124 Studies in Animal Behavior 



ably arose independently along a third line of de- 

 velopment. Intelligence makes its appearance at a 

 certain stage of organization along whatever line 

 such a stage may have been reached. 



Up to the point at which the power of associa- 

 tive memory becomes manifest there has been prog- 

 ress along many lines which has prepared the way 

 for the evolution of this new faculty. Behavior 

 has not only become more complex, but it has be- 

 come more plastic and capable of easy modification 

 to suit new conditions. The lower organisms do 

 not always react in a particular way to a given stim- 

 ulus. What reaction occurs may depend upon the 

 number of previous stimulations, the supply of food, 

 exposure to different environing conditions, and 

 numerous other factors which influence the internal 

 state of the organism. The behavior of many lower 

 animals is plastic and adaptive to a remarkable 

 degree, and to a superficial consideration often gives 

 the appearance of a considerable degree of intelli- 

 gence, without there being any detectable power of 

 associative memory. This plastic and varied be- 

 havior not only simulates intelligence, but it se- 

 cures for the organisms many of the advantages 

 which intelligence confers. It adapts the animal to 

 a more varied environment, and gives it the power 

 of meeting a given situation in more than one way, 

 so if one kind of response does not suit, another may 

 be more successful. Let us glance briefly at some 

 of the ways in which behavior may be modified. 



