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 f little brain ' type of organisation on quite a different 

 line of evolution from Vertebrates. Many spiders are read- 

 ily deceived if a vibrating timing 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- 



