CHAP, ix ANIMAL BEHAVIOUR 159 



to give a false simplicity to the facts. The sound prac- 

 tical rule is to try to re-describe the observed behaviour 

 in as simple terms as possible without leaving out any 

 essential feature. The simplest description is not neces- 

 sarily the true one, but we must hold to it till we are 

 forced to give it up. Prof. Lloyd Morgan has clearly 

 stated this canon : 



"In no case may we interpret an action as the outcome 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 psycho- 

 logical scale." 



We propose in this chapter to illustrate what may be 

 called the inclined plane or staircase of animal behaviour, 

 but the student should bear in mind that there are more 

 uncertainties than certainties. In the study of animal 

 behaviour naturalists and psychologists are still feeling 

 their way and most of the questions remain open. 



2. Restless Movements of Unicellular Animals. Many 

 of the so-called simplest animals or Protozoa exhibit a 

 restless movement which may be ranked at the foot of 



/ 



the inclined plane of behaviour, but not quite on it. So 

 long as certain combustible or explosive substances within 

 them last, and so long as certain outside provocations 

 continue, the little creatures keep on moving, and we see 

 them answering indifferently or distractedly to the 

 various calls that summon them now to one side and 

 now to the other. This locomotion is not much beyond 

 the level of some of the ordinary functions which go on 

 within higher animals, such as the beating of the heart 

 or the movements of respiration. It may be called 

 general organismal motor activity. There is a surplus 

 of energy, and the simplest, crudest disposal of it is to 

 keep moving on, but it should be noted that the restless 

 roving contains the raw materials of hunting and ex- 

 ploration, and that the mode of locomotion is often very 

 distinctive in detail. So soon is the note of individuality 

 struck. 



3. Organic Reactions of Unicellular Animals. Many 

 of the simplest animals respond by particular movements 



