Modifiability in Behavior. 459 



reaction of a different sort, that often comes on later, will be 

 mentioned in the next section. 



Experiment shows that the failure to respond is practically 

 universal if the drops fall three minutes or less apart. With 

 drops five minutes apart there is still marked evidence of acclimat- 

 ization, though irregularities appear. With drops falling at 

 intervals of more than five minutes I was unable to satisfy myself 

 with certainty that acclimatization occurs. 



Related to the present subject are changes in the reaction to 

 light. Aiptasia annulata is very sensitive to light, expanding in 

 darkness, but contracting after a few seconds when exposed to 

 strong light. In ordinary daylight the animal remains contracted 

 for some hours, but after such a period most specimens extend 

 in spite of the light. In comparative darkness the animals direct 

 the disk toward the source of light, through a contraction on the 

 side of the column exposed to the light. After remaining undis- 

 turbed for a long time in an aquarium that is fairly well lighted, 

 the animals give up their orientation with respect to the strongest 

 source of light; with less light they retain it. 



3. REACTIONS MODIFIED AS A RESULT OF THE PAST EXPERIENCES 



OF THE ORGANISM. 



Under this head will be considered all positive changes in 

 reaction, due to former stimuli or former reactions of the organism, 

 aside from those due to changes in metabolism. 



We have already described certain cases belonging here. In the 

 reaction by which the disk is kept clean in Stoichactis we find that 

 a mass of debris on the disk causes first one reaction, then another, 

 till one of these or a combination of several rids the animal of the 

 stimulating agent (see p. 451). In this case either the continua- 

 tion of the same stimulus, or the fact that a certain reaction has 

 been given, induces a new reaction, without change in the external 

 conditions. 



A similar phenomenon is often seen in the experiments with 

 falling drops of water, described above. To the first drop the 

 animal responds by a sudden sharp contraction, then to a consider- 

 able number of drops there is no response. Now if the drops con- 

 tinue, the animal usually begins to shrink slowly away from the 

 region where the drops are falling, so that in the course of time the 



