EVOLUTION OF INTELLIGENCE. 115 



away from the source of light. This inequality of muscular 

 action will last until the animal is so situated as to point 

 directly to or from the source of the stimulus. Thus when 

 a moth tends to fly straight into the flame of a candle it is 

 driven by its mechanism, not by an interfering emotion or 

 will. If one eye of a moth is darkened, the insect will be 

 unable to fly straight, but will continue to move in a circle. 

 Animals are more or less heliotropic according as they 

 possess more or less chemical substance capable of being 

 decomposed by light, and their susceptibility will vary 

 greatly according to temperature and other conditions 

 which hasten or retard the action. A weak acid added 

 to the water containing small Crustacea, like Cyclops, 

 can increase their heliotropi^m, or even change the 

 heliotropism from negative to positive. 



Another important element in the behaviour of 

 organisms is known as " differential sensibility," or the 

 reaction to sudden and marked changes in the strength 

 of the stimulus. A shadow passing across a worm will 

 cause it to contract ; other instances easily suggest 

 themselves. 



In the lower animals, as in plants, behaviour can 

 be almost completely analysed into manifestations of 

 tropisms and differential sensibility. Their general 

 motions when seeking food ; their habit of gathering 

 together in dark, warm, or damp places ; their sexual 

 instincts ; their instincts connected with reproduction 

 (the laying of the eggs on appropriate substances for the 

 young to feed on, and so forth), can all be interpreted as 

 corresponding to metabolic changes, though all the steps 

 in the physico-chemical processes are not yet known to 

 us. Interesting is the "instinct" shown by the plant 

 Limaria cymbalaria. At first positively heliotropic, it 

 becomes negatively heliotropic after pollination, and so 

 pushes its fruits into dark crevices, lodging them in 

 places favourable for the subsequent germination of the 

 seeds. 





