a squirrel is different from a cat, while the word
"kitty" is a response to the points of resemblance.

But response by analogy is not always so childish or comic as the
above examples might seem to imply. When we respond to a picture by
recognizing the objects depicted, that is response by analogy, since
the pictured object is only {407} partially like the real object; a
bare outline drawing may be enough to arouse the response of "seeing"
the object. Other instances of response by analogy will come to light
when, in the next chapter, we come to the study of perception.

[Illustration: Fig. 58.--Response by analogy. The letters, A, B, X,
Y, represent the several stimuli that make up the original object, and
each of them becomes well linked with their common response (seeing
the object, and perhaps naming it). When the linkage between X and Y
and the response has become strong, a similar object, presenting X and
Y along with other new stimuli, C and D, appears, and arouses the old
response, by virtue of the now-effective linkage from X and Y to this
response.]

[Illustration: Fig. 59.--Association by similarity. Everything here as
in the previous diagram, except that C and D get a response in
addition to that aroused by X and Y, and so the new object is seen to
be new, while at the same time it recalls the old object to mind.]

The machinery of response by analogy is easily understood by aid of
the law of combination. A complex object, presenting a number of parts
and characteristics, arouses the response of seeing and perhaps naming
the object. This is a unitary response to a collection of stimuli, and
each of the parts or characteristics of the object participates in
arousing the response, and the linkage of each part with the response
is thus strengthened. Later, therefore, the whole identical object is
not required to arouse this same {408} response, but some of its parts
or characteristics will give the response, and they may do this even
when they are present in an object that has other and unfamiliar parts
and characteristics.

The machinery of association by similarity is the same, with the
addition of a second response, called out by the new characteristics
of the present object.

II. SUBSTITUTE RESPONSE EXPLAINED BY THE LAW OF COMBINATION

The substitute response machinery is more complicated than that of the
substitute stimulus, as it includes the latter and something more.
What that somethin