284 A. S. PEARSE. 



external factors. It is important to discover, if possible, what 

 stimuli will cause these differences in behavior. When Thyoiie 

 extends its tentacles and feeds, the movements are brought about 

 by such factors as hunger, or the presence of food, or by a combi- 

 nation of two or more such stimuli. If we throw a shadow on 

 it as it feeds, the tentacles contract and the withdrawing reaction 

 takes place. Although all the stimuli which were effective in 

 producing the feeding reaction but a moment ago are present and 

 acting, we have introduced an additional stimulus which has 

 modified the response. This is an example of inhibition, as the 

 presence of one stimulus inhibits the response to certain other 

 stimuli. The periodic repetition of a stimulus is another means 

 by which responses may be changed. As Jennings (:05) says of 

 this method " the physiological state tends to resolve itself into 

 another and different state" after a stimulus has been received. 

 An individual will be in a different condition, and will really be a 

 different animal, after it has received the first stimulus and may 

 therefore give a different response the second time the same 

 stimulus is received. Some of the instances of variable behavior 

 which were observed will now be briefly considered. 



I . Repetition of a Stimuhis. — Responses usually vary in degree 

 when a stimulus is repeated at regular intervals. If an indi- 

 vidual is touched gently with a glass rod and then touched again 

 on the same spot at one minute intervals the withdrawing response 

 which was at first marked becomes gradually weaker and finally 

 ceases altogether. Similar results may be obtained by allowing 

 a drop of water to fall at regular intervals into the dish which 

 holds an animal, or by periodically throwing a shadow upon an 

 extended individual. By increasing the interval of time between 

 successive stimuli a larger number of responses may be obtained 

 but the result will be the same in the end. Individuals which 

 have been newly brought from the ocean contract at the slight- 

 est disturbance and give the withdrawing reaction whenev^er any- 

 one walks across the floor or opens a door or when any other 

 slight change occurs in their surroundings, but they soon cease 

 to respond to such stimuli. For example, one individual which 

 had been kept for two weeks on a table in the laboratory carried 

 on normal feeding and breathing reactions while people were con- 



