PROBLEM i y^hat Are the Simplest Forms of Behavior 



in Animals? 



The meaning of "behavior." To the biol- 

 ogist behavioi does not mean good or 

 bad conduct. It means the sum total of 

 all an organism does in the course of its 

 life. Stumbling as you stub your toe, 

 opening your umbr-^lla in the rain, de- 

 ciding to walk to the river for a swim, 

 feeling happy over an unexpected school 

 holiday, waking when the alarm rings, 

 going to sleep and dreaming, listening to 

 the radio, all these things and many more 

 make up your behavior. In this unit we 

 will study the behavior of organisms. 



Environment and sense organs. As you 

 cross a city street the honking of an 

 automobile horn may reach your ears. 

 You turn your head and see a red car 

 coming toward you. It passes you and 

 you feel the heat of the exhaust on your 

 skin; the fumes strike your nose. One 

 sense organ after another has received 

 something from the environment: your 

 ears, your eyes, special cells in your skin 

 and in your nose. Because of the way 

 they function the sense organs are called 

 receptors. 



Anything that excites a receptor is 

 called a stivmlus (plural stimuli — stim'- 

 you-lie). Light rays are a stimulus to the 

 eyes; sound waves are the stimulus re- 

 ceived by the ears. Each receptor is so 

 made that it picks up a different kind of 



stimulus from the environment. It is 

 because of your receptors that you be- 

 come aware of the environment. 



Stimuli call forth responses. If you are 

 crossing the street absent-mindedly, you 

 may get the loud blast of an automobile 

 horn close to your ear. You start; that 

 is, certain muscles in your body contract 

 violently and you move as a result. In 

 your kitchen you may carelessly brush 

 your hand against the hot oven; almost 

 instantaneously you jerk the hand away 

 from the stove. This, too, is the result of 

 muscular contraction. In both instances 

 a stimulus was received that aroused 

 some part of you to action. In both in- 

 stances there was a muscular contraction. 

 This contraction of the muscles and the 

 resulting bodily movement was your 

 response to the stimulus. The muscles 

 which contract are called effectors, as 

 distinguished from receptors which re- 

 ceive the stimulus. If the automobile 

 that blows a horn close to your ear 

 touches you and knocks you down, fall- 

 ing is not your response to the stimulus. 

 Can you explain how this differs from a 

 true response to a stimulus? 



Responses are not always muscular 

 contractions. A sharp cinder in the eye 

 (stimulus) causes two visible responses: 

 blinking of the eyelid, which is due to a 



