12 INTRODUCTORY 



from a caterpillar into a butterfly. By instinct, 

 animals and plants increase in size, and shape 

 their organs, until they attain maturity. They are 

 not conscious of the force within them : nor are 

 we conscious of the vital processes upon which 

 our lives, from hour to hour, depend. Directive 

 instinct needs no assistance from experience or 

 practice : the young chaffinch knows precisely the 

 materials for its first nest, and the method of inter- 

 weaving them : young birds generally know how 

 to fly without the need of practice : the bee is an 

 instinctive architect of hexagonal cells. By 

 instinct our organs perform their functions the 

 heart beats, the lungs pulsate, the liver and 

 kidneys distil their secretions. The direction of 

 these complicated processes owes nothing to 

 experience, and in this differs essentially from 

 the working of reason. But reason itself is a 

 development of processes that are fundamentally 

 instinctive, and are shared by man with the 

 animals below him. So also, we shall find, are 

 impulses which we do not ordinarily think of as 

 instinctive : such as the promptings of kindness, 

 and even the vague feelings of ecstasy which we 

 speak of as " aesthetic." Instinct underlies our 

 emotions. We love, are enraged, are terrified, 

 feel pity, as we are moved by impulses which lie 

 as deep as life itself. Our instincts may be com- 

 pared to a number of alarums, each of which is 

 set for a special stimulus and runs down im- 

 mediately upon being touched by it. The stimulus 

 may be a sensation of something that occurs out- 

 side us or inside us : it may be a recollection, or it 

 may merely be the lapse of a period of time. Once 

 stimulated, instinct acts forthwith, unless it is 

 checked, or inhibited, by another instinct, by 

 habit, or by an effort of will. 



The diversity which we perceive in the behaviour 



