949 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. Reflex 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 Ameba will avoid contact with the feel- 
ers or pseudopodia of another Amebéa, 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. 
