The term _instinct_ may be given to the act depending upon inherited
structure, an inherited bond, or it may be given to the inherited bond
itself. Similarly, the term _habit_ may be given to an act that we have
had to learn or to the bond which we ourselves establish between
response and stimulus. In this book we shall usually mean by instinct an
action depending upon inherited structure and by habit an act depending
upon a bond established during lifetime. A good part of our early lives
is spent in building up bonds between stimuli and responses. This
establishing of bonds or connections is called _learning_.

=Appearance of Inherited Tendencies.= Not all of our inherited tendencies
are manifested immediately after birth, nor indeed in the earliest years
of childhood, but appear at different stages of the child's growth. It
has already been said that a child, soon after birth, will close its
eyelids when they are blown upon. The lids do not close at this time if
one strikes at them, but they will do this later. The proper working of
an instinct or an inherited tendency, then, depends upon the child's
having reached a certain state of development.

The maturing of an instinct depends upon both age and use, that is to
say, upon the age of the animal and the amount of use or exercise that
the instinctive activity has had. The most important factor, however,
seems to be age. While our knowledge of the dependence of an instinct
upon the age of the animal is not quite so definite in the case of human
instincts, the matter has been worked out in the case of chickens.

The experiment was as follows: Chickens were taken at the time of
hatching, and some allowed to peck from the first, while others were
kept in a dark room and not allowed to peck. When the chickens were