d with the brake-stimulus, so that any occasion that
arouses the brake-response simultaneously arouses the clutch response.

[Illustration: Fig. 61.--Combining clutch-response with
brake-response. At first, the brake-stimulus has only a weak linkage
with the clutch-response, and an extra stimulus has to be found to
secure the clutch-response. But whenever the clutch-response is made
while the brake-stimulus is acting, the weak linkage between these two
is exercised, till finally the brake-stimulus is sufficient to give
the clutch-response, along with the brake-response.]

The combination of two responses is effected by linking both to the
same stimulus; thus the two become united into a cooerdinated higher
motor unit.

2. The word-habit in typewriting furnishes an example of _successive
cooerdination_, the uniting of a sequence of movements into a higher
unit. [Footnote: See p. 324.] The beginner has to spell out {412} the
word he is writing, and make a separate response to each letter; but
when he has well mastered the letter-habits, and, still unsatisfied,
is trying for more speed, it happens that he thinks ahead while
writing the first letter of a word, and _prepares_ for the second
letter. In effect, he commences reacting to the second letter while
still writing the first. This goes further, till he anticipates the
series of letters forming a short word while still at the beginning of
the word. The letter movements are thus linked to the thought of the
word as a whole, and the word becomes an effective stimulus for
arousing the series of letter movements.

[Illustration: Fig. 62.--Learning a word-habit in typewriting. At
first, besides the stimulus of the word, "_and_" it is necessary also
to have the stimulus "_a_" in order to arouse the response of writing
a, the stimulus "_n_" in order to arouse the writing of n, and the
stimulus "_d_" in order to arouse the writing of d. Yet the stimulus
"_and_" is present all this time, and its weak linkages with the
writing movements are used and strengthened, so that finally it is
sufficient, by itself, to arouse the whole series of writing
movements.]

Many other instances of learning can be worked out in the same way,
and there seems to be no difficulty in {413} interpreting any of them
by the law of combination. Even "negative adaptation" can possibly be
interpreted as an instance of substitute response; some slight and
easy response may be substituted for