p.16 The Adaptive Function of Behavior. Behavior may be defined as the process of responding to some form of energy in the environment by an activity generally useful to life. The energy, or less exactly the object from which it is derived, is known as the `stimulus,' while the resulting activity is called the 'response.'

p.147-148 Behavior... is the process of responding to a stimulus by an activity that is normally useful to life... A social stimulus is any movement, expression, gesture, or sound - in short, any reaction, made by an animal (human or infra-human) - which produces a response in another.

p.148 Social behavior may, therefore, be defined as behavior in which the responses either serve as social stimuli or are evoked by social stimuli.

p.148-149 Linear and Circular Social Behavior. When individuals respond to one another in a direct, face-to-face manner, a social stimulus, given, for example, by the behavior of individual A, is likely to evoke from individual B a response which serves in turn as a stimulus to A causing him to react further. The direction of the stimuli and of their effects is thus circular, the responses of each person being reevoked or increased by the reactions which his own responses called forth from others.

p.149 In situations which call for a mere transmission of stimuli, and where the effects are in one direction rather than back and forth, we find a simpler type of social behavior which we may describe as linear. The stalking of game illustrates a short linear series, the actions of the prey producing responses in the hunter. It is the aim, however, of the latter to prevent his responses from acting in turn as stimuli upon the quarry. Stealth and concealment prevent the circular reaction and diminish the likelihood of escape.