Bhagavad Gita 14.18ऊर्ध्वं गच्छन्ति सत्त्वस्था मध्ये तिष्ठन्ति राजसाः।जघन्यगुणवृत्तिस्था अधो गच्छन्ति तामसाः ॥

Those who abide in Sattva become the lords of heaven after giving up the physical body. The Rajasic are rorn on this earth as human beings. The Tamasic go downwards, i.e., they will be born in the wormbs of cattle and beasts. They may take their birth amongst the lowest grades of human beings. The lowest grades of human beings are only brutes though they have assumed human form. Their actions are brutal. Therefore it is not necessary for them to enter into animal incarnation.Man identifies himself with Nature on account of the force of ignorance or illusory knowledge and gets attached to the alities of Nature. This is the cause of his birth in the wombs of high or low creatures. He feels, I am happy, miserable or deluded, on account of the attachment to the Gunas.The nature of the Gunas, their functions, how they bind a man to the Samsara, the effects of each Guna when it is predominant, and the plane reached by the man when he is under the influence of a particular Guna are described in the previous verses. Now the Lord describes in the following verse that liberation comes when one knows Him Who is above the three Gunas.