242 ANIMAL LIFE 



method remains the same. This is simple reflex action, an 

 impulse from the environment carried to the brain and 

 then unconsciously reflected back as motion. The impulse 

 of fear is of the same nature. Strike at a dog with a whip, 

 and he will instinctively shrink away, perhaps with a cry. 

 Perhaps he will leap at you, and you unconsciously will try 

 to escape from him. Keflex action is in general uncon- 

 scious, but with animals as with man it shades by degrees 

 into conscious action, and into volition or action " done on 

 purpose." 



129. Instinct. Different one-celled animals show differ- 

 ences in method or degree of response to external influences. 

 The feelers of the Amoeba will avoid contact with the feel- 

 ers or pseudopodia of another Amoeba, while it does not 

 shrink from contact with itself or with an organism of un- 

 like kind on which it may feed. Most Protozoa will discard 

 grains of sand, crystals of acid, or other indigestible object. 

 Such peculiarities of different forms of life constitute the 

 basis of instinct. 



Instinct is automatic obedience to the demands of ex- 

 ternal conditions. As these conditions vary with each kind 

 of animal, so must the demands vary, and from this arises 

 the great variety actually seen in the instincts of different 

 animals. As the demands of life become complex, so may 

 the instincts become so. The greater the stress of envi- 

 ronment, the more perfect the automatism, for impulses to 

 safe action are necessarily adequate to the duty they have 

 to perform. If the instinct were inadequate, the species 

 would have become extinct. The fact that its individuals 

 persist shows that they are provided with the instincts 

 necessary to that end. Instinct differs from other allied 

 forms of response to external condition in being hereditary, 

 continuous from generation to generation. This suffi- 

 ciently distinguishes it from reason, but the line between 

 instinct and reason and other forms of reflex action can 

 not be sharply drawn. 



