ANIMAL BEHAVIOUR 179 



using psychological terms. We inquire into the creature's 

 power of profiting by experience, as to its pursuance uf trial- 

 and-error methods. The growtli of experimental psychology 

 has furnished many a welcome check to interpretations, show- 

 ing some to be too simple and others to be too generous. 

 Some weight will also be attached to the degree in which 

 the nervous system of tbe animal in question resembles ours, 

 or that of some types previously judged to be "capable of, 

 let us say, making an inference. 



The sound practical rule is to try to re-describe the ob- 

 served behaviour in as simple terms as possible without leav- 

 ing out any essential feature. As Prof. Lloyd Morgan has 

 put it, " In no case may we interpret an action as the out- 

 come of the exercise of a higher psychical faculty, if it 

 can be interpreted as the outcome of the exercise of one 

 which stands lower in the psychological scale " (1894, p. 53). 

 The simplest description is not necessarily the true one, as we 

 know in human affairs ; but the scientific method is to hold 

 to it till facts force us to give it up. 



§ 3. Activities of Unicellidar Organisms. 



Many unicellular organisms show restless movements 

 which may bo ranked at the foot of the inclined plane of 

 behaviour. As long as certain combustible materials within 

 the organism hold out, and as long as certain external stimuli 

 continue to act, the creatures continue moving; and we see 

 them responding to various calls which summon them now 

 to one side and now to the other. This is like the unworked 

 ore of behaviour, and not much beyond the level of everyday 

 internal activities. We may speak of it as general organismal 

 activity among unicellulars. 



But even among the simplest creatures we notice three 



