INSTINCTS AND HABITS IN CHICKS 41 



more self-dependent than we have hitherto supposed. The peck- 

 ing reaction suffers the supplementation of habit, one may well 

 believe, when it does not demand it. It remains to be shown 

 to what extent these instincts are typical of instincts in general. 

 But even if it should be established that acquisition contributes 

 relatively little, environment would still remain a powerful 

 factor in development. The animal begins life with an hereditary 

 endowment in interaction with the environment. One is neces- 

 sary to the other. In the economy of an organism there is no 

 reaction without stimulation. And not only is the environment 

 a system of energies without which natural tendencies of the 

 organism cannot be realized, but it is a selective system. What 

 tendencies shall be realized will be determined by what stimuli 

 are provided. Heredity and environment are not opposites, but 

 complements. 



PART II. ACQUIRED REACTIONS 



I. Introduction and Statement of the Problem 



From the study of the modifiability of reactions that are 

 made prior to experience and are supposed to be based on 

 inherited neuro-muscular co-ordinations, our interest now shifts 

 to a study of the development of reactions that depend upon 

 no such hereditary dispositions in the nervous system, but are 

 individually acquired. It was one of the aims of this investiga- 

 tion to discover not only whether reactions to certain optical 

 stimuli or stimulus-complexes are modifiable, but also to deter- 

 mine the rate of modification; in other words, to describe the 

 progress of habit-formation in quantitative terms. 



Modifiability was tested by the so-called discrimination method. 

 Never more than two possibilities of selection were offered to 

 the animal at one time. The conditions were so arranged that 

 the chick's natural tendency to react to confinement and soli- 

 tude by efforts to escape furnished the necessary random activity. 

 When, after a certain number of trials, the animal reacted 

 selectively to one of two constant form or size or color stimuli, 

 for neither of which it displayed a preference before training 

 began, the process of habit-formation was adjudged completed. 

 By selective reaction is here meant the ability to react without 

 error to a constant stimulus for an arbitrarily fixed number 

 of times. 



