ANIMAL BEHAVIOUR 193 



again equal illumination, the tension (or tonus) of syra- 

 metrical muscles becomes equal again, and the impulses for 

 locomotion will now produce equal activity in the symmetri- 

 cal muscles. As a consequence, the animal will move in a 

 straight line to the source of light until some other asym- 

 metrical disturbance once more changes the direction of mo- 

 tion " (Loeb, 1918, p. 14). Such, in outline, is Prof. 

 Jacques LoeVs ingenious and convincing theory of the tro- 

 pism or * forced movement ' which brings the moth into the 

 candle. 



Tropistic actions are obligatory in the sense that every 

 creature of the same kind and in the same physiological 

 state will in similar circumstances behave in the same wav ; 

 there is no alternative. But it is a very notable fact, to 

 be carefully thought over, that a tropism may be changed, 

 reversed, or annulled by changes in the physiological con- 

 dition of the body or by changes in the surrounding medium. 

 The common Amphipod Crustacean Gammarus of fresh- 

 water pools always moves away from light — that is its tro- 

 pism, but add the least trace of acid to the water, and it 

 moves towards the light — as if a drop of philtre changed 

 the creature's whole nature ! 



The tropistic movements often appear as if they had a very 

 definite external aim — such as the candle — but that is illu- 

 sory. The orientation is physiologically coerced. There is no 

 desire of the moth for the star. It should be noted that 

 their general adaptiveness is not contradicted by cases like 

 the moth flying into the candle, for organisms are not and 

 could not be adapted to the altogether exceptional and un- 

 natural. In some cases one tropism way thwart another, 

 and it may be that a tropistic movement is sometimes in- 

 terrupted by some strong internal stimulus such as a desire. 



