THE WAYS OF LIFE 243 



cuitous the result is inconstant. Pavlov showed that if 

 a whistle is always sounded when a dog gets something 

 to eat, then by and by the sound of the whistle will make 

 the dog salivate. An association between the sound and 

 the gustatory excitation has been estabhshed. 



The stimulus that ' suggests ' the sahvation may be 

 almost anything if the dog has the association estab- 

 lished — ^it may be, besides sight and sound, an odour, a 

 movement, a change of temperature or illumination, a 

 scratching of the skin, and so on. The method is useful 

 in definitely proving the animal's sensitiveness to various 

 stimuli — some of them well known to all who know dogs, 

 and others a httle surprising — ^but its chief value is in 

 showing the establishment of cerebral associations, and in 

 discovering their laws. The experiments leave in the mind 

 a vivid impression of the remarkable plasticity of the dog's 

 brain in forming associations. Thus, Orbeli succeeded in 

 estabhshing a reflex between the salivation and the shape 

 of the letter T (as distinguished from other shapes) thrown 

 on a screen. 



Bohn gives some other instructive illustrations. Many 

 fishes show no sign of hearing sounds, and yet they some- 

 times hear them. For Meyer taught some fishes in a couple 

 of months that whenever a certain sound was made they 

 would find some food in a dark chamber in their aquarium. 

 They acquired an interest in the sound and they came 

 gradually to associate it with their memory of food. 



There is a special interest in experiments with fishes, 

 since their brain, especially in bony fishes, or Teleosts, has 

 stopped at a low level. Some observers, like Edinger, deny 

 them even memory. M. Oxner has recently made some 

 instructive observations at the Oceanographical Museum 



