ILLUSTRATIONS 1 1 



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25. An ant which works at an intermediate rate 



may be speeded up if placed with an ant 

 which works more rapidly, or vice versa 141 



26. A simple maze used in training cockroaches 151 



27. Isolated cockroaches make fewer errors during 



training than if paired or if three are trained 

 together 152 



28. They also take less time 153 



29. Parrakeets learn equally well if trained when 



isolated, whether they are caged singly or in 

 pairs 156 



30. Parrakeets learn more rapidly if trained alone 



than if two are placed together in the maze 157 



31. Feeding a fish which has just come through the 



opening from the larger side of the aquarium 160 



32. Goldfish learn to swim a simple aquarium-maze 



the more readily the more fish there are 

 present 161 



33. Isolated goldfish learn the problem set for them 



less rapidly, and unlearn it more readily 162 



34. The aquarium-maze used in training part of 



the fish to come forward and part to go to the 

 rear to be fed 164 



35. Cyprinodon learn to move in a body more read- 



ily than to split into two separate groups 165 



36. Goldfish learn more readily if accompanied by 



a trained leader 166 



37. An aquarium-maze arranged to test the power 



of observation of fish 168 



38. Goldfish react more rapidly if allowed to watch 



others perform ^ 169 



39. Flocks of hens are organized into a definite so- 



cial hierarchy 178 



40. Cockerels also have a social organization 180 



