78 The Outline of Science 



trigger of another, and so on in a chain. The behaviour of the in- 

 sectivorous plant called Venus's fly-trap when it shuts on an 

 insect is like a reflex action in an animal, but plants have no 

 definite nervous system. 



What are Called Tropisms 



A somewhat higher level on the inclined plane is illustrated 

 by what are called "tropisms," obligatory movements which the 

 animal makes, adjusting its whole body so that physiological 

 equilibrium results in relation to gravity, pressure, currents, 

 moisture, heat, light, electricity, and surfaces of contact. A 

 moth is flying past a candle; the eye next the light is more 

 illumined than the other; a physiological inequilibrium results, 

 affecting nerve-cells and muscle-cells; the outcome is that the 

 moth automatically adjusts its flight so that both eyes become 

 equally illumined ; in doing this it often flies into the candle. 



It may seem baoT business that the moth should fly into the 

 candle, but the flame is an utterly artificial item in its environ- 

 ment to which no one can expect it to be adapted. These tropisms 

 play an important role in animal behaviour. 



2 

 Instinctive Behaviour 



On a higher level is instinctive behaviour, which reaches 

 such remarkable perfection in ants, bees, and wasps. In its typical 

 expression instinctive behaviour depends on inborn capacities; 

 it does not require to be learned ; it is independent of practice or 

 experience, though it may be improved by both; it is shared 

 equally by all members of the species of the same sex (for the 

 female's instincts are often different from the male's) ; it refers 

 to particular conditions of life that are of vital importance, 

 though they may occur only once in a lifetime. The female 

 Yucca Moth emerges from the cocoon when the Yucca flower 

 puts forth its bell-like blossoms. She flies to a flower, collects 



