Chapter 2) The Manifestations Of Infantile Sexuality of Three Essays on the Theory of Sexuality - Study Guide by Sigmund Freud

2) The Manifestations Of Infantile Sexuality

Thumb-sucking

Freud nominates this as manifestation due to both its rhythmic characteristics, and the incorporation of the mouth.

Auto-erotism

In sucking rhythmically at some part of the skin or mucous membrane, as in thumb-sucking the child is seeking to renew the pleasure first felt sucking at his mother's breast.

So it seems that infantile sexual manifestations have three essential characteristics: at their origin they attach themselves to one of the vital somatic functions, they have as yet, no sexual object - they are auto-erotic, and their sexual aim is dominated by an erotogenic zone.