68 HEREDITY AND ENVIRONMENT 



response may be the end of a long series of 

 internal changes which are started by the 

 original stimulus. 



(2.) The response to a stimulus may be 

 modified or inhibited in the following ways: 



(a) Through conflicting stimuli and 

 changed physiological states, due to fatigue, 

 hunger, etc. Many stimuli may reach the 

 organism at the same time and if they conflict 

 they may nullify one another or the organism 

 may respond to the strongest stimulus and 

 disregard the weaker ones. When an organ- 

 ism has begun to respond to one stimulus it is 

 not easily diverted to another. Jennings found 

 that the attached infusorian, Stentor, which 

 usually responds to strong stimuli by closing 

 up, may, when repeatedly stimulated, loosen 

 its attachment and swim away, thus respond- 

 ing in a wholly new manner when its physi- 

 ological state has been changed by repeated 

 stimuli and responses. Whitman found that 

 leeches of the genus Clepsine prefer shade to 

 bright light, and other things being equal they 

 always seek the under sides of stones and 

 shaded places ; but if a turtle from which they 



