THE LEVELS OF BEHAVIOUR 143 



unco-ordinated agencies and possibly also because its central 

 nervous system is normally unstable to a slight extent. In the 

 absence of vector stimuli we may therefore suppose animal 

 movements to occur at random. Let there be a '* stimulation 

 field," such as a drop of noxious acid in the water in which, 

 say, a Paramoecium is living. Round the drop the intensity of 

 the stimulus decreases in a roughly symmetrical way as the 

 acid diffuses outwards. The Protozoan swims, at random, into 

 this field of stimulation, experiences a noxious effect and changes 

 its direction of motion. Perhaps this change may carry it out- 

 side the field, but if it does not do so the animal again turns 

 away and repeats these changes until it avoids the noxious 

 stimulation. A definite behaviour is displayed by a Paramoecium 

 in these cases and, along with its structure, a definite behaviour- 

 pattern, or '' avoiding-reaction." 



(2) Most higher animals are bilaterally symmetrical, there 

 being similar receptors, say eyes, on the two sides of the body : 

 there are also similar locomotory apparatus, such as wings. 

 Action of the wings on one side of the body will turn the animal 

 to one side in its locomotion. Stimulation of the receptors on 

 one side, but not on the other, may thus be expected to turn the 

 direction of motion to one side or the other. Symmetrical 

 stimulation on both sides may be expected to maintain direct 

 forward locomotion. The classical examples are birds that fly 

 into the lanterns of lighthouses and moths that fly into candle- 

 flames. Such reactions are said to be " forced " ones. Another 

 much-quoted example is that of the caterpillar that climbs 

 vertically up a shrub and feeds on the young and tender shoots. 

 Here phototaxis is said to be associated with the habit. The 

 animal is stimulated optically on both sides of the head so that 

 if it turns to one side the other is more strongly stimulated and 

 the turning aside is thus corrected so that the animal preserves, 

 on the whole, an upward motion. Having fed, it then descends 

 the shrub and we are bound, by our hypothesis of pure taxis, 

 to assume that the changed " physiological state " of the animal, 

 which is consequent on its having fed, changes the sign of the 

 latter so that the caterpillar is now^ negatively phototactic. 



There are also many examples of vertical migrations of 

 Diatoms, Peridinians, micro-crustacea and other planktonic 

 marine organisms. These up-and-down motions are to be 



