192 ANIMAL BEHAVIOUR 



" Each stimulus causes as a rule not merely a single definite 

 action that may be called a reflex, but a series of ' trial ' 

 movements, of the most diverse character, and including at 

 times practically all the movements of which the animal is 

 capable" (Jennings, 1906, p. 280). 



6. Tropisms. 



From chains of reflexes, suffused with awareness, it is not 

 difficult to pass to the level of instinctive behaviour, but 

 before we pass to that level we have to recognise the important 

 role played by tropisms (see Loeb, 1918). Tropisms are 

 obligatory or forced movements of the creature as a whole, 

 which more or less automatically secure physiological equi- 

 librium in relation to outside stimuli, such as light or heat, 

 gravity or electricity, diffusing chemicals or water-currents. 

 When a moth, constitutionally adapted to nocturnal activity, 

 comes in its flight within the sphere of influence of a candle, 

 and has one eye much more illumined than the other, owing 

 to the direction in which it happens to be flying, more intense 

 chemical processes are set up in the illumined eye. On that 

 side there is therefore a relative increase in the mass of 

 certain chemical products. But messages, impulses, stimula- 

 tions, or waves of chemical reaction are always passing from 

 the brain of the flying moth to the contracting muscles, and 

 if the physiological symmetry of the brain has been disturbed 

 by the unequal illumination of the eyes, the muscles on the 

 more illumined side are thrown into a state of stronger ten- 

 sion or tonus, with the result that they will respond more 

 forcibly to stimulation from the brain, and will therefore 

 turn the head and body of the moth directly towards the 

 candle near which it is flying. " As soon as the plane of 

 symmetry goes through the source of light, both eyes receive 



