74 HEREDITY AND ENVIRONMENT 



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 

 normally suck blood is put into an aquarium 

 with the leeches, they at once leave the shade 

 and attach themselves to the turtle. They 

 prefer shade to bright light but they prefer 

 their food to the shade. The tendency to re- 

 main concealed is inhibited by the stronger 

 stimulus of hunger. On the other hand he 

 found that the salamander, Necturus, is so 

 timid that it will not take food, even though 

 starving, until by gradual stages and gentle 

 treatment its timidity can be overcome to a 

 certain extent. Here fear is at first a stronger 

 stimulus than hunger and unless the stimulus 

 of fear can be reduced the animal will starve 

 to death in the presence of the most tempting 

 food. 



(b) Responses may also be modified 

 through compulsory limitation of many pos- 

 sible responses to a particular one, and the con- 

 sequent formation of a habit. This is the 

 method of education emploj^ed in training all 



