INTRODUCTORY 21 



wings with which to carry themselves away, when neces- 

 sary, in an emergency like that, is in my opinion won- 

 derful. It is far superior to ours. 



Here is another illustration that Mr. Loeb gives of 

 heliotropism : "If small crustaceans of a fresh water pond 

 or lake are taken with a plankton net at noontime and 

 placed in an aquarium which is illuminated from one side 

 only, it is found that those animals move about in the 

 vessel pretty much at random and distribute themselves 

 irregularly. Some seem to go more towards the source 

 of light, others in the opposite direction, and the majority, 

 perhaps, pay no attention to the light. This condition 

 changes instantly if we add to the water some acid. If 

 the correct amount is added, all the individuals become 

 actively positively heliotropic and move in as straight a 

 line as the imperfection of the swimming movements per- 

 mits towards the source of light and remain there closely 

 crowded together on the illuminated side of the vessel. 

 How does the acid produce this result? We will assume 

 that it acts as a sensitizer." Mr. Loeb inquires, how does 

 the acid produce this result and suggests that the acid 

 probably is a sensitizer. We are forced to ask the ques- 

 tion, that if a man were in the water in place of these ani- 

 mals and someone should put enough acid into the water 

 so as to burn his skin and body, if he would not become 

 just as heliotropic as these crustaceans and if he would 

 not make a scramble to get out precisely in the same 

 manner. Being confined in a dark cave Hke the aquarium 

 with acid burning his body, his only thought would be to 

 escape from the place. The only thing to guide his ac- 

 tions to effect an escape from this dark cave would be the 

 light. It would be the only thing that could lead him or 

 indicate to him an opening to the outside world. These 

 cells or animals called crustaceans do nothing different 



