90 ROBERT M. YERKES 



Explanation of Plate V 



Orang utan, Julius, obtaining banana by piling boxes 

 or by using pole 



Figure 21. — Julius in act of setting larger box on end. 



Figure 22. — Placing smaller box on larger. 



Figure 23. — Balancing on larger box preparatory to reaching for 



banana. 

 Figure 24. — Balancing and reaching to the utmost. 

 Figure 25. — Standing on three boxes (after stacking them) and 



reaching for reward. 

 Figure 26. — Lifting smaller box up toward banana. 

 Figure 27. — The act of stacking the boxes. 

 Figure 28. — Sequel to figure 27. 

 Figure 29. — Box and pole experiment. Pushing the second pole 



into the box. 

 Figure 30. — Pushing pole into box. 

 Figure 31. — Enjoying the reward of success. 



