56 FUNDAMENTAL PRINCIPLES 



ductivity, which is the power to transmit this effect, are both properties 

 of Uving matter. 



87. Part Played by the Nervous System. — In animals possessing no 

 nervous system, behavior is summed up in the responses to stimuli or 

 in the tropisms which the animal exhibits. In animals that possess a 

 nervous system, the structure relations within the nervous system modify 

 the responses in a variety of ways. Two or more cells are involved 

 between the reception of the stimulus and the response, which accordingly 

 is said to be indirect. The presence of the nervous system also makes 

 possible more numerous and more varied effects due to internal stimuli. 

 The result is the production of the complex forms of behavior character- 

 istic of the higher animals. 



88. Physiological State. — The character of the response which an 

 animal will give to a stimulus is determined not only by the kind and 

 strength of the stimulus but also by the condition of the animal and 

 depends upon the state of the metabolic processes within its body. Thus 

 a one-celled animal in the body of which there is no food, which is hun- 

 gry, and which is at the end of a cycle of metabolism may give a response 

 different from that of an animal which has recently fed and in the body 

 of which the metabolic cycle has just begun. In higher animals different 

 parts contribute to the physiological state of the whole. Animals which 

 possess a nervous system exhibit physiological states dependent upon the 

 varying conditions of that system, which in turn have a metabolic basis. 

 Repeated and abnormal stimulation may throw an animal into a condition 

 of excitement in which it acts in a manner quite unusual. The different 

 feelings of which we are conscious at different times, the mental attitudes 

 which dominate us, and our varying ability to carry on our different 

 activities are all connected with different physiological states. Physio- 

 logical states are back of what is called temperament, or mood, and 

 explain one's ability to excel on one occasion and his inability to perform 

 creditably on another. It is a certain physiological state resulting from a 

 change in our ordinary routine which causes us to feel and act differently 

 after a holiday or after an unusual experience. This explains "blue 

 Mondays" and "off days." The word psychological is sometimes used 

 instead of physiological when the nervous system is involved. 



