EVOLUTION 995 



that insist on sharing in the game, and so making more of their 

 environment than did their predecessors. No doubt each receives 

 by heredity its particular hand of cards, beyond which it cannot go ; 

 but it is more or less open to each organism to play the hand for all 

 it is worth. 



New Associations. — Inborn in many animals are certain 

 reactions which always occur in answer to a particular stimulus. 

 When the earth vibrates the worm jerks itself back into its hole; and 

 when the action is as simple as this, it is called a reflex. The brain is 

 not needed, and the action is involuntary. As explained in the section 

 on Behaviour, a stimulus from the outside world is received by 

 sensory nerve-cells (S); the thrill passes to associative nerve-cells 

 (A); it is shunted to motor nerve-cells (M), which command the 

 effector (E) muscle-cells to contract. We may indicate the stages of 

 a simple reflex action (sensory, associative, motor, effector) by their 

 initial letters — S -> A -> M -> E ; and as it often happens that one 

 reflex action sets another at work, as sucking is followed by swallow- 

 ing, this might be conveniently indicated by S -> A -» M -> E, 

 followed by s -> a ^ m -> e, followed by s -> a -^ m-^ e. Such a 

 chain of reflex actions forms instinctive behaviour, considered from 

 the physiological side; but many instincts have probably more or 

 less of a psychological side as well, being suffused with awareness 

 and backed by endeavour. A hive-bee on one of its first open-air 

 explorations forces its way into a difficult flower, it collects the 

 pollen, it stows it away in its pollen-baskets; and, when these are 

 full, it makes a bee-line for home. It has not to leam to do these 

 things; the capacity of doing them is part of its hereditary make-up. 

 An instinct is an inborn ready-made power of doing apparently 

 clever things. Now the meaning of this digression is that in new 

 surroundings or circumstances animals have very little power of 

 changing their instincts ; what they may be able to change are their 

 individually learned associations, which may thus develop into 

 intelligent habits. The difference makes the whole problem of change 

 or no change more intelligible. When a young partridge or redshank 

 hears its parent's danger signal it is bound to squat and remain 

 motionless. This is instinctive behaviour. But when a mother otter 

 is teaching its cubs how to behave when the hound's bark is heard, 

 it is establishing an association; and when it is teaching them how 

 to guddle for trout or how to eat frogs, it is appealing to their 

 dawning intelligence. The habits built up on the basis of association 

 or of intelligence are more or less changeable if the conditions of 

 life change. In short, the more instinctive an animal is, the less 

 plastic it is ; and the more intelligent an animal is, the more educable 

 it is. When the schoolboys in the Mediterranean pine-forest adjust 

 a moving Indian file of the Procession Caterpillars so that the head 

 oi A is brought round to touch the tail of Z, the larvae may go 



