218 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



place in the organism, corresponding to the periodic 

 change of day and night, and that these changes con- 

 tinue for a time in the same periodicity, when the ani- 

 mal is kept in the dark. 



3. We will then consider the extent of associative 

 memory in the animal kingdom instead of the extent 

 of consciousness among animals. How can we deter- 

 mine whether an animal possesses the mechanism 

 necessary for associative memory ? The criteria for the 

 existence of associative memory must form the basis 

 of a future comparative psychology. It will require 

 more observations than we have made at present to 

 give absolutely unequivocal criteria. For the present, 

 we can say that if an animal can learn, that is, if it 

 can be trained to react in a desired way upon certain 

 stimuli (signs), it must possess associative memory. 

 The only fault with this criterion lies in the fact that 

 an animal may be able to remember (and to associate) 

 and yet may not yield to our attempts to train it. In 

 this case other experiments must be substituted which 

 will prove that the animal does associate or remember. 



We may conclude that associative memory is pre- 

 sent when an animal responds upon hearing its name 

 called, or when it can be trained upon hearing a 

 certain sound to go to the place where it is usually 

 fed. The optical stimulus of the place where the 

 food is to be found and the sensations of hunger and 

 satiety are not qualitatively the same, but they occur 

 simultaneously in the animal. The fusion or growing 

 together of heterogeneous but by chance simultaneous 



