Modification by Experience 287 



homing flight by the new and not the old landmarks. The 7 

 learning is essentially rapid and temporary. Where, as 

 for instance with the honey bee, the nest remains per- 

 manently fixed in one locality, guidance by visual land- 

 marks does not differ from the ordinary types of learning 

 where the process is gradual, where useless movements 

 are ehminated and useful movements organized into sys- 

 tems. We are still in possession of too few detailed obser- 

 vations on the homing flights of the wasp to draw positive 

 conclusions as to the nature of the learning process here. 



§ 77. The Memory Idea 



It is sufficiently clear that animals possess the power of 

 learning, in the sense of a power of reacting differently t< 

 a present stimulus because of their past experience with i' 

 Probably not a single animal form is so low that it lacks 

 this power. But there is another type of learning, of which 

 human beings make much use, whose existence in animals 

 we have yet to investigate ; namely, the abihty to recall 

 a mental image of an absent stimulus, a memory idea. A 

 dog shows clearly that he remembers his master, in the 

 sense of modifying his behavior in his master's presence 

 because of his previous experience. Can we be sure that 

 he has remembered him in his absence ; that he has had a 

 memory image of his master ? 



Most people, following the tendency to humanize ani- 

 mals and ignoring Lloyd Morgan's canon, interpret as evi- 

 dence of memory ideas certain features of animal behavior 

 which are susceptible of much simpler interpretations. 

 Dogs and cats, for instance, are supposed to dream because 

 they snarl and twitch their muscles in sleep ; but as Thorn- 

 dike (704) has pointed out, such movements may be purely 



