Symbols of Transformation (Collected Works of C.G. Jung, Volume 5) by C. G. Jung
So long as the child is in that state of unconscious identity with the mother, he is still one with the animal psyche and is just as unconscious as it.
The development of consciousness inevitably leads not only to separation from the mother, but to separation from the parents and the whole family circle and thus to a relative degree of detachment from the unconscious and the world of instinct.
Yet the longing for this lost world continues and, when diﬃcult adaptations are demanded, is forever tempt- ing one to make evasions and retreats, to regress to the infantile, which then starts throwing up the incestuous symbolism. Carl Jung, CW 5, Para 351