INSTINCTS AND HABITS IN CHICKS 23 



pecking reaction need not be some object of a size convenient 

 for eating. In line with this consideration is the further fact 

 that the bill of the chick is not only a feeding but a testing organ. 

 If anywhere, the " oral sense " has its organ here. The chick 

 often digs in the litter with its bill when the reaction is distinctly 

 not the pecking reaction. While trying to escape from the 

 reaction box of a discrimination apparatus in a dark-room, 

 chicks often approached a sheet of clean, smooth, lighted opal 

 glass and pecked it. In the same box they pecked the black 

 walls and especially the black cardboard closing the exit, w^hen 

 they happened to enter the wrong side and found their egress 

 barred. But, except where specially noted, no missing re- 

 actions were recorded when no object to be pecked was supplied. 

 Besides the occasional variation in the missing reaction in 

 which the animal pecked the bare cardboard when no other 

 stimulus seemed to be present, there were two other reactions 

 that were easily distinguishable from each other and from the 

 regular missing reaction, but which were, nevertheless, put down 

 in the records as reaction i. These variations in the missing 

 reaction were altogether so few in number, compared with the 

 many hundreds of reactions recorded, that the averages are not 

 materially affected by the classification of them with reaction i. 

 I refer here (i) to cases in which the chick pecked in the direc- 

 tion of the grain but did not reach it, and (2) to cases in which the 

 bill struck the cardboard almost exactly midway between two 

 grains. Of the first of these I have records of thirteen instances; 

 of the second, thirty-six instances. Case one needs no special 

 comment. The bill each time was going in the right direction, 

 but the innervation seemed to be insufficient. Case two is, if 

 anything, more interesting. When the typical instance of it 

 occurred, the grains were usually about i cm. apart, of about 

 equal size, and the line connecting the two ran about normal 

 to the direction in which the chick was oriented, the chick stand- 

 ing directly before them. One could say with little certainty that 

 this was an instance of pure missing. We have already agreed 

 with other observers that the chick from the first rarely missed 

 by more than a hair's breadth an object pecked at. But here 

 was a case where a chick from one to four weeks old missed a 

 grain by 4 to 6 mm. Not only that, but I have five records where 

 in a situation like that described above the chicks pecked twice 



