IRRITABILITY 41 



This illustration serves to show that it 

 is not always easy to distinguish between 

 tropism and instinct. In fact many psy- 

 chologists believe that tropisms are the 

 basis of instincts and that instincts gov- 

 ern human conduct to a very great de- 

 gree. According to this view we have a 

 gradually ascending scale from tropism 

 to intelligence with no hard and fast di- 

 viding line at any point. 



Of especial interest are the movements 

 of simple one-celled animals such as the 

 ameba. If food is placed near them 

 they may approach and try to engulf it 

 by putting out projecting pseudopodia 

 which surround it. This seems a highly 

 advantageous action but the writer has 

 found that it may be imitated by arti- 

 ficial cells which will approach and en- 

 gulf a variety of materials in the same 

 fashion. 



If the ameba comes in contact with a 

 food plant consisting of a filament many 

 times its own length a very extraordinary 

 reaction takes place. The ameba en- 



