thumbsucking we may gather a great many points useful for the
distinguishing of an erogenous zone. It is a portion of skin or mucous
membrane in which the stimuli produce a feeling of pleasure of definite
quality. There is no doubt that the pleasure-producing stimuli are
governed by special determinants which we do not know. The rhythmic
characters must play some part in them and this strongly suggests an
analogy to tickling. It does not, however, appear so certain whether the
character of the pleasurable feeling evoked by the stimulus can be
designated as "peculiar," and in what part of this peculiarity the
sexual factor exists. Psychology is still groping in the dark when it
concerns matters of pleasure and pain, and the most cautious assumption
is therefore the most advisable. We may perhaps later come upon reasons
which seem to support the peculiar quality of the sensation of pleasure.

The erogenous quality may adhere most notably to definite regions of the
body. As is shown by the example of thumbsucking, there are predestined
erogenous zones. But the same example also shows that any other region
of skin or mucous membrane may assume the function of an erogenous zone;
it must therefore carry along a certain adaptability. The production of
the sensation of pleasure therefore depends more on the quality of the
stimulus than on the nature of the bodily region. The thumbsucking child
looks around on his body and selects any portion of it for
pleasure-sucking, and becoming accustomed to it, he then prefers it. If
he accidentally strikes upon a predestined region, such as breast,
nipple or genitals, it naturally has the preference. A quite analogous
tendency to displacement is again found in the symptomatology of
hysteria. In this neurosis the repression mostly concerns the genital
zones proper; these in turn transmit their excitation to the other
erogenous zones, usually dormant in mature life, which then behave
exactly like genitals. But besides this, just as in thumbsucking, any