Modification by Experience 259 



on by the Peckhams reacted each day to the sound of 

 a tuning fork by dropping from its web until the 

 sound had been repeated some half dozen times, but 

 after the fifteenth day it would not drop at all (570). 

 Pier on (588, 590) found that snails, while at first respond- 

 ing to shadows by withdrawing the tentacles, on succes- 

 sive days stopped reacting after fewer and fewer trials; 

 and believed he could trace a parallel between the laws of 

 this learning and those of human memory. There is no 

 question in such cases of the reaction's being dropped off 

 in favor of some other reaction. It is dropped off, as it 

 were, by its own weight; simply because it is useless. 

 This same principle seems to enter as a cooperating factor 

 in cases where animals acquire a discrimination between 

 stimuli. The apparent ability of sea-anemones to dis- 

 tinguish between real food and filter paper soaked in food- 

 juice (see page 254) is, as we have seen, ascribed by some 

 to sensory adaptation, but the experiments of Fleure and 

 Walton (228), if their results are accepted, would indicate 

 that true learning is involved. They tested Actinia with 

 a scrap of filter paper once every twenty-four hours, plac- 

 ing it on the same tentacles, which usually carried it to 

 the mouth, where it was swallowed and later ejected. 

 After from two to five days the mouth would no longer 

 swallow the fragment, and in two more days the tentacles 

 refused to take hold of it. Other tentacles could be "de- 

 ceived" at least once or twice after this, but very soon 

 manifested the inhibition. All traces of the learning were 

 lost after from six to ten days interval. Another anemone, 

 Tealia, learned more quickly than Actinia. Again, Her- 

 rick (297) found that catfish, when the barbels were touched 

 with a bit of meat, immediately seized it. If a piece of 

 cotton wool were used instead of the meat, they made the 



