Facts and Factors of Development 51 



sponses. Whitman found that leeches of the genus Clepslne 

 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 aqua- 

 rium with leeches, they at once leave the shade and attach them- 

 selves to the turtle. They prefer shade to bright light but they 

 prefer their food to the shade. The tendency to remain 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) Compulsory Limitations. Responses may also be modi- 

 fied through compulsory limitation of many possible responses to 

 a particular one, and the consequent formation of a habit. This 

 is the method of education employed in training all sorts of ani- 

 mals. Thus Jennings found that a star-fish could be trained to 

 turn itself over, when placed on its back, by means of one pair 

 of arms simply by persistently preventing the use of the other 

 arms. Many responses of organisms are modified in a similar 

 way, not only by artificial limitations but also by natural ones. 



(3) Fixed and Plastic Behavior. Responses which have be- 

 come fixed and constant through natural selection or other means 

 of limitation may become more varied and general when the com- 

 pulsory limitation is relaxed. Behavior in the former case is fixed 

 and instinctive, in the latter more varied and plastic. Thus Whit- 

 man found that the behavior of domesticated pigeons is more var- 

 iable and their instincts less rigidly fixed than in wild species. If 

 the eggs of the wild passenger pigeon are removed to a little distance 

 from the nest the pigeon returns to the nest and sits down as if 

 nothing had happened. She soon finds out, not by sight but by 

 feeling, that something is missing, and she leaves the nest after a 



