In Section 109 of Book Five of the Masnavi, Rumi explores the concept of 'Spiritual Metamorphosis' (مسخ دل). He explains that while past transgressing nations, such as the People of the Sabbath, experienced physical metamorphosis (مسخ بدن) into apes and swine as a divine punishment for breaking their covenants, the punishment for breaking repentance in the current era is internalized. This 'metamorphosis of the heart' signifies a spiritual degradation where a person's inner nature becomes animalistic, despite retaining a human exterior. Rumi further warns that on the Day of Resurrection, this hidden inner reality will be exposed, and the physical body will take on the form of the corrupted heart.