58 L. W. SACKETT 



of a Nendel black. As the former had a smooth and the latter 

 a velvet finish, the difference in brightness-tone may have been 

 sufficient to confuse the animal, especially as the difference 

 between the stimuli was near the discrimination limen. When 

 gray 32 was inserted giving eighteen instead of nine shades of 

 difference he soon regained his ability. Later he succeeded 

 with a difference of nine shades and even made the loth series 

 perfect when gray 45 was paired with gray 50 but this habit 

 disintegrated when he was given newly papered boxes. Gray 41 

 was again restored as comparison and he had little difficulty ; 

 but he was lost when another attempt was made to test his 

 ability to discriminate between two grays differing by five shades. 

 '•■ Additional tests were then made toward the other end of the 

 scale of grays by varying the food-box stimulus and leaving the 

 comparison box constant, always feeding in the darker box. 

 The steps down to gray 1 1 gave no disturbance worthy of mention. 

 With these grays, however, both animals had difficulty. Like- 

 wise, both were somewhat confused when new boxes were pre- 

 sented after they had succeeded with the first pair. It. is sig- 

 nificant, however, that No. 3 made 23 errors with six series in 

 learning to distinguish grays i-ii while No. 10 made 25 errors 

 with eight series. The difference is sufiiciently slight to bring 

 them into the same category for the purjDoses of discussion. It 

 is evident that this is near the limen of the porcupine's ability 

 to .discriminate shades of brightness when presented as shaded 

 papers, — a limen very much higher than that reported by Cole 

 (10) for the raccoons. Following this, the shade of both the 

 food box and the comparison box were varied toward the middle 

 grays with food still in the darker box. Neither animal had 

 any great difficulty with grays 11 and 41 but both failed com- 

 pletely when grays 21 and 32 were paired. This, at first, seems 

 strange since there was a difference of eleven shades between 

 those two grays and the porcupines had learned to distinguish 

 a difference of nine shades at the dark end and of ten shades 

 at the light end. But even to the human eye discrimination is 

 easier where black or white are presented as one factor in com- 

 parison. It is not strange that the same thing is found in the 

 reactions of the porcupine. 



It is offered as a suggestion rather than a conclusion that 

 in discriminating the black and white boxes at first. No. 3 made 



