ely inactive at the outset, and would never make a start
towards any acquisition. His acquired reactions, then, are his native
reactions modified by use.

The vast number of motor acts that the individual acquires are based
upon the reflexes. They are modified reflexes. The simplest kind of
modification is the mere _strengthening_ of an act by exercise. By his
reflex breathing and crying, the new-born baby exercises his lungs and
breathing muscles and the nerve centers that control them, with the
result that his breathing becomes more vigorous, his crying louder.
The strengthening of a reaction through exercise is a fundamental
fact.

But we should scarcely speak of "learning" if the only modification
consisted in the simple strengthening of native reactions, and at
first thought it is difficult to see how the {298} exercise of any
reaction could modify it in any other respect. But many reflexes are
not perfectly fixed and invariable, but allow of some free play, and
then exercise may fix or stabilize them, as is well illustrated in the
case of the pecking response of the newly hatched chick. If grains are
strewn before a chick one day old, it instinctively strikes at them,
seizes them in its bill and swallows them; but, its aim being poor and
uncertain, it actually gets, at first, only a fifth of the grains
pecked at; by exercise it improves so as to get over half on the next
day, over three-fourths after another day or two, and about 86 percent
(which seems to be its limit) after about ten days of practice.
Exercise has here modified a native reaction in the way of making it
more definite and precise, by strengthening the accurate movement as
against all the variations of the pecking movement that were made at
the start. Where a native response is variable, exercise tends towards
constancy, and so towards the _fixation_ of definite habits.

A reflex may come to be _attached_ to a new stimulus, that does not
naturally arouse it. A child who has accidentally been pricked with a
pin, and of course made the flexion reflex in response to this natural
stimulus, will make this same reaction to the sight of a pin
approaching his skin. The seen pin is a _substitute stimulus_ that
calls out the same response as the pin prick. This type of
modification gives a measure of control over the reflexes; for when we
pull the hand back voluntarily, or wink at will, or breathe deeply at
will, we are executing these movements without t