The Reversal of Tropisms 95 



not common in the natural environment of either of 

 these species. Both of them, in common with most 

 other animals, high as well as low, respond nega- 

 tively in ordinary circumstances to an injurious de- 

 gree of temperature. Bring a hot needle near either 

 of them and it will turn away. But the remarkably 

 strong proclivity of these forms to go toward the 

 light is sufficient to draw them into a region which 

 they would ordinarily shun. If they turn away 

 at times from the more intense heat their phototaxis 

 quickly brings them back again. They behave as 

 if they were under a powerful spell which they are 

 unable to resist and which they obey as long as 

 any of their vital energy remains. 



While there are many animals which cannot be 

 made negatively phototactic by any increase of light, 

 it is not uncommon to find forms that are positive 

 in light of weak or moderate intensity and nega- 

 tive in strong light. Very intense light is often ac- 

 companied by an injurious degree of heat and even 

 where it is not, its effects on the organism are prob- 

 ably not good and are sometimes manifestly injuri- 

 ous. The power of change from positive to nega- 

 tive phototaxis in strong light is a serviceable en- 

 dowment with which we might expect a priori to find 

 many organisms equipped. Curiously enough, it is 

 not among the more highly organized phototactic 

 animals that this ability is most widespread. In the 

 insects reversal of the positive reaction is rather un- 

 common. In the crustaceans reversal in strong light 



