INSTINCT 107 
either in the spinal cord or the lower centers of the brain. 
It thus resembles the segmental reflexes which constitute 
much of the behavior of worms and arthropods. In the 
frog, as Schrader has observed, the central nervous system 
‘‘can be divided into a series of sections each of which is capa- 
ble of performing an independent function.” Its mode 
of action is hence of the same fundamental kind that we 
find in the nervous systems of the lower articulate animals. 
The relation of reflex action to instinct which is disclosed 
through operations on the nervous system is shown also 
by a study of the gradual development of instinct through 
the animal kingdom. The behavior of the protozoa, as we 
have seen, consists mostly of rather simple stereotyped 
activities which have all the directness of the simple reflex 
acts of higher forms. The behavior of the lower Metazoa 
falls largely within the same general type. Tracing the 
evolution of behavior upward we find a gradual increase in 
the number, complexity and perfection of reflex acts. 
Where instinct may be said to begin is an arbitrary matter. 
If instinct be, as Spencer defines it, ‘compound reflex ac- 
tion,” it begins of course where reflex action passes from the 
simple to the compound, but this point is not so easy to 
mark as theoretically it might appear. It is commonly said 
that in reflex action only a part of the body responds, as in 
winking the eye, or jerking back the foot, whereas in instinc- 
tive behavior there is a response by the organism as a whole. 
This distinction is at times difficult to draw, and it is not 
consistently adopted by most writers, but it is perhaps as 
useful a distinction as can be made. 
While instinct is most intimately related in its nature and 
origin to reflex action it would be an error to regard it as con- 
sisting of nothing but direct responses to external stimuli. 
The animal is not merely a machine responding to the 
