Modification by Experience 265 



case he makes no discrimination. The discrimination oc- 

 curs when the withdrawing reaction attaches itself to the 

 feature which distinguishes the dark passage from the rest 

 of the apparatus, namely, its darkness. It is probable 

 that in many cases the animal does not deHberately com- 

 pare the Ught with the dark passage, but merely learns to 

 distinguish the passage to be avoided from the rest of the 

 situation at large. We should expect this to be the case^ 

 where punishment is the only method of training used : 1 

 the case would not be one of "white preferred to black," / 

 but of "anything rather than black." 



The strength of the punishment appUed is of course an . 

 influential factor in the learning. Obviously it depends 1 

 not merely on the strength of the punishing stimulus, but j) 

 on the sensitiveness of the punished animal. Yerkes ' 

 was the first experimenter to employ the electric shock as 

 a means of training animals. He used it on the frog (805), 

 which he was trying to educate to make a turning to the 

 left rather than to the right : the frog showed a discouraging 

 tendency to sit motionless for long periods of time, and so 

 Yerkes placed electric wires on the floor, to induce by a 

 mild shock greater activity. In his work on the dancing 

 mouse (820), he substituted the giving of electric punish- 

 ments in the case of wrong choices, for the older method \>i 

 rewarding an animal's right choices, and one of the advan- 

 tages claimed for this method was that it seems to allow an 

 exact measurement of the strength of the stimulus, whereas 

 a reward, such as food, varies in strength with the animal's 

 physiological condition. But the effect of an electric shock 

 too varies with the temporary physiological state of the 

 animal, and with its general individual sensibihty. Yerkes 

 (827) carried out some interesting experiments on the re- 

 lation of the strength of the punishment to the difficulty 



