loo Studies in Animal Behavior 



exposure to light may affect organisms in two dif- 

 ferent ways. Continued stimulation may render 

 positively phototactic organisms negative. It may 

 also bring about a change of light attunement, 

 thereby dulling the sensitivity of the organism so 

 that it requires a stronger stimulus to evoke a nega- 

 tive reaction. These two opposed results are by 

 no means exceptional phenomena; they are quite 

 parallel to many other physiological changes which 

 render the organism less sensitive to habitual stim- 

 uli. The one is a change wrought in the organism 

 as a direct result of the stimulus. The other is 

 a response by the organism, a sort of defensive 

 measure, by which the organism becomes more or 

 less shielded from the action of an inimical force. 

 According to the relative potency of these two modi- 

 fying factors the effects of previous exposure to 

 light will naturally be varied. Should attunement 

 to increased intensity of light be quickly developed, 

 the effect of continued exposure might appear to 

 make the positive reaction more decided. The ef- 

 fects of exposure to light are notoriously different in 

 different forms, but many of these variations may be 

 the result of the varying potency of the two influ- 

 ences we have discussed. 



Exposure in certain cases, however, affects re- 

 versal in ways for which it is difficult to account. 

 When studying the light reactions of terrestrial am- 

 phipods I discovered that specimens of Orchestia 

 ag'ilis which were markedly positive quickly became 



