THE NERVOUS FUNCTIONS 31 



in two ways, both of which are universally present and which 

 throughout the remainder of this work we shall call the invariable 

 or innate behavior and the variable or individually modifiable 

 behavior. 



Every animal reaction, then, contains these two factors, the 

 invariable and the variable or individually modifiable. The 

 first factor is a function of the relatively stable organization of 

 the particular living substance involved. The pattern of this 

 organization is inherited, and these characteristics of the be- 

 havior are, therefore, common, except for relatively slight devia- 

 tions, to all members of the race or species; they are rigidly 

 determined by innate bodily organization so arranged as to 

 facilitate the appropriate reactions, in an invariable mechanical 

 fashion, to every kind of stimulation to which the organism 

 is capable of responding at all. In the strictly vegetative func- 

 tions, in all true reflexes (as these are defined on page 56), and in 

 purely instinctive activities in general this factor of behavior is 

 dominant. 



But in addition to this invariable innate behavior, all organ- 

 isms have some power to modify their characteristic action sys- 

 tems in adaptation to changed environmental relations. This 

 individual modifiability is known as biological regulation, a proc- 

 ess which has of late been very carefully studied. We cannot 

 here enter into the problems connected with form regulation, 

 that is, the power of an organism to restore its normal form after 

 mutilation or other injury. On regulation in behavior reference 

 should be made to the works of Jennings and Child. In lower 

 organisms Jennings recognizes three factors in the regulation of 

 behavior: First, the occurrence of definite internal processes; 

 these form part of the invariable hereditary action system re- 

 ferred to above. Second, interference with these processes 

 causes a change of behavior and varied movements, subjecting 

 the organism to many different conditions. Third, one of these 

 conditions may relieve the interference with the internal proc- 

 esses, so that the changes in behavior cease and the relieving 

 condition is thus retained. Lack of oxygen, for instance, would 

 interfere with an animal's internal processes; this leads it to move 

 about; if finally it enters a region plentifully supplied with oxy- 

 gen, the internal processes return to normal, the movement 



