a\NADA PORCUPINE 43 



the portion taken out. A circle seven inches in diameter was 

 taken from one. A square 6.2 inches was cut from another. 

 For the hexagon a side 3.8 inches was used. An equilateral 

 triangle having the same area as the circle was found to present 

 to the human eye a very common illusion of being much larger. 

 It was also found that the nine-inch block would not contain a 

 triangle of that size so a triangle with a side 8.2 inches was 

 used. This compensated for the illusion to the human eye and 

 allowed it to fall in the same general dimensions of the other 

 figures. A rectangle and an ellipse were also made approx- 

 imately the same area as the other forms. 



These forms were presented to the animals pair-wise and 

 six-wise by means of frames made to hold the forms of an angle 

 of about 75° from the horizontal, with the base of the form 10 

 inches above the ground. Back of the forms were stationary 

 food-boxes separated from one another by solid partitions and 

 the boxes were set in and down far enough to prevent the ani- 

 mal from seeing the food before he put his head through the 

 form. In addition, a secret pocket was made in each com- 

 partment where a piece of the kind of food being used was 

 concealed at the beginning of each day's work. The purpose 

 of this later precaution was to eliminate the olfactory sense 

 as a factor in the selection by distributing the food odor uni- 

 formly over the apparatus. To eliminate the possibility of the 

 animal's trailing himself and thus finding the food, the forms 

 were regularly turned so as to present a new base line and re- 

 versed to present a new surface surrounding the form. By 

 this means, the circle and the square could each have eight 

 symmetrical positions. The hexagon had two on each face 

 with an angle at the base and two with a side at the base. The 

 other figures did not lend themselves so well but were manipu- 

 lated with every possible precaution to prevent the animal from 

 using a cue other than form as a means of selection. Special 

 precaution was taken when the animal was nearing success but 

 there was no indication that secondary criteria were employed. 

 All the form boards were painted a uniform "French B" gray 

 so that brightness could not become a factor in the choice. An 

 added precaution was to wash the forms occasionally so as to 

 prevent their becoming soiled and thus characteristically marked ; 



