activity by individual experience. 

 Repeated stimulation alters ^ the physiological condition ' of 

 an animal so that it gives an intensified reaction to a mod- 

 erate stimulus, as in the case of an earthworm that has been 

 teased a little. Contrariwise, repeated stimulation that leads 

 to nothing may result in the suppression of a reaction, as in 

 sea-urchins that soon stop answering back to fruitless changes 

 in light and shade, or in sea-anemones that cease to respond 

 to the touch of false food. Even the carnivorous plant, 

 Venus's Ely Trap, refuses to be duped many times in suc- 

 cession. 



One of the marks of intelligence is profiting by experience 

 — learning. At a lower level there is temporary modification 

 of behaviour, and this passes, insensibly we think, into last- 



