REACTION TO EXTERNAL STIMULI 95 



which they then seize by a rapid movement. But this refine- 

 ment in the catching of prey is purely instinctive ; the fish acts, 

 not as the result of reflection, but when its instinct is awakened 

 by the sight of a victim. The utility of this behaviour is quite 

 unknown to the fish, which acts automatically in response to an 

 external stimulus affecting its sense organs. The psychical 

 factor, which Eibot sees in the original elaboration of every 

 instinct, is absent in the case of these fishes, as it is absent in the 

 case of the sea-cucujnber. In both cases it is natural selection 

 which has evolved the instincts, and adjusted an appropriate 

 reaction to an external stimulus. 



