256 The Animal Mind 



emphasized. As Jennings suggests, if the sea-anemone 

 that contracts at the first ray of light were to remain con- 

 tracted in steady illumination, it would lose all chance of 

 getting food under the new conditions (374). The negative 

 reactions ordinarily involve interruption of the food-taking 

 process, and it is important that they should not be 

 continued in response to stimulation that is relatively 

 permanent. Hargitt thinks that the loss of reaction to re- 

 peated shadows which he observed in marine worms may 

 be an adaptation to the varying illumination caused by 

 ripples at the surface of the water (285). 



A very important psychological question concerns the 

 permanence of the effects of adaptation. Sensory adap- 

 tation and lapse of attention to repeated or continuous 

 stimuli, as these 'phenomena are met in our own experience, 

 are not considered phenomena of learning at all. The 

 former is purely temporary in its effects : the person who 

 has become so used to an odor that he cannot smell it shows 

 no effects of this experience half an hour later. The effect 

 of familiarity on emotion and on attention is more lasting : 

 one's loss of attention to a clock ticking in one's room may 

 persist despite more or less prolonged absences from the 

 room, although a sufficiently long absence, during which 

 one encountered no ticking clocks, would cause the sound 

 to be noticed again. The loss of emotional response to a 

 familiar stimulus may persist for some time. Emotional 

 adaptation and lapse of attention to continued stimuli 

 may fairly be termed learning in proportion as their effects 

 are more than temporary. 



In many cases, the effects of adaptation on animal re- 

 actions last over a considerable interval between the stimuli. 

 This seems to be increasingly the case, the higher the animal. 

 Thus Hydra, which is only a ccelenterate, if it is allowed to 



