Modification by Experience 263 



tance, though great, cannot compete against that of the 

 negative response called forth by their injurious effect in 

 the special case. Learning by punishment is in most cases 

 especially rapid. Its effect may be to inhibit altogether, 

 for some time, a certain instinct. For example, the ex- 

 perience of receiving an electric shock when they seized a 

 certain kind of food prevented frogs from feeding at all 

 for several days (657). Rats which were being trained to 

 discriminate between a lighter and a darker passage with 

 the use of an electric shock acquired a distaste for the ap- 

 paratus as a whole (328). Mobius in 1873 (497) made 

 some experiments with a pike, afterwards repeated by 

 Triplett (720) with perch, which illustrate the same phenom- 

 enon. The fish was kept in one half of an aquarium, sepa- 

 rated by a glass screen from the other half, in which min- 

 nows were swimming about. The pike naturally dashed 

 at them, and whenever it did so bumped its nose on the 

 glass partition. After a considerable period of this sort of 

 experience, the glass screen was removed, and the min- 

 nows were allowed to swim freely around the pike, when it 

 was found that the latter's instinct to seize them had been 

 wholly suppressed by the harmful consequences of such 

 action. Here, again, the chances that a movement will 

 be suppressed in favor of the negative response depends 

 on how great the degree of its prepotency is. It was a 

 rash conclusion on Bethe's (49) part to deny the learning 

 ability of the crab because, although every time it went 

 into the darkest corner of its aquarium it was seized by a 

 cephalopod lurking there, it did not in six such experiences 

 learn to inhibit its innate tendency to avoid light : further 

 training would probably have been successful. Yerkes 

 (822) trained an earthworm, by giving it an electric shock 

 when it followed its innate inclination for turning towards 



