FACTS AND FACTORS OF DEVELOPMENT 75 



sorts of animals. 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. 



(c) Responses which have become fixed and 

 constant through natural selection or other 

 means of limitation may become more varied 

 and general when the compulsory limitation is 

 relaxed. Behavior in the former case is fixed 

 and instinctive, in the latter more varied and 

 plastic. Thus Whitman found that the be- 

 havior of domesticated pigeons is more vari- 

 able and their instincts less rigidly fixed than 

 in wild species. If the eggs are removed to a 

 little distance from the nest the wild passenger 

 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 few 

 minutes without heeding the eggs. The ring- 

 neck pigeon also misses the eggs and some- 



