The Soul Can Degrade into Lower Species.

The following is a verse from the Bhagavad-gita As It Is by His Divine Grace A.C.Bhaktivedanta Swami Prabhupada where Lord Krsna explains about the various bodies one attains if he is predominantly surcharged with the modes of passion and ignorance.

rajasi pralayam gatva

karma-sangisu jayate

tatha pralinas tamasi

mudha-yonisu jayate

When one dies in the mode of passion, he takes birth among those engaged in fruitive activities; and when he dies in the mode of ignorance, he takes birth in the animal kingdom.

PURPORT BY

HIS DIVINE GRACE A.C.BHAKTIVEDANTA SWAMI PRABHUPADA

Some people have the impression that when the soul reaches the platform of human life, it never goes down again. This is incorrect. According to this verse, if one develops the mode of ignorance, after his death he is degraded to the animal form of life. From there one has to again elevate himself, by evolutionary process, to come again to the human form of life. Therefore, those who are actually serious about human life should take to the mode of goodness and in good association transcend the modes and become situated in Krsna consciousness. This is the aim of human life. Otherwise, there is no guarantee that the human being will again attain to the human status.

(Bg 14.16 verse and purport)